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Investigation of arc processes: Relationships among deformation, magmatism, mountain building, and the role of crustal anisotropy in the evolution of the Peninsular Ranges Batholith, Baja California

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Investigation of arc processes: Relationships among deformation, magmatism, mountain building, and the role of crustal anisotropy in the evolution of the Peninsular Ranges Batholith, Baja California

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Content INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. 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Further reproduction prohibited without permission. INVESTIGATION OF ARC PROCESSES: RELATIONSHIPS AMONG DEFORMATION, MAGMATISM, MOUNTAIN BUILDING, AND THE ROLE OF CRUSTAL ANISOTROPY IN THE EVOLUTION OF THE PENINSULAR RANGES BATHOLITH, BAJA CALIFORNIA by Keegan Lee Schmidt A Dissertation Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (EARTH SCIENCES) December 2000 Copyright 2000 Keegan Lee Schmidt Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number 3041518 UMI* UMI Microform 3041518 Copyright 2002 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Mi 48106-1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UNIVERSITY OF SOUTHERN CALIFORNIA fH E GRADUATE SCHOOL UNIVERSITY PARK LOS ANGELES. CALIFORNIA 90007 This dissertation, written by ................................ under the direction of fe s Dissertation Committee, and approved by all its members, has been presented to and accepted by The Graduate School, in partial fulfillment of requirements for the degree of DOCTOR OF PHILOSOPHY Dean of Graduate Date ...P£cen^r>#18A->2000 DISSERTATION COMMITTEE Chairperson Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ii ACKNOWLEDGEMENTS This work was funded by the National Science Foundation (grant EAR9614682 to Scott Paterson), and the following grants in support of graduate student research: the Geological Society of America (grants 5706-95 and 5926-96); Sigma Xi, the Scientific Research Society; and two USC Earth Science Department grants. Td like to begin by thanking my patient and persistent advisor Scott Paterson, without who’s advising skills this study may well have never been completed. I appreciate his constant encouragement and mentorship and have enjoyed the camaraderie that he fosters in the structure group at USC. I’d also like to acknowledge the rest of my committee, Greg Davis, Doug Sherman, Jean Morrison, and Lawford Anderson, who, together with Scott, formed a solid foundation for my education at USC. I am grateful to have had the opportunity to see some of the geology of the Mojave with Greg. Chris Kopf kindly performed the Al-in-homblende analyses in this study. I appreciate his enthusiasm and expertise in the field, working-out careful metamorphic mineralogical constraints for my unbridled interpretations. I was endlessly entertained by his wonderful stories accompanied by ‘Reserva Del Patron’ around the campfire at night. Ann Blythe performed fission track analyses at USC and was a valuable source for advice on the geochronological aspects of this study. Marty Grove at UCLA patiently tolerated my bumbling in Mark Harrison’s Ar lab and was always willing to Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. discuss technicalities of the lab, *°Ar/i9Ar methods, and PRB geology. Oscar Lovera endured countless emails regarding his modeling programs. Many thanks are due to the San Diego State University tectonics group, particularly Gordon Gastil and his former students Chris Goetz and Mike Measures, for invaluable discussions and willingness to share their knowledge of the Sierra San Pedro Martir. Gary Girty was a continual source of advice, enthusiasm, and knowledge regarding age interpretations and the structural evolution of the PRB. Melissa Girty kindly provided U-Pb age analyses including re-analysis of a sample previously collected by their group. Thanks to Scott Johnson for many lengthy discussions regarding along-strike relationships in the PRB transition zone. I’m pleased to have had the privilege of experiencing a legendary “Scott J. Camp” in full operation. I also thank John Fletcher at CICESE for his hospitality and numerous discussions on Baja geology. I am grateful to the many ranchers whom I was privileged to meet in the Sierra San Pedro Martir. A life working harsh and lonely land that belongs to someone else isn’t easy, and I appreciate the open hospitality and light-heartedness that these resilient people show so remarkably well. Manuel at La Suerte provided me with valuable logistical advice, and on numerous mornings I enjoyed his company and coffee on his porch before the day’s traverse. He also kindly lent his expertise and mules to pack a spike camp for me in Arroyo Los Corrales. Many thanks are due to friends at USC who made life in L.A. enjoyable. Aaron Yoshinobu has been a constant inspiration with his boundless enthusiasm for geology Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and the simplest things in life. Celeste cooks the most incredible caldo de polio on this planet Other friends who added support and good humor include Ken, Nicole, Andrew, Sarah, Brian, Kristi, Paul, Geoff, Michael, Gunilla, and Markus. I thank my parents, Dwight and Carol for their unfaltering support and encouragement all the way through my education. Many thanks also go to my brother Lyman, with whom I have shared adventures, and who accompanied me on the 3-day hike across the Sierra San Pedro Martir that launched this study. And to Karen, thank you. All other pursuits pale in comparison to your love and friendship. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. V TABLE OF CONTENTS ACKNOWLEDGEMENTS................................................................................. ii LIST OF FIGURES..............................................................................................viii LIST OF TABLES................................................................................................xi ABSTRACT.........................................................................................................xii CHAPTER I: INTRODUCTION.........................................................................I Processes in arcs.............................................................................................3 The Peninsular Ranges batholith (PRB)...........................................................11 Overview of dissertation.................................................................................17 CHAPTER 2: A DOUBLY VERGENT, FAN STRUCTURE IN THE PENINSULAR RANGES BATHOLITH: TRANSPRESSION OR COMPLEX FLOW ALONG A CRUSTAL-SCALE DISCONTINUITY?........ 20 Introduction....................................................................................................20 Background on fan structures....................................................................22 Tectonic overview of Peninsular Ranges batholith (PRB)............................... 25 Fan structure in the southern SSPM segment of the PRB deformation b elt 30 Western footwall.......................................................................................33 Western domain.........................................................................................37 Central domain...........................................................................................42 Eastern domain...........................................................................................49 Eastern footwall.........................................................................................52 Folding of assemblages in the eastern domain and footwall...................... 54 Metamorphic and pressure/temperature constraints....................................56 Discussion.......................................................................................................62 Timing relationships and duration of deformation......................................63 Effects of inherited crustal heterogeneity...................................................69 Effects of magmatism on deformation.......................................................74 Partitioning of deformation in the fan structure..........................................78 Evolution of fan structure in the southern SSPM.......................................81 Conclusions....................................................................................................86 CHAPTER 3: THE ROLE OF CRUSTAL HETEROGENEITY IN DIFFERENTIAL EXHUMATION OF THE CENTRAL PENINSULAR RANGES BATHOLITH....................................................................................88 Introduction....................................................................................................88 Processes driving exhumation....................................................................91 Post-Early Cretaceous evolution of the PRB....................................................96 Sampling and analytical methods....................................................................98 Zircon U-Pb geochronology results.................................................................104 U-Pb TIMS...............................................................................................104 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. vi U-Pb SHRIMP............................................................................................109 Thermal history results......................................................................................Ill Biotite 40Ar/39Ar..........................................................................................112 K-feldspar^Ar/^Ar....................................................................................112 Apatite fission track.....................................................................................113 Integrated cooling history..................................................................................118 Discussion.........................................................................................................125 Cooling mechanisms...................................................................................125 Exhumation history.....................................................................................131 Exhumation mechanisms.............................................................................138 Conclusions......................................................................................................150 CHAPTER 4: TECTONIC SYNTHESIS OF THE MESOZOIC PENINSULAR RANGES BATHOLITH.............................................................153 Introduction .....................................................................................................153 Overview of the SW margin of North America........................................... 153 Tectonic models for the Peninsular Ranges batholith........................................ 157 Review of Peninsular Ranges batholith geology............................................... 160 Overview of the PRB................................................................................. 162 Geochemistry and petrogenesis.............................................................165 Geophysical studies..............................................................................168 Geochronology.....................................................................................170 Metamorphic studies..............................................................................176 Eastern zone......................................................................................................178 Stratigraphy.................................................................................................178 Magmatism.................................................................................................179 Deformation................................................................................................180 Cooling history............................................................................................182 Transitional zone...............................................................................................184 Stratigraphy.................................................................................................184 Magmatism.................................................................................................186 Deformation................................................................................................189 Cooling history............................................................................................192 Western zone....................................................................................................193 Stratigraphy................................................................................................ 193 Magmatism................................................................................................ 195 Deformation................................................................................................197 Boundaries between zones in the batholith........................................................199 Boundary between eastern and transitional zones.......................................200 Boundary between transitional and western zones......................................202 Summary and discussion..................................................................................204 Implications for existing tectonic models of the Peninsular Ranges batholith.................................................................................................. 204 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. vii Tectonic overview- a hypothesis for evolution of the Peninsular Ranges batholith...................................................................................... 208 Pre-Middle Triassic (>-241 Ma).......................................................... 208 Middle Triassic-Middle Jurassic (-241-164 Ma)..................................214 Middle Jurassic-Early Cretaceous (-164-132 M a)...............................214 Early- to mid-Cretaceous (-132-100 Ma)............................................. 216 mid- to Late Cretaceous (-100-65 Ma)................................................. 217 Conclusions..................................................................................................... 219 CHAPTER 5: CONCLUSIONS............................................................................ 221 Fan structure in the Peninsular Ranges batholith transition zone...................... 227 Shortening mechanisms..............................................................................229 Magmatism................................................................................................ 234 Exhumation history.................................................................................... 236 Jura-Cretaceous tectonic evolution of the Peninsular Ranges batholith.............238 Future directions.............................................................................................. 240 Western zone of the Peninsular Ranges batholith........................................240 Eastern zone of the Peninsular Ranges batholith.........................................243 Lithospheric evolution of the Peninsular Ranges batholith..........................244 REFERENCES CITED.......................................................................................... 246 APPENDIX A: ARE PROCESSES OF DEFORMATION AND MAGMATISM COUPLED? INSIGHTS FROM SPATIAL AND GEOMETRICAL ANALYSIS............................................................................272 Results of spatial and geometrical analyses...................................................... 276 Discussion and conclusions..............................................................................282 APPENDIX B: U-PB ZIRCON DATA................................................................. 284 Standard U-Pb isotope dilution methods (San Diego State University).............284 U-Pb zircon SHRIMP methods (Australian National University and Stanford University)...................................................................................... 287 APPENDIX C: 40Ar/39Ar BIOTITE AND K-FELDSPAR ANALYSIS................ 291 APPENDIX D: APATITE FISSION TRACK ANALYSIS...................................300 APPENDIX E: STRAIN ANALYSIS................................................................... 303 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. viii LIST OF FIGURES Figure 1.1 Series of diagrams depicting major processes that occur in continental margin arcs................................................................................... 4 Figure 1.2 Location and modem tectonic setting of the PRB................................12 Figure 1.3 Physiography of the region surrounding the Sierra San Pedro Martir...............................................................................................................15 Figure 2.1. Examples of doubly vergent fan structures......................................... 23 Figure 2.2. Map of Peninsular Ranges showing tectonostratigraphic assemblages.................................................................................................... 26 Figure 2.3. Generalized geologic map across the PRB.......................................... 31 Figure 2.4. Geologic map of study area in the southern Sierra San Pedro Martir.............................................................................................................. 32 Figure 2.5. Cross-section of study area. Equal area stereoplots of selected structural domains across fan structure............................................................35 Figure 2.6. Micrographs of orthogneiss from central domain................................45 Figure 2.7. Micrographs with crossed Nicols of hornblende tonalite from the sheeted complex.............................................................................................. 45 Figure 2.8. Down-plunge view of map-scale folds that deform both hangingwall and footwall in the eastern domain.............................................. 55 Figure 2.9. Metamorphic mineral assemblages and their location in a petrogenetic grid of the KFMASH system....................................................... 60 Figure 2.10. Diagram of timing relationships across the fan structure...................64 Figure 2.11. Block diagrams showing hypothetical structural evolution of the fan structure.................................................................................................... 82 Figure 3.1. Examples of processes in arc that may contribute to exhumation.........93 Figure 3.2. Generalized geologic map across the PRB at the latitude of the Sierra San Pedro Martir................................................................................... 99 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.3. Geologic map and cross-section of study area in the southern Sierra San Pedro Martir.....................................................................................101 Figure 3.4. Tera and Wasserburg (1972) U-Pb zircon concordia curves................. 105 Figure 3.5. Apatite fission track modeling results showing thermal histories..........114 Figure 3.6. Summary cooling history diagrams.......................................................120 Figure 3.7. Fission track ages and track lengths as function of elevation................. 127 Figure 3.8. Cross-section through PRB transition zone in the southern SSPM showing isosurfaces at various ages...................................................................132 Figure 3.9. Diagram showing compiled geophysical constraints across Baja California at -30.5° N latitude...........................................................................139 Figure 3.10. Isostacy models for simplified western and eastern PRB crust............ 141 Figure 3.11. Summary of possible central PRB uplift history..................................149 Figure 4.1 Map of western North America showing major Mesozoic Cordilleran batholiths........................................................................................154 Figure 4.2 Tectonic models suggested for the Mesozoic evolution of the PRB 158 Figure 4.3 Map of the Peninsular Ranges batholith showing the magnetiteilmenite line, Rare Earth Element belts, selected isopleths for initial ®7Sr/86Sr and 5 ,8Osmow and the gabbro/tonalite line...........................................163 Figure 4.4 Map of Peninsular Ranges batholith showing tectonostratigraphic assemblages......................................................................................................164 Figure 4.5 Very generalized, composite, crustal scale cross-section through the central PRB at ~30.5 °N latitude..................................................................169 Figure 4.6 Series of maps of the PRB showing major stratigraphic and structural relationships within the batholith.......................................................173 Figure 4.7 Tectonic models suggested for the PRB with constraints determined in this synthesis.............................................................................. 206 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.8 Series of time slices and generalized tectonic features for each time period....................................................................................................... 209 Figure 5.1 Simplified crustal cross-section through the PRB from Pacific to Gulf of California coasts across the Baja California peninsula at ~30.5° N .......222 Figure 5.2 Simplified end-member models for shortening at the lithosphericscale across the Mesozoic PRB margin............................................................. 230 Figure 5.3 A possible scenario for lithospheric-scale evolution of the PRB margin. See text for explanation....................................................................... 233 Figure A.1 Computer generated simulations of fault and pluton distributions illustrating problems with visual inspection of spatial relationships..................275 Figure A.2 Columns across the top: (a) maps of regions, (b) results of spatial analyses, (c) results of geometrical analyses for the following five orogens (listed down left side): (1) Armorican Massif; (2) Alleghenian Orogeny; (3) British Caledonides; (4) Maine Caledonides; and (5) Borborema Province........................................................................................................... 277 Figure A.3 Plots showing relationship between pluton length/width ratio and angle between pluton long axis and nearest fault (a and b), and relationship between pluton length/width ratio and distance to nearest fault (candd)........................................................................................................... 281 Figure E.l. Map of study area showing sample locations.......................................304 Figure E.2. Strain intensity and Lode’s parameter plotted against distance............308 Plate 1: Geologic map of the Southern Sierra San Pedro Martir............................ .......................................................................................................in back pocket Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES Table 2.1. Summary of plagioclase hornblende thermometry and Al-inhomblende barometry for plutons in the fan-like structure.............................. 58 Table 3.1 Summary of geochronology in the southern SSPM................................. 102 Table B.l U-Pb zircon TIMS analyses...................................................................286 Table B.2 U-Pb zircon SHRIMP analyses for orthogneiss.....................................289 Table B3 U-Pb zircon SHRIMP analyses for hornblende tonalite.........................290 Table C.l. Biotite ^Ar/^A isotopic measurements...............................................292 Table C.2. K-feldspar ^Ar/^A isotopic measurements..........................................297 Table D.l Apatite Fission Track Analyses............................................................301 Table E.l Results from strain analyses in the western domain of the fan structure..........................................................................................................307 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. xii ABSTRACT Detailed structural and geochronological studies of a spectacularly exposed region of Baja California have revealed the structural evolution and links between deformation, magmatism, and mountain building processes at mid- to upper crustal levels of a part of the Mesozoic Cordilleran magmatic arc system. The JuraCretaceous Peninsular Ranges batholith (PRB) of southern and Baja California contains one of the most remarkable examples of a crustal transition between oceanicand continental-floored arcs in the world. Within this transition is an enigmatic belt of contractional deformation that stretches at least 800 km along the batholith and corresponds with a dramatic eastward increase in exposed crustal depths. In the Sierra San Pedro Martir of northern Baja California a spectacular -20 km-wide, doubly vergent fan structure occurs across the PRB basement transition. This structure formed over a period of >40 m.y. during intrusion of the batholith and consists of mylonitic, inward-dipping marginal thrust sheets that gradually change to a steeply-dipping tectonized zone in the fan center that displays both mylonitic and magmatic fabrics. More than 15 km of differential exhumation occurred across the western side of the fan structure in Late Cretaceous time. The mechanical effects of inherited crustal heterogeneity, including the basement transition zone in the batholith and a buttress of more rigid crust to the east, played a major role in the evolution of the composite, largely contractional, fan structure. Intrusion of an extensive, sheeted plutonic complex in the center of the fan created rheological contrasts within the arc that led to partitioning of deformation for a Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. xiii limited period of time. Variation in crustal structure across the PRB also determined the extent and dynamics of uplift and denudation during mid- to Late Cretaceous evolution of the batholith, possibly including an Andean-style orogenic plateau across its eastern half. A portion of the western side of the batholith in this region appears to have collided with the North American margin in late Early Cretaceous time, but the effects of this event on the margin were masked by subsequent orogenesis that affected much of the southern Cordillera. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 CHAPTER 1: INTRODUCTION Arcs are one of the most geologically dynamic and diverse tectonic settings on Earth. They provide the major mechanism of continental growth in the Phanerozoic by means of lateral addition of accretionary prisms and pre-existing terranes at subduction zones and addition of nascent crustal material derived from underlying mantle by magmatic processes (e.g. Scholl et al., 1986; Stevenson, 1995). They also form the most extensive subaerial mountain belts in the world and encompass tremendous topographic variation from low standing regions, that are in some cases below sea level, to high relief mountain systems such as the central Andes that reach elevations comparable with those in continental collisional environments. Deformation is an integral process in these convergent plate boundary settings, and arc evolution may include contractional, extentional, as well as strike slip tectonics. Arcs are commonly the sites of complex partitioning of deformation that results from oblique plate motions. Structural complexity is inherent to continental arc systems for two reasons. First magmatism creates strong and highly transient rheological contrasts in crust due to the presence of melt and advection of heat Second continental margin convergent plate boundaries invariably develop at the site of previous plate boundaries and thus inherit crustal heterogeneity. Moreover, under certain conditions deformation may be strongly linked to other orogenic processes such as exhumation, magmatism, and even climate. Examining structural aspects of arcs and how deformation couples Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. with other orogenic processes is therefore crucial towards an increased understanding of processes of mountain building and crustal evolution. One of the most extensive arc systems in the world developed along the western Cordilleran margin of North America during the past 200 m.y. (million years). The Jura-Cretaceous Peninsular Ranges batholith (PRB) of southern and Baja California forms a >800 km-long segment in a series of batholiths in what was once a vast Andean-type arc system (Burchfiel and Davis, 1972; Lipman, 1992). The PRB is an excellent orogen in which to examine arc processes. Ancient crustal levels now exposed in this batholith vary from near-surface to > 20 km, permitting examination of processes at multiple levels in the batholith. The plutonic system in the PRB is spectacularly exposed, and a long history of deformation can be remarkably well constrained. The mechanical influences of both magmatism and inherited crustal heterogeneity are apparent in the central part of the batholith where deformation has been focused in a 20-40 km-wide zone during arc magmatism. In the following sections of this chapter I introduce processes that occur in arcs in the context of mass balance interrelations and discuss issues that bear on the mechanics of arc processes. Next I describe the Peninsular Ranges batholith from the standpoint of an ideal ancient arc in which to examine these processes and introduce the study area in the southern Sierra San Pedro Martir of Baja California. I conclude this chapter with an overview of the dissertation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 Processes in arcs Oceanic lithosphere subducts beneath continental lithosphere along a continental margin in the simplified view of a continental margin arc shown in Figure 1.1a. Coupling between the two plates results in deformation that may occur at a variety of locations within the subduction zone, and doubly vergent structures with fan-like cross-sectional geometry are common as discussed below. Magmatism arising from melt production during the subduction process is a significant source of mass and heat transfer within the arc. Typically a semi-continuous belt of high topography develops as a consequence of crustal thickening both by structural and magmatic means, thermal buoyancy induced by magmatism in the arc, and the construction of volcanic edifices. Erosional and tectonic exhumation of these mountain belts results in exposure of crustal sections as deep as lower crustal levels in some examples. The scenario described above is grossly oversimplified because a vast number of complications and feedback mechanisms among processes contribute to the great diversity of arcs around the world. I therefore limit the discussion below to a few of the major processes that occur in most arcs as depicted in Figure 1.1b (Beaumont et al., 1996). This diagram illustrates relationships among orogenic processes in terms of a 2-dimentional material balance problem. Crustal mass is added from the left of the diagram as a very simplified representation of material scraped or accreted from the subducting slab into the accretionary prism of a coupled subduction zone (Royden, 1993). The superimposed strain grid illustrates deformation and thickening of the orogenic zone that occurs during this deformation. In a doubly vergent orogenic belt Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 Figure 1.1 Series of diagrams depicting major processes that occur in continental margin arcs. A) Generalized crustal cross-section across continental margin subduction zone showing arc magmatism and deformation. B) View of the arc from mass balance perspective. Italicized text and thick arrows refer to additions and subtractions from system represented by crustal wedge bound by prowedge and retrowedge. Grid is a general representation of finite strain; thin arrows indicate material velocity. Modified from Willett et al. (1993) and Beaumont et al. (1996). See text for description. C) Series of cartoons depicting problems and questions among processes in arcs that are addressed in this thesis. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 4 Surface uplift denudatio traction afcfng base growth of crustal root plutonism subduction of lower crustal material magmatism and deformation -temporal -spatial -geometrical -mechanical Influence of crustal heterooeneitv on deformation and exhumation •crustal boundaries -magmatism Linkages between surface and deeper crustal levels •discrete faults? -distributed deformation? -crustal flexure? Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6 Material may also enter from the right, and in a transpressional orogen there may be a net gain or loss of material into/out of the section, parallel to the plate margin. Both of these complications considerably affect the geometry of topographic expression and exhumation of the arc (Koons, 1990; 1994). An additional and significant component of added mass and heat in subduction zones is derived from material melted in the mantle that is transported into the overlying arc by magmatic processes (e.g. Ague and Brimhall, 1988). Added crustal mass is accommodated by several processes within an orogenic belt. Thickening occurs by deformation of the crust, typically involving ductile flow of the lower crust and brittle faulting of the upper crust, and depends on the internal strength of crust which is predominantly a function of composition and temperature distribution (Beaumont et al., 1996). Crustal thickening results in any of the following (Fig. 1 .lb): (1) growth of a crustal root by displacing mantle; and/or (2) growth of topography either by surface elevation or depression of the surface adjacent to the orogen by formation of sedimentary basins. Mechanisms of mass removal in orogenic belts include: (3) erosional or tectonic denudation, whereby material is stripped from high-standing regions that correspond with thickened crust and redistributed to regions of lower elevation by erosion, or lateral spreading of thickened crust by normal faulting; (4) subduction of lower crustal material; and/or (5) delamination of the base of overly-thickened lithosphere. Many of these processes are complexly linked. For example coupling may occur between deformation and erosion by a feedback mechanism in which crustal shortening by contractional deformation thickens crust, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 leading to surface uplift, while erosion, enhanced by the development of topography, decreases gravitational stresses by removing crust, leading to increased shortening (Willett, 1999). Both the rheology structure and presence of crustal anisotropy at a variety of scales in an arc are important factors in determining how, and to what extent, mass transfer processes discussed above are linked. Thus a useful form in which to cast discussion of orogenic processes is in the context of how they contribute to, and/or are affected by, the mechanical character of orogenic zones. Crustal rheology is largely determined by the thermal configuration of crust, and temperatures in orogenic regions are anomalously high as compared with temperatures at comparable depths in adjacent crust. For example in the Ryukyu orogen in Japan at a depth of 20 km in the arc temperatures are about 700-800 °C but only 100-150 °C at comparable depths in the forearc (Uyeda and Horai, 1964), suggesting that overall strength of the crust at a given crustal level across an orogen varies by a factor of at least 10 (Jarrard, 1986). Magmatism in arcs such as the Ryukyu plate boundary plays a major role in this dramatic temperature variation. Heat is advected into the crust very efficiently by magmatic processes, at timescales on the order of 103 - 106 years (e.g. Jeager, 1968; Barton et al., 1988; Paterson and Tobisch, 1992), and can thus heat entire orogenic belts over the course of very short intervals. In fact, Barton et al. (1988) have suggested that the heat for metamorphism in most of the U.S. Cordilleran orogen was derived directly or indirectly from plutons. Emplacement of plutons may thus trigger Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8 deformation across broad regions as a result of thermal weakening of the crust (e.g. Hollister and Crawford, 1986; Collins and Vemon, 1992; Sandiford et al., 1992). Magmatism influences deformation in other ways at the scale of individual pluton complexes. The presence of melt results in strong and highly transient mechanical contrasts within the crust that produce spatially and temporally varying strain localization within magma-host rock systems (Miller et al., 1988; Karlstrom et al., 1993; Pavlis, 1996). Indeed magma viscosity varies by ~20 orders of magnitude during the interval from intrusion to crystallization (Cruden, 1990; Bergantz. 1991), resulting in tremendous rheological gradients in country rocks and the potential for strong partitioning of deformation. An alternative and popularly supported notion is that the magma-host rock relationships observed in field settings result from deformation itself providing space for rising plutons (e.g. Brun and Pons, 1981; Hutton, 1982; Soula, 1982; Guineberteau et al., 1987), or even providing a mechanism for the generation and ascent of melt (e.g. Strong and Hanmer, 1981; Hutton, 1988). Anomalously high temperatures also occur in active orogenic regions that lack significant magmatism, mainly as a result of the redistribution of radiogenic elements within structurally thickening crust coupled with the effects of surface erosion and accretion of continental crustal material from subducting plates (e.g. Huerta et al., 1996). This effect may be enhanced in arc regions because plutonic processes, independently of heat advection, can also redistribute radiogenic elements within the crust Ultimately the consequence of sustained anomalously high temperatures in thickened crust is weakening and lateral flow of lower crustal rocks (e.g. Royden, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 1996). Thus crustal mass is redistributed laterally; an effect that serves to diminish gravitational instability that may develop in overthickened regions, as well as widen the orogenic belt Broad regions of high average topography such as the Tibetan and Altiplano-Puna plateaus result from such processes. Deformation fronts may advance concomitantly with lower crustal flow in some cases as suggested for the Andean and Colorado plateaus (Pope and Willett, 1998; McQuarrie and Chase, 2000), or upper crustal shortening may not accompany lower crustal flow in other cases such as the Tibetan plateau because upper crust is strongly decoupled from lower crust (Royden et al., 1997). Thus far, 1 have focused discussion of orogenic processes around evolving crustal rheology during orogenesis and the resulting links among processes. However, an entirely separate and equally significant class of crustal heterogeneity that can influence arc processes occurs as a consequence of inherited crustal structure. Continental arcs are typically constructed along plate boundaries that have experienced an extensive tectonic history that commonly spans numerous changes in tectonic regime. The crustal heterogeneity inherited by subsequent orogenesis includes strong lithological contrasts between juxtaposed crustal blocks, fault systems, batholith-country rock contacts, and the margins of sedimentary basins. All of these features may serve to focus subsequent deformation, and thus have a profound influence on its geometry, leading to long-lived complex deformation systems such as the Mesozoic Western Idaho suture zone that accommodated continued contractional deformation along the western margin of the Idaho batholith well into the Cretaceous, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10 and long after Jurassic suturing was complete (Lund and Snee, 1988; Manduca et al.. 1993). Moreover, along-strike variation in crustal heterogeneity may lead to further contrasts in the evolution of an orogen. For example in the central Andean orogen the distribution of Paleozoic basins in the foreland of the orogen appears to have had a profound effect on deformation and exhumation in the adjacent arc (Allmendinger and Gubbels, 1996). In the northern part of the orogen the presence of a thick wedgeshaped Paleozoic basin has lead to underthrusting of the arc from the east and uplift of the Altiplano plateau since -10 Ma (millions of years ago). In contrast the southern part of the orogen, where thick Paleozoic basins are absent in the foreland, has largely thickened by pure shear over a similar period of time, contributing to uplift of the Puna plateau. A number of unresolved issues related to linkages among orogenic processes and the mechanical influences of crustal heterogeneity on the evolution of arcs remain (Fig. 1.1c). For example, what effect do inherited boundaries between compositionally and structurally distinct crustal blocks have on the character of deformation that develops within an orogen as well as exhumation processes that occur? Most of the numerical models that have been developed to understand deformation within orogens begin with homogeneous or very simplistic initial material conditions, and rheological contrasts develop in the models as they evolve from these simple starting conditions. None of these models incorporates the incredibly complex magmatic processes that characterize the majority of orogenic systems around the world. Thus a related question is what are the relationships between deformation and magmatism from the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 i standpoint of spatial, geometrical, temporal, and mechanical associations? Finally exhumation processes are comparatively well understood at upper crustal levels relative to mid- to lower levels, but what linkages connect coupled processes such as erosion and deformation at near-surface levels with processes in the deep crust? For instance is exhumation of the middle crust accomplished by uplift on discrete structures or does distributed deformation or crustal flexure prevail at these levels? In this dissertation I describe research that addresses some of these problems in a region of the Peninsular Ranges batholith of Baja California, Mexico that I introduce below. The Peninsular Ranges batholith (PRB) The Jura-Cretaceous PRB forms a >800 km-long segment of the once relatively continuous Mesozoic Cordilleran batholith along the western margin of North America (Fig. 12). The PRB is particularly fascinating because it consists of oceanic and continental floored arcs that are juxtaposed along a very well-defined crustal transition. The transition zone extends the known length of the PRB, from southern California through northern Baja California, and is the site of a number of distinctive features in the batholith including a sharp chemical transition in plutons, a series of elongate flysch basins, and a belt of persistent contractional deformation spanning >40 m.y. that overlapped with more than 60 m.y. of arc magmatism. A number of different ideas have been proposed for the origin of two juxtaposed arcs in the PRB including construction of the batholith across a pre-Mesozoic oceancontinental crust transition, formation and collapse of a marginal basin, and collision Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 12 Outcrop limit of the Mesozoic PRB San Qumon Gulf of California •Guzymas Lot Moctua c o a a r aaKoi MAJOR MESOZOIC BATHOUTHS OF WESTERN NORTH « AMERICA 11 Study S9AH0 M azatian U nitad Statai Figure 1.2 Location and modem tectonic setting of die PRB. Inset shows locations of major Mesozoic batholiths of western North America. The Jura-Cretaceous PRB is the largest segment of a magmatic arc that was once continuous throughout California. It extends for >1600 km from Riverside, California, USA to the southern tip of Baja California, Mexico. The northern 800 km are well exposed, particularly in Baja California Norte, whereas die southern 800 km are largely covered by Cenozoic rocks. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 3 of an exotic western PRB island arc complex. Associated with the crustal discontinuity in the batholith is striking variation in crustal exposures with near-surface crustal levels preserved in the western zone of the batholith and mid-crustal levels exposed across the eastern zone. Thick conglomeratic successions deposited along the west coast of Baja California record the sudden onset of denudation that corresponded with uplift of the eastern zone in mid-Cretaceous time. Like other Mesozoic Cordilleran batholiths the PRB was constructed on the Precambrian-Paleozoic passive margin of western North America that was initiated following Late Proterozoic rifting (Stewart, 1972). The southern half of the batholith intruded crust that, according to some tectonic models, formed the Paleozoic southern margin of North America, which extended across present northern Mexico (e.g. Stewart, 1988; Dickinson, 2000). Arc magmatism that dominated this part of the Cordillera in Jura-Cretaceous time was followed by relative quiescence as the arc shifted eastwards in the early Tertiary. The arc briefly returned to this region in the Early Miocene, and tectonics since that time have been dominated by extension that has culminated in rifting and transfer of the Baja Peninsula to the Pacific plate (e.g. Stock and Hodges, 1989). The modem tectonic setting of Baja California is shown in Figure 1.2. Extinct Tertiary trenches are preserved in the Cedros and Ulloa deeps off the Pacific coast of the peninsula. The eastern side of the peninsula is dominated by extensional tectonics associated with transtensional rifting in the Gulf of California. This is a highly oblique, northward-propagating continental rift that accommodates considerable Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14 dextrai relative plate motion. It is strongly asymmetrical, consisting of a relatively narrow western side located along the eastern side of Peninsular California and a broad eastern side that extends well into mainland Mexico and merges with the southern Basin and Range tectonic province to the north (e.g. Henry, 1989). The rift system links into the San Andreas transform margin to the north and East Pacific Rise to the south. The present physiography of Baja California is dominated by its recent extensional tectonic history. The main topographic feature is a series of mountain ranges that form a central spine down the peninsula with gently sloping western flanks and sharp escarpments along their eastern flanks (Figure 1.3). From the Pacific Coast, a coastal plain underlain by Cretaceous to Recent sedimentary rocks on western PRB basement rises gently to the east The western flank of the SSPM is marked by a topographic break that coincides with the approximate location of the western limit of the PRB transition zone. Much of the range is characterized by a plateau that dips gently to the west across which most of the transition zone geology is exposed. In ranges to the north similar geomorphic surfaces are as old as Late Eocene (Axen et al., 2000). Elevation of this plateau averages approximately 1500 meters and its apex reaches just over 3000 meters at Picacho del Diablo, the highest point in Peninsular California. Elevations drop precipitously down the eastern flank of the range to less than 500 meters in Valle Chico, forming a spectacular escarpment with >1 km of relief. This escarpment is controlled by the active Siena San Pedro Martir normal fault, the westernmost detachment system for the Gulf of California rift province Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 1.3 Physiography of the region surrounding the Sierra San Pedro Martir and location of study area. Major active faults as well as federal highways and major dirt roads used to access the study area are also shown. 16 (Stock et al., 1991). Basin and range physiography of lower elevation is apparent on the eastern side of the peninsula, a region that has undergone extensional tectonics since the Miocene. The study area transects the Sierra San Pedro Martir at its southern end and encompasses a region that includes the full width of the transition zone in the PRB and parts of the western and eastern zone to either side. Elevations reach 1700 meters in this part of the range and rise gently towards the center of the range to the north. To the south the Mesozoic geology is mostly covered by broad volcanic mesas of the Tertiary Puertecitos volcanic province. The area lies within the transition from chaparral to desert flora dominated vegitational zones in the Californian and southern Sonoran Central Desert phytogeographic regions respectively (Minch and Leslie, 1991), and thus exposures are significantly better than the more studied regions of the PRB to the north, which are generally heavily vegetated. Road access to the study area is from either side of the range as no roads transit the eastern escarpment, and much of the interior of the range remains roadless and remote (Fig. 1.3). Much of the road network linking with Highway 1 to the west and Highway 5 to the east is dirt track in variable condition and subject to frequent washouts. These are primarily ranching roads that connect a number of small outposts and are commonly transited by two wheel drive high-clearance vehicles, although four wheel drive is generally very useful. Geologic mapping of this region was primarily accomplished by day-long traverses from the ends of these roads, but much of the interior of the range required spike camps. The ranchers who live and work in this Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 17 remote region are incredibly hospitable folk who provided a valuable margin of safety during the course of this project, both in terms of routine fieldwork operations as well as up to date advice on avoiding areas of marijuana cultivation in the range. The latter activity is prevalent in regions of Baja with drainages that contain water for most of the year, but is generally easy to work around. Overview of dissertation This dissertation constitutes a multidisciplinary approach to understanding the structural aspects of continental arcs and relationships among orogenic processes. It represents a stage of continuing research in the understanding of these processes and forms part of a larger group effort undertaken by myself and researchers at USC. The focus of these efforts has been on the evolution of the Jura-Cretaceous Peninsular Ranges batholith, a spectacularly exposed but poorly known region of the North American Cordillera that exhibits a variety of crustal levels, unusually well preserved structural relationships, and one of the best examples in the world of the effects of inherited crustal heterogeneity on orogenesis. A number of other workers have been notably involved with this research including Scott Paterson and Ann Blythe (USC), Scott Johnson (University of Maine), Chris Kopf (University of North Carolina), Marty Grove (UCLA), Melissa and Gary Girty (San Diego State University), and Joe Wooden (USGS/Stanford). In this dissertation I describe investigations in the southern SSPM region of the Peninsular Ranges batholith designed to constrain some of the problems outlined Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 8 above. Because this region is only known in reconnaissance detail the results that follow also bear on the tectonic evolution of the PRB. The main body of this dissertation (chapters 2-4) is a collection of manuscripts at various stages of preparation for publication submittal. These chapters were written to stand alone, and thus some repetition exists among them, particularly in the introductory sections. In Chapter 2 1 explore the origin and evolution of a fan structure that occurs within the PRB transition zone in the southern SSPM. This chapter details the importance of inherited crustal heterogeneity as well as the effect of active magmatism on deformation processes in arcs. In Chapter 3 I investigate the exhumation history of the PRB and the role that the crustal discontinuity in the batholith played during exhumation. This chapter constrains some of the processes by which exhumation occurred and provides some evidence for speculation about topography and climate in the mid-Cretaceous PRB. Chapter 4 reviews the tectonic history of the PRB in more detail than is discussed in the first two chapters. In this chapter I integrate data collected in this study with existing data from the PRB and focus in particular on the structural development of the transition zone in the batholith. I then discuss the implications of new data reported in this dissertation on the tectonic evolution of the PRB. Processes as well as tectonic history are then summarized in Chapter 5 and I identify new directions for research in the PRB. In Appendix A, I digress from the PRB to briefly describe other research that I have undertaken with Scott Paterson as part of my PhD. work. This involves developing a test of spatial and geometrical relationships between deformation and magmatism from geologic maps that is applied Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 19 to five orogens around the world. The PRB was not used in this analysis because regional relationships are known only in reconnaissance detail. Appendices that follow contain most of the data from the southern SSPM that has been collected by myself and others listed above. Included as a separate plate is the geologic map of the southern SSPM, which shows detailed geological relationships and structural measurements. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 0 CHAPTER 2: A DOUBLY VERGENT, FAN STRUCTURE IN THE PENINSULAR RANGES BATHOLITH: TRANSPRESSION OR COMPLEX FLOW ALONG A CRUSTAL-SCALE DISCONTINUITY? In this chapter I describe in detail the lithologies, structures, and geological timing relationships across the PRB transition zone in the southern SSPM. In contrast to previous ideas regarding the structural evolution of the PRB that conclude a relatively simple and short-lived deformational system, this region preserves a complex and long-lived contractional history spanning >40 m.y. in the late Mesozoic. Despite its complexity, this location may be one of the best in the PRB for discerning structural relationships in the batholith. Exposures in this region are excellent, a considerable amount of prebatholithic strata are preserved, and relatively few of the late plutons that have obscured much of the earlier history elsewhere in the PRB occur here. It is also a spectacular example of the effects of crustal heterogeneity in terms of both crustal rheology and anisotropy on deformation processes in arcs. Introduction Continental arcs are dynamic, long-lived tectonic environments that typically accommodate complex deformation within strongly heterogeneous crust. Understanding the extent to which the mechanical nature of crust controls deformation in these systems is paramount to how we interpret complex structures that form in these environments and ultimately relate them to ancient plate interactions. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 21 The Jura-Cretaceous Peninsular Ranges batholith (PRB) preserves an excellent example of the structural complexity resulting from heterogeneous, mechanically active crust. The PRB developed across a continental margin established in the Late Precambrian that contains one of the most dramatic basement transitions preserved across the strike of a batholith. This transition zone between oceanic floored outboard and continental floored inboard zones of the batholith (western and eastern zones respectively) extends the known length of the PRB, some 800 km from southern California through northern Baja California, and is the site of a number of distinctive features in the batholith including a sharp chemical transition in plutons, a series of elongate flysch basins, and a belt of persistent contractional deformation spanning >40 m.y. that overlapped with arc magmatism. In the southern Sierra San Pedro Martir (SSPM) this belt of deformation in the transition zone is marked by a spectacular -20 km-wide, doubly vergent fan structure developed in tectonized plutons and country rocks of the PRB. Fan-shaped crosssectional geometry arises from mylonitic, moderately inward-dipping marginal thrust sheets that gradually transition to a steeply-dipping tectonized zone in the fan center that shows both mylonitic and magmatic fabrics. Mineral lineations display a similar transition from mylonitic, down-dip, and inward plunging on the sides to magmatic, strike-parallel, moderately N-plunging in the center. The deepest crustal levels known from the PRB (>15 km) occur in the center of the fan, reflecting mid-crustal exposures in the center of the batholith. Inverted metamorphic gradients on both flanks of the structure mark transitions outward to shallower crustal levels. The west side is Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. particularly impressive with >3 kbar of metamorphic pressure variation. A wealth of structural detail preserved in this region provides an exceptional opportunity to resolve the hitherto poorly known early structural evolution of the PRB that elsewhere has been largely overprinted, intruded-out, or covered by subsequent geologic events, and examine the complex interactions that may develop between deformation and magmatism in heterogeneous continental arc margins. Background on fan structures The double vergent nature of many orogens was first noted in the Alpine collisional belt (Argand, 1916), and since has been recognized in a variety of convergent tectonic settings. One of the classical examples of fan structure occurs in zones of transpression that accommodate oblique plate convergence (Harland, 1971). The archetypal “flower structure” is characterized by fan-shaped cross-sectional geometry in which kinematic partitioning occurs, involving reverse slip on moderately inward dipping faults on the sides of the structure and strike-slip on steeper faults in the center (Fig. 2.1a) (e.g. Sylvester, 1988). Equivalent structures have been identified at deeper crustal levels that involve more distributed deformation (Fig. 2.1b) (e.g. Hanson, 1989; Holdsworth and Strachan, 1991), and may include partitioning of additional aspects of deformation such as deformation path (e.g. Robin and Cruden, 1994; Goodwin and Williams, 1996). These systems have been numerically modeled by incorporating boundary conditions representative of plate motion parameters (e.g. Sanderson and Marchini, 1984; Robin and Cruden, 1994; Jiang and Williams, 1998). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 23 As e) e) Figure 2.1. Examples of doubly vergent fan structures, (a) Discrete faults forming flower structure in zone of transpression (from Sylvester, 1988). (b) Schematic diagram of transpressional fabrics that may form within a distributed deformation zone. Leaves represent cleavage, lines represent lineation, arrows show sense of shear parallel to cleavage and lineation with grayed arrows located behind cleavage in this perspective, (c) Mechanical model of Willett et al. (1993) in which synthetic (Ss) and antithetic (As) structures develop coevally above a point (pinned by screw) at which a lower layer subducts during convergence, (d) Structures developed above a tectonic wedge impinging on a basement ramp (after Colpron et al., 1998). (e) Example of a crustal-scale pop-up structure developed as a mechanical wedge formed by intersecting conjugate shear zones. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Another class of doubly vergent fan structures occurs in which only contractionai deformation is accommodated across the deformation zone. One mechanism of producing this geometry that has been applied to a number of orogens is by subducting a lower crustal or lithospheric mantle layer which, under specific boundary conditions and crustal rheology, results in two systems of reverse faults and folds in the overlying crust with opposed vergence that intersect to form a fan-shaped structure (Fig. 2.1c) (e.g. Willett et al., 1993; Beaumont et al., 1996). A similar style of deformation occurs by processes of tectonic wedging in which thrust fault zones bound the upper and lower surfaces of an intra-crustal wedge above which doubly vergent fan structures may form. These have been recognized in a variety of scales in the Canadian Cordillera and other orogens (Fig. 2.Id) (Price, 1986). Pop-up structures are another form of crustal wedge analogous to mechanical wedges formed by intersecting conjugate shear zones modeled in rock deformation experiments (Fig. 2.1e). These typically involve uplift of crustal-scale, wedge-shaped, basement blocks such as the Nanga Parbat massif (Schneider et al., 1999). Finally, some fan structures may result from the interaction of two unrelated fault systems associated with disparate tectonic events (e.g. Wahlgren et al., 1994). Many of the models introduced above are experimentally evaluated using homogeneously deforming materials. However numerous studies suggest that the mechanical nature of the deformation zone such as pre-existing heterogeneity and rheology exercise strong controls on deformation. For example, the Mesozoic Western Idaho suture zone accommodated continued contractionai deformation and plutonism Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 25 long after Jurassic suturing had ceased (Manduca et al., 1993). Moreover, arc environments in particular may be subject to highly transient strain localization resulting from rheological contrasts in the crust due to the presence of magma as well as magmatic advection of heat into broad crustal zones (e.g. Hollister and Crawford, 1986; Collins and Vemon, 1992; Sandiford etal., 1992; Pavlis, 1996). In this chapter I describe the fan structure in the southern SSPM in detail, including its structural and intrusive evolution, preliminary P-T constraints, and its relationship to the PRB deformation belt A prolonged history (>40 m.y.) of overlapping contractionai deformation and magmatism is apparent in the fan structure. This feature was focused in the transition zone of the PRB, a fundamental crustal discontinuity in the North American margin that controlled deformation through much of the Mesozoic. Additionally, a protrusion in the Precambrian-Paleozoic continental margin was manifest as a buttress in the Mesozoic that produced tectonic wedging within the PRB deformation zone and around which stresses refracted. Under limited circumstances rheological gradients formed within the fan structure as a result of extensive intrusion of magmatic sheets which led to partitioning of deformation. Thus the mechanical nature of this margin significantly influenced deformation during plate convergence. Tectonic overview of Peninsular Ranges batholitli (PRB) The Jura-Cretaceous PRB (Fig. 2.2), a >800 km segment of the Cordilleran magmatic arc that once stretched from Alaska to South America, intrudes a series of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. -32 Ensenad Quintin % Transitional PRB *, deformation belt 115' Outcrop limits of the Metosok PRB Tertiary (and Late Cretaceous?)' Sierra'san\pla; elXH7 ry Pedro Martir^ (Figure 3) HIT ''J ^ Tectonic assemblages [ \ \ * | J-K arc volcanics ITT1 TFK flysch [ \ \ 1 O-P slope-basin / \ 25*. |— { Pe-P miogeocline Geochemical parameters retlectlna transition in basement across batholith v Guerrero Negro X \G uaym asX Xx \ v \ \ \ ■ \ ' \ v MagnetiteIlmenite line 0.7040 and 0.7060 Sr, contours 756180 contour Transition in REE Figure 2.2. Map of Peninsular Ranges showing tectonostratigraphic assemblages, the Peninsular Ranges Batholith, and the transition between western and eastern zones of the batholith for various geochemical and isotopic parameters. Note that transitions in these parameters are poorly known in the southern SSPM. Patterns used for tectonostratigraphic assemblages reflect general structural trends in respective units. Note offset of contact between miogeocline and slope basin assemblages across modem Gulf of California plate boundary. Also shown are identified segments of the transitional PRB deformation belt that are referenced in text and include the Cuyamaca-Laguna Shear Zone (CLSZ), Calamajue (C), El Arco (A). From Gastil et al. (1990a; 1991a), Gastil (1993), Gromet and Silver (1987), Silver et al. (1979); Taylor and Silver (1978). 27 NW-trending, Pre-batholithic, lithostratigraphic assemblages in Peninsular California. From west to east these include: (1) Triassic-Cretaceous continental borderland assemblages (not shown in Fig. 2.2); (2) Jura-Cretaceous volcanic arc assemblage; (3) Triassic(?)-mid Cretaceous clastic and volcaniclastic flysch assemblage; (4) Ordovician-Permian slope-basin clastic assemblage; and (5) Upper ProterozoicPermian miogeoclinal carbonate-siliciclastic assemblage (Fig. 2.2) (Gastil, 1993). My choice of names for these assemblages follows historical usage in the PRB despite the incongruity of some terms with modem tectonic nomenclature. Plutonic rocks of the batholith have been subdivided into ‘western’ and ‘eastern' plutonic zones that reflect intrusion into oceanic and continental affinity crust, respectively (Gastil et al., 1975). The western zone of the batholith consists of gabbro to tonalite, mid-Jurassic to mid-Cretaceous plutons that intrude lower greenschist grade metamorphosed volcanic arc and flysch assemblages (Silver et al., 1979; Silver and Chappell, 1988; Todd et al., 1988; Waiawender et al., 1991). The eastern zone includes early Mesozoic to Late Cretaceous plutons of tonalite, granodiorite, and granite compositions that intrude flysch, slope-basin, and miogeoclinal assemblages (Silver and Chappell, 1988; Waiawender et al., 1991; Thomson and Girty, 1994). Country rock in this zone attained andalusite-sillimanite metamorphic conditions ranging from -475 °C at -3.2 kbar to -700 °C at -5.8 kbar (Rothstein, 1997). Regional metamorphism and deformation in Peninsular California occurred concurrently with intrusion of the PRB (Gastil et al., 1975; Todd et al., 1988). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 8 A number of geochemical and isotopic parameters vary sharply across a narrow transition zone (<20-40 km) between western and eastern plutonic zones that appears to reflect a fundamental basement boundary along the middle of the PRB. These include the following, listed for western and eastern zones respectively (Fig. 2.2): (I) the stable FeTi oxide phase- magnetite versus ilmenite (Gastil et al., 1990); (2) REE fractionation patterns- moderate REE fractionation versus HREE fractionation patterns (Gromet and Silver, 1987); (3) Sr; values- lower than 0.705 versus higher than 0.705 (Silver et al., 1979); and (4) 8180 values- +6.0 to +8.5 %o versus +9 to +12 %o (Silver et al., 1979). The isotopic compositions of the western PRB suggest a primitive island arc-like source for this part of the batholith, whereas the source regions of the transition and eastern portions of the PRB contain older, deeper reservoirs with eclogitic mineral assemblages (Silver and Chappell, 1988). A 5-25 km-wide belt of predominantly contractionai deformation of Mesozoic age occurs within the geochemically defined transition zone and appears to be continuous throughout the length of the PRB. This deformation zone is largely restricted to the flysch assemblage and plutons that intrude it, and includes deformation that is distinct from structures in western and eastern zones of the PRB to either side. Structures in the western zone include syn-depositional faults and gentle folds (Gastil et al. 1975; Busby et al., 1998). Structures in the eastern zone consist of folds, cleavage of variable development, and shear zones (Gastil et al. 1975; Todd et al., 1988). In contrast structures in the transitional deformation zone include steeplydipping, largely west-vergent reverse ductile shear zones, belts of folding, intensely Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 9 developed cleavage, and the fan structure described below. A sharp eastward increase in metamorphic grade and pressures coincides with this belt of deformation, and on the western side of the SSPM includes a spectacular inverted metamorphic gradient. In numerous places this deformation belt has been completely intruded out by post -100 Ma plutons or covered by Tertiary strata, leading to the discontinuous segments presently exposed in the Peninsular Ranges (Fig. 2.2). Previously recognized segments include the ~118-115 Ma Cuyamuca-Laguna Mountains Shear zone in southern California (Todd et al., 1988; Thomson and Girty, 1994), a ~115-108 Ma mylonite shear zone on the western side of the northern SSPM (Johnson et al., 1999a), >132 to <97 Ma deformation in the southern SSPM (Goetz, 1989; Measures, 1996; this study), a -103-100 Ma shear zone in the Calamajue region (Griffith and Hoobs, 1993), deformation >104.5 Ma in the El Arco region (Barthelmy, 1979), and Cretaceous(?) deformation in the region south of La Paz (A. Schuerzinger personal comm., 1998). Most of these studies examined one part of the deformation belt and resolved one or two episodes in its evolution. However, in the southern SSPM nearly the complete belt is exposed from western to eastern zones of the batholith and a much more extensive history of contractionai deformation can be discerned. The origin of the boundary between western and eastern basement in the PRB and the deformation belt within it has been controversial for some time, and three basic models have emerged. Thomson and Girty (1994) proposed that the transition is a pre-Triassic crustal boundary that possibly corresponds to the ancient North American rifted margin. They based this conclusion on their interpretation that the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 30 flysch assemblage overlaps the batholith discontinuity in southern California and is Triassic or older as determined from a Triassic age obtained from a pluton that intrudes part of the flysch assemblage. Other workers have suggested that the transition was initiated as far back as Jurassic time with formation of a back-arc basin and development of a fringing western arc that was sutured back to the continent following collapse of the basin in the mid-Cretaceous (Gastil et al., 1981; Rangin, 1978; Todd et al., 1988; Griffith and Hoobs, 1993; and Busby et al., 1998). In contrast Johnson et al. (1999a), working in the northern SSPM, suggested that the western zone of the PRB represents an exotic island arc terrane based on paucity of continental components in both volcanics and plutons. They proposed suturing of this terrane on the Main Martir thrust between I IS and 108 Ma, although they are uncertain as to the extent of this suture. Fan structure in the southern SSPM segment of the PRB deformation belt Exposed in the southern SSPM are the major Paleozoic and Mesozoic lithotectonic assemblages of the PRB, the lithological transition from western to eastern plutonic zones of the batholith, and the transitional PRB deformation belt, which in the southern SSPM is manifest as the fan structure (Fig. 2.2,2.3,2.4). I divide the fan structure into western, central and eastern domains, reflecting the composite nature of deformation and magmatism that define it The fan structure is juxtaposed by the Rosarito fault on the west with rocks of the Jura-Cretaceous Alisitos arc volcanic assemblage that underlies much of the western zone of the PRB. The Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 1 31*30* insional Pacific Ocean 30*30* Km 115*00* □ Quaternary Sediment and volcanic rocks □ Tertiary volcanic rocks □ Cretaceous-Quaternary sedimentary rocks Cretaceous r~ -- --‘l gabbro plutons , Early Cretaceous Alisitos volcanic assemblage ■ Jura-Cretaceous [V 'sM tonalite, granodiorite and granite plutons Jun-Cretaceous volcanic rocks and plutons in fan-like structure |J "I Precambrian-Paleozoic I— rl miogeodinal assemblage Figure 2.3. Generalized geologic map across the PRB at the latitude of the Sierra San Pedro Martir. Formlines denote general structural trends in the fan structure which occurs in the transition zone in die batholith. Note change in orientation for these trend lines within and north of study area. Only faults with Tertiary history are shown. Modified from Gastil et al. (1975). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. • Al in Hblnd (kbar; C. Kopf) < zirc U/Pb SHRIMP age (Ma) ■ zirc U/Pb age (Ma) Not mapped Rinconada Tentf 18+3 Nue\ Rinconada complex / Agua Caliente Pluton Rqsarito faulty J qLaPosta- 'i type(?) x J / Pluton z / // V / / / '' < V ■ r - ( \ / : ■ ■■ ' ' ■■ ' \ / / Plutons of Peninsular Ranges batholith Volcanic and sedimentary assemblages West FW Alisitos volc-rich Alisitos sed-rich Flysch Metavolcanic Miogeocline *SferraSan? , Pedro’ Martie 'fault' y | | bi Tonalite p | Granite | [ Orthogneiss | | Gabbro CentralV East domain\FW I 5 km / East Homain Figure 2.4. Geologic map ofstudy area in the southern Sierra San Pedro Martin Formlines show general strike ofsolid state foliations (long lines), and magmatic foliation (short lines). Cross-section A-A”’ shown in Figure 2.5. QQt cover fault —J--------normal/ reverse fold I synform/ Iff overturned ||| antiform/ | U overturned western domain consists of a mylonitic package of flysch assemblage and plutonic rocks with moderately NE-dipping foliation and NE-plunging lineation first described by Goetz (1989).1 These fabrics continue structurally upwards and eastwards into the central domain where they grade into nearly vertical NNW-striking foliation and moderately N-plunging lineation within hornblende tonalite sheets that intrude Jurassic orthogneiss. The fabrics are mostly magmatic in the tonalite sheets and mylonitic gneiss in orthogneiss where they overprint older fabrics oriented parallel to those in the eastern domain. In the eastern domain, as first described by Measures (1996), a mylonitic package with moderately WNW-dipping foliation and WNWplunging lineation in orthogneiss and flysch assemblage rocks is faulted against miogeoclinal assemblage footwall rocks that occur in much of the eastern PRB. Below I describe the fan structure and its footwails in more detail followed by a discussion of pressure constraints and timing relationships. Procedures and results from U-Pb analyses for units discussed in this chapter are presented in Chapter 3. Western footwall The western footwall of the fan structure consists of a volcanic-rich unit of the Alisitos Fm. that includes lower greenschist grade flows and tufis with minor tuff breccias and poorly bedded tuffaceous limestone. Similar lithologies are intruded by 113-117 Ma gabbro-tonalite ring complexes west of the northern SSPM (Johnson et 1 Here and henceforth I use the term "mylonitic” and "mylonite” to indicate microstructural evidence for grain-size reduction during deformation and "mylonite Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 34 al., 1999b). At distances >0.5 km from the Rosarito fault, bedding in the Alisitos Fm. is folded by map-scale open, upright folds with SE and NW shallowly-plunging hinges (Fig. 2.5a). These folds tighten and hinges steepen near to the Rosarito fault, where steeply-dipping fault-parallel cleavage becomes common. The Rosarito fault is a steep to moderate NE-dipping fault zone across which a sedimentary-rich unit of the Alisitos Fm. in the western domain of the fan structure has been thrust over a volcanic-rich unit of the Alisitos Fm. in the western footwall. Deformation associated with the Rosarito fault overprints moderately-dipping mylonitic fabrics in the western domain (Fig. 2.4). The fault zone is characterized by steep NE-dipping spaced to locally penetrative cleavage, shear bands showing predominantly NE-over-SW shear sense, discrete chloritized faults with centimeter- to meter-scale displacements, and in places reverse faults that juxtapose rocks with contrasting metamorphic grade and fabrics. A variety of folds occur within and to both sides of the Rosarito fault zone. The dominant set consists of outcrop- to map-scale, shallow NW-SE-plunging, tight folds with moderate NE-dipping axial planes that are commonly sheared with SWvergence. Chlorite and white mica in fold hinges are typically recrystallized, bent, and broken. A less common, moderate to steep NE-plunging, upright, open to tight fold set also occurs, and typically shows a box-style character with steep NW- and SE-dipping axial planes. Complex interference patterns between the two fold sets are common, gneiss” to indicate a recrystallized rock fabric that includes stretched mineral grains. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35 Figure 2.5. Cross-section of study area. Equal area stereoplots of selected structural domains across fan structure. Patterns same as Figure 2.3. a) Poles to bedding in Alisitos volcanic-rich unit measured at distances greater than 300 m from Rosarito fault; b) Dominant lineation and foliation measured in western and central domains showing transition in fabric orientations; c) Poles to conjugate shears in hornblende tonalite sheets in central domain; d) Poles to dominant foliation and layering in eastern domain and western part of eastern miogeociinal footwall in map-scale NW-trending folds; e) Lineation and poles to foliation outside of NW folds in eastern domain showing pre-folding orientation of eastern thrust sheet; f) Small fold hinges and poles to foliation and layering in eastern part of the miogeociinal assemblage showing prefan structure deformation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Lineation Lineations Rosarito Western domain Western PRB Eastern PRB Mean fold axis 35/323 -400 g .-800 £ SE 1600)2 1200 s 800 E 400 - Mean fora 25/257 Mean fod 34/265 Mean meatier 46/307 fonation 191 50 Western domain so Flysch data •■La Suerte data mean foliation 322 35 mean lineation 37/037 Central domain Tonalite sheets ^^Orthogneiss mean foliation 348 81 mean lineation 345/44 Poles to foliation Poles to beddin Mean fold axis 09/147 SW A Poles to shears Vertical scale = horizontal scale Poles to foliation and layering dextral shears mean = 204 71 N=27 Small fold hinges eastern domain metavolcanic orthogneiss assembl. Eastern__ footwall miogeoclinal assembl. Poles to foliation and layering sinistral shears mean = 137 90 N = 12 Poles to foliation (f) Eastern miogeoclinal footwall [Central domain \ Eastern domain Transitional PRB Break in section Cross-section A-A'A"-A" LU 37 and both show mutually cross-cutting relationships with cleavage and shears in the Rosarito fault zone. Western domain The western domain consists of a moderate NE-dipping mylonitic package that has overthrust the volcanic-rich unit of the Alisitos Fm. on the Rosarito fault, and includes (from structurally lower to higher levels) the sedimentary-rich unit of the Alisitos Fm., flysch unit, -132 Ma tonalite-gabbro La Suerte plutonic complex, and numerous granite sheets. The -101 Ma tonalite-gabbro Rinconada complex intrudes this mylonite package and is slightly deformed (Fig. 2.4). The sedimentary-rich unit of the Alisitos Fm. lies within a tectonic sliver bordered to the west by the Rosarito fault and structurally overlain to the east by the flysch unit. This sequence consists of greenschist grade metamorphosed tuffaceous sandstones, volcaniclastics, and tuffs with minor argillite and calcareous sandstone. This assemblage is similar to strata in the northern SSPM correlated by Silver et al. (1963) and Johnson et al. (1999a) with the late Aptian-early Albian type-section of the Alisitos Fm. (Allison, 1974). The overlying flysch unit consists of well-bedded basalts, silicic lithic crystal tuffs, tuff breccias, and tuffaceous sandstones. A high strain zone juxtaposes the Alisitos and flysch units and is marked by upwards-increasing mylonitic fabric intensities, higher metamorphic grade with the appearance of biotite, garnet, and green amphibole (gedrite?), and increasingly abundant leucocratic veins and stringers. In Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 38 contrast to the underlying unit, bedding features are rare, concomitant with increasing metamorphic grade and development of metamorphic layering. The high strain zone is equivalent to the Main Martir thrust of Johnson et al. (1999a), which they considered to be the suture between an exotic western PRB island arc and eastern PRB continental arc. The flysch unit is similar to a metavolcanic sequence in the hangingwall of the Main Martir thrust dated by Johnson et al. (1999a) at 127.9 ± 1.2 Ma using U-Pb SHRIMP techniques. However, these authors are uncertain as to whether a volcanic flow or sill was dated. Several lines of evidence suggest that the flysch unit is older, including a I32±7 Ma age from the Suerte complex that intrudes it in the southern SSPM and correlative ~156 Ma volcanic and epiclastic strata described in the Calamajue area, 160 km along strike to the SE (Fig. 2.2) (Griffith and Hoobs, 1993). The Suerte plutonic complex consists of predominantly biotite tonalite intruded by hornblende gabbro (Fig. 2.4). Biotite tonalite of this complex is compositionally and texturally sheeted at scales varying from centimeters to 10’s of meters in width and 100’s of meters to kilometers in length. Gabbro bodies within the Suerte complex are compositionally and texturally heterogeneous, consisting of dominantly hornblende gabbro with minor homblendite and anorthositic gabbro. These phases form sheeted to irregular geometries, range in texture from fine-grained to pegmatitic, and show both mutually cross-cutting and gradational contacts. Co-genetic relationships between tonalite and gabbro in the complex are indicated by mutually cross-cutting relationships, gabbro globules in tonalite with crenulated margins, and Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 39 microgabbro enclave swarms that occur at the ends of many elliptical gabbro bodies concomitantly with modal increase in hornblende in tonalite. The eastern and southern parts of the complex have been intruded by hornblende tonalite of the sheeted complex in the center of the fan structure. A U-Pb zircon age of 132 ± 7 Ma was obtained from the Suerte complex. Coarse- and fine-grained granite sheets and stocks crop-out across a wide belt in the western and central domains of the southern SSPM (Fig. 2.4). In the western domain cm-scale sheets of fine- and coarse-grained granite and fine-grained hornblende gabbro are commonly intricately interleaved and intensely mylonitized. The granite pluton in the northern part of the area is relatively homogeneous except for some sheeting at its southern end. Granitic plutons are strongly diachronous based on ages and cross-cutting relationships. A U-Pb age of 118±3 Ma was obtained from a mylonite granite sheet that intrudes the flysch unit in the southern SSPM, and Johnson et al. (1999a) reported an age of 133.6±1.9 Ma from a granite pluton that intrudes orthogneisses in the northern SSPM. In the central domain of the southern SSPM, mutually cross-cutting relationships are apparent between granite and -100 Ma hornblende tonalite sheets, suggesting that granite intruded over a period spanning at least ~ 133 to less than —100 Ma. The Rinconada pluton is a strongly heterogeneous complex consisting primarily of anorthositic gabbro cut by minor gabbro sheets in its interior and gabbro and hornblende tonalite units that form ring-like sheets near the margins (Fig. 2.4). Within 1 km of the southern margin of the pluton numerous wall rock screens varying Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 0 from 1-3 m in width occur between these ring-like sheets. A U-Pb SHRIMP age of 101.3 ± 0.6 Ma was obtained from hornblende tonalite in the pluton. The Rinconada complex is similar to ring-like plutons that have been described from numerous areas in the western zone of the PRB (referenced in Johnson et al., 1999b), including some in the northern SSPM with ages of 104-108 Ma (Johnson et al., 1999a). The oldest fabrics in the western domain occur in the upper part of the flysch unit. These include uncommon to rare crenulations and mm-scale isoclinal folds that are overprinted by mylonitic fabrics described below, as well as folded internal fabrics in garnet porphyroblasts around which mylonitic foliation is tightly wrapped. These older fabrics were not observed in either the Suerte complex nor granite that intrudes the flysch unit, indicating that deformation was probably initiated on the west side of the fan structure before -132 Ma. Nearly the entire western domain comprises a spectacular package of welldeveloped LS-myionite, over 5 km in overall structural thickness, which extends from the Rosarito fault through much of the Suerte complex. NW-striking foliation is subparallel to bedding in the Alisitos sedimentary-rich unit, to metamorphic layering in the flysch unit, and to igneous sheets in the Suerte complex. Lineation are dominantly NNE-trending (Fig. 2.5b). Kinematic indicators including s-c fabrics, sigma-clasts, and extensional crenulations are common in the mylonite package and show consistent NE-over-SW shear sense in sections parallel to lineation. Gradients in mylonite fabric intensity are apparent across the western domain. Moderate intensity fabrics that include both stretched and unstretched feldspar grains Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 1 occur in the Alisitos sedimentary-rich unit Fabric intensity increases upward in the flysch unit to include strongly stretched feldspars and L>S fabrics in the upper part of the unit. This intensity continues ~3 km into the Suerte complex where it grades into mylonite gneiss fabrics. Fabric orientations are more variable in the central and eastern parts of the Suerte complex. However, neither overprinting nor folding relationships were observed, and lineation throughout the complex are consistently NE trending. Both magmatic and subsolidus foliation and lineation fabrics are developed in gabbro bodies in the Suerte complex. Magmatic fabrics and layering typically show inconsistent orientations among outcrops, whereas solid state fabrics are commonly oriented subparallel to mylonitic fabrics in surrounding tonalites. A variety of important structural relationships are preserved in the Rinconada complex and surrounding wall rocks. In granite and flysch host rocks along the eastern margin of the pluton intensely developed mylonitic fabrics overprint older mylonitic structures. The younger fabrics continue into gabbro and tonalite lithologies in the pluton and show consistent NE-over-SW shear sense, suggesting that host rocks have been thrust over the Rinconada complex after emplacement Small tonalite and gabbro plutons associated with the Rinconada complex show a similar relationship near its SE end. These recorded a solid state fabric that along with host rocks, has been folded by a series of map-scale, NE-trending, upright folds. The folds tighten dramatically and become completely transposed within mylonitic fabrics along the eastern margin of the pluton (Fig. 2.4). Similar relationships occur along the western margin of the pluton where an elongate tonalite pluton contains moderate intensity solid state fabrics Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 2 and kinematic indicators consistent with thrusting of the Rinconada complex over its western wall rocks. NE-plunging folds in host rocks are also overprinted by thrust fabrics at this location. Central domain The central domain consists of a sheeted plutonic complex of hornblende tonalite and gabbro that intrudes Jurassic orthogneiss and the Suerte complex (Fig. 2.4). A penetrative cleavage and lineation are developed in orthogneiss, parallel to associated, predominantly magmatic, foliation and lineation in hornblende tonalite sheets. These structures overprint older fabrics in orthogneiss associated with deformation in the eastern domain. A final cleavage-forming event produced contrasting fabrics in orthogneiss and tonalite sheets, followed by intrusion of undeformed plutons into the sheeted complex. The sheeted plutonic complex consists of predominantly vertical, NNWstriking hornblende tonalite sheets of variable mode and texture that range in width from meters to 10’s of meters, and in length from 100’s of meters to kilometers. Elliptical gabbro plutons intrude these sheets and display mingling textures and extensive microgabbroic enclave swarms along some contacts. Gabbro plutons show similar compositional heterogeneity to gabbros in the Suerte complex and contain mostly magmatic fabrics that occur in highly variable orientations. A U-Pb SHRIMP zircon age of 100.1 ± 0.5 Ma was obtained from the mingled contact between the end of one of these gabbro plutons and hornblende tonalite sheets. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 43 Belts of biotite muscovite orthogneiss form screens within the tonalite sheeted complex in the central domain (Fig. 2.4). These contain gamet-bearing amphibolite pods and sillimanite, garnet, K-feldspar pelitic domains. A U-Pb SHRIMP zircon age of 164.4 ± 1.4 Ma was obtained from one of the screens. This unit extends at least another 35 km northwards where it has been mapped by Woodford and Harriss (1938) near the SSPM pluton (Fig. 2.3), and similar orthogneiss occurs in southern California (Todd et al., 1988; Thomson and Girty, 1994). Orthogneiss contains a highly composite gneissic fabric with <25% relict plagioclase containing oscillatory zoned cores. Kinematic indicators are poorly developed in these fabrics. As described below, three deformation events are recognizable in orthogneiss, and felsic selvages and stringers typically occur in lit-parlit association with gneissic layering formed in each of these events. The sheeted plutonic complex is intruded along its eastern margin by hornblende tonalite and biotite hornblende granodiorite that are lithologically similar to the SSPM pluton 35 km to the north, one of a regionally extensive La Posta-type plutonic suite that has been dated between 93 and 97 Ma (Fig. 2.4) (Gastil et al., 1991a; Walawender et al., 1990). The hornblende tonalite phase contains sheeted margins that are similar in terms of both lithology and fabrics to the sheeted hornblende tonalite complex and a lithologically homogeneous interior that shows little evidence of sheeting and contains weakly developed magmatic fabrics with variable orientations. The biotite hornblende granodiorite phase has gradational Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 44 contacts with hornblende tonalite, weak magmatic margin-parallel fabrics, and lithologically homogeneous interior with poorly developed fabrics. Three fabric-forming events are recognizable in the central domain. The oldest is apparent only in orthogneiss screens and includes rarely preserved NNE-striking, moderately W-dipping foliation and gneissic layering and moderately WNW-plunging lineation, similar in orientation to structures in orthogneiss in the eastern domain (Fig. 2.5e). Rare folds are also preserved in these domains that are transected and transposed by the two younger cleavages. A second and dominant fabric consisting of steeply-dipping, NNW-striking foliation and moderately NNW-plunging mineral lineation is strongly developed in both orthogneiss screens and hornblende tonalite sheets (Fig. 2.5b and 2.4). In orthogneiss, penetrative cleavage is typically axial planar to common outcrop-scale, tight folds with moderately NNW-plunging hinges. Plagioclase and quartz are dynamically recrystallized and define a lineation that shows evidence of polygenic origin (Fig. 2.6a). Biotite grains show evidence of both dynamic recrystallization, formed during this main fabric-forming event (Fig. 2.6a), and an intersection lineation resulting from crenuiation of the older foliation (Fig. 2.6b). Lineation-parallel stretch is defined by rare boudins. Older layering associated with the oldest fabric in the orthogneiss is commonly transposed to form a new gneissic layering. In hornblende tonalite, fabrics associated with this event are largely magmatic with some high temperature, solid-state overprint and include equally well developed foliation and lineation. Hornblende is almost completely igneous, plagioclase shows Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 45 Figure 2.6. Micrographs of orthogneiss from central domain, (a) Section oriented perpendicular to foliation and parallel to lineation with crossed Nicols showing lineated quartz and plagioclase. Groundmass is largely recrystallized feldspar, quartz, and biotite. Larger grains are plagioclase with relict cores. Note difference in orientation between relict grains and recrystallized domains (white versus black lines respectively) indicating polygenic origin, (b) Section oriented perpendicular to lineation in plane polarized light showing crenulation fabrics in biotite that corroborate older history. Figure 2.7. Micrographs with crossed Nicols of hornblende tonalite from the sheeted complex in the central domain oriented perpendicular to foliation and parallel to lineation. Compare magmatic-high temperature solid state textures in (a) from the western side of the complex with mostly magmatic textures in (b) from the eastern side. Arrows in (a) show examples of relict plagioclase surrounded by mostly recrystallized feldspar and quartz groundmass. Arrows in (b) show plagioclase phenocrysts with slightly recrystallized margins in groundmass that is largely igneous. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2.6a 2.7a 0.5 cm A l some recrystallization, and quartz and biotite are variably recrystallized. Foliation are subparallel to the sides of tonalite sheets shared with orthogneiss screens, and the orientations of foliation and lineation are continuous across the ends of sheets into gneissic fabrics in orthogneiss. In contrast to the abundant folds involved with formation of this fabric in orthogneiss, folding in hornblende tonalite is rare and largely restricted to sheets with stronger solid-state overprinting fabrics. Enclaves in the tonalite typically show oblate to plane strain fabric shapes that are oriented parallel to mineral foliation and lineation. Shear sense indicators in hornblende tonalite fabrics are variably developed and include s-c and extensional shear bands. Biotite and quartz commonly define csurfaces, and hornblende and plagioclase typically define s-surfaces. Shear direction is inconsistent among outcrops; of 38 locations in the central domain displaying locally consistent kinematic indicators, 17 showed SW-side-up (sinistral) sense of shear parallel to mineral lineation, and the remaining 21 showed the opposite shear sense. No spatial pattern of outcrops showing one shear sense versus the other is apparent across the domain. Kinematics are best developed on surfaces oriented parallel to lineation and perpendicular to foliation. Other surfaces oriented at angles to mineral lineation were examined on outcrops, but kinematic indicators were rarely observed. Fabric gradients, including systematic variation in the orientation and amount of solid state overprint in fabrics, occur in hornblende tonalite sheets across the strike of the complex (Fig. 2.5b and 2.7). On the western side of the central domain, the orientation of foliation and lineation and the sides of tonalite sheets dip and plunge Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 48 moderately to the NE, subparallel to fabrics and igneous sheets in the western domain. Farther east a 1-2 km-wide transition zone occurs in which these fabrics gradually change to NNW-striking, steeply-dipping foliation and moderately NNW-plunging lineation that are prevalent across the rest of the central domain. The amount of solid state overprint also decreases eastward across the domain. In the western hornblende tonalite sheets approximately 40% plagioclase, 100% quartz, and 5% hornblende are recrystallized (Fig. 2.7a). The amount of recrystallization decreases eastward to approximately 10% plagioclase and 80% quartz in the center of the domain; hornblende is completely igneous (Fig. 2.7b). Solid state overprint occurs parallel to original magmatic fabrics, even in the fabric transition zone. This is corroborated in thin sections, which show relict plagioclase, and hornblende crystals in full contact and few intervening solid state deformed minerals. Albite twins in relict plagioclase are preferentially aligned parallel to mineral lineation (c.f., Paterson et al., 1989). Locally developed conjugate shears cut fabrics and sheets within hornblende tonalite. Fabrics within these shears include shear zone-parallel foliation and subhorizontal lineation. The shears bisect the igneous foliation they cut, and form steeply-dipping NNE- and WNW-striking sets. High-temperature conditions are apparent for this deformation as indicated by dynamic recrystallization of plagioclase and hornblende grains. (Cinematics determined from 3-D geometry of fabric bending show horizontal displacement in a dextral sense for the NNE-striking set, and sinistral sense for the WNW-striking set (Fig. 2.5f). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 9 A third cleavage-forming event is apparent in the central domain, with deformation strongly partitioned into orthogneiss and granite, and only minor cleavage development in hornblende tonalite. In orthogneiss and granite, fabrics include moderately NNE-plunging folds, steeply NNE-striking crenulation cleavage, and domains of penetrative axial planar cleavage that locally completely obscure the two earlier formed fabrics. These latest fabrics are very well developed in the easternmost orthogneiss screen, and are variably developed in the other screens. In contrast hornblende tonalite shows only rare crenulation and cleavage development that is characterized by high-moderate temperature solid state fabrics showing evidence for recovery deformation mechanisms in plagioclase and passive rotation of hornblende. Thus hornblende tonalite was still at relatively high temperatures (>450 °C) (e.g. Simpson, 1985) when this third deformation event initiated. Eastern domain The eastern domain consists of a thick package of mylonitic Jurassic orthogneiss and metavolcanics of probable Jura-Cretaceous age that have been thrust SE-ward over Late Proterozoic-Early Paleozoic miogeociinal rocks and Jurassic plutons of the eastern zone of the PRB described below (Fig. 2.4). Missing from the eastern domain are rocks of the slope-basin assemblage. This thrust package has been intruded by the La Posta-type pluton and hornblende tonalite sheets described above in the central domain and folded by a series of map-scale, NW-plunging folds described in a separate section below. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50 Orthogneiss that is lithologically similar to -164 Ma orthogneiss in the central domain forms the upper part of the thrust sheet in the eastern domain. Pelitic and garnet amphibolite gneisses occur in distinct, semi-continuous layers. Much of this unit contains well-developed gneissic and mylonitic layering that lies concordantly within the WNW-dipping mylonitic thrust sheet. Near the top of the sequence orthogneiss is migmatitic, containing greater than 50% leucosome material that occurs as both concordant and discordant stringers in discontinuous gneissic layering. This transition to migmatitic rocks at structurally highest levels in the eastern thrust package suggests that an inverted metamorphic sequence also occurs on the eastern side of the fan structure, albeit with more poorly defined metamorphic facies transitions than the western side. Domains of less recrystallized biotite tonalite occur in parts of the orthogneiss indicating that the assemblage may consist of intrusions of various ages. A well-layered assemblage of metavolcanic and rare metasedimentary rocks lies below and in sharp fault contact with orthogneiss. No relict intrusive relationships were observed between orthogneiss and metavolcanic units. The metavolcanic sequence consists of gneiss, schist and amphibolite that contain a well developed mylonitic fabric throughout A distinctive feature of many metavolcanic layers is a strong blastocrystic texture consisting of ~20% feldspar and quartz porphyroclasts with relict igneous cores in a fine-grained gneissic groundmass and cm-sized schistose inclusions. An uncommon sedimentary component in the sequence includes relatively pure quartzite and marble, and psammitic layers. These features strongly suggest that Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 1 the sequence is largely volcanic in origin, consisting of mostly siliceous crystal-rich lithic tuffs or flows with a minor mafic component The overall succession is continuous for more than 10 km but individual layers are only continuous for 10’s to 100’s of meters. Measures (1996) obtained a discordant U-Pb zircon age from the metavolcanic sequence that he interpreted as -123 Ma. However, the two fractions he analyzed are almost identical, within error, and yielded U-Pb ages o f-123 Ma and Pb/Pb ages of -143 Ma, implying that the sample has suffered Pb loss. Thus, the Pb/Pb age may be more appropriately interpreted as a minimum age. Furthermore this would permit correlation with a very similar succession, the -156 Ma Cafion de las Palmas unit which lies unconformably on Mississippian sedimentary rocks and is unconformably overlain by Albian-Aptian sedimentary-rich Alisitos equivalents, in the Calamajue area (Griffith and Hoobs, 1993). Several generations of structures are present in the eastern domain. Both the orthogneiss and metavolcanic units contain uncommon outcrop-scale domains of older folds. These are mostly moderate NE-plunging, tight, upright folds in the former and isoclinal to tight, rootless folds with axes subparallel to mylonitic lineation in the latter. The dominant fabric in the eastern domain is mylonitic, NNE-striking, moderately WNW-dipping foliation that is subparallel to metamorphic layering, and moderately WNW-plunging lineation (Fig. 2.5c). A fabric gradient is apparent across the domain with mylonite fabrics well-developed throughout the metavolcanic sequence and continuing 3-4 km into the overlying orthogneiss. This fabric gradually Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. dies-out and is replaced by gneissic foliation and lineation fabrics in the migmatitic upper part of the orthogneiss. Mylonitic fabrics in this package contain shear sense indicators including sigmoidal porphyroclasts, and s-c and extensional shear band fabrics that display consistent top-to-the-ESE kinematics in sections parallel to lineation. Ptygmatically folded pegmatites are locally common in the metavolcanic sequence, and axial planes are typically oriented parallel to foliation. Groundmass in the metavolcanics has undergone extensive annealing following mylonitization. Eastern footwall The footwall of the eastern thrust package includes miogeociinal rocks intruded by the Jurassic Agua Caliente pluton (Fig. 2.4). Lithologies in the miogeociinal assemblage include well-layered carbonate and quartzite with screens of uncommon amphibolite and rare quartzo-feldspathic gneiss. Carbonate lithologies consist of marble and calc-silicate mineral assemblages containing quartz, calcite, diopside, tremolite, wollastonite, grossular, and graphite. Siliciclastic rocks include biotite and muscovite with rare sillimanite, andalusite, and garnet in schistose parts of the assemblage. Both deformed and undeformed pegmatite dikes occur locally. The Agua Caliente pluton consists of moderately recrystallized, tourmalinebearing, biotite tonalite with uncommon, concordant mafic sheets. Quartz and plagioclase porphyroclasts with partially recrystallized rims occur in >90% recrystallized medium-grained groundmass of quartz, feldspar, biotite, and muscovite. Ia places biotite tonalite is finer-grained and more gneissic. A U-Pb SHRIMP zircon Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 53 age of 164.3 ± 2.3 Ma was obtained from the southern end of the pluton where it obliquely cross-cuts host rock layering in the miogeociinal assemblage (Fig. 2.4). The western margin has been overthrust by the eastern domain of the fan structure. The miogeociinal assemblage has been polydeformed to produce a complexly folded region (Fig. 2.4). The oldest apparent structural subdomain occurs in the eastern part of the assemblage where well developed, NW-dipping metamorphic layering and mineral foliation occur on the limb of a shallowly SW-plunging fold (Fig. 2.5d). Similar structures and orientations occur in miogeociinal rocks 15 km further east in the Sierra San Felipe (Fig. 2.2; my unpublished mapping). Younger deformation occurs in the western part of the assemblage, which is intensely deformed by moderately NW-plunging, upright, tight to isoclinal folds and associated steeplydipping, axial planar layering and mineral foliation. Interference patterns are common between eastern and western subdomains in the miogeociinal assemblage (Fig. 2.4). Fabrics in miogeociinal rocks show extensive recrystallization to produce moderate intensity foliation and rare lineation that includes locally occurring intersection lineation in fold hinges commonly defined by coaxial fibrolite. Within -20 m of the thrust contact between miogeociinal and overlying metavolcanic assemblages, stretching lineation appear and increase in intensity towards the contact. A strong annealing event is evident in these rocks. Fabrics in the Agua Caliente pluton are characterized by mostly moderate intensity solid state foliation and lineation, and these vary in orientation relative to distance from the faulted NW contact of the pluton. Within one km of the thrust, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 4 fabrics are similar in orientation to fabrics in the thrust hangingwall. Top-to-ESE shear is indicated by s-c and extensional crenulation kinematic indicators, and some mylonitic shear zones occur. Metavolcanic rocks in the hangingwall above the pluton show moderately increasing fabric intensity within a 25 m-wide zone next to the thrust. Farther east in the pluton, foliation and lineation fabrics are variably developed and inconsistently oriented. All fabrics show evidence of later annealing. At the SW end of the pluton several tonalite sheets extend horn the pluton into miogeociinal rocks near the thrust fault and contain strong mylonitic fabrics associated with the thrust. Folding of assemblages in the eastern domain and footwall Both footwall and hanging wall assemblages on the eastern side of the fan structure are folded by a map-scale, NW-plunging antiformal-synformal fold pair (Fig. 2.4). These folds show strong contrast in geometry among miogeociinal, metavolcanic, and orthogneiss units as illustrated in a down-plunge reconstruction (Fig. 2.8). In the miogeociinal footwall the axial surfaces of these folds interact complexly with pre-established folds that include the older NW- and NE-plunging sets described previously to form steep fabrics that completely transpose older fabrics in many areas. In contrast this antiformal-synformal fold pair has a much more open interlimb angle in the hanging wall assemblages. The folds continue into the orthogneiss assemblage and die out ~5 km above the thrust contact The antiformal hinge hooks into a set of S W-plunging overturned folds that are cut by the La PostaReproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Trace of axial surfaces, Strike slip fault, in/out of section Thrust fault, teeth on hangingwall Contact — Trace of layering I I hb Tonalite Q f Metavolcanic [3^] Orthogneiss |=jMk>geocline Figure 2.8. Down-plunge view of map-scale folds that deform both hangingwall and footwall in the eastern domain. i-n Ln 5 6 type pluton. Although some of the folding may result from emplacement of the La Posta-type pluton, strong contrasts in shortening that are apparent among folded units are independent of their location relative to the pluton contact, indicating they largely resulted from regional deformation that followed thrusting in the eastern domain. The SW side of the miogeociinal assemblage is a particularly complicated region (Fig. 2.8). The thrust fault has been folded to a vertical orientation in a NWplunging antiform. Lineation in metavolcanic and orthogneiss rocks in this region plunge moderately to shallowly NW. Sinistral shear sense indicators occur on subhorizontal surfaces that are commonly at angles to lineation. These consist of asymmetric folds, s-c fabrics, and sigma clasts that are distributed over a broad area. A narrower, apparently younger, high strain zone that splays out to the NW from the thrust fault shows dextral shear sense indicators including shearbands, s-c fabrics, and sigma and delta clasts on horizontal surfaces. This region appears to have been influenced by a number of events discussed below including thrusting that juxtaposed eastern domain and footwall assemblages, possible sinistral transpression, dextral flexural slip during subsequent folding, and intrusion of La Posta-type plutons. Metamorphic and pressure/temperature constraints Metamorphic grade and Al-in-homblende determined pressures vary systematically across the fan structure. Higher P-T conditions are apparent for the central domain and upper parts of both the western and eastern domains. Pressures Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 57 decrease dramatically down-structure on the western side and moderately on the eastern side of the fan structure. Emplacement depths of selected plutons were calculated using Al-inhomblende barometry summarized in Table 2.1. All samples contain the buffer assemblage plagioclase, K-feldspar, quartz, hornblende, biotite, titanite, and Fe-Ti oxides. Mineral compositions were determined by C. Kopf using the JEOL JXA-8900 electron microprobe at the University of Minnesota with analytical conditions of 15 kV accelerating voltage, 20 micro-amp sample current, and counting times of 10-20 seconds. Temperature determinations are based on calibration B of the amphiboleplagioclase thermometer presented by Holland and Blundy (1994), and hornblende structural formulae (and calculated FeO, Fe20j are based on the 23 oxygen formulation utilized in that calibration. Al-in-homblende barometry is based on the calibration of Anderson and Smith (1995) at specified temperature, with temperature and pressure calculated by iteration. Errors in calculated pressure are on the order of ±1 kbar. The discussion below incorporates observed mineral assemblages, relationships between metamorphic minerals and microstructures, hornblende barometry results, and the results of Rothstein’s (1997) metamorphic reconnaissance study. Alisitos volcanic rocks of lower greenschist grade in the western footwall of the fan structure contain chlorite, white mica, and prehnite-pumpellyite assemblages, and host hornblende tonalite plutons that yielded pressures of 2.0 and 22 kbar and temperatures o f747 and 725 °C. Although these pressures are at the lower limit for the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 58 Table 2.1. Summary of plagioclase hornblende Thermometry and Al-in-homblende Sample* A1T§ T (° C )t P (kbar)# W-139 1.05 725 22 W-138 1.1 747 2.0 R-306 0.85 685 5.8 R-971 1.90 718 5.4 S-579 1.75 702 5.3 S-591 1.85 665 5.7 S-555 1.96 676 6.4 L-413 1.57 690 5.0 L-564 1.63 709 5.0 L-637 1.53 733 4.8 E-850 1.67 686 4.8 'Letter proceeding sample number indicates plutonic suite: W-westem zone plutons, R-Rinconada piuton, S-sheeted complex, L-La Posta-type plutons, E- eastern zone piuton §A1T: total A1 atoms per 23 oxygen in hornblende t Temperature calibration: Holland & Blundy (1994) ^Pressure calibration: Anderson & Smith (199S) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 59 empirically determined barometer, they are in strong agreement with a pressure of 2 kbar determined by Johnson et al. (1999b) for the Zarza piuton in the western zone of the PRB west of the northern SSPM. Immediately east of the Rosarito fault in the Alisitos sedimentary-rich unit chlorite-dominated mineral assemblages change abruptly to phyllite-rich assemblages. Sparse garnet and biotite occurs farther to the east, and rare andalusite occurs near the top of the sequence suggesting temperatures <550 °C and pressures <4 kbar in the KFMASH system (Fig. 2.9). The Rinconada complex that intrudes just above this sequence yielded pressures of 5.4 and 5.8 kbar (temperatures of 718 and 685 °C), consistent with eastward increasing pressures. Structurally above the eastern margin of the Rinconada complex amphibole (gedrite?), garnet, fibrolite, and staurolite occur in gneisses and schists of the flysch unit suggesting temperatures >550 °C and pressures >2 kbar. In the central domain, pelitic layers in orthogneiss are commonly migmatitic and contain the assemblage biotite, garnet, fibrolite, K-feldspar, suggesting temperatures >600 °C (Fig. 2.9). Al-in-homblende determinations from hornblende tonalite sheets yielded pressures of 5.3,5.7, and 6.4 kbar (temperatures of 702,676, and 665 °C), suggesting even higher pressures in the central domain. These are similar to pressures of -5.8 kbar and temperatures of -670 °C determined by Rothstein (1997) for migmatites in the Canon Baroso area 15 km along strike to the north using metamorphic mineral equilibria constraints in the CNKASH system, and are among the highest pressures determined in the PRB. Pressures of 4.8 and 5.0 kbar at 733,709, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 350 450 550 650 750 Temperature (°C) Figure 2.9. Metamorphic mineral assemblages and their location in a petrogenetic grid of the KFMASH system from Rothstein (1997). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 61 and 690 °C were determined for samples from the La Posta-type piuton in the central domain, indicating that this domain may have been exhumed <1 kbar by the time these plutons were intruded. These are comparable to an Al-in-homblende 5.2 ± 0.6 kbar pressure estimate for the ~97 Ma SSPM piuton in the northern SSPM from which orthogneiss host rocks yielded a 5.8 ± 0.6 kbar pressure (Ortega-Rivera et al., 1997). The eastern domain of the fan structure contains the assemblage biotite, garnet, fibrolite, K-feldspar, again suggesting conditions >600 °C that are corroborated by strongly migmatitic rocks in the upper part of this package. Rocks in the footwall of the eastern domain contain andalusite and sillimanite in a variety of relationships including andalusite only, sillimanite (and/or fibrolite) only, and andalusite overprinted by fibrolite. In the strongly folded western part of the miogeoclinal assemblage fibrolite is abundant and occurs as the only aluminosilicate mineral. The eastern part of the miogeoclinal assemblage contains all three of the above aluminosilicate relationships in different regions, and andalusite preserves folded internal inclusion trails that are the oldest discernible fabrics in the southern SSPM. The southern aureole of the Agua Caliente piuton contains andalusite without fibrolite, suggesting relatively shallow emplacement levels that are further corroborated by evidence for porphyritic textures in the piuton. Elsewhere, sillimanite occurs in the assemblage staurolite, biotite, and garnet, constraining temperatures to 600-700 °C and pressures to 2-8 kbar in the KFMASH system (Fig. 2.9). This is corroborated by peak metamorphic conditions of —4.1 kbar and -610 °C obtained by Rothstein (1997) from the miogeoclinal assemblage in this region. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6 2 Reconnaissance mapping in the Sierra San Felipe, 15 km across Valle Chico to the east, identified the metamorphic assemblage andalusite, sillimanite, actinolite, biotite, muscovite in psammitic country rocks with orthogneiss that is lithologically similar to the Agua Caliente piuton (W. Marko and K. Schmidt, unpublished mapping). A hornblende tonalite piuton from the Sierra San Felipe yielded a 4.8 kbar pressure at 686 °C for Al-in-homblende (Table 2.1), and Rothstein (1997) obtained mineral equilibria results of -5.0 kbar and ~610 °C in the same region. This suggests that peak metamorphic conditions were similar across this portion of the eastern PRB. Discussion To date structural studies have largely concentrated on specific parts of the PRB. This study has examined a nearly complete and well-exposed transect across the transition zone in the PRB and thus has important implications for the following: (1) the timing and duration of magmatism and deformation in the PRB; (2) the distribution of Mesozoic strain across the arc; and (3) the relationship of the western zone of the PRB to the North American continental margin and origin of the crustal discontinuity that separates them. Below I review timing relationships in this part of the batholith followed by a discussion of factors that may have lead to the development of the fan including the influence of pre-existing heterogeneity, syntectonic magmatism, and transpression. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 63 Timing relationships and duration of deformation Figure 2.10 summarizes timing relationships in the southern SSPM and the discussion below addresses the development of deformation within the fan structure (from oldest to youngest) and implications for the tectonic evolution of the PRB. The oldest structures evident in this part of the PRB are the E-W trending folds and cleavage in the miogeoclinal footwall that are probably of Permo-Triassic age and associated with ~N-S contraction that has been described from miogeoclinal rocks in Sonora (Stewart et al., 1990). These rocks are intruded by tonalite plutons such as the Agua Caliente piuton that are now recognized as Mid-Jurassic in age using U-Pb SHRIMP techniques, and they have important implications for the age of the batholith. This unit underlies an extensive area of the SSPM, indicating that the Jurassic batholith continues well south from southern California, where it has been previously recognized, into Peninsular California and may underlie a substantial region of the transitional and possibly eastern zones of the PRB. The next discernible deformation involves thrusting in the eastern domain (Fig. 2.10). Top-to-ESE kinematics and increasing metamorphic grade across the fault that juxtaposes the eastern domain and footwall indicate thrust relationships, yet the structure places metavoicanic flysch assemblage rocks of probable Jura-Cretaceous age on miogeoclinal assemblage rocks of Precambrian-Early Paleozoic age. Moreover rocks of the Paleozoic slope basin assemblage, a regionally extensive unit in Baja California, are absent from this part of the PRB. I speculate that this fault originated as a basin-bounding normal fault during Jurassic extensional arc development that has Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 64 WESTERN FOOTWALL WESTERN DOMAIN ■FAN STRUCTURE CENTRAL DOMAIN EASTERN DOMAIN EASTERN FOOTWALL » 80 o 8 s i 90 S o itxr- »110 ^ o s 8 e S120 i B p -=130 _ (0UJ 140 _ 8150 ® a 5 3 W = S § 170 Rosarito NW folds ring mnyHams1 Thrusting in westen domain Thiusting in a n domain Steep NNE fabric Deposition | of volc-rich | Alisitos Fm " sfseckfch Alisitos unit LaSuam complex ■Deposition *of ttysch lassembL v LaPoata \ plutons (center hbtonaUa MM Thiusting associated wttheasten domain ^nnggnasT NW I folds Thiusting ineasten [ domain! I ■ of metavolc. ■ assembl. | Deposition ■ NW folds I Agua CaBanie piuton -h E-W foldsI • Tie-line h Dated piuton showing error Regional plutonic suites dated in other studies >118to~80Ma 164-132 to <97 Ma 132-100 to >132 to -80 Ma -98 Ma Western Central Eastern /E ast FW >164 and « 1 6 4 to <97 Ma WestFW Figure 2.10. Diagram of timirtg relationships across the fan structure in the southern SSPM. Tie-lines denote mutually exclusive events. Time lines solid where constrained, dashed where unknown. Ages for ring complexes in western footwall from Johnson et al. (1999b). Ages for La Posta plutonic suite from Walawender et al. (1990). Simplified cross-section A-A"’ shows very generalized ages of deformation for each domain. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 65 been proposed for the SW margin of North America (e.g. Busby-Spera. 1988). Thus, Jura-Cretaceous flysch may have been deposited on Jurassic batholithic and slope basin rocks within trough basins whose geometry were controlled by crustal heterogeneity in the transition zone of the PRB. Amphibolite layers in the Agua Caliente piuton, the orthogneiss unit, and the miogeoclinal assemblage may represent basaltic dikes that were intruded during extension; thick amphibolite layers in the flysch and metavolcanic units may have once been basaltic flows. Subsequent contraction during latest Jurassic- Early Cretaceous time inverted this fault and juxtaposed Jurassic batholithic basement on miogeoclinal basement with an intervening sliver of the Jurassic flysch basin. The development of rift basins within, and presumably controlled by, crustal heterogeneity within the PRB transition zone provides a compelling explanation for the restricted distribution of at least some of the flysch assemblage within the PRB transition zone. Mylonite fabrics associated with thrusting in the eastern domain occur in orthogneiss in the central domain but are not found in the Suerte complex, suggesting that earliest thrusting on the eastern side of the fan structure occurred after -164 Ma and before ~132 Ma. Some of the older fabrics in the flysch unit may correspond to this event, but if so they have been completely overprinted by subsequent thrusting on the west side. This sequence of events implies that both the flysch and metavolcanic units were deposited, buried, and thrust between Late Jurassic and earliest Cretaceous time. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 66 Thrusting on the western side of the fan structure appears to have been initiated before intrusion of the ~132 Ma Suerte complex. Evidence for syn-tectonic intrusion of the piuton includes the development of igneous sheets and high-temperature solid state fabrics in tonalites that parallel regional thrust fabrics. Moreover, fabrics are continuous across tonalite-gabbro contacts in the complex implying that parts of the piuton were still at near-solidus temperatures during this deformation. Early Cretaceous intrusive ages from sizable plutons such as the Suerte complex in the fan structure indicate that arc magmatism during this time was not restricted to the western zone of the batholith as implied by some island arc collision models for the PRB. Such models require two subduction zones. Continued thrusting on the western side of the fan structure deformed granite that was intruded at ~118 Ma and the Alisitos sedimentary-rich unit that was deposited at -115 Ma in either an intra-arc or intra-collisional setting (Fig. 2.10). The Rinconada complex was intruded during this deformation at -101 Ma as shown by magmatic to high-temperature solid state fabrics developed in tonalite lithologies that parallel mylonite thrust fabrics in country rocks. Some of this deformation may correspond with the proposed suturing of the western zone of the PRB between 115-108 Ma (Johnson et al., 1999a). It is unknown when deformation associated with the steeply-dipping NNWstriking cleavage in orthogneiss in the central domain began. Culmination of this deformation episode is recorded by the ~100 Ma elliptical gabbro piuton in the center of the study area, which shows extensive mingling with hornblende tonalite sheets Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6 7 containing magmatic fabrics that parallel steeply-dipping cleavage in orthogneiss screens (Fig. 2.4). Little deflection of these fabrics is apparent around the gabbro piuton margins, implying that shortening across the central domain during this episode of deformation was largely complete. In contrast, the smaller gabbro body one km to the south shows strong deflection of contacts around its margins, implying that it had been emplaced at an earlier time. The central and western domains experienced coeval deformation during this period and were mechanically coupled as suggested by the smooth fabric transition between them (Fig. 2.5b). Partitioning of deformation across this transition was probably related to Theological contrast between the two domains resulting from wholesale intrusion of hornblende tonalite sheets as discussed further below. Steeply-dipping, NE-striking crenulation cleavage formed in the central domain closely following emplacement of hornblende tonalite sheets as shown by high temperature fabrics. This deformation was relatively short lived as it is not developed in the -93-97 Ma La Posta-type plutons (Fig. 2.10). It is also apparent in the upper part of the thrust sheet in the eastern domain where WNW-dipping fabrics are developed in both migmatitic orthogneiss and adjacent hornblende tonalite sheets. In the western domain, open NE-trending folds also formed coevally with this deformation; within and around the Rinconada complex this deformation folded fabrics that originally formed parallel to thrust fabrics during emplacement. Significant deformation continued across the western domain following the generation of these NE-trending folds. The Rinconada complex was overthrust by its Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 68 eastern wall rocks, and post-intrusive, ductile and brittle, west-vergent thrusts occur along the Rosarito fault on its western margin. This thrusting accommodated up to 15 km of uplift of the fan center relative to the western footwall across the western domain between ~100 and 85 Ma, as constrained by thermochronology studies that are discussed in Chapter 3 (Fig. 2.10). Thrust faults in the Rosarito fault zone associated with this deformational episode both cut and are folded by folds in the Alisitos volcanic-rich unit, indicating coeval deformation of the western footwall and domain at this time. La Posta-type plutons also intruded the central domain during this period, and thus, although commonly considered post-tectonic (e.g. Walawender et al.. 1990). at least part of the La Posta magmatic suite is coeval with significant deformation in the batholith. Extensive denudation of the entire eastern PRB also occurred at this time, commonly facilitated by westward-vergent structures within the PRB transitional deformation belt (Todd et al., 1988; Grove, 1994; Lovera et al., 1999). One of the implications of the prolonged history of contractional deformation that is apparent for the PRB is that deformation was by no means restricted to a specific period of time. Moreover, the existence of a crustal discontinuity in the batholith is apparent as far back as Triassic time and appears to have controlled a number of early features in the Peninsular Ranges including Triassic(?)- Jurassic rifting and basin formation, and subduction-related contractional deformation that may have extended back as far as Jurassic time. These constraints do not preclude Early Cretaceous suturing in the PRB, but require a persistent crustal discontinuity within the batholith through Mesozoic time. Thus, if the present western zone of the batholith Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 69 originated as an exotic island arc, then a crustal element of transitional to oceanic composition must have existed adjacent to the eastern zone prior to suturing. Either this proto-western crust is unrecognized and occurs in fault slivers within the transition zone, was displaced relative to the PRB by strike slip faulting, or was subducted during suturing. Alternatively, the western zone may represent an oceanic element that remained attached to the North American margin during Mesozoic subduction (e.g. Thomson and Girty, 1994) or evolved as a fringing arc that collapsed against the margin in Early Cretaceous time (e.g. Busby et al., 1998). A second implication for the long-lived contractional deformation described above is that this example of doubly vergent fan structure is indeed strongly composite, having formed over the course of >40 m.y., and, as indicated in Figure 2.10, contractional deformation in the fan was at times diachronous. Moreover, this structure was localized within the PRB transitional zone, forming part of a relatively narrow, regionally extensive belt of contractional deformation. This particular structure, therefore, does not fit a single simple model for the formation of doubly vergent fan structures, but may have evolved from a number of processes as discussed below. Effects of inherited crustal heterogeneity Crustal structure varies strongly across the trend of the PRB as corroborated by passive seismic experiments in southern and Baja California. A ~3% variation in Pwave velocities is apparent across the transition zone to a resolvable depth of -20 km Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 70 (Magistrate and Sanders, 1995), and Moho depths vary across the transition zone (Ichinose et al., 1996; Lewis et al., 2000; J. Lewis and S. Day personal comm., 2000). implying that a significant change in crustal type occurs across the basement boundary in the batholith. Thus the eastern zone of this part of the PRB is underlain by continental crust that is highly folded and faulted by structures that apparently pre-date the trend of the batholith. In contrast the western zone is underlain by crust of oceanic affinity that has been only weakly deformed into broad folds. Within transitional crust between them lies a zone of complex, protracted deformation that extends along the length of the PRB. The fan structure in the southern SSPM has thus been localized in a specific zone between crustal blocks of contrasting rheology. In this respect it is similar to many orogenic belts in which deformation is controlled by laterally continuous orogen-parallel basement heterogeneity. For example in the Canadian Cordillera a belt of structural divergence that is focused within the transition from deep water to shelfal facies in Precambrian-Paleozoic sedimentary successions can be traced for nearly 2000 km (Price, 1986). The basement transition in the PRB has continued to influence deformation since Mesozoic time. In southern California, this boundary has localized segments of the Quaternary Elsinore and San Jacinto faults, both of which form part of the modem San Andreas transform system (Magistrate and Sanders, 1995). The along-strike extent of fan structure geometry in the transitional PRB deformation belt is poorly known. The western to central domains of the fan structure appear to continue into the northern SSPM (Johnson et al., 1999a; S. Johnson, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 1 personal comm., 1999); however, the eastern side of the range at this location has not been described. Similar features also occur near Ojos Negros, east of Ensenada, and include a psammitic succession with SW-vergent cleavage west of a sheeted hornblende tonalite complex that is lithologically and texturally similar to the sheeted complex in the central domain of the southern SSPM (my unpublished mapping). Elsewhere in the PRB westerly thrust vergence in steeply dipping cleavage is common within the transitional PRB deformation belt (e.g. Thomson and Girty, 1994; Todd et al., 1988; papers collected in Gastil and Miller, eds., 1993). Structures with easterly vergence appear to be rare, although large tracts of the PRB are unknown, intrudedout, and/or covered. Reconnaissance work of Gastil et al. (1975) suggests that fan structures may be exposed in other regions of northern Baja California, such as the Sierra Juarez near 32° N latitude. The presence of a crustal break across the PRB explains why the fan structure was localized in this part of the PRB, but another feature is apparent that causes the batholith to bend at this location and strike of the sides of the fan structure to be nonparallel. This bend is apparent throughout the evolution of the fan structure: the eastern thrust sheet strikes NNE, similar to structural trends in the batholith to the north, while the western thrust package strikes NW, similar to orientations to the south. The central domain contains structures with intermediate orientations and crosscutting relationships between these two trends that are as old as ~132 Ma (Fig. 2.3; 2.4). Thus the antiquity of this feature in the evolution of the fan structure precludes doming or warping of the batholith late in its history. Moreover, its apparent Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 72 uniqueness in the PRB indicates that it probably did not result from alternating regional stress fields. Instead this influence may be caused by variation in the structure of North American crust as reflected in the distribution of Paleozoic assemblages further east. The contact between miogeoclinal and slope basin assemblages is oriented almost EW across the Gulf of California and Sonora, where rocks of the slope basin assemblage have been thrust northwards over the miogeoclinal assemblage in PermoTriassic time (Stewart et al., 1990). This contact has been offset nearly 300 km by Neogene tectonism in the Gulf of California, but in the Mesozoic it intersected the eastern side of the batholith near the southern SSPM (Fig. 2.2) (Gastil et al.. 1991b). North of the southern SSPM the contact is oriented NNE-SSW, and its origin has not been defined because it is largely intruded-out and/or covered, but further north it joins the Permo-Triassic Golconda thrust system (Dickinson, 2000). Within the southern SSPM the slope basin assemblage is not exposed and flysch assemblage rocks have been thrust over miogeoclinal rocks on the shear zone in the eastern domain that parallels this NNE-SSW oriented contact. Contrast in North American crustal rheology is apparent at this location as reflected by variation in the age and orientation of structures developed in miogeoclinal and slope basin assemblages across the contact at -30.5° N latitude. In the eastern zone north of this latitude my unpublished mapping in the Sierra San Felipe and papers collected in Gastil and Miller, eds. (1993) indicate folds and cleavage that are oriented nearly E-W, similar to structures developed in miogeoclinal Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 73 rocks in Sonora. Structures of this orientation also occur in the miogeoclinal footwall of the fan structure as described above, and are overprinted by fan structure deformation within only a narrow zone adjacent to the eastern thrust fault (Fig. 2.4). In contrast, workers in slope basin rocks in Baja south of 30.5° N report NW-SE to N-S oriented folds and cleavage, parallel to the structural trend of the PRB. indicating that a broad region of the eastern zone that is underlain by the slope basin assemblage was affected by deformation associated with Jura-Cretaceous tectonism along the North American margin. (Fig. 2.2) (Gastil and Miller, eds., 1993). Thus, during Mesozoic tectonism, crust underlying the miogeoclinal assemblage may have behaved more rigidly than that underlying the slope basin assemblage further south (Gastil et al., 1991b). This condition suggests that a protrusion of miogeoclinal crust that is apparent in the southern SSPM (Fig. 2.2) may have served as a crustal indentor during evolution of the fan structure. I propose that regional subduction-related stresses were refracted around the indentor to form N-S and NW-SE structural trends respectively to the north and south of this buttress. Furthermore, the presence of a strong miogeoclinal buttress in the eastern footwall may have concentrated strain in the fan structure relative to other locations in the transitional PRB deformation belt to the south where slope basin rocks accommodated a possibly significant proportion of the total strain during Mesozoic contraction. This variation in strain may further explain why the deepest known levels of the batholith are exposed in the SSPM. Greater shortening in Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 4 this zone would be expected to correspond with greater crustal thickening and thus more buoyant crust that was subject to greater denudation. I suggest that the origin of reversal in tectonic vergence in the southern SSPM may partially result from this buttress. Basement to this region may have undergone a process of tectonic wedging such that crust within the arc impinged against the buttress from the west and was forced upwards (e.g. Price, 1986). This model has been suggested for other fan structures such as the Selkirk fan in the Canadian Cordillera which occurs across the corresponding Paleozoic miogeoclinal/slope basin assemblage transition (Price, 1986; Colpron et al., 1998). Although unconstrained it is also plausible that lower crust or lithospheric mantle to the western zone was subducted beneath the more rigid miogeoclinal crust in a manner suggested by numerical simulations of doubly vergent orogens (e.g. Willett et al., 1993; Beaumont et al., 1996). Effects of magmatism on deformation Arc magmatism in the PRB developed over a period of >60 m.y. and overlapped in both time and space with deformation. However, plutons in the PRB in general are not restricted to strongly deformed regions such as the fan structure; extensive plutonism also occurred in areas where few Mesozoic-aged faults and/or folds occur. Thus, there appears to be no one to one correlation between the distribution of plutons and tectonic structures, indicating that these processes largely operated independently of each other. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 75 Indeed within the fan structure most piuton-host rock relationships suggest that intrusion produced little effect on deformation. However, once crystallized, plutons typically behaved as more rigid objects than host rocks during subsequent deformation. For example, the Rinconada complex, a piuton with small length/width ratio, was intruded during thrusting in the western domain. Thrusting continued around the piuton following its emplacement, resulting in the strong wrapping of wall rocks around the sides of the complex. Similarly, folding in the eastern domain is strongly localized between the La Posta-type complex to the SW and Agua Caliente piuton to the NE. In contrast to many other plutons within the fan structure, the sheeted hornblende tonalite complex in the central domain does show evidence of strong interaction with tectonically deforming wall rocks, as indicated by close geometrical and structural ties between magmatic sheets and orthogneiss screens. A compelling difference between this tonalite complex and others in the PRB is that intrusion here occurred at the deepest exposed levels in the PRB, suggesting that relationships between deformation and magmatism may vary with crustal level. Similar syntectonic sheeted plutonic complexes have been described from a variety of other regions in which mid-lower crustal levels are exposed in the Cordilleran orogen including the Cascades Mountains (Paterson and Miller, 1998), Coast Ranges batholith (Ingram and Hutton, 1994), and Idaho batholith (Manduca et al., 1993). At these depths host rocks are highly ductile and, thus, intruding magma is more strongly influenced by regional Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 6 stresses (e.g. Hand and Dirks, 1992), and host rock heterogeneity (e.g. Weinberg, 1999) than at shallower crustal levels. I, therefore, suggest magma-host rock processes in the sheeted tonalite complex were synergistic, with both deformation and plutonism mutually influenced by heterogeneity and rheology to which each contributed. Pre-existing cleavage in orthogneiss provided a direction of weakness that was probably exploited by processes such as magma wedging (Weinberg, 1999). Once intruded, plutonic sheets may have provided strong heterogeneity along contacts that influenced the geometry of subsequent structures and fabrics in the orthogneiss. For example, domains of older NE-striking fabrics in the central domain occur at distances greater than -300 m away from tonalite sheets, indicating that regions adjacent to the sheets were more strongly deformed than at significant distances away from them. This interplay of heterogeneity between deformation and plutonic sheets is apparent across the central domain; both orthogneiss fabrics and tonalite sheets vary to some extent in orientation from one region to another, but they maintain parallelism in any given area. Compare, for example, sheets and cleavage in the western versus eastern parts of the central domain in Figure 2.4 that have on average NNW and NNE orientations respectively. In terms of rheology, orthogneiss at crustal depths now exposed in the central domain was probably already ductile enough to favor sheeted piuton geometries, and subsequent intrusion of tonalite sheets would have further weakened host rocks in this domain. Evidence for migmatite in NNW-striking cleavage and associated folds in the orthogneiss indicates that the orthogneiss was at minimum melt temperatures during Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 77 intrusion. Moreover, fabrics in tonalite and orthogneiss indicate that these lithoiogies had similar rheologies at this time. Magmatic and solid state fabrics in these respective lithoiogies are oriented parallel to one another, even across contacts such as the ends of tonalite sheets with no apparent fabric refraction. Because this region was at or near melting conditions it was much weaker than the rest of the fan structure, resulting in partitioning of strain into this region both during and shortly following intrusion of tonalite sheets. This variation in rheology among domains in the fan structure was not sustained for long. As the central domain cooled strong rheological contrasts become apparent among individual hornblende tonalite sheets and orthogneiss screens even while the tonalite was still near its solidus. NE-striking cleavage and folds that formed during this time are highly localized in the orthogneiss and strong fabric refraction is apparent across contacts with tonalite, indicating that tonalite sheets had attained considerable strength relative to orthogneiss host rocks. Thus, plutons in the fan structure were largely intruded independently of deformation and, once crystallized, behaved as more rigid objects than host rocks during deformation. Extensive intrusion of tonalite sheets at deeper crustal levels in the central domain of the fan structure, however, weakened this domain to the extent that deformation was partitioned, and possibly transpressive, a phenomenon noted in other orogens such as the Coast Plutonic Complex of British Columbia (Chardon et al., 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 78 Partitioning of deformation in the fan structure Deformation path was partitioned between western and central domains during intrusion of the hornblende tonalite sheeted complex at ~ 100 Ma. The western domain deformed by a simple shear-dominated mechanism as shown by asymmetric fabrics with consistent top-SW shear sense indicators. In contrast, a pure shear mechanism is inferred for the central domain based on largely symmetrical magmatic fabrics with poorly developed kinematics and high temperature conjugate shears that bisect magmatic foliation. This partitioning resulted from Theological variation within the fan structure. Tabular shaped, vertically oriented plutons that thermally weakened the central domain caused the development of pure shear dominated deformation path. Whether or not this permitted the fan structure to accommodated transpression is discussed below. The smooth variation of fabrics with no apparent cross-cutting or overprinting relationships suggests that the western and central domains were indeed mechanically coupled during this time, and thus kinematic partitioning may have also occurred across these two regions (e.g. Goodwin and Williams, 1996). The change in fabric orientations, particularly the variation in plunge of Iineation from moderately ENE to moderately NNW between the western and central domains (Fig. 2.5b), is similar to fabric transitions described from other examples of transpression at mid-crustal levels (e.g. Robin and Cruden, 1994). If the flow direction during deformation was parallel to Iineation in the central domain, then the western domain would have had a SW flow velocity relative to the central domain, and thus, a sinistral displacement component Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 79 would be apparent across the fan structure. This shear sense is in agreement with sinistral oblique convergence predicted by relative plate motion estimates for the Early Cretaceous Cordilleran margin (Kelley and Engebretson (1994) refinement of Engebretson et al., (1985)). Moreover the development of late NE-striking cleavage in the central domain and NE-plunging folds in the central and western domains closely followed intrusion of the tonalite sheeted complex, suggesting quite strongly that shortening across the PRB had changed to a more NW-SE direction, and was thus oblique to the margin at this time. Importantly, intrusion of the tonalite sheeted complex appears to have controlled the lateral strength geometry of crust within the fan structure as discussed above and, thus, determined whether oblique plate motion was manifested as transpressional deformation in the arc or whether contractional structures formed oblique to the orogen. As long as the central domain remained relatively weak due to melt presence and/or near solidus temperatures, kinematic partitioning occurred within the arc such that contraction was accommodated in the western domain while transpression occurred in the center. Once tonalite sheets cooled and strengthened, partitioning could no longer be sustained and NE oriented structures formed across all domains in the fan structure. The orthogneiss unit occurs across the fan structure and provides some constraints that limit the amount of possible transcurrent displacement in the PRB transition zone. Orthogneiss screens are continuous from the eastern domain into the sheeted tonalite complex and show no appreciable deflection. Additionally, where Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 80 older fabrics in the orthogneiss are preserved, they are oriented similar to fabrics in orthogneiss in the eastern domain, again indicating little to no rotation of the orthogneiss. These constraints are consistent with less than a few kilometers of possible strike-slip displacement across the central domain. Finally the period for which strong strength contrasts developed between western and central domains was probably only a few million years, the time for tonalite sheets to intrude, crystallize, and cool slightly below their solidus, and, thus, little time was available in which to accumulate significant strike-slip displacement. At the very most a few 10’s of km of displacement could have accumulated across the central domain during this time, assuming typical transform plate boundary displacement rates. Consequently, transpression may be a localized feature in the SSPM resulting from local crustal heterogeneity effects rather than a margin-wide phenomenon. Several observations suggest exercising additional caution with regards to a transpressional model for part of the evolution of the fan structure. Fabrics in the western domain continued to record deformation long after -100 Ma, the time at which fabrics ceased to form in the central domain. Thus the finite fabric in the western domain may not be comparable with fabrics in the central domain even though they form a smooth transition and lack overprinting relationships. Furthermore, fabrics in the southern SSPM contrast with transpressional features described from other regions. The possible transcurrent component of transpression in the fan structure occurs in the central domain for which pure shear dominated deformation is apparent Thus, sinistral kinematic shear indicators are not developed in this domain, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8 1 contrary to most transpression models and descriptions of transpressive structures which emphasize simple shear dominated deformation and well-developed kinematics associated with domains that accommodate largely transcurrent deformation. An alternative explanation for the fabrics that occur in the central domain is that they represent complex flow within the rheologically weakened interior of the fan structure. As brought out by a number of recent structural studies, deformation in high strain zones is typically characterized by complex three-dimensional flow in which flow velocity vectors are difficult to predict (e.g. Jiang and Williams, 1998). Consequently, lineations may not necessarily indicate either the finite stretching or transport directions within rocks. The presence of crustal heterogeneity such as the crustal boundary within the PRB and the likelihood of a buttress in the North American margin may have induced a complex response within the weakened central domain. Thus, my constraints suggest that transpression in the PRB was minor if it occurred at all, and remains to be tested as the geology of the Peninsular Ranges is further explored. Evolution of fan structure in the southern SSPM A likely evolution of the southern SSPM is illustrated in Figure 2.11. Figure 2.1 la is a cartoon showing the basic configuration of this part of the Cordilleran margin in Jura-Cretaceous time. Crustal heterogeneity inherent to the North American margin produced at least two effects in the evolving deformation zone. First, a crustal boundary between outboard oceanic and inboard continental basement within the PRB Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 82 Figure 2.11. Block diagrams showing hypothetical structural evolution of the fan structure, (a) Overview of setting in which fan structure evolved including heterogeneity imparted by transition zone in batholith and buttress created by relatively rigid North American crust underlying miogeoclinal assemblage rocks, (b-d) Series of time slices illustrating evolution of the fan structure. Map surfaces correspond with present erosion level. Note that significant difference in crustal level occurs between across western flank of the structure previous to 100 Ma. Units discussed in text labeled when they first appear. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 00 LU 84 served to localize deformation in the transitional PRB deformation belt. Second, a buttress in Precambrian-Paleozoic North American crust caused stress refraction, resulting in a bend in the evolving batholith. The presence of these influences during evolution of the fan structure suggests that tectonic wedging may have been an important process leading to fan geometry. The fan structure also evolved within an active arc, and as shown in Figure 2.1 la, plutonism occurred across a broad region. For the most part magmatism operated independently of deformation, with minor postintrusive effects created by more rigid behavior of some plutons relative to host rocks during subsequent deformation. A notable exception occurred within a few km-wide crustal zone in the center of the fan structure where extensive sheeted tonalite intrusions created strong rheological gradients across which deformation was partitioned. This zone accommodated complex flow within the fan structure that may have included minor sinistral transpression for a period of time limited to the time at which the sheeted tonalite complex was at near-solidus conditions. Block diagrams in Figure 2.11 b-d show progressive development of the fan structure within the PRB deformation belt. Prior to the first time step in Figure 2.11 b Jurassic orthogneiss intruded miogeoclinal assemblage rocks that contained older E-W trending structures. Following intrusion flysch successions were deposited in troughshaped basins that formed along the PRB transition zone. Most likely these were extensional basins that were active while the Jurassic arc was extensional (e.g. Rangin, 1978; Gastil et al., 1981; Busby-Spera, 1988), and were localized along the crustal discontinuity in the Peninsular Ranges. Later, in Jurassic and/or Early Cretaceous Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 85 time, the arc became compressional and thrusting initiated on the eastern side of the fan structure, reactivating some of these basin-bounding normal faults. Crust located west of the fan structure at this time consisted of either basement to the present oceanic western zone or oceanic to transitional forearc crust that has since been subducted. By ~132 Ma thrusts on both sides of the fan structure were active, resulting from tectonic wedging between Theologically contrasting crustal blocks in the PRB (Fig 2.1 lb). Arc magmatism was also active in this region as represented by the syntectonic La Suerte complex. Contractional deformation continued within the fan structure in the PRB transition zone even as arc-parallel extension apparently occurred at near surface levels from ~130-120 Ma in the western zone, forming intra-arc basins, caldera complexes, and dike swarms (Busby et al., 1998). Several tectonic settings are possible for the apparently extensional western zone at this time, including that it was: (1) an exotic island arc complex; (2) a fringing arc that had been rifted to an outboard arc position in the Jurassic (Busby et al., 1998); or (3) a forearc element to the eastern zone. Alternatively, upper crustal levels may have experienced extension as middle crustal levels underwent contractional deformation at this time. Thrusting continued on the sides of the fan structure as deformation progressed into the fan interior by at least -100 Ma (Fig. 2.1 lc). Most granite had been intruded and the Rinconada and sheeted tonalite complexes were emplaced during contractional deformation at this time. Partitioning of deformation is apparent between the western side and center of the fan structure due to strong Theological gradients created by Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8 6 intrusion of the extensive sheeted tonalite complex. The weakened center thus deformed by a more pure shear-dominated mechanism and may have accommodated limited sinistral transpression. Collision of the western zone may have occurred closely preceding this time (Johnson et al., 1999a), with the sedimentary-rich facies of the Alisitos Fm. deposited in an intervening basin. Alternatively this unit may have been deposited in an intra-arc or backarc basin. By -90 Ma thrusting continued in the western domain, including more brittle deformation along the Rinconada fault that accommodated uplift of the fan structure and possibly the entire eastern zone of the PRB (Fig. 2.1 Id). The La Posta-type plutons had intruded the central domain, and much of the folding in the eastern domain had occurred by this time. Conclusions The transitional PRB deformation belt in the southern SSPM is an excellent example of focused deformation by crustal heterogeneity within continental-margin arc systems. Deformation in these tectonic environments can be long-lived and contractional through much of their evolution, overlapping in both time and space with arc magmatism. During relatively short time intervals lasting a few m.y., strong rheological gradients may form within the arc due to intrusion of large sheeted plutons that can further localize and partition of deformation. Complex, composite structures such as the doubly vergent fan structure in the southern SSPM can result Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8 7 A number of relationships identified in this study indicate that the PRB transition zone is a fundamental crustal boundary that existed for at least most of the Mesozoic. This transition between oceanic and continental crustal blocks has controlled the following features in the Peninsular Ranges: (1) the source and host rock characteristics that provided first-order controls on pluton compositions across the batholith; (2) the location of margin-parallel, elongate basins; (3) differential exhumation of the batholith; and (4) the location and geometry of complex contractional deformation that, in the southern SSPM, formed a fan structure that evolved over >40 m.y. This long history of focused contractional deformation implies that oceanic or transitional arc crust lay to the west of the eastern zone of the batholith for much of the Mesozoic. Thus the western PRB has either replaced pre-existing forearc crust or evolved entirely within a forearc position relative to the eastern zone of the PRB. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 88 CHAPTER 3: THE ROLE OF CRUSTAL HETEROGENEITY IN DIFFERENTIAL EXHUMATION OF THE CENTRAL PENINSULAR RANGES BATHOLITH Introduction The highest rates of surface uplift and erosion on Earth occur at convergent tectonic plate boundaries that are characterized by regions of high average elevation, significant topographic relief, and/or rapid exhumation. These boundaries include arc environments, which are one of the most dynamic tectonic settings, distinguished by internal orogenic processes such as crustal deformation, metamorphism, and plutonism. The connections between these internal and surficial orogenic processes are enigmatic and complex, but significant strides have been made within the past few decades towards understanding them. These studies have used a wide variety of techniques such as low temperature thermochronology and geomorphology at surficial and near-surface crustal levels and tomography and mantle xenolith studies at lower lithospheric levels. However the role of mid-crustal processes in mountain building is less well understood because, for example, pressure-time paths are more difficult to constrain for higher temperature thermochronology systems and deformation history is typically very complex due to extensive structural overprinting during exhumation. Moreover, at these crustal levels in particular, the effect of crustal heterogeneity on mountain building processes in arcs is controversial, yet there is abundant evidence of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 89 heterogeneity, both from active magmatism as well as inherited structures and terrane boundaries in most of these settings. In Chapter 2 I explored the structural evolution of part of the crustal transition in the PRB and identified several forms of crustal heterogeneity that strongly influenced deformation in this region. In this chapter I incorporate thermochronologic data to constrain the cooling history of this part of the PRB. Early geochronologic studies in the northern PRB identified distinct differences in cooling histories between the western and eastern zones of the PRB. These studies noted variable, mostly Early to mid-Cretaceous, K-Ar ages in the western zone of the batholith and a systematic eastward younging of Late Cretaceous K-Ar ages across the trend of the eastern zone of the batholith (Krummenacher et al., 1975). Subsequent studies utilizing a variety of other geochronologic systems have bome-out these features in a number of other areas in the PRB (e.g. Silver et al., 1979; George and Dokka, 1994; Grove, 1994; OrtegaRiveraetal., 1997; Rothstein, 1997). In the study presented here, I specifically targeted the transition zone in the batholith where these disparate cooling histories change in conjunction with a sharp transition from upper to mid-crustal exposure levels. The fan structure that developed across the transition zone in the SSPM through much of Early Cretaceous time accommodated rapid uplift of the eastern zone of the batholith in the mid- to Late Cretaceous. ^Ar/^Ar and fission track thermochronology systems show a distinct step in cooling ages across the inverted metamorphic gradient on the western side of the fan structure, constraining uplift and denudation of the region to the east to —100 - <85 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 0 Ma. Thick conglomeratic sequences along the west coast of Baja California record the sudden onset of this exhumation. In this chapter I focus on gradients in cooling ages that are evident across the fan structure in the PRB transition zone, use apatite fission track annealing models to detail temperature-time paths across the structure, and tie this data set to the region's well-constrained structural and plutonic history (Chapter 2), sedimentary record (Bottjer and Link, 1984; Dorsey and Bums, 1994), metamorphic history (Rothstein. 1997; C. Kopf, in prep.), and geophysical constraints (Lewis et al., in prep.). From these data sets I conclude that contrasts in crustal composition and structure between the western and eastern zones of the batholith determined the extent and dynamics of uplift and denudation across the PRB. The crustal discontinuity between these two zones played a fundamental role during exhumation. The western side of the fan structure that occurs along this discontinuity accommodated >10 km of differential exhumation across the batholith in mid- Late Cretaceous time. The western zone of the batholith remained relatively stable and at low average elevation during the Late Cretaceous and Tertiary. In contrast the eastern zone probably achieved >55 km crustal thicknesses by mid-Cretaceous time and experienced dramatic uplift, possibly forming a high plateau across much of its extent The concentration of contractional strain into the fan structure probably led to even greater crustal thicknesses in this part of the transitional zone, and it is very likely that crustal depth varied by >20 km across a narrow belt between western and transitional zones, leading to a sharp topographic break on the western margin of this plateau. Although unconstrained, the presence of a Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 1 dramatic escarpment facing a large ocean basin such as the Pacific, suggests that rapid erosion may have ensued, possibly coupling with deformation in the fan structure, leading to further thickening of crust beneath the plateau followed by eastward migrating erosional retreat. The abrupt onset of exhumation in the PRB at ~ 100 Ma coincides with an increase in plate convergence along the North American Cordilleran margin that may have initiated surface uplift by increasing the rate of structural thickening in the orogen. Rapid intrusion of the volumetrically significant La Postatype magmatic suite in the PRB at this time indicates that mantle processes such as lithospheric mantle delamination were also important, but restricted to the transitional and eastern zones of the batholith. Processes driving exhumation High topography results from crustal buoyancy, the internal strength of the orogen, and the extent to which the sides of the orogen are supported by orogen normal compressional stress and loading of crust to either side of an orogen by sedimentary basins (e.g. Small and Anderson, 1995). Crustal thickening resulting from contractional deformation and addition of crust by magmatism are two m echanisms of increasing crustal buoyancy. For instance in the Himalayas, regions of greatest surface elevation, exhumation, and denudation by low-angle normal faulting correspond with regions of most pronounced thrust faulting (Treioar and Rex, 1990; Burchfiel et al., 1992a), and in much of western North America crustal thickening by m agmatic processes has been proposed (Johnson, 1991). However, in order to contribute to 3*V- * ... :: ...... . Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 2 crustal buoyancy magmas must either be derived from eclogite facies regions, and thus transform denser lithospheric components to lower density derivatives upon intrusion into overlying crust, or lower lithosphere containing dense melt residuals must be removed (England and Houseman, 1988; Willett and Beaumont, 1994). Other mechanisms that affect crustal buoyancy involve lithospheric mantle dynamics that include coupling of lithospheric mantle and downward flowing asthenosphere leading to subsidence of the overlying lithosphere (Mitrovica et al., 1989; Stem and Holt, 1994), impingement of upward circulating asthenosphere on the base of the lithosphere, leading to uplift of the overlying region (e.g. Houseman and England. 1986), or outright sinking (delamination) of lithospheric mantle, leading in most circumstances to increased buoyancy of the remaining crustal lithosphere (Fig. 3.1a; e.g. Wernicke et al., 1996). Specific mantle processes are typically difficult to constrain and require innovative approaches such as detailed geophysics and mantle xenolith petrology that has recently been applied to the high Sierra Nevada (e.g. Jones et al., 1994; Ducea and Salebee, 1996; Wernicke et al., 1996). High standing topography is subject to higher gravitational stresses than surrounding crust. This commonly results in localized collapse of regions with high average elevation by normal faulting at upper crustal levels (Fig. 3.1b; e.g. Dewey, 1988). This may even occur during contraction in an orogen, resulting in extrusion of mid- to upper crustal material by a combination of reverse and normal faulting (Fig. 3.1b; e.g. Burchflel et al., 1992a; Johnston et al., 2000). However some orogens appear not to Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 93 Figure 3.1. Examples of processes in arc that may contribute to exhumation, a) Lithospheric mantle processes including slab delamination (e.g. Kay and Kay, 1993) and ablative subduction (Tao and O’Connell, 1992). b) Mechanisms of tectonic denudation including extrusion (left) and extension of gravitationally unstable region of high elevation (right), c) Contractional orogen depicting denudation by erosion and coupling between climate and deformation (Willett, 1999). 94 reach this stage of development because erosion is effective enough to keep up with uplift (Fig. 3.1c; e.g. Small and Anderson, 1995; Willett, 1999). The thermal structure of crust plays a critical role in crustal strength, and thus the support of high topography. Temperatures in orogenic regions are anomalously high as compared with temperatures at comparable depths in stable continents. As discussed in Chapter I, this results from: (1) crustal thickening and redistribution of radiogenic elements within orogenic crust coupled with the effects of erosion at the surface and accretion of sialic material at the base of orogens during subduction (Huerta et al., 1996); and (2) advective heating of large orogenic regions by intrusion of magma (Barton et al., 1988), as well as further enrichment of crust in radiogenic elements by magmatic processes. Significant thermal weakening of orogens leads to lateral flow of lower crust to form broad orogenic welts of high average elevation that are apparent in regions such as the Tibetan and Altiplano-Puna plateaus (Royden, 1996). Finally denudation strongly influences not only the thermal configuration, but also the deformation dynamics, of orogens by the following mechanism. Contractional deformation within an orogen leads to thickening of crust, which in turn results in isostatically compensated surface uplift and the creation of high topography. However, the development of topography leads to an increase in erosion by orographic enhancement of precipitation. Thus, a feedback mechanism forms between erosion and deformation that is linked through material mass balance and gravitational stresses and is directly influenced by the internal strength of the evolving orogen (Fig. 3.1c; ■'■X' Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 95 Willett, 1999). Other climatic effects such as the direction and strength of prevailing weather patterns may have additional and profound influences on the geometry and structural evolution of orogens (Willett et al., 1993; Hoffman and Grotzinger, 1993). Numerical modeling of young orogens such as the southern Alps of New Zealand suggests that coupled deformation and exhumation processes may form a predictable pattern of structures and crustal exposures across an orogen (Willett, 1999). Clearly the best regions to study coupling between erosional and deformational processes are actively forming mountain belts. However, to understand how deeper crustal levels are involved in this coupling requires examination of deeply exhumed ancient orogenic regions. Thus, a comparison between patterns of deformation and exhumation identified at surface level in active orogenic regions and at deeper crustal levels in ancient arcs is a valuable exercise towards understanding coupling between near-surface and deeper crustal processes, particularly when integrated with other geological, geochronological and geophysical data sets. Below I describe an ideal region to do this, in the Mesozoic Peninsular Ranges batholith, where the deformational, magmatic, metamorphic, sedimentary, and exhumation history are unusually well constrained from mid-crustal levels of what once was probably an Andean-style and -scale orogenic system. Moreover, abundant evidence for the influence of crustal heterogeneity in this orogen affords an opportunity to explore the control this factor has on mountain building processes in active tectonic margins. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 96 Post-Early Cretaceous evolution of the PRB The crustal boundary that strongly localized deformation along the axis of the PRB through much of the Early Cretaceous continued to influence orogenesis through Late Cretaceous time. Regional metamorphism and deformation in Peninsular California occurred concurrently with intrusion of the PRB in the mid-Cretaceous (Gastil et al., 1975; Todd et al., 1988). A sharp eastward increase in metamorphic grade and pressures coincides with the crustal boundary in the PRB, resulting in juxtaposition of upper and mid-crustal levels in the western and eastern zones of the batholith respectively. On the western side of the fan structure in the SSPM this metamorphic break includes a spectacular inverted metamorphic gradient associated with SW vergent thrusting. Evidence for extensional deformation in the fan structure at any time is notably absent Arc magmatism that had persisted since early Mesozoic time culminated in intrusion of the volumetrically significant La Posta magmatic suite in the transitional and eastern zones of the batholith in the mid-Cretaceous. By Late Cretaceous time arc magmatism had migrated eastward to the present mainland of Mexico (Coney and Reynolds, 1977; McDowell and Mauger, 1994), and denudation of the eastern PRB had begun, producing an eastward-younging pattern of cooling ages for all thermochronologic systems (e.g. Krummenacher et al., 1975). Deposition of thick conglomeratic sequences on a narrow coastal plain along much of the western coast of the present Baja peninsula initiated coevally with this denudation, and the lithology, chemistry, and metamorphic grade of clasts in this sediment indicates that it was derived from the eastern batholith (Bother and Link, 1984; Lovera Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 7 et al., 1999). By late Paleogene time the ancestral Peninsular Ranges had been extensively denuded and powerful extraregional rivers were transporting distinctive exotic sediment (Poway clasts) derived from Jurassic volcanic deposits in Sonora (Minch, 1979; Abbott and Smith, 1983). In late Eocene time rejuvenated uplift on Cretaceous structures disrupted this fluvial system (Axen et al., 2000), an event that may correspond with a regional unconformity on which locally-derived alluvial material of pre-Early Miocene age was deposited in the SSPM (Dorsey and Bums, 1994). Arc magmatism was reestablished between -24-16 Ma and rift-related volcanism initiated in the northern Gulf of California by -12 Ma (Sawlan, 1991; Martin-Barajas et al., 1995). The eastern flank of the SSPM forms a spectacular escarpment up to 2.5 km high that is controlled by the 11 Ma to Recent Sierra San Pedro Martir normal fault, a rift-bounding structure to the Gulf extensional province to the east that has accumulated at least 5 km of displacement (Stock et al., 1991). A geomorphic surface(s) is preserved on the western side of the range which dips gently towards the west coast of the peninsula in a manner similar to ranges to the north that preserve surfaces incised by large Eocene paleoriver channels (Fairbanks, 1893; Minch, 1979). A thick pile of Miocene and younger voicanics and locally derived sediment lap onto the SSPM from the Puertecitos volcanic field to the south (Dorsey and Bums, 1994). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 8 Sampling and analytical methods Samples were systematically collected across a traverse of the fan structure in the southern SSPM and its western and eastern footwalls (Fig., 3.2 and 3.3). Care was taken to sample across specific structures within the PRB transition zone in order to characterize their role in exhumation. Where possible, separates of several minerals were made from the same sample. Zircon, biotite, K-feldspar, and apatite were separated using conventional density and magnetic techniques followed by hand selection. Detailed analytical procedures are given in Appendix B, C, and D. Zircon was analyzed by standard U-Pb isotope dilution methods at San Diego State University and by SHRIMP and SHRIMP-RG methods at the Australian National University and Stanford University respectively (Appendix B). Analyses were performed by Drs. Melissa Girty, Mark Fanning, and Joseph Wooden at respective labs. I analyzed biotite and K-feldspar by ^Ar/^Ar step-heating methods in Dr. Mark Harrison’s lab under the direction of Dr. Marty Grove at the University of California, Los Angeles (Appendix C), and apatite was analyzed by fission track methods by Dr. Ann Blythe at the University of Southern California (Appendix D). Irradiation constants, experimental conditions, and complete data tables are given in respective appendices. Table 3.1 summarizes results from all methods. Standard U-Pb zircon isotope dilution methods were performed on samples of the Suerte complex, sheeted tonalite complex, and a sample of granite from the upper part of the western thrust package that was originally collected by Goetz (1989) and reanalyzed in this study (Appendix B, Table B.l). U-Pb SHRIMP analyses were Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 9 Figure 3.2. Generalized geologic map across the PRB at the latitude of the Sierra San Pedro Martir. Formlines denote general structural trends in the fan structure which occurs in the transition zone in the batholith. Only faults with Tertiary history are shown. Modified from Gastil et al. (1975). Geochronology from Ortega-Rivera et al., 1997 (many results are averaged); Rothstein, 1997; Johnson et al., 1999a; Dorsey and Cerveney, 1991; Cerveney et al., 1991; and P. Cerveney, R. Dorsey, J. Corrigan, B. Bums unpublished data). Representative analyses are shown from the southern SSPM. Samples collected from outside of area of Figure 3.3 and analyzed as pan of this study are indicated with sample numbers in parentheses. Geobarometry from Rothstein (1997)(metamorphic mineral phase equilibria) and C. Kopf (Al-in-homblende). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 100 116° 00' 115° 00' a Geobarometry (in kb) Geochronology (in Ma) ♦ 101 zirc U/Pb SHRIMP ■ 103 zirc U/Pb • E Hornblende 40Ar/39Ar • ® Biotite 40Ar/39Ar • so Biotite K/Ar • CED Apatite fission track I Quaternary Sediment J and volcanic rocks Early Cretaceous Alisitos volcanic assemblage ] Cretaceous-Quaternary sedimentary rocks Jura-Cretaceous and granite plutons r i , \i Jura-Cretaceous volcanic frX VI rocks and plutons in fan-like structure [..... 1 Precambrian-Paleozoic I I miogeoclinal assemblage Rinconada \gua 118+3 West FW West .domain Volcanic and sedimentary assemblages Agua Caliente Pfuton Rinconada complex / synform/ overturned antiform/ overturned Sierra-San-; -Reiko Martie ’fault* ::' /,La Posta- ,'i type(?) s J / Pluton QT cover fault normal/ reverse Alisitos volc-rich Alisitos sed-rich Flysch Metavolcanic Miogeocline • Al in Hblnd (kbar; C. Kopf) ♦ zircU/Pb SHRIMP age (Ma) ■ zirc U/Pb age (Ma) • biotite40Ar/39Artotal gas age (Ma) • <^> K-felds40Ar/39Artotal gas age (Ma) • (83 ) apatite fission track age (Ma) Not mapped KF Rpsarito fault . [ CentralV domain\FW 5 km / East Plutons of Peninsular Ranges batholith | | hb Tonalite | | bi granodiorite | | bi Tonalite | | Granite | [ Orthogneiss | | Gabbro Figure 3.3. Geologic map of study area in the southern Sierra San Pedro Martir. Formlines show general strike of solid state foliations (long lines), and magmatic foliation (short lines). Results from U-Pb, Ar/Ar, apatite fission track, and Al-in-hornblende analyses are shown. 101 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 3.1 Summary of geochronology in the southern SSPM. Distance Sample Unit Location U-Pb age *’Ar/"Ar biotite "Ar/wArK- Apatite fission Mean track (km)* number Lat. (N) Long. (W) (Ma) age (Ma) felds age (Ma) track age (Ma) length (pm) (n) Western innc •29.3 SPI40a lib tonalitc-EI Ciprcs 30* 22.19’ 115° 38.93’ 108.0 ±2.7 101.7 ± 15.6 13.6 ±0.4 (7) •24.7 SPI40b lib tonalitc-EI Ciprcs 30° 22.55’ 115° 35.99’ 52.2 ± 7.4 13.4 ±0.3 (10) •77 SPI39 Hb tonalite 30° 26.92’ 115° 26.55’ 102.3 ± 05 76.8 ± 6.2 14.4 ± 0.1 (III) •6.9 ZP40 Hb tonalite- Zarza 30° 53.0' 115° 49.5’ 110.9 ± 1.4 -5.2 SPI38 Hb tonalite 30° 29.04’ 115° 26.17’ 100.9 ± 0.6 94.7 ± 0.5 82.8 ± 9.7 13.7 ±0.2 (45) Western domain of fan structure 0.7 SPI36 Hb tonalite 30° 31.20’ 115° 23.71’ 87.9 ± 0.5 67.1 ±4.9 13.2 ±0.2 (29) 2.0 SP820 Hb tonalite- Rinconada 30° 32.13’ 115° 23.51' 101.3 ±0.6 86.3 ± 4.2 14.4 ±0.1 (105) 3.4 SPI33a Granite 30° 29.68’ 115° 19.15' 89.6 ± 1.6 85.5 ± 0.2 80.3 ± 6.7 13.5 ±0.1 (87) 3.4 Goetz Granite 30° 29.55’ IIS’ 18.80' 118.2 ±2.6 3.6 SPI97 Schist- flysch 30° 31.26’ 115° 22.19’ 89.7 ±0.5 5.6 SPSI9 Hb tonalite-La Suerte 30° 30.80' 115° 18.03’ 85.7 ± 1.9 7.1 SP73 Hb tonalite- La Suerte 30° 32.56’ 115° 18.28’ 83.8 ± 0.6 74.7 ± 3.8 13.0 ±0.2 (72) 7.1 SP90 Gabbro- La Suerte 30° 32.56’ 115° 18.28’ 132 ±7 Central domain of fan structure 9.6 SPS40 Bi orthognciss- sheeted complex 30° 32.64' 115° 16.09’ 70.9 ± 5.4 13.8 ±0.1 (86) 10.1 SP835 Bi orthogneiss- sheeted complex 30*33.85’ 115° 17.62' 164.4 ± 1.2 11.0 SP68 Bi ortbogneiss- sheeted complex 30° 32.86’ 115° 16.59’ 89.9 ± 0.4 12.1 SPS43 Bi orthogneiss- sheeted complex 30° 33.92’ 115° 15.25’ 65.9 ± 3.7 13.7 ±0.1 (100) 12.2 SP832 Gabbro- sheeted complex 30° 35.28’ 115° 16.74’ 100. I± 0.5 13.8 SPS69 Hb tonalite- La POsta type 30° 34.92’ 115° 14.46’ 85.2 ± 2.0 69.2 ± 4.7 13.5 ±0.1 (102) 17.3 SP7 Bi orthogneiss- sheeted complex 30° 38.58’ 115* 15.46’ 82.8 ± 0.5 Eastern domain of fan structure 17.5 SP409 Hb tonalite- La Posla type 30° 37.94’ 115° 15.10’ 85 7 ± 1.4 70.3 ±4.1 13.3 ±0.1 (108) 19.3 SP9 Bi orthogneiss- east side 30° 39.03’ 115° 14.09’ 81.6 ±0.4 20.4 SP374 Schist- metavolcanic 30° 39.34’ 115° 13.62’ 65.9 ±4.4 13.1 ±0.6(5) 20.5 SP448 Bi orthogneiss- east side 30° 40.35’ 115° 14.41’ 87.3 ± 1.6 20.8 SPI7 Schist- metavolcanic 30° 39.40’ 115° 13.61' 82.9 ± 0.7 21.2 SP453 Bi orthogneiss- cast side 30° 41.71’ 115° 15.28’ 85 3 ± 14 22.2 SP44I Bi orthogneiss- cast side 30° 40.63’ 115° 13.22’ 81.3 ± 14 Eastern rone 23.6 SP20 Bi orthognciss- Agua Calicntc 30° 39.47’ 115° 10.71’ 83.4 ± 0 5 71.7 ± 0 3 51.5 ±2.4 13 2 ±0.1 (105) 23.6 SP849 Bi orthogneiss- Agua Calicntc 30° 39.58’ 115° 10.63’ 164.3 ± 2.3 * Distance from Rosarito fault, east is positive. 102 103 completed on samples from orthogneiss host rock in the tonalite sheeted complex, the Agua Caliente pluton, Rinconada pluton and a gabbro pluton in the tonalite sheeted complex. Procedures and data are listed in Appendix B and Tables B.2 and B.3. Ar isotope systematics for biotite and K-feldspar samples from plutons in the batholith were analyzed using step furnace heating methods to constrain the intermediate temperature thermal history of this part of the PRB. Procedures and data are listed in Appendix C and Tables C.l and C.2. Apatite fission track apparent ages and apatite confined track length measurements were used to define the lower temperature thermal history across the fan structure. Fission tracks are the elongate damage zones in crystals or glass that result from the fission of 238U nuclei. In apatite crystals these tracks are stable at low temperatures and begin to anneal with increasing temperature (e.g. Naeser, 1976; 1979; Gleadow and Duddy, 1981). The temperature at which track annealing initiates is dependent on cooling rate and mineral composition, and I conservatively interpret the apatite partial annealing range of my samples to be 110-60 °C (Gleadow and Fitzgerald, 1987; Fitzgerald and Gleadow, 1988). Apparent ages are calculated by measuring the density of fission tracks and the U concentration of samples (Naeser, 1976). For moderate to rapid cooling rates (e.g. >12 °C/m.y.) apatite apparent ages can be considered the time at which samples cooled below 110 °C. Methods and data are listed in Appendix D and Table D .l. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 104 Zircon U-Pb geochronology results Results from U-Pb analyses of zircon provide critical constraints on the structural evolution and magmatic history of this part of the PRB. In this section I describe results from the 6 samples analyzed in this study starting with those determined by isotope dilution methods (TIMS), followed by SHRIMP methods. Sample locations are shown in Figure 3.3, and data from analyses are given in Appendix B. Results are plotted on Tera and Wasserburg (1972) concordia plots in Figure 3.4. U-Pb TIMS Four zircon fractions from a sample of the Suerte complex (sample SP90) cluster about an age of ~132 Ma (Fig. 3.4a). A Model 2 regression, which weighs each datum equally, provides a lower intercept of 132 ± 7 Ma and upper intercept o f324 ± 503 Ma. This regression is strongly influenced by the smallest size fraction, which weighed only 0.0001 g and produced the greatest uncertainty. The possible cause(s) of discordance is difficult to interpret from scatter among the positions of the other fractions, leading to the relatively large total uncertainty. Four fractions from a sample of hornblende tonalite in the tonalite sheeted complex (sample SP555) plot in a cluster off of concordia (Fig. 3.4b). The data do not permit calculation of a meaningful regression, and thus, all that can be interpreted from this diagram is that the sample is likely 100-130 Ma. However, sample SP832 from a gabbro pluton that shows mingling textures with the tonalite from which this Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 105 SP90 La Suerte complex: 132 ± 7 Ma 0.0490 Intercepts at 132±7Ma& 324 ±503 Ma' (MSWD = 2.1) 144 140 136 0.0486 128 •325 124 0.0484 44 4846 50 52 238U /206p|) Figure 3.4. Tera and Wasserburg (1972) U-Pb zircon concordia curves for following units and analyses: a) La Suerte pluton (thermal ionization mass spectrometry (TIMS)); b) sheeted tonalite complex (TIMS); c) granite (TIMS); d) orthogneiss magmatic fraction (SHRIMP); e) Agua Caliente pluton magmatic fraction (SHRIMP); f) Rinconada pluton magmatic fraction (SHRIMP-RG); g) sheeted tonalite complex magmatic fraction (SHRIMP-RG). MSWD- mean square of weighted deviates. Note exaggerated ordinate scale on SHRIMP Tera and Waserburg (1972) diagrams. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 106 b) 0.0488 SP555 sheeted tonalite complex: '130-100 Ma 0.0486 140 •3251 T.200,+325 0 . 0.0484 120 Inheritance £ 0.0482 110 trend 0.0480 100 0.0478 54 70 238y/206pb C) C. Goetz granite sample: V 118.2 ± 2.6 Ma 0.052 +200 Inheritance trend 0.0.050 200 Pb loss trend ~ +325 .3 2 5 . 160 0.049 120 0.048. 238U/206pb Figure 3.4 (continued) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 107 _____________________1o error ellipses SP835- orthogneiss in sheeted tonaBte complex ■164.4 i 1.2 Ma MSWD»0.41.n»11 a. m 0017 Age (Ma) E 3 1ct error ellipses ooc OOtt Age (Ma) Figure 3.4 (continued) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 108 f) 0.065 0063 0061 0.059 0.057 £ O-OK ^ 0.053 0- OjOSI S 0049 ■ 0047 0045 0043 • 0041 0039 110 1 a error bars _ t- SP820- Rinconada pluton 101.3±0.6M a 105 100 95 90 55 57 59 61 63 65 67 23»U/20*pb 69 71 73 75 1a error bars SP832- sheeted tonafite complex 100.1 ±0.5M a 0.055 - 110 105 0043 55 57 59 63 65 67 69 71 73 7561 aau/2«pb Figure 3.4 (continued) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 109 sample was collected, was analyzed by SHRIMP methods and yielded an age of-100 Ma as discussed below, suggesting that inheritance is the major factor leading to discordance. Goetz (1989) analyzed three zircon fractions, separated by magnetic means only, from a sample of mylonitized granite in the upper part of the western thrust package. He interpreted his data as providing a concordant U-Pb age of 108 ± 1 Ma. Melissa Girty recovered some of the leftover material from this analysis, split the sample by size fraction, and used air abrasion techniques on one fraction. The results from analysis of three fractions are shown in Figure 3.4c. The +325 fraction produced an internally concordant age of 118.2 ± 2.6 Ma. The other two fractions show complex but systematic lead loss and inheritance. The +200 size fraction was air abraded and likely represents inherited core material that provides a well-defined inheritance trend. U-Pb SHRIMP U-Pb SHRIMP analysis of zircons from a sample of orthogneiss host rock in the sheeted tonalite complex (sample SP835) yielded 11 rim analyses that cluster near concordia (Fig. 3.4d). These data provided a weighted mean age of 164.4 ± 1.2 Ma that is interpreted as a magmatic age. Three additional core fractions yielded ages of -166,1010, and 1100 Ma. The latter two probably represent an inherited component that is very similar in age to a ~1100-1300 Ma inherited lead age range commonly found in plutons of the eastern zone of the batholith (Gastil, 1993). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 110 A sample of orthogneiss from the Agua Caliente pluton (sample SP849) in the eastern footwail of the fan structure yielded 11 rim analyses that assemble near concordia and produce a weighted mean age of 164.3 ± 2.3 M a. This age is identical to orthogneiss in the fan structure, indicating that Jurassic magmatism was widespread in this region. Core analyses of 3 fractions yielded ages o f-156,910, and 975 Ma suggesting a slightly younger inherited component than orthogneiss within the fan structure. Sixteen fractions from a sample of hornblende tonalite in the Rinconada pluton (sample SP820) yielded a weighted mean age of 101.3 ± 0.6 Ma (Fig. 3.4f). The tonalite body shows mingling textures near to where the sample was collected and thus likely represents an approximate age for gabbro in the complex as well. Gabbro from an extensive zone showing well-developed mingling textures with hornblende tonalite in the tonalite sheeted complex in the center of the fan structure was collected from a location near to the sample site for SP555 (described above). SHRIMP analysis of 10 fractions yielded a weighted mean age of 100.1± 0.5 Ma (Fig. 3.4g), confirming that the lower age intercept for sample SP555 likely represents the age of the tonalite sheeted complex. This age is virtually identical to that of SP820 from the Rinconada complex, indicating that these systems were comagmatic. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ill Thermal history results I describe results from individual thermochronologic systems as well as integrate them to produce cooling histories in this section, then address the significance of these ages in the discussion that follows. I assume monotonic cooling of the southern SSPM for several reasons. First biotite and K-feldspar 40Ar/39Ar age spectra generally show relatively well-behaved release of 39Ar during heating steps, suggesting that samples are undisturbed. Second, the youngest plutons in the southern SSPM are the La Posta-type complex that was intruded at 4-5 kbar in the center of the fan-structure, and are probably -97-93 Ma in age, consistent with the La Posta magmatic suite. These postdate intrusion across a broad region in the fan by hornblende tonalite and gabbro at -100 Ma. Thus, conditions in the region within the fan likely did not approach biotite closure temperatures until after emplacement of La Posta-type plutons. Finally the culminating magmatic event in this region produced a swarm of nearly horizontal pegmatite dikes that cut the La Posta suite. Rothstein (1997) obtained a -77 Ma *°Ai/i9Ai K-feldspar age from one of these pegmatites in the northern SSPM and a -80 Ma ^Ar/^Ar biotite age from La Posta-type plutonic rock within -20 m of the pegmatite. Thus the pegmatite dikes appear to have reheated a limited area as further corroborated by the systematically eastward younging progression of cooling ages across the fan-structure that occurs independently of the area intruded by pegmatite. Therefore this region likely did not experience significant reheating following intrusion of La Posta-type plutons at -97-93 Ma, and I interpret Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 112 thermochronologic apparent ages as the time at which samples cooled through appropriate mineral closure temperatures. Biotite40 Ar/39 Ar Biotite 40Ar/39Ar step-heating results from this study range from 111 to 82 Ma and generally young eastwards (Fig. 3.2 and 3.3; Table 3.1). Ages in the western footwall of the fan structure range from 111 to 100 Ma. Ages range from 90 to 84 Ma for the western side of the fan structure, and an age step of >10 Ma is thus indicated across the Rosarito fault. Farther east, from the center through eastern footwall of the fan structure ages generally decrease systematically from 90 to 82 Ma regardless of structural position. Most biotite ^Ar/^Ar spectra show reasonably well behaved degassing during step heating (Appendix C), and I interpret these ages as the time at which samples cooled through 330 ± 20 °C (Grove and Harrison, 1996). K-feldspar 40Ar/39Ar Step-heating ^Ar/^Ar results from three samples of K-feldspar show a similar (to biotite) eastward progression of younging and steps in cooling ages, yielding 94.7 ± 0.5 Ma for the western footwall, 85.5 ± 0.2 Ma for the western side of the fan structure, and 71.7 ± 0.3 Ma for the eastern footwall (Fig. 3.3; Table 3.1; Appendix C). A difference of ~9 Ma is apparent across the Rosarito fault; ages between the western side and eastern footwall differ by 14 Ma. These samples show reasonable step heating systemadcs, and generally accepted estimates of closure temperature for Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 113 K-feldspar are approximately 230 °C for samples that cooled at rates of ~5 °C/Ma (e.g. Berger and York, 1981). Apatite fission track Fission track apparent ages are shown in Table 3.1 and Figure 3.3; track length distributions are displayed in Figure 3.5. In general, ages young to the east across the transect Two well-defined ages, 82.8 ± 9.7 Ma and 76.8 ± 6.2 Ma, were obtained in the western zone of the batholith near to the fan structure. Two additional samples, located -4 km apart were analyzed from a pluton located approximately 20 km farther west yielded disparate ages of 101.7 ± 15.6 and 52.2 ± 7.4 Ma (Fig. 3.2). This region is known only in reconnaissance, and the disparity in ages is difficult to interpret. A sample from the western side of the fan structure, near the Rosarito fault yielded an age of 67.1 ± 4.9 Ma. Farther up in the western hangingwail, significantly older ages are apparent Two samples yielded ages of 80.3 ± 6.7 and 86.3 ± 4.2 Ma (Table 3.1; Fig. 3.3). Analyses from samples collected in the center of the fan structure range from 74.7 ± 3.8 Ma to 65.9 ± 3.7 Ma. Two samples from the eastern fan domain yielded ages of 70.3 ± 4.1 and 65.9 ± 4.4 Ma. A sample from the eastern footwall produced an age of 51.5 ± 2.4 Ma. Fission track lengths were also measured in all samples. Samples for which a statistically significant population of tracks could be measured yielded track lengths varying from 13.0 ± 0.2 to 14.4 ±0.1 pm and show no systematic relationship to structural position. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 14 Figure 3.5. Input track length distributions and resulting thermal histories for apatite fission track modeling. The Laslett et al. (1987) Durango apatite annealing model and an initial mean track length of 15.0 pm were specified along with three timetemperature ranges shown by boxes and the modem mean annual surface temperature of 10 ± 10 °C. The genetic algorithm approach (e.g. Gallagher, 1995) was used in modeling to generate 50 thermal histories for 10 iterations. Light gray lines on the thermal history plots represent all solutions; black line is best-fit solution for all iterations. Ages and temperatures for 4 points on best-fit solution are listed in the upper left of plots. Dashed lines on track length histograms represent measured track length distributions in each sample. Smooth black curve is predicted distribution for best-fit thermal history. Observed and predicted ages, measured and predicted mean track lengths and standard deviations are also listed to the right. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 115 Obs. Age : 51.50 Ma Pred. Age : 51.59 Ma Obs. Mean length : 13.2 Pred. Mean length: 13.2 Obs. S.D. : 1.2 Pred. S.D. : 1.2 Obs. Age : 74.70 Ma Pred. Age : 73.70 Ma Obs. Mean length : 13.0 Pred. Mean length: 13.0 Obs. S.D. : 1.5 Pred. S.D. : 1.5 Obs. Age: 80.30 Ma Pred. Age: 80.34 Ma Obs. Mean length: 13.5 Pred. Mean length: 13.3 Obs. S.D. : 1.16 Pred. S.D.: 1.21 Obs. Age : 67.10 Ma Pred. Age : 67.01 Ma Obs. Mean length : 13.2 Pred. Mean length: 13.2 Obs. S.D. : 1.2 Pred. S.D. : 1.2 Obs. Age : 82.80 Ma Pred. Age : 83.10 Ma Obs. Mean length : 13.7 Pred. Mean length: 13.3 Obs. S.D. : 1.5 Pred. S.D. : 1.4 1 16 Obs. Age : 76.80 Ma Pred. Age : 76.85 Ma Obs. Mean length : 14.4 Pred. Mean length: 14.2 Obs. S.D. : 1.3 Pred. S.D. : 1.7 Obs. Age : 70.30 Ma Pred. Age : 70.69 Ma Obs. Mean length : 13.3 Pred. Mean length: 13.3 Obs. S.D. : 1.2 Pred. S.D. : 1.2 Obs. Age : 65.90 Ma Pred. Age : 66.37 Ma Obs. Mean length : 13.7 Pred. Mean length: 13.7 Obs. S.D. : 0.9 Pred. S.D. : 1.2 Obs. Age : 69.20 Ma Pred. Age : 69.39 Ma Obs. Mean length : 13.5 Pred. Mean length: 13.4 Obs. S.D. : 1.2 Pred. S.D. : 1.3 Figure 3.5 (continued) Obs. Age: 86.30 Ma Pred. Age: 86.70 Ma Obs. Mean length: 14.4 Pred. Mean length: 14.4 Obs. S.D. : 0.9 Pred. S.D.: 1.1 117 Forward thermal modeling of apatite-confined fission track length data was employed to further constrain thermal histories in the 110-60 °C range using the program Monte Trax (Gallagher, 1995) and the fission track annealing model of Laslett et al. (1987). The model calculates the degree of fit of measured track-length distributions and fission track ages to input hypothetical thermal histories with wide ranges of possible ages and temperatures. All samples except SP140a, SP140b, and SP374, which each yielded less than 50 tracks, had sufficient numbers of measurable track lengths to permit modeling. Results from thermal modeling are displayed in Figure 3.5. Apatite fission track model results show systematic variation in cooling patterns among samples across the PRB transition zone for Late Cretaceous to early Tertiary time. In most modeled track length distributions a transition to significantly slower rates of cooling can be constrained. In samples from the western footwall and hangingwall of the fan structure this takes place at 70-80 Ma. For the central to eastern flanks of the structure this transition occurs from 80 to approximately 60 Ma, and generally a systematically eastward-younging trend is apparent. The eastern footwall records a much more gradual transition to slower cooling rates ranging from -55 to 40 Ma. Apatite fission track length modeling also yields interesting information regarding the latest Cretaceous to Recent cooling history of the southern SSPM. Modeling for samples from the western zone of the PRB and the western side of the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 118 fan structure suggests that samples remained within the apatite fission track partial annealing temperature range from -70-20 Ma followed by slow cooling to present conditions (Fig. 3.5). Samples from the center to eastern footwall of the fan structure show possible reheating within the partial annealing temperature range from -70-40 Ma to 25-10 Ma. This feature overlaps in timing with, and may be explained by, arc rejuvenation in the eastern zone of the PRB from -24 to 16 Ma (Martin-Barajas et al., 1995). In all samples from the fan structure a final stage of rapid cooling below -10 Ma is apparent relative to samples from the western PRB, and this feature is most pronounced in the easternmost samples. This region now lies in the footwall of the active Sierra San Pedro Martir fault, a major, 100 km-long, rift-bounding normal fault with at least 5 km of dip-slip displacement that initiated at -11 Ma (Stock et al., 1991). Thus, final cooling probably reflects uplift of the modem San Pedro Martir range across this range-bounding fault Moreover, the first sedimentary record of uplift of the SSPM in the eastern zone is seen in the Santa Rosa basin on the eastern side of Valle Chico (Fig. 3.2) The basin records deposition of granitic and metamorphic material derived from the range beginning around 8-9 Ma (Bryant 1986). Integrated cooling history Variation in cooling ages across the PRB transition zone for the thermochronologic systems described above implies differential cooling among structurally distinct crustal blocks in the PRB. In the following discussion these data are integrated with constraints on peak metamorphic pressures and temperatures as Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 119 well as the structural and intrusive history from west to east across the fan structure of the southern SSPM (Fig. 3.6). My results are consistent with previous thermochronology studies in the PRB, which have noted that age isopleths in the western zone are locally deflected around plutons intruded into shallow crust, reflecting localized cooling around plutons. Isopleths are increasingly more regular to the east where they cut across individual plutons, consistent with regional cooling during denudation of the more deeply buried eastern zone (Krummenacher et al., 1975; Silver and Chappell, 1988; Grove, 1994). The western footwall, west of the Rosarito fault, shows relatively rapid rates of cooling for plutons that were probably emplaced ~ 108-97 Ma (Johnson et al., 1999a) at relatively shallow levels (<2 kbar). These plutons cooled through -330 °C at -102 Ma, and -110 °C at -80 Ma and cooling curves are consistent with thermal histories of shallow level plutonic terranes that experience slow denudation (Fig. 3.6, e.g. Morgan, 1984). East of the Rosarito fault a much more rapid episode of cooling is apparent for rocks that reached peak P-T conditions at much deeper crustal levels. The -101 Ma Rinconada pluton was emplaced at 5-6 kbar and had cooled to -330 °C by -89 Ma, and -110 °C by -86 Ma. An anomalously young fission track cooling age of -67 Ma is apparent from a sample collected just east of the Rosarito fault (Fig. 3.3). This sample occurs at the lowest structural level in the western thrust sheet and may record loading by structurally higher levels in the thrust package. Further east in the center of the fan structure -100 Ma sheeted plutons were emplaced at nearly 6 kbar and had cooled to -330 °C by -87 Ma, and -110 °C by -75 Ma. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 120 Figure 3.6. Summary cooling history diagrams for the western zone, western and central domains of the fan structure, and eastern side and footwall of the fan structure in the eastern zone of the PRB. The 700-735 °C constraint is determined from U-Pb zircon ages from plutons except for easternmost domain, which is a U-Pb monozite age, determined by Measures (1996). The 330 °C constraint is 40Ar/39Ar biotite data. Bars at 110 °C are apatite fission track ages. Thermal modeling envelopes for 110-10 °C are from apatite fission track length data. Also shown for each domain is one possible cooling path interpolated among all data. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0) cu 0) Q. E CD o CD o CD <D 120 100 80 60 40 20 Age (Ma) Fan Structure 122 In the eastern footwall peak metamorphic conditions decrease to -4 kbar and 610 °C (Rothstein, 1997), and metamorphic ages are poorly constrained. However, Measures (1996) obtained a single-fraction, slightly reverse discordant U-Pb monazite age of~96 Ma from the Agua Caliente pluton in the eastern footwall. In the absence of data from the analysis and a SHRIMP U-Pb zircon age of-164 Ma from the same part ofthe Agua Caliente pluton I speculate that the monazite sample incorporated excess 206Pb, leading to reverse discordant results (e.g. Scharer, 1984; Hawkins and Bowring, 1997; Parrish, 1990), and interpret this age to loosely constrain the upper Tt path of the eastern footwall for 725 ± 25 °C (Parrish, 1990) at -96 Ma (Fig. 3.6). Eastwarddecreasing cooling ages are apparent across the eastern flank and into the eastern footwall ofthe fan structure. The highest structural levels on the eastern side cooled through -330 °C by -86 Ma and -110 °C by -70 Ma while the footwall cooled through -330 °C by -83 Ma and -110 °C by -52 Ma. Intermediate cooling ages are apparent between these two extremes. The gradients in Tt paths described above closely correspond to structures within the transition zone in the PRB. A sharp gradient occurs across the Rosarito fault for -330 °C with a -13 m.y. eastward decrease in cooling age and rates of cooling that differ by 10 °C/Ma from 101 Ma, versus 13 °C/Ma from 88 Ma from the western footwall and side ofthe fan structure respectively. However, it is important to note that metamorphic pressures change dramatically across the lower part ofthe western side ofthe fan structure from <2 kbar in the western footwall to >5 kbar in central parts ofthe fan structure. Other than the anomalous age (—67 Ma) immediately 123 adjacent to the Rosarito fault, the apatite fission track system shows little variation in cooling ages across this structure, indicating similar temperatures of-110 °C at -85 Ma (Fig. 3.6). A moderate cooling age gradient is then apparent eastwards across the center ofthe fan structure and into the eastern footwall, with ages decreasing systematically from -88-82 Ma. Cooling rates also decrease for the fan center and eastern footwall respectively, from ~20 °C/Ma from 84 Ma and -10 °C/Ma from 82 Ma. Apatite fission track ages also show moderate gradients with an eastward decrease of 85-66 Ma across the fan structure for -110 °C. A sharper gradient then occurs between the eastern flank and footwall ofthe fan structure with a -14 m.y. eastward decrease in ages across this thrust fault. The gradients in cooling ages across the PRB in the southern SSPM are consistent with those found in other studies in the PRB to the north. Thermochronology studies in the northern SSPM have focused on the -97-94 Ma Sierra San Pedro Martir La Posta-type pluton and show systematically west to east younging ages across the pluton from 98-90 Ma for Ar/ Ar hornblende, 96-88 Ma for 40Ar/39Ar biotite, and -72 to 57 Ma for apatite fission track systems (Fig. 3.2; Ortega-Rivera et al., 1997). Other reconnaissance fission track studies show similar results through the pluton and across the eastern side ofthe northern SSPM. Fission track ages range from —104 to 76 Ma for zircon and —76 to 36 Ma for apatite (Fig. 3.2, Cerveny et al., 1991; Dorsey and Cerveny, 1991). None ofthese studies examined the region farther west which corresponds structurally to the western side ofthe fan structure in the southern SSPM in which the prominent age step occurs. 124 In the northern PRB K-Ar, 40Ar/39Ar, and fission track thermochronology shows similar west to east gradients, ranging from ~120 to 70 Ma for hornblende ^Ar/^Ar, 120 to 64 Ma for biotite 40Ar/39Ar, and 100-80 to 60-40 Ma for apatite fission track systems across a much broader region than that underlain by the SSPM (Krummenacher et al., 1975; Silver et al., 1979; George and Dokka, 1994; Grove, 1994; Naeser et al., 1996; Rothstein, 1997; Premo et al., 1998). Some authors have shown that this west to east progression is systematic (Krummenacher et al., 1975; Premo et al., 1998). Others have suggested that steps occur in the age progression (Silver et al., 1979; Naeser et al., 1996) and still others have proposed that these steps occur across east-side-up structures in the PRB (Todd et al., 1988; Grove, 1994). These gradients in cooling ages across the PRB have been interpreted in a variety of ways. Butler et al. (1989) suggested that the batholith was differentially exhumed during west-directed tilting caused by thickening of the eastern zone during batholith emplacement that resulted in deeper erosional levels to the east. O’Connor and Chase (1989) proposed that flexural isostacy mechanisms controlled by plutonism in the eastern PRB determined exhumation patterns across the batholith with greater exhumation of the eastern zone that diminished as the crust cooled and gained flexural strength. Denudation patterns have also been attributed to gravity-driven extension resulting from overthickened and thermally weakened crust during intrusion (Gastil, 1979; Walawender et al., 1990; George and Dokka, 1994; Thomson and Girty, 1994), and syn- to post-intrusive west-vergent reverse faults (Todd et al., 1988; Grove, 1994). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 125 As developed further in discussion below, my results are consistent with this latter observation. Discussion This study specifically targets the thermal history across the PRB transition zone in a region where the structural evolution of the crustal discontinuity can be particularly well constrained. Below I discuss specific cooling mechanisms that are apparent in the PRB followed by discussion of exhumation mechanisms and coupling of a number of factors including deformation, climate, and deep lithospheric processes that are apparent during exhumation in the PRB. My results have implications for placing initial constraints on exhumation mechanisms in arcs, where geothermal gradients may be notoriously variable making exhumation histories difficult to interpret from cooling histories. Cooling mechanisms The sharp gradient in cooling ages across the Rosarito fault and lower part of the western side of the fan structure occurs across a reverse fault zone that was active during much of the cooling history of the fan structure and that bounds distinct western and eastern crustal zones with strongly contrasting levels of crustal exposure in the PRB. The -100 Ma initiation of cooling in this part of the PRB corresponds with the age of the oldest mid-Cretaceous conglomeratic sediments preserved along the west coast of Baja California. Detrital *°Ar/39Ar K-feldspar ages from clasts in Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 126 these sediments are slightly older than the depositional age of the deposits, but overlap within analytical and age uncertainty, indicating that material from the eastern zone of the batholith was entrained in sedimentary systems within a few m illion years of cooling through ~280 °C (Lovera et al., 1999). Moreover, m axim um metamorphic pressures were attained at this time in the fan structure consistent with coeval development of maximum crustal thickness in the transition and eastern zones of the batholith. Collectively this evidence strongly indicates that rapid denudation that initiated in the mid-Cretaceous is likely the dominant mechanism of cooling in the transition and eastern zones of the PRB. The more moderate eastward-younging cooling gradient that is apparent east of the center of the fan structure may result from secondary factors including the partial preservation of a Late Cretaceous geothermal gradient either across the Gulf escarpment or due to tilting of the batholith, gradients in erosional denudation and/or uplift, and lateral geothermal gradients. The apatite fission track system is particularly sensitive to variation in temperature across relatively short vertical distances and in some cases may partially preserve ancient geothermal gradients. However, in contrast to a positive correlation between elevation and fission track age normally found in these fission track partial annealing zones (Fitzgerald et al., 1995), ages from the southern SSPM, albeit sparse, show little relationship to elevation (Fig. 3.7a). The best possible case can be made for the three samples from the high-relief eastern side and footwall of the fan structure for which ages do vary positively with elevation. However, even in this subset, ages show poor sensitivity to elevation; the greatest Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2000 7 Elevation vs fission track age 1800 - 127 cn CD o 1600 1400 ' 1200 iooo 800 600 I 400 ? 200 1 0 ■ 40 ♦west side ■ East side Age (Ma) E l e v a t i o n ( m e t e r s ) 2000 1800 1600 1400 1200 1000 800 600 400 200 0 Elevation vs track length 16 Track length (micrometers) Figure 3.7. Fission track ages and track lengths as function of elevation. 128 variation in age among samples occurs between the two sample locations with the least variation in elevation (SP20 and SP374). Even more compelling is the distinct lack of correlation between average fission track length and elevation in my samples (Fig. 3.7b). Within a preserved partial annealing zone average track length values among samples should also correspond positively to elevation. I thus conclude that my data do not define a partial fission track annealing zone(s), an inference that was also reached by George and Dokka (1994) from a fission track study in the northern PRB. Rather than varying with elevation, cooling ages in the eastern PRB appear to vary systematically with distance eastwards across the trend of the batholith, irrespective of elevation (e.g. Krummenacher et al., 1975). Data from the southern SSPM are consistent with this observation as indicated in Table 3.1. A widely favored mechanism for this eastward-younging gradient is SW tilting of the batholith on a NW-SE axis, either coincident with or prior to, onset of denudation to produce a northeastward-younging gradient in cooling ages concomitant with eastward-deepening crustal exposures (e.g. Ague and Brandon, 1992). A postmid-Miocene SW tilt of 2.5° related to the development of a rift shoulder to the Gulf of California extensional province is apparent from volcanic deposits that lie on a Tertiary erosional surface in the SSPM (Gastil et al., 1975). However, the distribution of peak metamorphic pressures across the eastern zone of the batholith limits the amount of SW tilting in the southern SSPM. Metamorphic pressures are highest in the western side and center of the fan structure, then fall slightly (<l kbar) in the eastern Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 129 footwall. Pressures further east in the Siena San Felipe are equivalent to slightly higher than those in the footwall of the fan structure (Fig. 3.2), and ^Ar/^Ar cooling ages decrease steadily across this region while fission track ages show a sharper gradient across the eastern side-to-footwall transition of the fan structure, and are unknown from regions further east. I thus infer that the eastern zone in this region is not significantly tilted, a conclusion also reached by Ortega-Rivera et al. (1997) for the 97-94 Ma SSPM pluton in the northern SSPM. Tilting up to 15° SW is permissible, although, tilts at the higher end of this range generally imply greater displacement on both Mesozoic and Cenozoic structures in the eastern zone of the batholith in order to maintain relatively high metamorphic conditions across the eastern zone. For example, tilts on the order of 15° would suggest greater throw on the eastern thrust of the fan structure than is presently deductible from the <1 kbar pressure variation apparent across it Moreover, significant throw with minor tilting would also be necessary on the Neogene SSPM normal fault in order to maintain similar crustal exposures farther east. Gradients in denudation rates and/or timing of exhumation across the eastern zone of the PRB provide a more plausible explanation for the eastward-younging cooling ages coupled with little apparent variation in metamorphic pressures. Thus, similar crustal levels were exposed across this region, but the time at which they reached the present surface varied systematically eastward. Several factors may combine to produce these gradients. First erosional gradients, particularly along continental margins, may exercise extensive control on erosional denudation as well as Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 130 deformation of crust in an orogenic region (Willett et al., 1993). Prevailing weather patterns coupled with orographic effects are typically strongest on one side of an orogen. This effect alone may cause differential denudation, and thus, cooling gradients that extend inwards from a topographic front Additional processes such as erosional retreat may progressively erode a topographically high region across an orographically-induced erosional gradient (e.g. Beaumont et al., 1999). Moreover, the progressive unloading of crust across this erosional gradient causes crust to respond isostatically by continued uplift to maintain a topographically high region. Thus, gradients in flow velocity of crust in a deforming orogenic welt are created with fastest uplift rates corresponding with greatest total erosion that ultimately exhumes the deepest structural levels in the orogen (Fig 1; e.g. Beaumont et al., 1996). As discussed further below, the structural and thermal evolution of the southern SSPM are consistent with such a model. Some of the apparent gradient in cooling ages across the fan structure may also result from exhumation in a region with high topographic relief. For example denudation rates of 0.5-1.0 mm/yr. in an area with average topographic relief of 3 km will produce a minimum relief of 0.4-1.0 km on the 100 °C isotherm (Stttwe et al., 1994). Higher temperature isotherms will show progressively less amplitude. Thus a sharp topographic gradient in the mid-Cretaceous southern SSPM may have produced corresponding variation in low temperature isotherms of at least 1 km, a factor that would contribute to the eastward-younging age gradient. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 131 Finally, regional lateral thermal gradients can not be ruled-out as a plausible explanation for at least some of the variation in cooling ages across the eastern zone of the batholith. This factor is supported by an eastward progression of pluton ages that indicate that the locus of arc magmatism migrated eastward in the Late Cretaceous (e.g. Silver and Chappell, 1988). However, to explain the eastward cooling gradient across the eastern zone of the batholith with this mechanism alone would require a lateral temperature gradient of ~10 °C/km across a >50 km-wide region for timescales on the order of >10 m.y, an implausible scenario. Thus, denudation was by far the most important mechanism for cooling in this part of the Cretaceous Peninsular Ranges and occurred at contrasting rates and times across the transition zone in the batholith. Unless extensional faulting occurred exclusively at crustal levels above 15 km depth, denudation was by erosional means. Other mechanisms such as crustal tilting and lateral temperature gradients resulting from eastward migration of the Late Cretaceous arc may have contributed to cooling, but to a much lesser degree. Below, I further explore constraints on the degree and timing of denudation. Exhumation history An estimate of denudation through time is shown in Figure 3.8, a diagram showing isotherms at depths below paleotopography. The figure is constrained by Al-inhomblende pressures determined from plutons with U-Pb zircon ages and ^Ar/^Ar and fission track thermochronology coupled with ranges of geothermal gradients. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. E10 £ +■> Q. O Q Western Fan structure Eastern zone zone 95 Ma\ "lOoW & U/Pb zirc 85W No exaggeration fission Strack —-bi Ar/Ar Figure 3.8. Cross-section through PRB transition zone in the southern SSPM showing isosurfaces at various ages as constrained by Al-in-hornblende barometry coupled with U-Pb ages on plutons, Ar/Ar biotite and apatite fission track thermochronology (constrained using geothermal gradients of 20-50 C/km corresponding with depths shown by gray band). Isosurfaces solid where constrained, dashed where inferred. Note rapid uplift of almost 15 km between -100-85 Ma in transition zone accommodated by reverse shear zone on western side of the fan structure. 132 133 Present topography and highly simplified structures within the fan structure are also shown in this diagram. Paleotopography may have varied considerably from modem topography, as discussed below, and would have influenced the position of isotherms, particularly for the apatite fission track system. Figure 3.8 thus represents one possible configuration, among others, but gross features such as the sharp gradient along the western side of the fan structure and moderate eastward-younging gradient across the eastern zone of the PRB are unavoidable in any reasonable representation. Pre-~100 Ma metamorphic conditions across the SSPM are poorly constrained. Sediments and volcanics of the flysch assemblage were deposited in trough basins that formed in the PRB transition zone in Triassic-Jurassic time, possibly as a result of crustal thinning and subsidence of this region during backarc extension (Rangin, 1978; Gastil et al., 1981). Arc magmatism was initiated by -164 Ma and contractional deformation is apparent by -132 Ma. Peak metamorphic pressures were recorded at -100 Ma. Thus, crustal thickening probably commenced by -132 Ma and maximum crustal thickness was attained by -100 Ma as corroborated by other evidence. First, kyanite has not been reported from pelitic assemblages in the Peninsular Ranges, indicating that rocks presently exposed were probably exhumed from no deeper than 6-8 kbar (assuming temperatures o f600-700 °C). Second, Kimbrough and Gastil (1997) have argued, based on petrological similarities between the -97-93 Ma La Posta plutons and other tonalite-trondhjemite-granodiorite magmatic suites that occur in strongly thickened orogens such as the Andes, that the voluminous La Posta magmatic event was produced by melting the base of continental arc crust that had Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 134 attained anomalous thickness by 100 Ma. Thus, peak metamorphic conditions recorded at ~100 Ma indicate the maximum pressures attained in the PRB, corresponding with the timing of greatest crustal thicknesses. Rapid denudation closely followed peak metamorphic conditions as constrained by thermochronology. The sedimentary record preserved in CretaceousPaleocene forearc basins along the west coast of the Baja Peninsula corroborates this rapid denudation of the central and eastern batholith beginning at ~100 Ma. In the Viscaino region, lower Cenomanian (—99 Ma) strata show a dramatic transition from marine silt and mud dominated deposition to channelized coarse conglomerate containing clasts that are lithoiogically similar to the eastern batholith and consistently southwest-directed paleo-current indicators (Busby et al., 1998). In the coastal region to the SW of the SSPM a thick Turanian (93.5-89 Ma) to Paleocene sequence of fluvial to shallow marine conglomerate, sandstone, and shale marks rapid erosion of the batholith to the east, and sediments of similar age occur along the coast to the north (Botq'er and Link, 1984). The -100 and -95 Ma isosurfaces in Figure 3.8 are constrained by Al-inhomblende barometry from the Rinconada pluton on the western flank of the fan structure and tonalite sheeted complex and La Posta-type plutons in the center of the fan, and record maximum depths in this part of the PRB. Plutons in the western footwall of the fan structure were emplaced at -2 kbar at 115-104 Ma and had cooled below 330 °C by 100 Ma. Thus, a significant gradient in crustal depth is apparent across a <5 km-wide distance on the western flank of the fan structure at 100 Ma that Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 135 corresponds with a spectacular inverted metamorphic gradient. Moreover, abundant structural evidence such as overthrusting of the Rinconada pluton by its eastern country rocks and post-intrusive, brittle, west-vergent thrusts along its western margin suggests that thrusting accommodated uplift of the fan center relative to the western footwall across this inverted metamorphic gradient An inverted metamorphic gradient also occurs on the eastern flank of the fan structure, although metamorphic field gradients are not as great as on the western side and its age is not as well constrained. However, migmatitic rocks at structurally highest levels on this side are strongly deformed and continue into orthogneiss screens within the ~ 100 Ma sheeted tonalite complex, suggesting that they were at similar depths at the time of intrusion of the sheeted complex. By the next time step at -95 Ma, the approximate age of La Posta-type plutons in the center of the fan structure, the apparent depth gradient across the western side of the fan structure had diminished but was still significant Thrusting on the west side of the fan structure thus continued to uplift the center of the fan relative to the western footwall, an implication that contrasts with previous studies that consider the La Posta plutonic suite post-tectonic (e.g. Todd and Shaw, 1979). Time steps that follow rely on assumed geothermal gradients coupled with closure ages for thermochronology systems, and thus are more poorly constrained. The depth inferred for biotite 40Ar/39Ar ages in particular is very sensitive to geothermal gradients, and a depth intermediate between the Al-in-homblende data and less sensitive apatite fission track ages was selected for the 85 Ma isotherm in Figure 3.8. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 136 This uncertainty makes relative crustal depths of rocks on either side of the Rosarito fault difficult to resolve from the -13 m.y. cooling age step that occurs across it. This step may have resulted from any of the following: (1) 90-85 Ma reverse displacement on the Rosarito fault; (2) a strong lateral thermal gradient due to earlier thrusting of deeper eastern crust on shallow western crust; or (3) significant topographic relief across the western thrust sheet However, all of these imply a significant gradient in erosional denudation, uplift, and /or topographic relief across the western thrust In contrast across the fan structure and into the eastern footwall, biotite 40Ar/39Ar ages are internally highly consistent and young systematically eastwards from 88 Ma in the western-most side of the fan structure to 83 Ma in the eastern footwall with no apparent sharp gradients. This well-behaved part of the dataset is consistent with fairly constant relative denudation rates and smooth topography across the rest of the fan structure and into the eastern footwall. Thus, the center to eastern footwall transition in the fan structure behaved as a fairly coherent crustal block with little apparent topographic relief, consistent with the formation of a plateau at this time as discussed further below. Depths for ~75 and -50 Ma time steps are better constrained due to lower closure temperatures in the apatite fission track system. Shown on Figure 3.8 is a range of depths for temperatures at which fission tracks would be preserved in apatite assuming geothermal gradients ranging from 20 to 50 °C. Thus the center of the fan structure resided between 2 and 5 km depth a t-75 Ma. In contrast the eastern side and footwall of the fan structure remained above —110 °C while the western side and Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. footwall were at shallower crustal levels. The absence of age gradients across the Rosarito fault at -85 Ma and anomalously long average fission track lengths (14.4 ± 0.1 tun) in SP820 from the Rinconada pluton indicates that this system corresponds with a region of rapid denudation and was no longer accommodating significant throw by this time. By ~50 Ma the eastern footwall passed through this depth zone in the eastern PRB at comparatively slow denudation rates. The cooling age gradient across the eastern thrust at this time probably does not result from displacement on the fault Mean fission track lengths are identical within analytical error (13.3 ± 0.1 and 13.2 ± 0.1 pm respectively) among samples across this structure indicating that the fission track age gradient does not reflect juxtaposition of rocks with drastically different thermal histories. Moreover peak metamorphic pressures are not significantly different across this structure, and notable brittle deformation is not apparent, indicating that little displacement occurred across this structure at temperatures low enough to perturb apatite fission track systematics. Maximum average denudation rates also varied dramatically across the transition zone in the PRB as calculated from maximum and minimum depth constraints in Figure 3.8. The western zone was denuded at rates of <0.3 km/m.y. from ~l00-80 Ma. Peak denudation rates of 1.0-1.2 km/m.y. from 100-85 Ma are apparent on the western side of the fan structure. Apparent denudation rates then drop to 0.5-0.7 km/m.y. from 100-70 Ma in the center of the structure and <0.3 km/m.y. from -96-55 Ma in the eastern footwall. Thus, deep crustal levels were exposed rapidly along the western side of the eastern zone of the batholith beginning in mid-Cretaceous time. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 138 Similar levels were exposed further east, but apparently at a slower rate. Below, I explore the implications for removing >15 km of crust from the fan structure and eastern zone of the PRB relative to the western zone, and mechanisms that may have lead to this differential denudation. Exhumation mechanisms Recent geophysical studies in the PRB of southern California and the northern SSPM have provided critical constraints on crustal thickness and P wave velocity structure across the PRB. In both southern and Baja California Moho depths vary from 37-43 km across the western zone and decrease sharply across the PRB transition zone to <30 km across the eastern zone. Crustal thickness decreases eastwards across the eastern zone to a depth of 18 km near the Gulf of California as a result of NeogeneQuatemary extension. (Fig. 3.9; Ichinose et al., 1996; Lewis et al., 2000; J. Lewis and S. Day personal comm., 2000). Moreover in southern California P-wave velocities differ by -3% between faster crust underlying the western zone and slower crust of the eastern zone of the batholith for all crustal levels (Magistrate and Sanders, 1995). Thus crust of contrasting composition and thickness characterizes the two zones of the PRB, an anomalously thick western oceanic-floored arc and anomalously thin eastern continental arc that has been recently extended. A simple isostatic model of a pre-Miocene reconstruction of the PRB in the southern SSPM using constraints described above shows that average topographic elevation and crustal thicknesses across the transition zone in the SSPM can be Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Crustal structure across the Peninsular Ranges at -30.5N Figure 3.9. Diagram showing compiled geophysical constraints across Baja California at -30.5° N latitude. Also shown is my interpretation of crustal structure. 139 140 explained by variation in crustal densities alone (Fig. 3.10a). This model assumes that average topographic elevation in the SSPM is representative of pre-Gulf elevations in the eastern zone, the entire eastern zone was initially as thick as its westernmost part today, average topographic elevation in the western zone has not changed since the initiation of Gulf extension, crust in the transition zone has no flexural strength, and the lithosphere in both zones has similar velocities below mid-crustal levels where they are presently unconstrained. Most likely the contrast in seismic velocities persists to these depths, an effect that would further enhance isostatic contrasts across the transition zone. The Pratt model results in average elevations of 800 and 1300 m in western and eastern zones respectively, and a relative root of -3 km for the eastern zone. These values closely match elevations and crustal depths inferred across the SSPM for the time preceding Gulf extension. Mid-crustal levels are presently exposed in the eastern zone of the batholith, and -15 km of differential denudation has occurred since 100 Ma between the two crustal zones. Isostatic models showing the results of adding back this 15 km of crust to the eastern zone are also displayed in Figure 3.10. This crustal volume was added to the uppermost crustal level to represent denudation strictly by erosion (Fig. 3.10b) and distributed among crustal levels to represent crustal thinning (Fig. 3.10c). Both analyses result in eastern zone crust nearly 55 km thick, a relative crustal root of-10 km, and average topographic elevation of -4000 m. Thus, eastern zone crust in the PRB at -100 Ma certainly had the potential, depending on surface processes and crustal strength, to produce high mean elevations that contrasted dramatically with Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 141 _0 _-10 _-20 —30 -40 km a ) Pre-Miocene Gulf extension Western Zone Eastern Zone Elevation 800 m Elevation 1300 m ■2620BI 2610 2800 2700 2880 2750 2920 ||||||2810 2940 r ; 2940 Root -2.9 km * 300 : K ■■■ > **• « st* * » a* * * a* * «#3\ C) Pre 100 Ma, replace 15 km uppermost crust (2700) -0 -10 -20 -30 _-40 km Western Zone Eastern Zone Figure 3.10. Isostacy models for simplified western and eastern PRB crust. See text for explanation. Crustal layer densities determined from P wave velocities in Magistrale and Sanders (1995). 142 western zone crust based on reconstructed crustal density and thickness alone. As discussed below, if lithospheric mantle varied between western and eastern zones, then most likely lithospheric mantle underlying the western zone would have been thicker and denser than beneath the eastern zone, further enhancing topographic contrast between the two zones. Differential exhumation of the PRB in the SSPM occurred across active shear zones in the fan structure. The western side of this structure accommodated the majority of throw associated with uplift of the eastern zone of the batholith across a shear zone that is presently <5 km-wide. The eastern side of the fan structure was probably also actively deforming at this time, predominantly by folding that occurred during or after emplacement of the La Posta-type plutons. However, the apparent throw on this side of the structure is considerably less than on the western side, imparting an asymmetric geometry to the structure during denudation. Thus, contrasts in crustal density and thickness between western and eastern zones of the batholith in mid-Cretaceous time resulted in dramatic contrasts in the amount of exhumation experienced by these two crustal belts. The crustal boundary between them had the potential to produce a sharp topographic break as well, depending on how efficiently erosion could keep pace with uplift If uplift outpaced erosion, then the presence of a topographic front along the western side of the fan structure could have induced orographic effects that would have further enhanced coupling between deformation and erosion. Moreover, if the prevailing wind across the paleo-Penmsular Ranges was from the Pacific basin in midReproduced with permission of the copyright owner. Further reproduction prohibited without permission. 143 Cretaceous time, as suggested by the direction of asymmetry in the fan structure and distribution of syn-exhumadon sediments, then erosion may have exerted an even greater effect on deformation and uplift Thus, the fact that the greatest total exhumation is apparent along the western side of the transition zone may result, in part, from coupling between prevailing weather systems, strong orographic effects, and deformation on the western side of the fan structure. The absence of mid- to Late Cretaceous foreland basin deposits in the western zone of the batholith adjacent to the PRB transition zone further indicates that western zone crust was not loaded by thickening and uplifting crust to the east, either because flexural strength across the transition zone was minimal, and/or because erosion kept up with deformation in the fan structure at this time. Similar conclusions have been reached in other deeply denuded ancient orogens based on evidence for high rates of erosional unloading that in some cases may even have driven contractional deformation (Hoffman and Grotzinger, 1993). Nevertheless, significant exhumation was not restricted to the region within the fan structure. An extensive part of the eastern zone of the batholith underlying the region east of the southern SSPM was denuded to almost the same degree as the fan structure despite experiencing little if any Cretaceous deformation. A considerable distance farther east, in south-central Arizona and northern Sonora, low- to medium grade metamorphism is apparent that is probably largely Jurassic (Tosdal et al., 1989), suggesting that relatively shallow crustal levels were exposed there by Late Cretaceous time. The eastward extent of the more deeply denuded eastern zone of the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 144 PRB has not been defined in central to western Sonora, and may have extended >100 km east of the PRB transition zone in the Late Cretaceous. This implies that a broad belt of thick orogenic crust characterized this part of the North American margin as corroborated by coeval intrusion of La Posta-type magmas during this time that were probably derived from unusually deep lower crust (Kimbrough and Gastil, 1997). As discussed above erosional denudation was probably the major cause of cooling in the eastern PRB, and thus, moderate, progressively eastward-younging gradients in cooling ages across this region indicate progressive eastward denudation of a thickened crustal welt These features bear strong resemblance to modem continental plateaus such as the Altiplano-Puna and Tibet and I therefore speculate that a continental plateau formed in the eastern PRB at this time. Thickening of crust to form a plateau may have resulted from lateral flow of thermally weakened lower crust (e.g. Royden, 1996) following more than 30 m.y. of structural crustal thickening within the PRB transition zone by mid-Cretaceous time. Blocked by strong, cooler lithospheric mantle to the west, this lower crust in the deeper eastern crustal root could have been forced eastward, thickening a region that was experiencing little deformation, and ultimately leading to progressive uplift of the entire eastern zone of the batholith. In some regions such as the modem Andean orogen and Laramide Colorado plateau, contractional deformation propagates concomitantly with lateral flow of weak lower crust (Pope and Willett, 1998; McQuarrie and Chase, 2000). However, in other areas such as the eastern margin of the Tibetan Plateau lower crustal flow and thickening may not be Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 145 accompanied by upper crustal shortening (Royden et al., 1997), presumably because lower crust is strongly decoupled from upper crust (Royden, 1996). The lack of evidence for eastward propagating deformation in the eastern batholith of the PRB indicates that crust may have been internally decoupled, possibly leading to midcrustal detachments along parts of the margin such as in southern California where crustal extension accompanied Late Cretaceous exhumation (George and Dokka, 1994). Finally, although some denudation could have occurred earlier than 100 Ma, contractional deformation, and thus potential to thicken crust, initiated before -132 Ma along the Peninsular Ranges margin, more than 30 m.y. subsequent to the onset of perceivable denudation. Thus, although lower crustal flow may have been an important mechanism to thicken crust across the eastern zone of the batholith, another factor must have initiated rapid denudation of the transitional and eastern PRB after -100 Ma. Moreover, this event is coincident with the 97 to 93 Ma pulse of voluminous La Posta plutonism in the transition and eastern zones of the batholith, which must also be explained. Mechanisms that could trigger this exhumation include changes in crustal dynamics along the margin resulting from collision of the western batholith, a change in relative plate motions, and/or mantle/lower crustal delamination. Collision of the western zone against the North American margin has been suggested for the period 115-108 Ma in the SSPM (Johnson et al., 1999b), a time that is -10 m.y. earlier than the initiation of rapid denudation. Collision in the PRB may have proceeded over many millions of years before significant crustal thickening was Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 146 attained, as seen in other collisional belts such as the Himalaya where collision between India and Asia initiated between 65 and 45 Ma, but significant denudation did not occur until after ~21 Ma (Sorkhabi and Stump, 1993). However, if indeed collision was an important factor in the exhumation of the eastern PRB, then collision models must account for a common origin of the entire western PRB because a similar denudation history is apparent along the entire length of the PRB as described further in Chapter 4. Thus, tectonic models that propose collision of only part of the western PRB are not compatible with rapid denudation in the entire PRB resulting primarily from collision. Variation in relative plate motions provides a more compelling possibility for initiating abrupt denudation in the PRB. Although there is some controversy over relative convergent rates and vectors between the North American and Farallon plates at this time, it appears that convergence rates increased and become more orthogonal at ~100 Ma along this part of the Cordilleran margin, followed by the establishment of dextral relative motions by -85 Ma (Kelley and Engebretson, 1994- a refinement of Engebretson et al., 1985). The result of such an increase in convergence rates may be seen in the Puna-Altiplano plateau of the Andes where increased subduction rates at -10 Ma corresponded with increased strain rates and development of the orogenic plateau, despite the fact that contractional deformation in the region began at -30 Ma (Pardo-Casas and Molnar, 1987). Analogous increased convergence rates between the Farallon and North American plates at -100 Ma may have induced greater coupling across the subduction interface that ultimately led to higher strain rates focused in the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 147 PRB transition zone. Moreover, robust arc magmatism that produced the La Posta plutonic suite may also have resulted from accelerated subduction beneath this rapidly thickening continental margin. It could also have led to, or contributed to, decoupling of mafic lower crust or lithospheric mantle beneath the eastern batholith as discussed below. Removal of mafic lower crust and/or lithospheric mantle (e.g. Kay and Kay, 1993) also provides an appealing explanation for the abrupt onset of exhumation across a broad region in the eastern batholith. Such an event would rapidly produce highly buoyant crust across a wide belt to form an orogenic plateau due to previous crustal thickening resulting from earlier deformation as well as magmatism. Continued deformation coupled with rapid erosion of the western flank of the orogen might have provided a feedback mechanism that maintained the topographic plateau for a time, followed by eastward erosional retreat It also provides an attractive mechanism to produce the relatively abrupt and voluminous pulse of tonalite-trondhjemitegranodiorite type magmatism from a deep crustal source that led to the La Posta magmatic suite. Delamination has been proposed for the Puna plateau of the Andes orogen (Kay et al., 1994), where geophysical studies show that felsic continental crust extends some 65 km below the surface and mantle lithosphere is anomalously thin (e.g. Beck et al., 1996). Numerical modeling of this process indicates that removal of lithospheric mantle within even a limited zone allows the overlying crust to preferentially shorten and thicken, which in turn leads to heating and weakening of the lower crust and growth of an orogenic plateau (Pope and Willett, 1999). This proposal Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 148 remains to be tested in the PRB, potentially using P-T-t constraints on mantle xenoliths from pre-Gulf volcanics or geophysical studies that may be able to image vestiges of Mesozoic lithospheric mantle. Thus, the central part of the Mesozoic PRB may have developed in a manner analogous to the Late Cenozoic central Andean orogen, possibly including the formation of a continental plateau during part of its history. A simplified possible evolution of the central PRB is displayed in Figure 3.11. During the Jurassic, this part of the Cordilleran margin may have developed an extensional arc (e.g. Rangin, 1978; Busby-Spera, 1988) characterized by subsidence and deposition of much of the flysch assemblage into extensional basins in the transition zone between western and eastern belts in the batholith. Contractional deformation that was focused in the PRB transition zone was initiated before —132 Ma during continued or rejuvenated arc magmatism, and lasted for the next 30 m.y. resulting in thickened crust in the PRB transition zone (Fig. 3.1 la). At -100 Ma Farallon-North American plate convergence velocities increased and became more orthogonal, leading to elevated strain rates in the Peninsular Ranges margin and possible removal of mafic lower crust and/or lithospheric mantle (Fig. 3.1 lb). Peak metamorphic conditions were reached in the southern SSPM at this time and exhumation commenced, accommodated on the western side of the fan structure by a major reverse shear zone that juxtaposed strongly contrasting crustal depths across the transition zone in the batholith. Coupling between deformation and erosion induced by prevailing westerly weather patterns and orography may have enhanced exhumation along the western margin of the orogen. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Pre-100 Ma 100-90 Ma melting and lateral flow of lower crust Eastward propagating plateau coupling with prevailing weather patterns | Wind > B remove of eastern PRB lithospheric mantle La Posta . magmatism D Post 65 Ma Western batholith crust Western batholith lithospheric mantle Oceanic crust Eastern batholith crust 100 km Eastern batholith lithospheric mantle Figure 3.11. Summary of possible central PRB uplift history. Lithospheric mantle structure unconstrained. 150 Thickened lower crust within the PRB transition zone was thermally weakened and flowed eastward to form an orogenic plateau. Melting also began at the base of this thick crustal welt to form the voluminous La Posta magmatic suite. This was accompanied by coeval deposition of eastern batholith-derived sediments along the western coast of Baja California in basins that formed independently of crustal processes attendant with mountain building further east (Fig. 3.1 lb). By latest Cretaceous time a significant portion of eastern lithospheric mantle may have been removed. The arc migrated eastward into present day Sonora in tandem with shallowing subduction (e.g. Coney and Reynolds, 1977; McDowell and Mauger, 1994). Deformation had largely halted on the western side of the fan structure and erosional retreat of the western margin of the plateau ensued, continuing to fill basins along the Pacific coast (Fig. 3.1 lc). By the end of the Cretaceous the eastern zone of the batholith was a deeply denuded low-standing region, and large river systems planed across it, transporting extraregional material from the active arc some 300 km distant in Sonora to the Pacific (Fig.l2d, Minch, 1979; Abbott and Smith, 1983). Conclusions The exhumation history of the central PRB of Baja California in mid- Late Cretaceous time is an excellent example of the extreme contrast in crustal dynamics that can develop in contmental-margin arcs constructed across chemically and structurally distinct lithospheric blocks. These contrasting margin-parallel crustal Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 151 zones in the PRB, consisting of western, oceanic floored crust and eastern, continental floored crust, behaved as coherent crustal blocks during exhumation. The present —40 km thick western crustal zone of the arc remained stable, and probably at low average elevation, throughout the Mesozoic. In contrast the eastern zone may have achieved >55 km crustal thicknesses in mid-Cretaceous time and subsequently experienced dramatic denudation across the entire region. The transition between these crustal zones was the site of focused contractional deformation through much of the Cretaceous and accommodated >15 km of differential uplift within the PRB across a <5 km-wide reverse shear zone in mid-Cretaceous time. Gradients in cooling histories across the PRB in the southern SSPM coupled with preliminary geophysics and a well-defined structural, magmatic, and sedimentological history constrain a number of details regarding the evolution of the mid-Late Cretaceous Peninsular Ranges. Maximum structural thickening following -30 m.y. of contractional deformation in the transition zone of the PRB was attained by -100 Ma, coincident with increased convergence rates between Farallon and North American plates. Eastward flow of thermally weakened lower crust in the transition zone may have thickened crust across a broad region in the eastern batholith, leading to the construction of an Andean-like continental plateau in Late Cretaceous time. The apparently sudden onset of denudation at -100 Ma and intrusion of a voluminous, lower crustal-derived, La Posta magmatic suite across the region may result from the removal of lithospheric mantle that would have further amplified orogenic plateau formation. Rapid denudation ensued, possibly involving coupling between Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 152 contractional deformation and heightened erosion rates across the western edge of this plateau, which formed along the crustal transition zone in the batholith. Subsequent eastward propagating denudation of the plateau is apparent, probably as a consequence of eastward erosional retreat Contrasts in crustal structure that so dominated many aspects of the Mesozoic evolution of the PRB continue to influence modem crustal dynamics in this region. Neogene lower crustal thinning in the Gulf of California extensional province is apparent under the eastern zone of the PRB to the ancient batholith transition zone in the SSPM where crustal thicknesses increase dramatically, and thereby retain probable Mesozoic crustal thicknesses beneath the western batholith. Uplift of the modem Peninsular Ranges to form the western rift shoulder to the Gulf extensional province has ensued just east of this inherited lithospheric boundary. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 153 CHAPTER 4: TECTONIC SYNTHESIS OF THE MESOZOIC PENINSULAR RANGES BATHOLITH Introduction Thus far, discussion has focused on the transect undertaken as part of this study across the transition zone in the Peninsular Ranges batholith in the southern Sierra San Pedro Martir. In this chapter I synthesize a number of regional geological, geochemical, and geophysical data sets, including data from this study, that bear on the Mesozoic evolution of the PRB as a whole. I close the chapter with a hypothetical Mesozoic history and discuss the implications of key relationships, both those already established and those that require further testing, for tectonic models of PRB evolution. Overview of the SW margin of North America The Peninsular Ranges batholith formed as part of a nearly continuous series of Cordilleran batholiths along the western margin of North America during the Mesozoic era (Fig. 4.1). This margin was previously rifted and received thick sequences of passive margin sediments during Late Proterozoic to mid Paleozoic time. By the late Paleozoic to Triassic the southwest edge of North America was truncated, possibly by large scale sinistral strike-slip faults that translated crustal slivers into present Mexico (e.g. Davis et al., 1978). Prebatholithic strata of Ordovician and late Paleozoic-early Mesozoic age at two locations in Baja California have been tentatively Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 154 Alaska I V s' ’ .V ALASKA-ALEUTIAN RANGE £ Canada • i l 500 km IDAHO. P-H OuachitaMarathon suture P-R Qolconda thrust Pz mtofleodinecraton hinge line J Mojave--' Sonora % megasear or older structure . - Gulf of Mexico Mexico SIERRA S ^ NEVADA \ .x . PENINSULAR' RANGES Rgure 4.1 Map of western North America showing major Mesozoic Cordilleran batholiths and selected Paleozoic>Mesozoic tectonic features of the southwest North American margin discussed in text Possible continuation of Ouachita-Marathon suture across Mexico from Stewart (1988). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 155 linked with strata of equivalent age in Nevada, suggesting that some of these fragments may now be located in the Peninsular Ranges. Subduction commenced along the Cordilleran margin during or following truncation. Along the southern margin of Paleozoic North America, in present day northern Mexico, rifting also fragmented the margin in Late Proterozoic-early Paleozoic time (Stewart, 1972). By the late Paleozoic-Triassic many of these fragments had been sutured back to North America along with other exotic assemblages including the South American continent in the Ouachita orogeny (Fig. 4.1, e.g. Stewart, 198S). The central part of the PRB may have been empiaced across this older collisional margin, where it once intersected the western margin of North America (e.g. Gastil et al., 1991b). South America and many microplates in eastern Mexico have since been translated southward and rotated during the formation of the Gulf of Mexico, but most models maintain that western Mexico remained fairly intact during the Mesozoic (see Sedlock et al., 1993 for alternative models). One exception is the controversial Mojave-Sonora megashear model that contends much of NW Mexico, including the PRB, has been displaced ~800 km in a sinistral sense relative to North America in Jurassic time (Fig. 4.1, Silver and Anderson, 1974). By Late Permian-Early Triassic time an Andean-type subduction complex was well established along the SW margin of North America (Burchfiel et al., 1992b). Arc magmatism continued through the Mesozoic possibly (and controversially) punctuated by collision of island arc terranes such as Sierran western foothills assemblage in Late Jurassic time (Nevadan orogeny, e.g. Schweickert and Cowan, 1975). Alternatively, at Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 156 least some of the ophiolitic assemblages in the Cordillera that have been attributed to remnants of the suturing process of exotic terrenes may have formed in a sinistral transtensional setting that produced intra-arc, margin-parallel, rifting along the northern Cordilleran margin that linked via the sinistral Mojave-Sonora megashear to opening of the Gulf of Mexico in Jurassic time (e.g. Dickinson, 1981). Busby-Spera (1988) suggested that the upper plate of the southern Cordilleran subduction margin was extensional as well, leading to a series of intra-arc rift basins that received both arc-derived and mature continental sediments. Robust arc magmatism characterized Early to mid-Cretaceous time along much of the southern Cordillera. By late Cretaceous time the subduction angle had shallowed and the axis of arc magmatism had moved eastward (e.g. Coney and Reynolds, 1977; McDowell and Mauger, 1994). Arc magmatism returned to the margin briefly in the Miocene, followed by extension in the same region that culminated in creation of the Salton trough-Gulf of California rift system which is still active today (e.g. Stock and Hodges, 1989). The focus of the tectonic synthesis in this chapter will be on the Mesozoic evolution of the Peninsular Ranges batholith of southern California and northern Baja California. As is the case throughout the Cordillera, one of the major issues in the PRB is whether assemblages along the margin evolved in their present positions or were displaced from other regions in which they originated. Components within the outermost assemblage in the PRB, the Continental borderland or Franciscan subduction assemblage, are certainly exotic. However, this issue is also relevant for Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 157 the next inboard assemblage, that is, the western zone of the PRB, which appears to have been constructed on oceanic lithosphere. Discussion below will focus on the Jura-Cretaceous batholith proper in southern California and Baja California Norte, where zones that have been defined in the PRB are contiguous and reasonably well exposed. Thus the continental borderland assemblage is not examined, nor is the region south of the 28th parallel, where eastern, transitional, and presumably much of the western zones of the batholith continue offshore in the Gulf of California, and Mesozoic rocks that remain on the peninsula are covered by extensive Tertiary volcanics. Tectonic models for the Peninsular Ranges batholith Three basic tectonic models attributing varying degrees of allochthoneity to the western zone have been proposed for the origin of the transition zone in the PRB, and each of these is actively advocated by various research groups working in different parts of the Peninsular Ranges (Fig. 42). The most fixist model for the PRB, largely originating from workers in southern California, suggests that the batholith was constructed across an inherited pre-Mesozoic transition between oceanic and continental crust along the western North American passive margin (Fig. 42a). Accordingly, Walawender et al. (1991) suggested that the PRB formed above a single subduction zone based on pluton ages and petrogenesis, the change in character between western and eastern zones resulting from shallowing of subduction angle beginning at -105 Ma. Thomson and Girty (1994) established an early Mesozoic tie Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 158 a) Arc built across pre-Mesozoic crustal join b) Backarc extension, development of western fringing arc, followed by collapse of marginal basin c) Collision of exotic western island arc Western batholith crust Transitional batholith crust Eastern batholith crust Oceanic crust Extinct/ active magmatism late Early Cretaceous flysch basin Figure 4.2 Tectonic models suggested for the Mesozoic evolution of the PRB. See text for historical development of models and corresponding references. A) Western zone evolved adjacent to the rest of the batholith, separated by an inherited crustal boundary. B) Backarc basin formation rifted western zone into a fringing arc position in Late Jurassic-Early Cretaceous time followed by collapse of intervening marginal basin and reattachment to North America. C) Development of the western zone as an exotic island arc during Jurassic and early Cretaceous time followed by collision and suturing of the exotic arc with North America in the latest Early Cretaceous. 159 between the western and eastern zones of the batholith from a pluton that yielded a Triassic age and intrudes the Julian Schist, which in turn overlaps western and eastern zones in the batholith. A second model that involves partial evolution of the western zone as a fringing arc has been advocated by a number of researchers working in both southern and Baja California (Fig. 4.2b; Gastil et al., 1975,1981; Rangin, 1978; Todd et al„ 1988; Griffith and Hoobs, 1993; and Busby et al., 1998). They suggest that back-arc extension initiated in Jurassic time, rifling the western arc outboard to a fringing arc position relative to the eastern arc. Collapse of a marginal basin located between the two arcs occurred in the mid-Cretaceous and initiated a mid- Late Cretaceous contractional phase in the batholith. Arc magmatism migrated eastward from the western zone following back-arc basin collapse. Gastil et al. (1975, 1981) further suggested that the northern half of the western zone in the batholith (Santiago Peak segment) was sutured to the margin in Early Cretaceous time while the southern half (Alisitos segment) collided in mid- to Late Cretaceous time. They proposed that the two segments were separated by an ancestral transform fault that has been reactivated to form the presently active Agua Blanca fault A final model that proposes that the western zone is an exotic tectonic element that collided with the North American m argin has been advocated by researchers working in the northern SSPM (Johnson et al., 1999a). Based on preliminary evidence for strong oceanic lithospheric affinities with little influence of continental crust in rocks of the Alisitos segment of the western zone, these authors suggested that the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 160 western arc originated as an island arc that evolved completely separate from North America and was sutured to the continent in latest Early Cretaceous time (Fig. 4.2c). Evidence includes a lack of zircon inheritance in both plutonic and volcanic rocks, paucity of quartz and K-feldspar components in sediments, and plutons with strong island arc geochemical and isotopic affinities. Moreover, plutons that intruded the PRB transition zone immediately following proposed suturing at 115-108 Ma show evidence for mixed continental/island arc lower crustal sources whereas previously intruded plutons in the west do not Johnson et al. (1999a) acknowledged that the Alisitos segment is more poorly understood than the western zone to the north and that relationships between the western and eastern zones of the PRB may change along strike. Review of Peninsular Ranges batholith geology The PRB remains a relatively poorly known part of the North American Cordillera. Although extensive work has been accomplished, for the most part these studies have been localized, and far more attention has been focused on the northern part of the batholith in southern California than in Baja California where >75% of the exposed area of the PRB occurs. Accordingly, most of the published data on the PRB is from work in southern California and, in general, less information is available progressively farther south. Important compilations include Silver and Chappell (1988), a synthesis of more than 2 decades of early geochemical and geochronological work by L. Silver and colleagues, as well as Todd et al. (1988) who described the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 161 lithological, metamocphic, and structural aspects of the PRB in southern California. The work of Gastil et al. (1975), published more than a quarter century ago, remains by far the most important dataset on the geology of the PRB in Baja California. This landmark publication as well as subsequent work by researchers at San Diego State University has yielded >90% of our present knowledge of the geology of this region. Sedlock et al. (1993) provided an important contribution to our understanding of the tectonics of Baja California in the context of the rest of Mexico. Investigators early on recognized a number of transitions across the strike of the batholith in the prebatholithic stratigraphy (e.g. Gastil et al., 1975; Gastil and Miller, 1983; Gastil, 1993), structures (e.g. Gastil et al., 1975; Todd et al., 1988; Thomson and Girty, 1994; Johnson et al., 1999a), and petrology and geochemistry (e.g. Silver and Chappell, 1979; DePaolo, 1981; Gastil et al., 1990; Walawender et al., 1991). Some also noted significant along-strike changes (e.g. Silver et al., 1963; Gastil et al., 1975). These transitions are a hallmark of the PRB, perhaps better developed here than in any other batholith in the world. However, as will be elaborated on below, a problem has become apparent in our collective view of the PRB. The transitions that occur across the trend of the batholith, although sharp, are difficult to represent by a single line on a map that divides distinct east and west zones in the PRB. Rather, these transitions occur across a minimum width of 5 km and as much as 40 km, depending on which parameters one chooses to define zones in the batholith (compare, for example, the distance between 0.704 and 0.706 initial strontium isopleths versus REE zones or 5180 isopleths in Fig. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 162 4.3). It, thus, may be more accurate to define a transitional zone, which occurs between western and eastern zones and that encompasses most of these geochemical and geophysical transitions. Below I will further argue that this transitional zone is geologically distinct from the crustal zones to either side, and variation in geochemical, geophysical, and geological parameters by which it is defined along strike are critical to understanding the tectonic evolution of the PRB. O verview o f the Peninsular Ranges batholith The Jura-Cretaceous Peninsular Ranges batholith extends >800 km from Riverside California to the 28th parallel in Baja California. The batholith intrudes a series of NW-trending, pre-batholithic, lithostratigraphic assemblages (Fig. 4.4) that include from east to west (Gastil, 1993): (1) a Triassic-Cretaceous continental borderland assemblages (not shown in Fig. 4.4); (2) a Jura-Cretaceous volcanic arc assemblage; (3) a Triassic(?)-mid Cretaceous clastic and volcaniclastic flysch assemblage; (4) an Ordovician-Permian slope-basin clastic assemblage; and (5) an upper Proterozoic-Permian miogeoclinal carbonate-siliciclastic assemblage. Pluton lithologies in the batholith vary from mostly tonalite with gabbro, quartz gabbro, and diorite in the west to mostly tonalite and granodiorite with some granite and rare gabbro in the east. Gastil (1983) delineated “gabbro” and “tonalite” belts on western and eastern sides of the peninsula respectively, and “granodiorite-granite” and “alkaline” belts further east in mainland Mexico (Fig. 4.4). These designations Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 163 116° 114° 112° 0.706 0.704 REE zones ' &56180 isopleth \ y^Sry^Sr isopleth \ Magnetite/llmenite US; Me: ^ gabbro/tonalite line San Diego Ensenada* San iFelipe 0.706 Puertecitos San\>>. Quintin' 115* 110* LSan 100 km Rosario XT' liade LA Rgure 4.3 Map of the Peninsular Ranges batholith showing the magnetiteilmenite line, Rare Earth Element belts, selected isopleths for initial 37Sr/06Sr and 6 18O SMq w . and the gabbro/tonalite line. From Gastil etal. (1990), Grometand Silver (1987), Taylor and Silver (1978), Silver etal. (1979), and Silver and Chappell (1988). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 164 114' 112' Tectonic assemblages J and/or K arc volcanics ■p-K flysch O-P slope-basin Pe-P miogeocline Puertecitos -30° 116° El Rosario zircon age step gabbro/tonalite line \ biK-Ar&40Ar/39Ar \ agecontour 100 San Quintin Figure 4.4 Map of Peninsular Ranges batholith showing tectonostratigraphic assemblages, "gabbro" and "tonalite" belts west and east of gabbro/tonalite line respectively ("granodiorite-granite" belt lies to east in Sonora), the U-Pb age step between western and eastern zones, the La Posta pluton (largest recognized pluton in the batholith), and K-Ar and 40Ar/39Ar biotite apparent age isopleths in the Peninsular Ranges Batholith. From Gastil et al. (1990; 1991a), Gastil (1993), Krummenacher et al. (1975), Silver et al. (1979), Rothstein (1997), Axen et al. (2000). 165 characterize the PRB to a first approximation, but the main differentiation of variation across the batholith is by the chemical and isotopic compositions of plutons. Geochemistry and petrofenesis Nearly all geochemical and isotopic work in the PRB has been focused on the northern part of the batholith in southern California and the northern Sierra Juarez, with only a few samples analyzed from the central part, and virtually none from the southern part. Thus, geochemically and isotopically defined zones in the batholith have been projected as far south as the Sierra San Pedro Martir, but caution is warranted in interpreting this data because geochemical and isotopical trends this far south are only loosely constrained. Baird et al. (1974) were among the first to note major element compositional gradients across the PRB in southern California. They described a distinct eastward increase in SiCh and K.2O, and decrease in CajO, Fe total, and MgO that they interpreted to reflect variation in source lithologies from upper mantle in the west to an unspecified, non-primitive source in the east (Baird and Miesch, 1984). Work by G. Gastil and colleagues (summarized in Gastil et al., 1990) identified a petrologic boundary in the PRB dividing plutons containing magnetite+ilmenite in the west and plutons largely bearing only ilmenite in the east (Fig. 4.3). They suggested that changes in oxide mineralogy across the batholith reflected variation in source region oxygen fugacity and speculated that shallower, more oxidized sources at the depth of active subducted slab dehydration were apparent in the western zone, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 166 whereas deeper, more reduced continental sources, possibly containing graphite, occurred in the eastern zone. Gromet and Silver (1987) identified systematic variation in rare earth element (REE) patterns in tonalite plutons across the trend of the northern PRB. They divided the batholith into three REE belts (Fig. 4.3). The western belt shows nearly flat heavy REE values that are 10-15 times chondritic values, moderately enriched light REE values, and a moderately negative Eu anomaly. The central belt displays significantly lower heavy REE values, similar enrichment in light REE, and no Eu anomaly. The eastern belt shows similarly depleted heavy REE values, strongly enriched light REE, and a very minor negative Eu anomaly. Gromet and Silver (1987) interpreted REE zoning in the batholith to indicate an abrupt west-to-east transition from gabbroic sources to garnet eclogite sources. Early and Silver (1973) presented some of the first isotopic work in the northern PRB. They identified eastward-increasing gradients in initial ^Sr/^Sr isotopes across the batholith that are generally independent of pluton Ethologies and range from values of <0.7030 in the west to 0.7080 in the east (Fig. 4.3). DePaolo (1980, 1981) added a limited number of neodymium and strontium analyses from the northern PRB to this dataset and found that Em ranges from +8.0 in the west to -6.4 in the east and closely corresponds with initial “’Sr/^Sr values. DePaolo (1981) interpreted these isotopic trends as reflecting assimilation of variable crustal components by mantle-derived magmas across the batholith, from island arc material in the west to Early Proterozoic-Paleozoic continental crustal material in the east Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 167 Taylor and Silver (1978) measured the whole-rock oxygen isotope compositions of plutons in the northern PRB, and deduced from careful sampling and uniform A1S0 quartz-feldspar values that their results largely reflect undisturbed magmatic values (Taylor, 1986) with some regions showing the effects of hydrothermal interaction. Data interpreted as magmatic 618Osmow values range from +6.0 to +8.5%o across the western to central parts of the batholith, followed by an abrupt step to values of ~9.0%o, then a steady eastward rise to a maximum o f+1 l%o (Fig. 4.3). Silver and Chappell (1988) argued that fractional crystallization and assimilation models do not account for the overall variation in chemical and isotopic systems across the batholith because observed gradients in oxygen as well as strontium and neodymium isotopes and rare earth element patterns are independent of pluton lithologies. Rather, variation in source regions must be the major factor in these trends, with primitive island arc sources in the western PRB, and older, deeper reservoirs containing eclogitic mineral assemblages in the central and eastern PRB. All available petrologic, geochemical, and isotopic data from plutons in the PRB indicate that the western zone formed in largely oceanic lithosphere, whereas the central to eastern parts of the batholith evolved in transitional to continental lithosphere. A distinct compositional boundary divides the two zones. Nonetheless, this boundary in the batholith involves a change in chemical and isotopic parameters across an appreciable distance, implying that a region of transition exists that has been influenced by processes in both oceanic as well as continental lithosphere. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 168 Geophysical studies Crustal structure beneath the batholith is only known in detail in the northern part of the PRB where geophysical studies have identified a distinct crustal boundary in the batholith that corresponds with the compositional boundary described above. The compositional boundary is marked by a 60 mgal Bouguer gravity step (Oliver, 1980) that has been explained by large density contrasts (>0.1 g cm*1) in both the deep crust (>10 km, Fuis et al., 1982) and upper crust (<15 km, Weslow, 1985; Jachens, 1986). However, 3D imaging of P wave velocities from earthquakes has been used to determine that a -3% velocity contrast across the compositional boundary, corresponding with a -0.035 g cm'1 density contrast, extends to at least 20 km depth (Fig. 4.5), leading Magistrate and Sanders (1995) to hypothesize that the compositional boundary is continuous through the entire crust They also suggested that this boundary focused not only Mesozoic contractional deformation in the PRB, but strike-slip faulting along the Neogene San Jacinto fault as well. Moho depths across the batholith in southern California and the Sierra San Pedro Martir were estimated by Ps minus P seismic arrival times in passive seismic experiments (Ichinose et al., 1996; Lewis et al., 2000, personal comm., 2000). These studies determined a relatively deep and fiat Moho (37-41 km depth) beneath the western PRB and an eastward-shallowing Moho that decreases to -25 km depth beneath the eastern PRB that they attributed to crustal thinning resulting from Neogene extension in the Salton trough- Gulf of California extensional province (Fig. 4.5). The Moho geometry determined in these studies is corroborated by reflection Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Crustal structure across the Peninsular Ranges at ~30.5°N Figure 4.5 Very generalized, composite, crustal scale cross-section through the central PRB at ~30.5 °N latitude emphasizing features derived mostly from geophysical studies across the batholith. References noted on diagram. 169 170 seismic (Richards-Dinger and Shearer, 1997) and teleseismic receiver function studies (Zhu and Kanamori, 2000) in southern California, although absolute Moho depth determinations vary by as much as 7 km among these studies. Both geopotential as well as seismic studies thus strongly corroborate the presence of a crustal scale compositional boundary between dense, -40 km thick western zone crust and less dense, eastward-thinning (37-25 km thick) eastern zone crust along the PRB (Fig. 4.5). The width of this boundary can only be estimated within the resolution of the data, which is on the order of -5-20 km. Hence, these techniques do not resolve the width of the transition zone beyond geochemical techniques. Geochronology Silver et al. (1979) and Silver and Chappell (1988) reported some of the first U-Pb zircon geochronology studies in the northern PRB that focused on a transect across the PRB centered on the international border. They defined an older, static, western belt that ranges in age from -140 to 105 Ma and a younger, eastward migrating, eastern belt in which largely undeformed plutons range in age from -105 to 90 Ma. They defined an age boundary between these two belts that coincides with the chemical, isotopic, and geophysical transitions described above (Fig. 4.4). Silver and Chappell (1988) reported U-Pb zircon ages of 80 to 57 Ma for the batholith farther east in Sonora. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 171 A potential problem with the age-step hypothesis is that the transect undertaken by Silver et al. (1979) encompasses the largest pluton that has been defined in the PRB, the La Posta pluton, which is almost as long as the width of their transect (Fig. 4.4). Thus, the age step is defined along, and is potentially strongly influenced by, the presence of this single, very large 95-93 Ma pluton. Moreover, since their work, studies by Walawender et al. (1990) have determined that magmatism in the eastern belt is static rather than eastward migrating based on additional dates and the reinterpretation of a single pluton in the transect as a “western” rather than “eastern” zone pluton. This underscores the problem that has developed in the PRB of representing the crustal scale boundary in the batholith as a single line. Not only do geochemical and isotopic parameters change across a perceptible distance on maps of the batholith, but individual plutons within the transition zone reach sizes of >1500 km2 and, thus, may strongly influence geochemical, isotopic, as well as age gradients that are regionally defined for the batholith. Other studies have yielded Jurassic, and possibly Triassic ages from orthogneiss units in both the western and eastern zones as they were originally defined in southern California (Todd et al., 1991; Walawender et al., 1991; Thomson and Girty, 1994). This point may be further emphasized by a more recent geochronologic transect across the northernmost PRB (Premo et al., 1998) that found an apparently smooth progression of pluton ages across the transition zone from ~125 Ma in the west to -83 Ma in the east Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 172 Krummenacher et al. (1975) reported K-Ar hornblende and biotite ages from plutons across an extensive region of the northern and central PRB and integrated their work with previous K-Ar studies in southern California by Evemden and Kistler (1970) and Armstrong and Suppe (1973). They found that both hornblende and biotite ages young systematically from west to east across the batholith; hornblende ages vary from -120 to ~75 Ma and biotite ages vary from ~115 Ma to -65 Ma (Fig. 4.4). Differences among homblende-biotite K-Ar cooling age pairs also vary across the batholith, a feature that Krummenacher et al. (1975) attributed to interplay among parameters such as pluton emplacement depths and timing and rates of denudation that vary between western and eastern zones. Silver et al. (1979) also reported K-Ar hornblende and biotite ages from them transect on the international border and noted that these ages are concordant and consistently -5 m.y. younger than U-Pb zircon ages from the same plutons in the western zone. This age discordance increases eastward where the difference between U-Pb zircon and K-Ar biotite ages may reach -25 m.y. Silver et al. (1979) suggested that plutons in the 97-93 Ma La Posta magmatic suite may have reheated the eastern zone, causing cooling to lag in this region. More recent thermal studies in the PRB have confirmed the general age trends previously recognized. However, Grove (1993; 1994) also argued that east-side-up faults in the batholith played an important role in its cooling history. For example, he noted abrupt changes in cooling times and rates derived from modeling 39Ar diffusion in K-feldspar across a reverse fault (Chariot Canyon, Fig. 4.6b) that lies on the boundary between the transition and eastern zones of the batholith. The transition zone Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 173 Figure 4.6 Series of maps of the PRB showing major stratigraphic and structural relationships within the batholith. (a) Major stratigraphic units identified in the PRB. Note distribution of Albian-Aptian sedimentary-rich strata of the Alisitos Fm. BCBedford Canyon Fm. FV- French Valley Fm. JS- Julian Schist. From references in Gastil and Miller, eds. (1993), Todd et al. (1988), and Silver et al. (1963). (b) Major identified Mesozoic structures in the PRB including the inferred trace of the transitional PRB deformation belt: EP- Eastern Peninsular Ranges Mylonite Zone (Todd et al., 1988), CL- Cuyamaca-Laguna Mountains Shear Zone (Todd et al., 1988), CCF- Chariot Canyon Fault and CGF- Carrizo Gorge Fault (Grove, 1994), ABancestral Agua Blanca Fault (Silver et al., 1963; Gastil et al., 1975), SJ- shear zone(?) in Siena Juarez (Gastil et al., 1975), SSPM- shear zone in the Sierra San Pedro Martir (Johnson et al., 1999a; this report), C- shear zone at Calamajue (Griffith and Hoobs, 1993); A- Shear zone at El Arco (Barthelmy, 1979). Also shown is estimated location of transition from sub- to lower greenschist grade rocks in the west to upper greenschist-amphibolite grade rocks in the east PRB (from references cited above). Selected Cenozoic dextral strike-slip faults also shown: EF- Elsinor Fault, SJ- San Jacinto Fault, AB- Agua Blanca Fault Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 174 A) 175 B) Figure 4.6 (continued) 176 shows rapid cooling from ~88 to 85 Ma, whereas rapid cooling is apparent in the eastern zone from -76 to 72 Ma. Near the Chariot Canyon fault samples showed both cooling episodes. Rothstein (1997) defined both along-strike as well as across-strike variation in K-Ar cooling ages in the eastern zone of the batholith in Baja California. He noted that ages north of the Puertecitos Volcanic Field (Fig. 4.6a) are consistently younger than ages to the south. Thus, regional U-Pb age trends in the PRB show a general tendency towards eastward-younging for plutons less than -100 Ma. Earlier ages appear to overlap across the transition zone in the batholith. K-Ar isotopic systems exhibit variable cooling consistent with shallow pluton emplacement depths in the western zone. They also display systematic eastward-younging cooling trends across the transitional to eastern zones of the batholith that generally correspond with deeper denudation and are punctuated by abrupt transitions that coincide with individual structures. Possible causes for this eastward decrease in ages were discussed in detail in “Cooling mechanisms” in Chapter 3. Metamorphic studies Metamorphic studies in the PRB have perhaps been the most neglected aspect of investigations of this margin. Nevertheless, several significant advances have been made. In southern California, Todd et al. (1988) reported distinct variation across the strike of the batholith from lower greenschist grade metamorphism in the westernmost part of the western zone, to andalusite-siUimanite-bearing lower amphibolite grade in Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ill the eastern part of the western zone (i.e. the transition zone of this paper), to sillimanite-bearing upper amphibolite grade metamorphism in the eastern zone. Grove (1989; 1994) provided some of the first quantitative metamorphic constraints in the batholith and determined that pressures and temperatures increase from 2.5 ± 1.2 kbar and 550 ± 35 °C along the eastern side of the transition zone to 4.5 ± 1.5 kbar and 650 ± 50 °C along the western side of the eastern zone across the Chariot Canyon fault Reconnaissance Al-in-homblende studies on plutons in southern California by Ague and Brimhall (1988), Butler et al. (1991), and Ague and Brandon (1992) indicated pressures up to 6 kbar in the eastern zone and less than 2 kbar in the western zone, with intermediate pressures between. These studies concluded that the PRB experienced east-side-up tilting of 12-24° about a NW-trending axis. In Baja California, Rothstein (1997) conducted a reconnaissance metamorphic study of the eastern zone of the batholith and noted a distinct along-strike break in metamorphic grade in the eastern zone that coincides with the cooling age break he defined across the Puertecitos Volcanic Field. The region to the north of this latitude attained pressures of -4.0-5.8 kbar at -570-720 °C, whereas the region to the south recorded pressures of 32-3.5 kbar at-475-590 °C. C. Kopf is presently conducting a detailed metamorphic study along a transect that crosses all zones in the batholith at the latitude of the SSPM. Preliminary findings include metamorphic assemblages in the western zone consistent with pressures <2 kbar and a sharp increase in pressure to 5-6 kbar at the boundary between the western and transitional zones along the western side of the fan structure (Kopf and Whitney, 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 178 In summary three discrete zones can be defined in the PRB that contain distinct sedimentary assemblages, chemical and isotopic pluton compositions, geophysical parameters, and thermal and metamorphic histories. Below I discuss relationships both within and among these zones in more detail, starting with the eastern zone that most likely evolved on North American crust, and finishing with the western zone that has questionable heritage. The origin of the transition zone between these two assemblages is critical to understanding the tectonic evolution of the batholith. Eastern zone Stratigraphy The eastern zone of the batholith consists of Proterozoic-Paleozoic passive margin sedimentary rocks, most likely of North American origin, that are intruded by Mesozoic plutons. The passive margin assemblage has been divided into two facies (Fig. 4.4), a continental shelf sequence of limestone, quartz arenite, and shale and a deeper water slope-basin facies consisting of argillite, chert, siltstone, shale, limestone, and basalt (Gastil, 1993). The shelf unit is lithologically similar to Proterozoic-Early Cambrian miogeoclinal strata in Caborca, Sonora (e.g. Anderson, 1993), although, the oldest ages reported in the Peninsular Ranges come from Early Ordovician conodonts in southern California (Miller and Dockum, 1983). The slope-basin sequence has yielded conodonts of Devonian-Mississippian age (Gastil, 1993). Passive margin assemblage rocks have been metamorphosed to greenschist-amphibolite grade. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 179 The contact between shelf and deeper marine facies is largely obscured by intrusions and younger cover in the Peninsular Ranges. However, its inferred trace on the peninsula forms a protrusion near San Felipe on the Gulf coast (Fig. 4.4). The boundary between the two facies trends nearly E-W along -30.5° N latitude (near Puertecitos), and, after restoring -300 km of Neogene offset in the Gulf of California, connects with the trace of a contact between sim ilar facies in Sonora (Gastil, 1993). In Sonora, rocks of the slope basin assemblage were thrust northwards over the miogeoclinal assemblage in Permo-Triassic time. This thrust system could be the westward continuation of the late Paleozoic-early Mesozoic Ouachita-Marathon orogen (Fig. 4.1, Stewart, 1988; Stewart et al., 1990, however, see Sedlock et al., 1993, for alternative views). From the central PRB northwards the facies boundary between shallow and deep marine Paleozoic strata is oriented NNE-SSW, and may join the Permo-Triassic Golconda thrust system in southern Nevada and southeastern California (Dickinson, 2000). Magmatism Pluton lithologies in the eastern zone mostly vary from tonalite to granite. Pb inheritance in zircon is common in the few plutons that have been dated, and inherited components are typically ~1100-1300 Ma, ages that are comparable to the lower end of the 1200-1700 Ma age spectrum of crust that occurs in northern m ainland Mexico (Gastil, 1993). This inheritance may reflect either Precambrian crust at depth in the Peninsular Ranges or interaction of plutons with Phanerozoic sediments that contain Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 180 Precambrian zircons sourced from farther east Very little age control is available from eastern zone plutons outside of the region along the international border discussed above, where plutons range from -105-80 Ma. However, orthogneiss in the eastern footwall of the fan structure in southern SSPM has yielded one -164 Ma U-Pb zircon age and similar lithologies are present in the Sierra San Felipe to the east suggesting that plutons comparable in age to Jurassic plutons in the PRB transition zone may also occur across the eastern zone of the PRB. The -97-93 Ma La Posta suite, which features prominently in the transition zone and is discussed in more detail below, also occurs in the eastern zone (Kimbrough and Gastil, 1997). The only evidence for the remnants of an extensive pile of volcanic strata that is inferred to have once blanketed the eastern zone occurs as -108-106 Ma volcanic clasts in Turanian (<94 Ma) conglomerate in the western zone of southern California (Herzig and Kimbrough, 1998). This sequence was completely stripped off the top of the eastern batholith during Late Cretaceous denudation. Deformation Little is known about structures in the eastern zone of the batholith. However, marked variation in structural style are apparent along-strike, and much of this contrast appears to coincide with the inferred contact between mostly miogeoclinal Paleozoic rocks north of —30.5° N latitude and mostly slope-basin rocks to the south. North of this latitude, in the San Felipe region, tight folds and spaced to penetrative cleavage are oriented nearly E-W (Anderson, 1993, Leier-Englehardt, 1993; W. Marko and K. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 181 Schmidt, unpublished mapping), similar to structures developed in miogeoclinal rocks in Sonora (e.g. Stewart, 1988). In contrast, workers in slope-basin rocks south of 30.5° N report NW-SE to N-S oriented folds and cleavage, parallel to the structural trend of the batholith (Griffith and Hoobs, 1993; Campbell and Crocker, 1993; Buch and Delattre, 1993, Goldfarb, 1996). Gastil et al. (1991b) attributed this contrast in structural style to greater basement rigidity in crust underlying miogeoclinal rocks compared with that underlying slope-basin strata to the south. Thus, E-W PermoTriassic deformation along the southern margin of North America may be preserved in the eastern zone of the PRB north of 30.5° N, whereas rocks to the south were heavily overprinted by deformation associated with orogenesis along the Mesozoic Cordilleran margin. An important structure in the eastern zone in southern California is the Eastern Peninsular Ranges Mylonite Zone (EPRMZ), a moderately east-dipping shear zone that strikes SE some 100 km from the vicinity of Palm Springs, and has been offset by Neogene to Recent strike-slip faults (Fig. 4.6b). Lithologies include granodiorite and granite of the batholith and a package of migmatized metasedimentary and orthogneiss rocks (Palm Canyon complex of Todd et al., 1988). Proto- to ultramylonite foliation is commonly well-developed, strikes parallel to the zone, and contains E- to NEplunging Iineation. The history of the EPRMZ is controversial. Most workers agree that an older contractional deformation phase that produced top-to-west shear sense indicators (Simpson, 1984) is overprinted tty a younger extensional phase that produced low-angle imbricate shear zones with top-to-NE shear sense (Erskine and Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 182 Wenk, 1985). Dynamic metamorphic conditions o f600-800 °C at 3-5 kbar have been determined by Todd et al. (1988). However, the timing of these events is disputed. Goodwin and Renne (1991) suggested that contractional deformation began by -80 Ma in the mylonite zone, and that thermal equilibration of the zone’s hangingwall and footwall occurred by -62 Ma during extension. George and Dokka (1994), on the other hand, argued that the extensional deformation phase in the EPRMZ was largely complete by -92 Ma based on sphene and zircon fission track ages from the footwall. Although its timing is debated, this shear zone clearly played an important role in the thermal history of this part of the eastern batholith. Cooling history The eastern zone of the batholith has been deeply denuded, and cooling of this region has been attributed to episodes of rapid unroofing (e.g. Lovera et al., 1999). As described above eastward-younging cooling ages are common across the eastern zone for most thermochronologic systems. Periods of rapid cooling determined by 40Ar/39Ar biotite and K-feldspar systematics (-300 °C) include 76-72 Ma in southern California (Grove, 1994) and -83-73 in northern Baja California (Rothstein, 1997). In the Sierra El Mayor (Fig. 4.6b), a period of rapid cooling is recorded to -65 Ma (Axen et al., 2000). Farther south, changes in structural style across the Puertecitos Volcanic Field at 30.5° N also appear to coincide with contrast in crustal depths and Late Cretaceous cooling history of the eastern PRB. Regions to the north show metamorphic pressures on the order of 4-5 kbar and cooling through biotite ^Ar/^Ar closure temperatures Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 183 (-330 °C) at <85 Ma whereas areas to the south yield only 3-4 kbar pressures and cooled below this temperature at >85 Ma (Rothstein, 1997). The eastern zone of the batholith also preserves an extensive history of Paleogene denudation and Neogene extensional tectonics. Periods of rapid cooling at -35 Ma and 45-33 Ma have been determined, respectively, from apatite fission track systematics (-110 °C) on the eastern side of the SSPM and the Sierra El Mayor (Dorsey and Cerveney, 1991; Axen et al., 2000). Evidently this cooling episode corresponded with uplift of the eastern region and consequential disruption of fluvial systems that flowed across the batholith before this time. Arc magmatism returned to the eastern zone of the batholith by -24 Ma (Sawlan, 1991, Martin-Barajas et al., 1995). Extension associated with the Gulf of California province was initiated by -12 Ma in response to transition of the North American southern Cordilleran margin from a subduction zone to transform plate boundary (e.g. Stock and Hodges, 1989). Both high-angle faults, such as the SSPM fault with over 5 km of cumulative throw, and lower-angle normal fault systems, such as the Cafiada David detachment in the Sierra El Mayor, which accommodated 5-7 km of denudation and 10-12 km of horizontal extension, were active early in this extensional history (Stock et al., 1991; Axen et al., 2000). Marine sedimentation in the Gulf of California is evident by 6-7 Ma, and -270 km of oblique dextral displacement has been measured across the Gulf since that time (Oskin et al., 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 184 Transition zone Stratigraphy The transition zone in the PRB consists of a series of mostly Mesozoic flysch basins that formed on continental to transitional crust and were intruded by plutons that span the range of chemical variation between western and eastern zone plutons. The Mesozoic flysch sediments are inferred to have been deposited on Paleozoic North American margin sedimentary rocks and early Mesozoic plutons. However, in only one location, at El Marmol in Baja California (Fig. 4.6a), has this relationship actually been described (discussed below). In southern California, metamorphosed rocks of the flysch assemblage (Bedford Canyon Fm., French Valley Fm., and Julian Schist) and their correlatives in northern Baja California (Rancho Vallecitos Fm. and strata near Rancho Santa Clara, east of Ensenada) consist of pelitic and psammitic schist and gneiss with minor calc-silicate rocks interpreted as continentally derived turbidite to deltaic deposits (Fig. 4.6a; references cited in Gastil and Miller, eds., 1993). Few volcanic protoliths have been identified in these northern assemblages, with the notable exception of a poorly known and undated amphibolite-grade metavolcanic unit that lies southwest of these units in San Diego county and is depositionally overlain by western zone strata of the Santiago Peak Volcanics (Todd et al., 1988). Age control for the metasedimentary flysch units is mainly from Jurassic (Bajocian-Callovian, -177-159 Ma) fossils in the Bedford Canyon Fm. This unit is Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. unconformably overlain by -127 Ma volcanic strata of the Santiago Peak Volcanics in the Santa Ana Mountains (Herag and Kimbrough, 1998), Fossils are either absent or provide only tentative age relationships in other units. Thomson and Girty (1994) interpreted 234 ± 39 Ma Harper Creek orthogneiss as intrusive into Julian Schist, providing a pre-Triassic-earliest Jurassic age constraint on the Julian Schist Similar relationships occur in northern Baja California where possible equivalents may occur in sheared unconformable contact with Santiago Peak Volcanics. The flysch assemblage units in southern California contain zircon populations derived from Late Proterozoic, and Paleozoic-early Mesozoic sources (Gastil and Girty, 1993). Abundant plutonic and volcanic rocks of these general ages occur in Arizona and Sonora; hence, these sedimentary basins appear to be well tied to North America during their formation. An additional tie is suggested by allochthonous Ordovician strata containing 2000 Ma zircons that occurs on Julian Schist equivalents at Rancho Santa Clara in northern Baja (Fig. 4.6a). This strata has tentatively been linked to the Valmy and Vinini Fms. in north-central Nevada of similar lithology, age, conodont provenance and zircon age population (Lothringer, 1993; Gastil, 1993). In the SSPM, the flysch assemblage includes a substantial volcanic component. Age constraints here are somewhat uncertain. Johnson et al. (1999a) obtained a U-Pb SHRIMP zircon age of -128 Ma from the western side of die transition zone, but are unclear as to whether or not the unit dated is a sill or volcanic flow. This assemblage is faulted against a sedimentary-rich facies of the Alisitos Fm. of western zone provenance as described below. Measures (1996) obtained discordant zircon U-Pb Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 186 ages from a metavolcanic unit in the eastern side of the transition zone that he interpreted as ~123 Ma. This package lies in fault contact with shelf facies rocks of the eastern zone of the batholith. In the transition zone of the southern PRB, flysch assemblage rocks occur in fault slivers that are juxtaposed with both Paleozoic assemblages and Alisitos equivalents. At some locations, depositional contacts between flysch strata and both of these units have been reported, in the El Marmol area (Fig. 4.6a), a Lower Permian to Lower Triassic sequence (Zamora, El Volcan, and De Indio Fms.) conformably overlies Lower Permian strata (Buch and Delattre, 1993). Buch and Delattre consider this package to be the southern extension of a Permo-Triassic miogeocline belt that occurs in southern Nevada and east-central California. These units are unconformably overlain by Alisitos Fm. equivalents (Olvidada Fm. of Phillips, 1993) that contain Aptian-AIbian (-121-99 Ma) fossils. In the Calamajue region (Fig. 4.6b), -156 Ma volcanogenic flysch assemblage rocks (Cafiyon de las Palmas Fm. of Griffith and Hoobs, 1993) unconformably underlie -103 Ma Alisitos equivalents. M agmatism Since the work of Silver et al. (1979), who reported U-Pb zircon ages ranging from -120-95 Ma from plutons in the transition zone of southern California, a few important studies have identified older plutons in the same region. The oldest dated arc plutonic suite in the Peninsular Ranges occurs in southern California where two samples of the largely granodioritic Harper Creek orthogneiss yielded 234 ± 39 and Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 187 156 dt 12 Ma U-Pb zircon ages (Thomson and Girty, 1994). The Cuyamaca Creek gneiss, another arc plutonic suite in this region yielded U-Pb zircon ages of 161 ± 12 and 149 ± 21 Ma (Thomson, 1994). Recent progress in the northern and southern SSPM has delineated an extensive history of arc magmatism (Johnson et al., 1999a; present study). The oldest plutonic unit in the batholith of this region, which is lithologically similar to the Cuyamaca Creek gneiss in southern California, produced a 164.4 ± 1.2 Ma U-Pb SHRIMP zircon age (Chapter 3). This orthogneiss unit is intruded by plutons that have yielded U-Pb zircon ages of 133.9 ± 1.5,133.6 ± 1.9, and 127.8 ± 1.2 Ma from the northern SSPM (Johnson et al., 1999a), and a 132 ± 7 Ma age from the Suerte pluton and 118 ±3 Ma age from a granite sheet on the western side of the fan structure in the southern SSPM (Chapter 3). Johnson et al., (1999a) noted a switch to deeper sources determined from petrogenetic modeling of 108-102 Ma plutons that occur along the border between transitional and western zones in the batholith of the northern SSPM. They speculated that this was caused by crustal thickening resulting from collision of the western zone just prior to this age. Slightly younger pluton ages have been obtained from the Rinconada pluton and hornblende tonalite sheeted complex in the southern SSPM (-101 and 100 Ma respectively, Chapter 3). Plutons from the transition zone farther south in Baja California have received only limited attention and yielded U-Pb zircon ages of -100 Ma and younger. The 97-93 Ma La Posta plutonic suite occurs along the length of the PRB (e.g. Walawender et al., 1990; Gastil et al., 1991a). Most of the larger plutons of this suite Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 188 that have been identified in the batholith are centered in the transition zone, but numerous smaller bodies have been mapped across the eastern zone. These plutons reach diameters of 40 km and are typically concentrically zoned from hornblende tonalite margins to muscovite-bearing monzogranite cores. Many geochemical and isotopic systems also show zoning from parameters that are considered “western PRB” in their margins to “eastern PRB” in their cores, including increasing zircon Pb inheritance in pluton centers (Walawender et al., 1990). The La Posta plutons have generally been considered post-tectonic (e.g. Todd and Shaw, 1979) and their petrological, geochemical, and isotopical characteristics are considered type-examples of eastern zone plutons. This plutonic suite is considered to have been derived from either deep amphibolite-eclogite sources and modified by interaction with continental crust during ascent (Walawender et al., 1990), or unusually thick continental crust underplated by basaltic magmatism, as has been suggested for magmatism in the high Andes (Kimbrough and Gastil, 1997). Although clearly a significant plutonic suite, I question the designation of the La Posta suite as the type example of eastern zone magmatism. The age range that has been determined for this suite is far too restricted, and it appears that a substantial region of the transitional and eastern zones of the batholith are underlain by variably deformed and metamorphosed granitoid plutons that span an age range from the Middle Jurassic through Early Cretaceous. Few petrological or geochemical studies have been conducted on these units. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 189 Deformation Initial studies in the PRB inferred that deformation and metamorphism largely occurred in latest Early to mid-Cretaceous time (e.g. Todd et al., 1988). However, more recent work has identified an earlier period of deformation. In southern California, this initial deformation is apparent in flysch assemblage rocks that were folded, penetratively cleaved, and metamorphosed in the period 151-127 Ma, an event that has been attributed to the Nevadan Orogeny (Kimbrough and Herzig, 1994). In the SSPM, flysch assemblage rocks were folded and cleaved prior to intrusion of the -132 Ma La Suerte pluton as described in Chapter 2. Thus, available constraints suggest an early regional episode of contractional deformation in the PRB transition zone within, but not restricted to, the age range -151-132 Ma. The most prevalent episode of deformation and metamorphism in the PRB transition zone occurred in late Early- mid Cretaceous time. This event appears to have occurred along the known extent of the PRB, and the most intensive deformation is restricted to a distinct belt within the transition zone that I have informally designated the transitional deformation belt Recognition of the initial continuity of this deformation zone has been hampered by the extent to which it has been posttectonically intruded by large plutonic suites such as the La Posta series and covered by extensive Tertiary volcanic strata. Moreover, the character and, to some extent, age of this deformation appears to vary along strike as described below. In southern California, the Cuyamaca Laguna Mountains shear zone represents the northernmost extent of the transitional PRB deformation belt (Fig. 4.6b). It Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 190 consists of penetrative NW-striking, steeply NE-dipping mylonitic to magmatic foliation and steeply pitching lineation with top-to-SW shear sense indicators (Todd et al., 1988; Thomson and Girty, 1994). This episode of deformation has been constrained to -118 to 105 Ma and occurred under amphibolite grade metamorphic conditions. An overprinting deformation event that produced NE-side down shear sense on nearly vertical NW-striking mylonitic foliation and steeply pitching lineation in a ~12 km-long section of the main shear zone has been bracketed between —105 and -94 Ma (Thomson and Girty, 1994). The CLMSZ extends >100 km in two segments through the PRB. The northern, southern, and eastern extents of this major structure in the Peninsular Ranges are obscured by younger plutons and alluvial cover (Todd et al., 1988). The structural character of the transitional PRB deformation belt in the Sierra Juarez (Fig. 4.6b), is virtually unknown. Reconnaissance mapping by Gastil et al. (197S) confirms that amphibolite grade metamorphism and steeply-dipping NWstriking structures, possibly including a fan structure, occur in the transitional zone of the batholith. South of the Agua Blanca fault the transitional PRB deformation belt is spectacularly exposed across the Sierra San Pedro Martir as described in detail in Chapter 2 (Fig. 4.6b). Although parts of the region are poorly known, it appears that the fan structure is developed in most of the range (Johnson et al., 1999a; S. Johnson, personal comm., 1999). In this region complex composite deformation occurred over a protracted period of time, with much of the deformation apparent between 118 and 85 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 191 Ma. Structural development of the fan includes extensive mylonitic shear zones, minor brittle overprinting, and multiple episodes of folding with contrasting styles and orientations. All identifiable deformation is contracdonal, with the exception of possible, but limited, sinistral transpression during a restricted period of time around -100 Ma. South of the SSPM the central and eastern parts o f the PRB are almost completely covered by volcanic strata of the Puertecitos Volcanic Field. The batholith re-emerges in the El Marmol region (Fig. 4.6a and 4.6b) where upper greenschist to lower amphibolite metamorphic grade rocks have been multiply folded, with a change from production of NNE-plunging to WNW-plunging fold sets apparent during Albian-Aptian time (Buch and Delattre, 1993; Phillips, 1993). Much of the early batholithic and pre-batholithic rocks have been intruded-out or overlain by younger cover in this area and significant shear zones in the transitional zone may be obscured. In the Calamajue region even further south, a major NW-striking shear zone occurs, which forms a series of SW-vergent thrust slices that interleave slivers of Alisitos Fm. and flysch assemblage rocks (Fig. 4.6b, Griffith and Hoobs, 1993). Steeply NE-dipping mylonite foliation and steeply pitching lineation are welldeveloped. Foliation is axial planar to steeply-plunging isoclinal folds that have progressively rotated during deformation. Flattening strains and >60-70% shortening have been ascertained in parts of the shear zone, and deformation has been constrained to -103-100 Ma as determined from deformed Alisitos equivalent strata and crosscutting undeformed granite. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 192 The southernmost reported exposure of the transitional deformation belt in the northern half of the Baja peninsula occurs at El Arco before the belt disappears offshore into the Gulf of California (Fig. 4.6b). Amphibolite grade flysch assemblage rocks occur in disconnected wall rock screens in a region that has been extensively intruded. Deformation and amphibolite grade metamorphism continue to the west into Alisitos rocks (Barthelmy, 1979). Strata, structures, and upper greenschist to amphibolite grade metamorphism similar to the PRB deformation belt in the north, and possibly a fan structure, occur in the La Paz crystalline complex on the southern cape of the Baja peninsula (A. Schuerzinger personal comm., 1998), where some tectonic models contend that the PRB reemerges from the Gulf. This region exposes rocks similar to much of the eastern and central zones of the PRB; western zone rocks have not been reported. Cooling history Along much of the transition zone, a sharp step in cooling ages is superimposed on the generally eastward-decreasing thermal ages that extend across the eastern side of the batholith. This age-step typically corresponds with eastward increasing metamorphic gradients across east-side-up structures, and has been interpreted as resulting from rapid denudation associated with rock uplift on discrete structures in the deformation belt (e.g. Lovera et al., 1999). A commonly cited example in southern California is the Chariot Canyon fault discussed above. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 193 In the Sierra Juarez (Fig. 4.6b), a sharp gradient in K-Ar cooling ages determined by Krummenacher et al. (1975) corresponds with the location of fan-like structural geometry ascertained in reconnaissance mapping by Gastil et al. (1975). Rothstein (1997) inferred a rapid cooling episode from -85 to 72 Ma using K-feldspar 39Ar diffusion studies. Farther south, in the fan structure of the Sierra San Pedro Martir, he determined rapid cooling in the range -91-87 Ma, consistent with results in the present study from the southern part of the range. As detailed in Chapter 3, the western side of the fan structure in the southern SSPM continued to thrust westward over the western zone of the PRB after -100 Ma, producing -15 km of structural throw and juxtaposing rocks with strongly contrasting cooling histories that differ by at least 10 m.y. Western zone Stratigraphy The western zone of the PRB consists of an assemblage of Jura-Cretaceous volcanic arc sequences intruded by Cretaceous plutons. Pre- to syn-batholithic stratigraphy of the western zone is subdivided into the Santiago Peak Volcanics (Larson, 1948) north of the Agua Blanca fault and the Alisitos Formation (Allison, 1955; 1974) to the south (Fig. 4.6a). The Santiago Peak Volcanics consist predominantly of andesite and quartz latite flow and volcaniclastic rocks with subordinate rhyolite and basalt (Larson, 1948). A largely subaerial depositional Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 194 environment has been interpreted for this volcanic sequence. Latest Jurassic fossils in the Santiago Peak Volcanics of southern California have been found at three localities (Fife et al., 1967); U-Pb zircon ages range from 138-118 Ma (Silver and Chappell, 1988; Walawender et al., 1991; Carrasco et al., 1995). The Alisitos Fm. consists of volcanic breccia, tuff, flows, tuffaceous mudstone and wacke with sparse biostromal limestone (Santillan and Barrera, 1930; Silver et al., 1963; Allison, 1955,1974). Volcanics range in composition from basalt to rhyolite; andesite and dacite are the most abundant lithologies. Near the Agua Blanca fault in the type-section of the Alisitos Fm. the sequence is particularly rich in sedimentary strata that is at least 6500 m thick, with no top or bottom defined (Fig. 4.6a). Sedimentary detritus of exclusively volcanic origin and organic carbonate occurs in the sequence between the SSPM and Punta China where the Agua Blanca fault continues offshore (Fig. 4.6a, Silver et al., 1963). hi contrast to the Santiago Peak Volcanics, a dominantly submarine depositional environment has been interpreted for the Alisitos Fm. Near El Rosario large stratovolcanos with associated caldera complexes have been identified that fed volcanic flows and debris into fault-bounded marine basins on their flanks (Busby et al., 1998; Fackler-Adams and Busby, 1998). Age constraints on the Alisitos Fm. include Albian-Aptian fossils (121-99 Ma; Silver et al., 1963; Allison, 1974) and U-Pb zircon ages of 116 ± 2 Ma (Carrasco et al., 1995) and 115.7 ± l.l Ma (S. Johnson, personal comm., 1999). In Arroyo San Jose, -220 km SE of El Rosario, slightly metamorphosed clastic and pyroclastic strata containing Jurassic-Early Cretaceous fossils has been reported that is unconformably Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 195 overlain by upper-Lower Cretaceous strata equivalent to the lower part of the Alisitos Fm. (Minch, 1969). Both the Santiago Peak Volcanics and Alisitos Fm. show lower to sub-greenschist grade metamorphism. Thick sequences of Late Cretaceous-Paleocene clastic sedimentary rocks overly both the Santiago Peak Volcanics and Alisitos strata along the Pacific Coast They were derived from the eastern side of the batholith and deposited in forearc basins beginning abruptly at -100 Ma. In the Viscaino region (Fig. 4.6a), lower Cenomanian (-99 Ma) strata show a dramatic transition from marine silt- and muddominated deposition to channelized coarse conglomerate containing clasts that are lithologically similar to plutons and host rocks of the eastern batholith. The conglomerates exhibit consistently southwest-directed paleo-current indicators (Busby et al., 1998). In the coastal region SW of the SSPM a thick Turonian (93.5-89 Ma) to Paleocene section of fluvial to shallow marine conglomerate, sandstone, and shale marks rapid erosion of the batholith to the east; sediments of similar age occur along the coast to the north (Botq'er and Link, 1984). Magmatism Plutons in the western zone vary from mostly gabbro to tonalite lithologies, and geochemical and isotopic values are characteristic for primitive island arc sources (Silver and Chappell, 1988). Walawender and Smith (1980) suggested that gabbroic complexes in southern California were derived from fractionation of high-alumina basalt at pressures <5 kbar. They argued based on trace element geochemistry that Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 196 granitoid magmas were derived from separate sources. Tate et al. (1999) proposed that the bimodal Zarza pluton, located just west of the northern SSPM, is a representative western zone pluton in which gabbro and andesite were derived from andesitic calcalkaline magmas produced from mantle sources fluxed by subduction related fluids. Tonalite compositions in the complex were subsequently produced by fractionation of the resulting andesite, and trondhjemite liquids resulted from partial melting of wall rocks. The Zarza pluton is particularly interesting because it is a rare example of a cone-sheet-bearing ring complex that probably represents the ~8-5 km deep roots of an ancient caldera complex such as those still preserved further south (Johnson et al., 1999b). Zircon U-Pb ages for plutons in the western zone of the batholith range from 120 to 105 Ma in the Santiago Peak segment (Silver and Chappell, 1988). An age of 116.2 ± 0.9 Ma has been determined for the Zarza pluton in the Alisitos segment (Tate et al., 1999). Basement to western zone volcanic sequences is not seen except in the north where eastern exposures of the Santiago Peak Volcanics lie unconformably on flysch assemblage rocks. Volcanic rocks yield zircons with some inheritance from the Santiago Peak segment (e.g. Meeth, 1993), but volcanics and plutons in the Alisitos segment have not, as yet, been shown to contain zircons with inherited Pb (e.g. Johnson et al., 1999a). Thus, at least the Alisitos segment of the western zone may not contain continental crustal basement and may have evolved in an environment that was isolated from continental influence until ~115-108 Ma (Johnson et al., 1999a). Moreover, if this lack of inheritance proves correct, then it is unlikely that the flysch Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 197 sequences containing conspicuous inherited zircon components in the transition zone are present in the basement of the Alisitos segment of the western zone, unless magma that intruded this crust somehow avoided chemical and mechanical exchange with a portion of its host rock. Deformation Rocks of the western zone are for the most part only weakly deformed, but along-strike and across-strike variation is apparent within it. In southern California much of the Santiago Peak Volcanics are nearly horizontal, including strata that unconformably overlies flysch rocks of the transition zone. In contrast, in Baja California north of the Agua Blanca fault, Santiago Peak Volcanics near the boundary with the transition zone are tightly folded- in some places by two generations of folds containing moderate to steep axes- and cleaved by steeply-dipping cleavage. South of the Agua Blanca fault a deformation gradient is apparent across a west-to-east transect of the Alisitos segment at the latitude of the SSPM. Alisitos strata are horizontal to gently folded near the Pacific Coast and become more folded eastward, characterized by open folds with nearly horizontal, NW-SE trending axes. Within a few km of reverse shear zones that occur along the border between western and transition zones Alisitos strata is tightly folded with moderately- to steeply-plunging axes and well developed axial planar cleavage. These folds trend parallel, to 20° oblique, to the strike of reverse faults that juxtapose western and transitional zone strata. Ages of deformation in the western volcanics are poorly constrained. Johnson et al., (1999b) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 198 determined that folding of Alisitos Fm. rocks in part of the northern SSPM occurred pre- to syn-emplacement of 117-113 Ma plutons. The contact between the Santiago Peak Volcanics and Alisitos Fm. across the Agua Blanca fault appears to be a major tectonic boundary in the batholith (Fig. 4.6b, Gastil et al, 1975,1981; Wetmore and Paterson, 2000). The Agua Blanca fault is currently active and shows ~6 mm of annual dextral displacement (Allen et al., 1960; Rockwell et al., 1989). However, a pre-Tertiary sinistral history has been inferred based on discordances in strati graphic and structural trends in both the Santiago Peak Volcanics and Alisitos strata. A marked change in lithology occurs across the length of the fault, from a sequence dominated by volcanic flows and tuffs to the north to a mixed volcanic/epiclastic and carbonate sequence to the south. This mixed volcanic/sedimentary unit is continuous with strata to the south that occurs along the border between western and transition zones of the batholith (Fig 4.6a). Moreover, the structural trend of the largely margin-parallel Santiago Peak Volcanics is highly discordant to the trend of the Agua Blanca fault, whereas Alisitos strata south of the fault bend from a margin-parallel (and highly fault-oblique) orientation at distances further away from the fault to fault-parallel near to it Ductile deformation in Alisitos rocks in the package near the Agua Blanca fault is cross-cut by undeformed western zone plutons, indicating a Mesozoic age of deformation (Wetmore and Paterson, 2000). The westward bending geometry of strati graphic and structural trends in the Alisitos segment near the Agua Blanca fault suggest drag-folding of the Alisitos block in a sinistral sense. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 199 Farther south in the Alisitos segment, east of El Rosario, normal faults that strike nearly perpendicular to the trend of the batholith bound small marine basins and link with faults in caldera complexes (Busby et al., 1998; Fackler-Adams and Busby, 1998). The ages of syntectonic strata and resurgent intrusions are in the range 120-130 Ma (Busby personal comm., 1999). Thus, an older extensional history may precede contractional deformation that is apparent farther north. Additionally, variable crustal levels may be exposed along-strike in the Alisitos segment The caldera complexes in the south near El Rosario appear to give way northward to deeper levels where ring complexes, interpreted as the subvolcanic roots of calderas at -5-9 km depths in the crust occur (Johnson et al., 1999b). Massive intrusions farther north represent even deeper plutonic levels in the arc. Thus it appears that a difference in exposure level of ca. 5-12 km exists over a north-south strike-length of -200 km in the mid-section of the western zone. Boundaries between zones in the batholith As described above, the eastern, transitional, and western zones of the batholith are distinct entities in terms of their pluton Iithologies and chemistry, host rock Iithologies, structures, and metamorphic histories. The boundaries between these belts are variable and different geological criteria define them along-strike in the batholith. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 200 Rnimdarv betw een eastern and transitional zones The boundary between eastern and transitional zones in the batholith is perhaps the least well defined, not only because changes are less dramatic than on the western side, but also because more of this boundary has been overprinted by Neogene tectonic events. In fact, several researchers have noted Late Cretaceous features that commonly coincide with the western boundary of the Gulf Extensional Province (Grove, 1994; Rothstein et al., 1995). The modem topographic break across this boundary is mostly controlled by a series of down-to-the-east normal faults that produce deep basins in their hangingwalls and, therefore, commonly cover preTertiary relationships. In southern California contacts between flysch and slope basin lithological units are largely intraded out. Nevertheless, Todd et al. (1988) suggested that they may be gradational, as inferred from rare exposures in strongly migmatitic screens within the batholith. Metamorphic breaks have been reported from the Chariot Canyon and Carrizo Gorge areas (Fig. 4.6b) in which upper amphibolite grade rocks have been thrust westward over phyllite of the Julian Schist between 76-72 Ma and possibly earlier, and discontinuities in cooling age gradients occur across these faults (Grove, 1994). Thus, the northern part of the eastern zone of the batholith shows higher metamorphic conditions than the transition zone, and the boundary is, at least in part, defined by west-directed reverse faults. In the Sierra Juarez, little is known about the eastern to transitional zone boundary as it is largely covered by Neogene sediments in the Laguna Salada area to Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 201 the east Much of the boundary in the Sierra San Pedro Martir appears to be similarly covered. An exception occurs in the southern part of the range where migmatitic flysch assemblage rocks in the eastern side of the fan structure have been thrust over miogeoclinal rocks. The relationship is similar to the boundary in southern California in that higher metamorphic grade rocks lie in the thrust hangingwall, but in the SSPM the thrust is east-vergent and metamorphic conditions across the eastern zone appear to remain slightly lower than in the transition zone. The age of thrusting is poorly constrained to a range between intrusion of ~164 Ma orthogneiss and ~97-93 Ma plutons (Chapter 2). Biotite ^Ar/^Ar apparent ages are similar across this boundary, but a significant gradient occurs in apatite fission track ages. In the poorly known area south of the Puertecitos Volcanic Field, features that may be associated with this boundary have not been described. Thus, preliminary observations from the eastem-to-transitional zone boundary show that it may largely correspond with major structural and lithological changes that coincide with some variation in west-to-east cooling age gradients. However, crustal exposure levels do not change much across this boundary. Sedimentary as well as plutonic criteria indicate that the transitional and eastern zones of the batholith to either side of the boundary have been joined since pre-Mesozoic time. Therefore, this boundary represents a pre-Mesozoic crustal join along which deformation has been focused. In fact, this older boundary commonly corresponds with the location of the modem Main Gulf Escarpment, a rift shoulder to the Gulf Extensional Province (e.g. Grove, 1993), and may have been reactivated in Neogene time. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 202 RnnnHarv between transitional and western zones In contrast to the eastern boundary of the transition zone, the western boundary is in places impressively exposed and defined. However, the contacts that define the boundary are strongly variable along-strike, ranging from unconformities to faults. In southern California deformed and metamorphosed late Middle Jurassic Bedford Canyon rocks depositionally underlie nearly horizontal -127 Ma strata of the Santiago Peak Volcanics (Herzig and Kimbrough, 1998). A similar relationship occurs near San Diego, where Santiago Peak Volcanic strata lies depositionally on folded and cleaved amphibolite grade metavolcanic rocks. Farther east of these locations, in the transition deformation belt, metamorphism and deformation continued into midCretaceous time along significant deformation zones such as the CLMSZ. The contact between Santiago Peak Volcanics and flysch assemblage rocks in the Sierra Juarez appears to remain west of the main belt of deformation in the transition zone (Fig. 4.6b). The contact here is gradational across a broad region displaying complex folding and strong cleavage development It thus appears that the western volcanic-flysch assemblage contact is a sheared unconformity. South of the Agua Blanca Fault, in the Sierra San Pedro Martir, the boundary between the transitional and western zones of the batholith is strikingly defined. Here, a shear zone on the western side of the fan structure juxtaposes sedimentary rich units of the Alisitos Fm. and flysch assemblage rocks, and corresponds with a dramatic metamorphic and cooling age gradient Plutons stitching this structure are as old as Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 203 -108 Ma in the northern SSPM (Johnson et al., 1999a), and -101 Ma in the south (Rinconada pluton, Chapter 2). South of the Puertecitos Volcanic Field the boundary between the western and transitional zones is more poorly known because it is largely intruded out and covered by extensive Tertiary volcanics. However, in parts of the boundary that have been examined, significant differences with the boundary to the north are apparent. At Calamajue -103 Ma Alisitos and -156 Ma flysch assemblage strata of similar metamorphic grade are structurally interleaved across a broad region that is stitched by -100 Ma plutons (Fig. 4.6b, Griffith and Hoobs, 1993). Farther south at El Arco the structural and metamorphic boundary occurs entirely within western zone strata (Barthelmy, 1979). Alisitos Fm. rocks are progressively more deformed and metamorphosed from chlorite to amphibolite grade towards the NE across a <5 kmwide zone. The contact between Alisitos rocks and strongly deformed and metamorphosed flysch assemblage rocks is completely intruded out or covered. Deformation in the Alisitos Fm. near to this inferred contact includes isoclinal folds and penetrative, NW-WNW-striking, steeply-dipping fracture cleavage intruded by plutons dated >104 Ma. Thus, stratigraphic, structural, and metamorphic relationships between the western and transitional zones appear to change along-strike in the batholith. The main break along this trend is the Agua Blanca fault that cuts obliquely across the western zone, hi general, unconformities, both depositional and sheared, define the boundary north of the Agua Blanca fault, whereas the boundary to the south is largely defined Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 204 by fault contacts for at least pre-Albian-Aptian time. The transitional deformation belt lies east of this boundary in the northern batholith, and adjacent to it in the south. Summary and discussion The geological relationships described above indicate that the eastern, transitional, and western zones of the batholith are distinct crustal entities with dissimilar basement and varying geologic histories. Below, I integrate these observations into a discussion of tectonic models that have been proposed for the PRB and propose a new testable hypothesis for the evolution of the PRB. Implications for existing tectonic models of the Peninsular Ranges batholith Several stratigraphic, magmatic, and structural relationships outlined in sections above have important implications for tectonic models for the PRB. A number of criteria indicate that the eastern and transitional zones of the batholith evolved together during the Mesozoic. These include Triassic-Jurassic flysch strata in the transition zone that apparently thin to the east and contain detritus that is most likely of North American origin. Moreover, plutons of similar lithology and identical Middle Jurassic age (-164 Ma) intrude both the transition and eastern zones of the batholith in the SSPM, and many plutons in the transition zone show inherited Pb ages of ~1100-1300 Ma, a common basement age to the east in Sonora (Gastil and Girty, 1993; Gastil, 1993). The main issue in PRB tectonics concerns whether the western and transition zones evolved together during the Mesozoic, and if not then when, and Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 205 by what process, were they juxtaposed? One issue that bears prominently on this problem is that the Santiago Peak and Alisitos segments of the western zone appear to have been juxtaposed during Cretaceous time by sinistral displacement on the Agua Blanca fault (Gastil et al., 1975; Wetmore and Paterson, 2000). In the northern part of the PRB, western zone strata were deposited on flysch basement, strongly suggesting that a single, broad arc formed across an inherited continental-oceanic lithospheric boundary. This interpretation is corroborated by Pb inheritance in zircons from volcanics and plutons in the Santiago Peak segment, reflecting continental-derived sources in its basement Thus, model A in Figure 4.7 seems to appropriately describe the Jura-Cretaceous evolution of the northern PRB. In contrast the Alisitos segment of the western zone does not show stratigraphic ties with the batholith transition zone until Albian-Aptian time (-121-99 Ma), when sedimentary-rich facies of the Alisitos Fm. were deposited and interleaved in reverse faults that bound the two zones. Moreover, petrological studies suggest a primitive island arc setting for plutons in the Alisitos segment, and both plutons and volcanics appear to lack inherited continental components. Thus, the Alisitos segment appears to have originated as an exotic island arc that collided with the North American margin in the period -115-108 Ma (model C in Figure 4.7). A difficulty with this model is the fate of the older forearc element that must have existed prior to collision. On Figure 4.7c I speculate that a portion of the colliding Alisitos arc was obducted over the old forearc, and consequently obscures it At present there is no evidence for this event However, virtually no information exists Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 206 a) Arc built across pre-Mesozoic crustal join Santiago Peak segment in northern PRB? b) Backarc extension, development of western fringing arc, followed by collapse of marginal basin Doesn't fit Late Jurassic through Early Cretaceous history in transitional zone robust arc Late Jurassic - magmatism Early Cretaceous contraction >132-85 Ma extension? c) Collision of exotic western island arc Alisitos segment in southern PRB? Figure 4.7 Tectonic models suggested for the PRB with constraints determined in this synthesis. Explanation is the same as for Fig 4.2. 207 regarding the geology of the Alisitos Fm. adjacent to the transition zone of the batholith, and our present knowledge does not preclude the existence of structures that may be associated with this event or preserved remnants of the hypothetical older forearc. Understanding the geology of this region is, therefore, critical to testing collisional models for the Alisitos segment A collisional model for the Alisitos segment in the period ~115-108 Ma has important implications for orogenesis to the east that followed collision. During the period ~100-90 Ma a major orogenic pulse is apparent along the length of the PRB. The transitional and eastern zones of the batholith adjacent to both the Santiago Peak and Alisitos segments experienced similar histories of heightened deformation, exhumation, and extensive La Posta magmatism during this time. Thus, if the Alisitos segment was collisional, then the effects of this collision on the North American margin have been masked by orogenic processes such as mantle delamination or increased subduction zone coupling that affected an extensive part of the southern Cordillera as discussed at the end of Chapter 3. Finally, there is no evidence in the transitional and eastern zones of the PRB for backarc extension of the arc in latest Jurassic-Cretaceous time. Furthermore, the transitional zone of the batholith experienced voluminous arc magmatism for an extensive portion of this period, an issue that is difficult to explain by subduction of a backarc basin. Thus, Early Cretaceous extension documented in the Alisitos segment (Busby et al., 1998; Fackler-Adams and Busby, 1998) is more easily reconciled with an extensional Alisitos island arc system that later collided with North America than Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 208 backarc basin formation within the contiguous PRB (Fig. 4.7b). Alternatively, extension at higher crustal levels represented by this part of the PRB could have occurred synchronously with contraction at deeper levels in the arc now exposed in the transitional zone. Tectonic overview- a hypothesis for evolution of the Peninsular Ranges batholith Pre-Middle Triassic (>~241 Ma) The remnant Precambrian-Paleozoic passive margin of North America is preserved in the present eastern zone of the batholith is (Fig. 4.8a). Older and shallower marine miogeoclinal facies lie east of deeper marine facies strata in the northern part of the batholith. Near 30.5° N latitude the Proterozoic-Early Triassic southern margin may be preserved where the contact between shallow and deep facies rocks is oriented nearly E-W. The eastern zone in the southern part of the batholith appears to be underlain entirely by Paleozoic deeper facies strata. The oldest structures in the eastern zone near this transition may result from Permo-Triassic -N-S contraction associated with collision of micro-continents that make-up present day Mexico. This inherited transition in the batholith possibly controlled a number of Mesozoic processes including deformation and exhumation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 209 A) PredVIiddle Triassic (^241 Ma) Figure 4.8 Series of time slices and generalized tectonic features for each time period. See text for explanation. 210 B) Middle Triassic-Middle Jurassic (~241-164 Ma) Arc in Arizona & Sonora Some arc activity! in PRB? \ western PRB zdin I. Future western PRB zone? Unknown distance 100 km Figure 4.8 (continued) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 211 C) Middle Jurasslc-Earlv Cretaceous (-164-132 Mai Santiago Peak segment evolves adjacent to margin? \ Extension >151 Ma? Contractional deformation -151-132 Ma 9 * a 1 " " ■ \ Location \ of AllsKos xsegment? ^ Island arc • n r t n a t f i i O Basins collecting continental- and arc-derived detritus 100 km Figure 4.8 (continued) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 212 D) Eartv- to mid-Cretaceous (-132-100 Ma1 Series of volcanogenic Albtan-Aptian (~121-99Ma) basins along north & west borders of Allsitos segment Collision of Allsitos segment at 115-108 Ma? Figure 4.8 (continued) 100 km Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 213 E) mid- to Late Cretaceous f-100-65 Mai Voluminous La Posta magmatism in transition and eastern zones High plateau across transition and eastern zones? 100 km Figure 4.8 (continued) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 214 Middle Triassic-Middle Jurassic (—241-164 Ma) Jurassic (and possibly Triassic) flysch basins in the presently exposed transition zone in the northern part of the batholith received detritus from North American sources, apparently with little material from volcanic arc(s) that could have been located to either the west or east (Fig. 4.8b). These deposits are interpreted as deltaic to distal submarine fan facies that deepened to the west across the transition zone (Gastil, 1993). Arc magmatism was initiated in southern California in Triassic to earliest Jurassic time as indicated by the 234 ± 39 Ma Harper Creek orthogneiss (Thomson and Girty, 1994). Middle Jurassic-Early Cretaceous (~164-132 Ma) Sedimentation continued within and to the west of the PRB transition zone in Baja California, and detritus was derived from both cratonal North America as well as proximal volcanic arc(s) (Fig. 4.8c). These basins, and the apparently older basins in southern California, were not necessarily restricted to a narrow belt represented by the present transition zone. In southern California, sedimentary rocks that were deposited in these basins continue into the western zone of the batholith (Santiago Peak segment) and sedimentary studies indicate deeper marine facies to the west In contrast, it is likely that the eastern zone was not a significant site of deposition in Mesozoic time because the eastern zone has undergone a similar degree of exhumation to the transition zone, yet flysch rocks are not exposed there. Moreover, in Chapter 2 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 215 speculated that the eastern side of the fan structure in the southern SSPM reactivated an older normal fault that formed the eastern boundary of one of these basins. Middle to earliest Late Jurassic arc magmadsm occurred in a belt within the transitional and eastern zones stretching from southern California through the southern SSPM, and probably farther south (Fig. 4.8c). Plutons of this episode intruded Triassic(?) flysch strata in the transition zone of southern California (Thomson and Girty, 1994) and miogeoclinal rocks in the eastern zone of the southern SSPM. Speculatively, plutons of this age were unconformably overlain by Jurassic and/or Cretaceous flysch strata in the transition zone of the southern SSPM as discussed in Chapter 2. The earliest discemable episode of contractional deformation, including folding and cleavage development, occurred in the transition zone within the period -151-132 Ma, and apparently affected at least the northern as well as central sections of the batholith that are presently adjacent to both the Santiago Peak and Alisitos segments. Metamorphism and exhumation attended this event, at least in southern California, and very possibly farther south as well. Contractional deformation continued in these regions through the Early Cretaceous. Initial Volcanism in the Santiago Peak segment initiated by at least -138 Ma, and it is likely that these volcanics were deposited on flysch basement Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 216 Early- to mid-Cretaceous (—132-100 Ma) During this period the Alisitos segment of the western zone apparently collided with the Peninsular Ranges margin within the interval ~115-108 Ma (Johnson et al. 1999a). Relationships along the Agua Blanca fault suggest its role as a Cretaceous transform fault that juxtaposed Alisitos and Santiago Peak segments (Wetmore and Paterson, 2000). In the interval of time leading up to collision, sedimentation and volcanism continued in flysch basins of the transition zone in the central part of the batholith (Fig. 4.8d). The Santiago Peak Volcanics continued erupting until at least -118 Main the north. Deposition of Alisitos Fm. strata in the south began by at least -116 Ma and ended before 99 Ma, thus, bracketing collision. Distinctive Albian-Aptian sedimentary-rich facies of the Alisitos Fm. were deposited in both the transition zone and Alisitos segment This event occurred previous to, or overlapped with, the time of collision. In southern California, evidence for what is inferred to have been an extensive pile of volcanic strata that once blanketed the eastern zone occurs as -106- 108 Ma volcanic clasts in Late Cretaceous conglomerate in the western zone (Herzig and Kimbrough, 1998), indicating that volcanism was also active in the eastern zone at this time. Cretaceous plutonism in the transitional zone of the SSPM is known to have occurred within the periods -135-128, -118, and -108-100 Ma during Early- to midCretaceous time. In the transitional zone of southern California, plutonism is apparently restricted to -120-100 Ma for the same time frame. These magmatic events Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 217 produced large plutonic complexes, indicating robust arc magmadsm. Comparable pluton ages occur in the western zone of the batholith, indicating significant overlap in arc magmatism between the western and transitional zones. In the Santiago Peak segment of the western zone, pluton ages of-120-100 Ma are apparent, and preliminary studies in the northern Alisitos segment have indicated 116-113 Ma pluton ages. Contractional deformation was apparently focused within the transitional deformation belt that extends along the length of the batholith over a prolonged period of time (Fig. 4.8d). Little difference in deformation is apparent between the transition zone adjacent to the Santiago Peak and Alisitos segments, despite inferred collision of the Alisitos segment A subsequent pulse of deformation in mid- to Late Cretaceous time appears to have masked the effects of Alisitos segment collision. In the eastern zone of the batholith, deformation that includes tight folding and cleavage development is evident across a broad region underlain by Paleozoic deep marine strata south of 30.5° N. However, no age constraints exist other than undeformed Late Cretaceous plutons that intrude this deformed region. Areas to the north of this latitude, underlain by mostly miogeoclinal shelf strata, appear to have escaped much of this deformation. mid- to Late Cretaceous (-100-65 Ma) Throughout the PRB dramatic exhumation of the transitional and eastern zones occurred during this time. Plutons in the transitional zone o f the southern SSPM that Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 218 were emplaced at >15 km depth at -100 Ma had reached near surface conditions (<5 km) by -85 Ma. Thick sequences of coarse detritus accumulated in forearc settings along the present western coast of Peninsular California beginning at -105-100 Ma (Fig. 4.8e). Detrital ^Ar/^Ar K-feldspar studies of these sediments corroborate rapid denudation of the transition and eastern zones o f the batholith. An episode of voluminous La Posta plutonism occurred in the transitional and eastern zones along the length of the batholith from -97-93 Ma. This was followed by an apparent eastward migration of magmatism, reaching present day Sonora by latest Cretaceous-earliest Tertiary time. The La Posta plutonic suite shows little evidence for tectonic deformation, although, in some of the regions intruded by these plutons, synto post emplacement contractional deformation is known to have occurred. In the southern SSPM thrusting, which continued on the western side of the fan structure during La Posta time, accommodated nearly 15 km of exhumation across the transition zone. In southern California extension in the Cuyamaca Laguna Mountains Shear Zone may have also overlapped with early La Posta magmatism. Structures that accommodated exhumation and are known to have been active after La Posta time in southern California include the contractional Chariot Canyon and Carrizo Gorge faults, and both contractional and extensional deformation episodes in the Eastern Peninsular Ranges Mylonite Zone (Fig. 4.6b, Grove, 1994; Lovera et al., 1999). Cretaceous denudation continued until -70 Ma in the transition zone of the batholith. The eastern zone continued to be denuded into the Paleogene, exposing crustal levels similar to those in the transition zone. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 219 Conclusions Our present understanding of geologic relationships in the PRB indicate that the eastern, transitional, and western zones of the batholith are distinct crustal entities constructed across dissimilar basement belts that are continuous along the length of the batholith. The eastern belt appears to include two distinct types of continental crust- one beneath mostly Proterozoic-Paleozoic miogeoclinal rocks north of -30.5° latitude, and the other beneath deeper water Paleozoic facies rocks to the south. These two crustal blocks appear to have experienced disparate deformation and exhumation histories, possibly because the northern block is more rigid and thicker than the southern block. The transition belt, which evolved across the interface between western and eastern zone crust, was a depositional center, the site of focused contractional deformation for much of the Mesozoic, and ultimately a crustal break separating deeply denuded crust to the east from high crustal levels to the west It is tied by sedimentological and petrological criteria to the eastern zone throughout the Mesozoic. However, where exposed, the boundary between the two zones is commonly expressed by distinct shear zones. The transition zone is a long-lived entity in the batholith, and tectonic events along-strike within it appear to have had similar magnitudes and are nearly synchronous. The western zone of the batholith was constructed on basement of oceanic crustal affinity and appears to consist of two segments juxtaposed by an early phase of the stOl-active Agua Blanca fault The Santiago Peak segment is clearly tied by r ..-. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 220 depositionai contacts to the transition zone since -127 Ma, and Pb zircon inheritance in igneous rocks within the terrane is consistent with its evolution adjacent to the transition zone (Fig. 4.7a). The less well understood Alisitos segment to the south, on the other hand, shows no ties to the transition zone until -1 15-108 Ma. Moreover, igneous rocks from this segment, as of yet, have yielded no inherited Pb in zircons that would be consistent with the terrane’s origin adjacent to North America, and petrological studies strongly suggest an island arc environment Finally, the geometry of stratigraphy and structures in the Alisitos segment adjacent to the Agua Blanca fault suggests sinistral drag folding, consistent with docking of the Alisitos terrane south of the Santiago Peak segment An interesting problem is, therefore, apparent in the PRB transition zone adjacent to the Santiago Peak and Alisitos segments. The transition zone shows little variation in evolution along-strike between parts that are adjacent to these respective non-collisional and collisional terranes. Thus, it appears that the effects of collision of the Alisitos segment with North America at -115-108 Ma have been overprinted by subsequent deformation that affected a large part of the southern Cordilleran margin. Collision was, consequently, not as effective at driving orogenesis as other processes that operated along much of the margin by -100 Ma to produce the later pulse of deformation and exhumation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 221 CHAPTER 5: CONCLUSIONS In this dissertation I have reported on structural and geochronological studies from a nearly complete and well-exposed transect across the transition zone in the PRB that has played a fundamental role in sedimentary, structural, exhumation, and plutonic processes throughout the Mesozoic history of the margin. This region has important implications for the following: (1) the relationship of the North American continental margin to the western zone of the PRB and origin of the crustal discontinuity that separates them; (2) resolution of the hitherto poorly known early structural evolution of the PRB that elsewhere has been largely overprinted, intrudedout, or masked by subsequent geologic events; (3) the nature of the flysch basin assemblage that occurs along this discontinuity; and (4) the age of the batholith in Baja California. This project also set out to examine the influence of crustal heterogeneity and coupling among arc processes that is apparent in this region, and thus has important implications for the general evolution of arcs. Below I summarize these implications and outline potential future areas of research that might significantly improve our understanding of the Mesozoic PRB and provide further insight into arc processes in general. Figure 5.1 shows some of the major constraints that have been determined for the central part of the PRB, the basic framework of which also applies to the rest of the batholith. At the crustal scale, three zones are distinguishable across the ancient arc margin that contain distinct tectonostratigraphic assemblages and contrasting Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 222 Figure 5.1 Simplified crustal cross-section through the PRB from Pacific to Gulf of California coasts across the Baja California peninsula at ~30.5° N. Very generalized structural styles and density/rheology contrasts are shown for various crustal zones in the batholith. Total denudation since -100 Ma depicted by line across top of diagram (relate to scale on left). Reconstruction of mid-Cretaceous Moho (minimum depth relative to paleosurface shown above) in transition and eastern zones shown at bottom of diagram. Note that isostacy has not been taken into account Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Total material removed from top of orogen since mid-Cretaceous (-100 Ma) +20 124 km to <------- paleotrench +10 sw 52 km to modern - rift center -+3 -0 —10 -20 -30 —40 km El Socorro Moho (Mesozoic?) Surficial units Mesozoic Peninsular Ranges batholith Tectonic assemblages Q alluvium tonalite and granodiorite J-K arc volcanic Sierra San Pedro Martir Sierra San Felipe Punta San NE Felipe \ Transition Eastern Zone ’ basinal afesemblaJ transitional \crust \ Paleozoic assemblages on N. American crust (Proterozoic) No exaggeration loho (Neogene) Minimum mid-K crustal thickness: 55 km Late Cretaceous Moho? T volcanic and sedimentary rocks K-T gravels diorite and gabbro ^-K flysch Paleozoic miogeocline 2 2 3 224 structural and exhumation histories. Dense, 35-40 km thick crust occurs in the western zone that has strong oceanic arc geochemical character and appears to have been little modified since mid-Cretaceous time. This crustal block has experienced little deformed, except near contacts with the transition zone, and it preserves near-surface crustal levels of the Jura-Cretaceous arc. In marked contrast the eastern zone consists largely of North American continental arc crust that, as the consequence of Neogene transtension, thins eastward to a thickness of ~15 km beneath the Gulf of California. This region experienced a dynamic Phanerozoic tectonic history that includes thick Paleozoic passive margin sedimentation, late Paleozoic- early Mesozoic contraction oriented at high angles to the Jura-Cretaceous margin, strongly variable crustal rheology during deformation associated with the evolving PRB, dramatic Late Cretaceous exhumation, and strong Neogene extension and crustal thinning during formation of the Gulf of California. This crustal block may have reached a thickness exceeding 55 km prior to Late Cretaceous exhumation, and more than 15 km of crust has been removed from the top of the orogen since that time. The initiation of exhumation in this region coincided with voluminous emplacement of batholithic rocks over a period of >10 m.y. Between the western and eastern zones lies a transition zone that is intermediate in composition between crustal types to either side, contains a distinct tectonostratigraphic assemblage, and had contrasting Mesozoic deformation and exhumation histories. During much of the evolution of the PRB this transition zone was the site of focused deformation in contractional shear zones and belts of complex Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 225 folding along a narrow zone that apparently was weaker than the zones to either side (Fig. 5.1). In the Sierra San Pedro Martir, this deformation produced a doubly vergent composite fan structure that formed during intrusion of a large portion of the batholith and accommodated subsequent mid-to-Late Cretaceous exhumation amounting to >15 km of denudation within ~15 m.y. Along the western side of this zone, an impressive gradient in crustal exposure levels is evident It might have corresponded with a major west-facing topographic front behind which an orogenic plateau may have formed. The transition zone was a major elongate depositional center in the Mesozoic that consisted of a diverse collection of flysch basins. Some of these basins may have been in part bounded by normal faults as suggested, for example, by a younger-on-older fault relationship on the eastern side of the fan structure in the SSPM. These sedimentary sequences continue, at least to some extent, beneath the Santiago Peak segment of the western zone, but appear to be absent from beneath the Alisitos segment based on a lack of inherited zircons in plutons and volcanics in this segment that are abundant in the adjacent flysch assemblage. The reconstructed crustal thickness across this orogen is anomalous. Crust of oceanic character underlying the western zone of the batholith is ~40 km thick, an unusual thickness for oceanic arc systems, which are commonly on the order of 20-30 km thick (e.g. Uyeda, 1978). Assuming that the western zone of the batholith attained the higher end of this range, a simple constant cross-sectional area calculation demonstrates that even minor amounts of shortening could increase its thickness to 40 km. Taking the present 40 X 40 km cross-sectional area of the western zone in Figure Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 226 5.1 as the final width and thickness of the belt and assuming an initial crustal thickness of 30 km requires an initial width of 53.3 km and 25% shortening. Thus, a minor amount of shortening, which is apparent from gentle, upright folds that occur across much of the region, can account for the 40 km thick crust presently underlying the western zone of the batholith. The hypothesized >55 km mid-Cretaceous thickness of the eastern zone of the batholith is also anomalously thick. In Paleozoic time this region was the continental shelf of a passive margin and should have thinned towards the west as a result of Proterozoic-Paleozoic rifting. Relatively little is known about the early Mesozoic history of the eastern zone. This region appears to have been shortened an unknown amount in a N-S sense and experienced some plutonism before mid-Cretaceous time. Both of these processes may have thickened it by the mid-Cretaceous. Moreover, similar crustal thicknesses have been hypothesized for Paleozoic passive margin crust to the east of the Sierra Nevada batholith at similar times (Wernicke et al., 1996). Finally, a notable feature that is apparent in Figure 5.1 is the enormous amount of material has been eroded off of the top of the orogen in the transitional and eastern zones since mid-Cretaceous time, amounting to >1000 km2 in cross-sectional area (Fig. 5.1). The -100 km-long segment of the PRB represented by the SSPM region thus produced a volume of sediment on the order of 1.2 x 105 km3 (corrected for a change in density from 2.6 to 2.1 gm/cm3 respectively for crystalline rock and compacted sediment), most of which must have been deposited along the Pacific coast to the west Assuming a similar degree of denudation for the -800 km-long known Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 227 extent of the PRB results in 1.2 x 106 km3 of sediment By comparison the 1000 kmwide by 3000 km-long Bengal fan, which has been estimated to contain 1016 tons of post-India/Asia collision sediments (Curray, 1991), drains an orogenic segment of the Himalaya that is nearly twice as long as the PRB. This amounts to a sediment volume of 5 x 106 km3 (assuming an average sediment density of 2.1 gm/cm3, Curray and Moore, 1971). Thus, the amount of sediment produced by exhumation of the PRB is approximately 0.25 that of a region double in length in the Himalaya that feeds the Bengal Fan. Deposits of this size are not presently apparent along the Continental Borderland of southern and Baja California, and much of this sediment must have been subducted during Late Cretaceous and Tertiary time. Fan structure in the Peninsular Ranges batholith transition zone The strongly heterogeneous crust that is apparent along this ancient margin appears to have profoundly controlled deformation during orogenesis, leading to the formation of a composite fan-like structure in the SSPM. The largely inherited transition from continental to oceanic lithosphere in the PRB provided a first order control on the location of contractional structures and the extent of deformation that occurred in the batholith. This boundary focused contractional deformation over practically the entire Mesozoic history of PRB orogenesis, and its control appears to have largely prevailed over other influences such as rheological contrasts that may have formed in response to the location of active arc magmatism at any given time in the batholith. Other, less significant, mechanical influences are also evident along the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 228 margin. For example, in the SSPM a promontory in the Precambrian-Paleozoic continental margin created a buttress that further localized deformation in the transition zone and resulted in a bend in the trend of structures and units in the zone. Older faults also appear to have influenced contractional deformation that occurred during intrusion of the batholith. For instance, possible inversion tectonics are apparent along the eastern side of the fan structure where younger flysch strata have been thrust over miogeoclinal strata across a structure that is inferred to have once bounded a flysch basin on the east In my discussion of deformation in the fan structure I have inferred that the total displacement field across the PRB transition zone is largely vertical with a component of west-directed flow. This inference is consistent with mostly steeplypitching lineation across the fan structure and at other locations in the PRB deformation belt The moderately N-plunging lineation in the center of the fan structure is the main exception to this observation. However, the fabrics in the center formed over a period of time limited to durations of intrusion and crystallization of the tonalite sheets, and, therefore, possible strike-slip displacement is limited to a relatively short interval in the >40 m.y. overall structural evolution of the fan structure. Other observations further limit the amount of possible strike-slip displacement in the fan. Jurassic orthogneiss wall rocks occur on both sides of the fan, indicating that there is no major change in host rock lithology across the central zone, changes that would be consistent with large lateral displacements within the batholith. Furthermore, the oblique orientation and lack of deflection of the easternmost Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 229 orthogneiss screen that extends across much of the central domain of the fan structure limits significant strike-slip displacement to a zone <3 km-wide between its tip and the —132 Ma La Suerte pluton. It is more likely that fabrics in the sheeted tonalite complex record a local deviation horn the overall vertically oriented displacement field. One possibility is that strongly localized and relatively brief weakening of the arc by intrusion of the arc-parallel tonalite sheets may have caused material in the central domain to flow laterally as well as upward around the miogeoclinal buttress to the east. This hypothesis may be tested further in the future as more is learned about the distribution and character of Mesozoic structures in the batholith. Shortening mechanisms The belt of Jura-Cretaceous deformation in the PRB transition zone was thus largely contractional with shortening perpendicular to the paleo-margin. Two endmember mechanisms for lithospheric shortening during orogenesis may apply to this zone: simple shear and pure shear deformation. The predominant west-vergence of structures within the transitional deformation belt in the PRB and associated sharp metamorphic gradients suggests that the western zone has been underthrust beneath the transition zone in a simple shear-dominated system (Fig. 52a). Such a mechanism also provides a means by which parts of the pre-Alisitos forearc assemblage could have been subducted beneath the transition zone during hypothesized collision of the Alisitos segment. Important shear zones occur on the eastern side of the transition zone as well, but these are less common and show less consistent shear sense as Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 230 Simple shear- underthrust western zone Pure shear- vise tectonics Figure 5.2 Simplified end-member models for shortening at the lithospheric-scale across the Mesozoic PRB margin. 231 compared with the western side. For example, in the SSPM, an east-vergent shear zone occurs on the eastern side of the fan structure, but in southern California, the Chariot Canyon fault displays west vergence on the eastern side of the transition zone. Along-strike variation in crustal strength may have largely dictated the structural geometry of the eastern side of the transition zone. Thus, in the SSPM region, the presence of more rigid miogeoclinal crust to the east of the transition zone caused a local backthrust to form along the eastern side of the fan structure, whereas fan-like geometries are apparently absent in southern California where the transition zone lies west of the miogeoclinal assemblage. What the simple shear model does not account for are the near-vertical orientations of shear zones in the PRB transition zone throughout much of the batholith. An alternative explanation for the style of shortening in the PRB transition zone is a pure shear-dominated deformation model (Fig. 52b). Crust in the transition zone that was weakened by intrusion of vertical magmatic sheets may have deformed in a situation analogous to material in a vise with relatively strong ‘jaws’ of western and eastern zone crust to either side. Modeling such vise tectonic systems shows that discrete, typically outward-vergent, thrust zones form at the interface between weak and strong crust, and that material between deforms in a more distributed fashion (Ellis et al., 1998). Depending on Theological contrasts between crust outside and within the vise, these models also predict relatively steeply-dipping structures in the center of the vise. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 232 These simple and pure shear end-member models explain different observations in the PRB and it is likely that a progression from one to the other occurred during evolution of the batholith as shown in Figure S.3. A possible pre-Late Jurassic setting that was largely determined during Paleozoic massive margin evolution is shown in Figure 5.3a. The future batholith transition zone in this figure is the site of extension, forming intra-arc basins into which flysch sediments accumulated. Also shown is the future location of a major intra-lithospheric shear zone that will form within the transition zone. Figure 5.3b shows a hypothetical initial, simple shear-dominated orogenic phase in the PRB during Late Jurassic- Early Cretaceous time. Western zone lithosphere is partially subducted beneath eastern zone lithosphere across the comparatively weak transition zone. At this early stage, lithosphere along the margin may still have been relatively cool and strong, and, thus, could sustain discrete shear to relatively deep levels in the arc. Robust arc magmatism during the mid-Cretaceous could have led to overall weakening of a broad region in the PRB, resulting in accentuated Theological contrasts between crust in the transition zone and crust to either side. Additionally, extensive intrusion of vertical magmatic sheets such as those that occur in the central domain of the fan structure, further weakened along-strike belts in the transition zone. This led to overall pure shear conditions, as recorded in the central domain of the fan structure, and the initiation of vise-style tectonics in the PRB (Fig. 53c). Previously established thrust zones at interfeces between assemblages of contrasting theology on the sides of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 233 Pre-Late Jurassic setting transition zone west zone east zone crust lithospheric mantle Late Jurassic - Early Cretaceous- simple shear system mid-Cretaceous- pure shear system Figure 5.3 A possible scenario for lithospheric-scale evolution of the PRB margin. See text for explanation. 234 the fan structure continued to be active, and in these regions the fan developed moderately-dipping orientations. New structures that formed during this time were largely steeply-dipping and older shear zones were rotated to steep orientations. Thus, the overall triangular shape of the original fan structure was deformed to produce a flower-like structure with a region of outwardly-fanning shear zones near the surface that steepen at depth in the crust Steeply-dipping structures that characterize much of the rest of the transition zone in the PRB could have largely resulted from this midCretaceous pure shear deformational phase. The initiation of exhumation towards the end of this phase suggests that vise tectonics may be a very effective mechanism to shorten and thicken crust, resulting in large scale exhumation. M agmatism Plutonism in the batholith appears to have operated largely independently of faulting. Plutons occur both within and outside of shear zones in the PRB. In Appendix A I explore spatial and geometrical relationships between faults and plutons using statistical methods in five orogens around the world that have been mapped in more detail than the PRB. I conclude that the spatial relationships between faults and plutons in these orogens is weak, but that a geometrical relationship may exist, indicating that plutonism is generally decoupled from faulting processes in orogens but may be influenced by factors such as regional stress fields that are common to both processes. For the most part, these conclusions appear to apply to the PRB, and at Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 235 some point in the future when the region has been mapped in greater detail a statistical analysis would be interesting. One exception to this general observation is the relationship between tonalite sheets and wall rock screens in the sheeted complex of the fan structure. This was an actively deforming region during intrusion, as indicated by the development of gradients in magmatic to high-temperature solid state fabrics in tonalite intrusives and parallelism among fabric orientations in tonalite sheets and orthogneiss host rock screens. This domain in the fan structure deformed very differently than other domains; a pure shear-dominated deformation mechanise and orogen-parallel lineation are apparent here. Thus, only under limited circumstances involving intrusion of extensive magmatic sheets at mid-crustal levels in the arc did deformation appear to have been significantly influenced by magmatism. Sheeted magmatic complexes such as these are common in exhumed mid-crustal levels of other arcs that underwent syn-intrusive contractional deformation. Possibly the extremely long tabular geometries of these complexes, which extend for 10’s to 1000’s of km alongstrike (e.g. Great Tonalite Sill of the Coast Plutonic complex, Ingram and Hutton, 1994), create conditions that result in sharp, but highly transient, rheological gradients that influence deformation. Such gradients may have led to partitioning of deformation within the fan structure over an interval of a few m.y. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 236 Exhumation history The exhumation history of the PRB is an important example of the effects of inherited contrasts in crustal type and structure on crustal dynamics during orogenesis. These contrasts among the distinct crustal zones in the basement to the batholith exerted fundamental control on the extent and dynamics of exhumation across the margin in mid- to Late Cretaceous time. Moreover, contractional structures located in the narrow transition zone between strongly disparate western and eastern crustal blocks accommodated nearly 15 km of differential uplift that produced this exhumation. The western zone of the batholith remained relatively stable and at low average elevation during the Late Cretaceous and Tertiary. In contrast the eastern zone may have achieved >55 km crustal thicknesses by mid-Cretaceous time and experienced dramatic uplift, possibly forming a high continental plateau across much of its extent Exhumation histories derived from thennochronologic studies in this part of the PRB were both internally consistent and provided fairly tight constraints. This is surprising considering that active arc environments are characterized by broad variation in thermal structure and, thus, a wide range of geothermal gradients had to be assumed to arrive at depth estimates for assemblages within the fan structure. To some degree the tightness of the exhumation history derived from this region may be serendipitous. Exhumation was rapid, as confirmed by independent means determined in sedimentological and detrital thermal history studies. Moreover, intrusion of both the Rinconada and sheeted tonalite complexes at -100 Ma occurred approximately Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 237 coevally with initiation of exhumation, thus, providing a valuable pressuretemperature-time constraint on the exhumation that followed. Depth determinations for intermediate temperatures were fairly poor in this study, but once near-surface conditions were attained, apatite fission track systematics provide tight and internally consistent constraints on depth. Therefore, the initial and final points for the majority of exhumation in the fan structure were determined with a relatively high degree of confidence. Uplift in the central part of the PRB was accommodated largely by contractional structures. Unless extensional faulting occurred exclusively at crustal levels above 15 km depth in this part of the orogen, denudation was limited to erosional processes. Furthermore, there is little indication of crustal loading on the eastern side of the western zone by thrust sheets in the transition zone. Sediment derived from erosional exhumation to the east appears to have been efficiently transported farther outboard to the Continental Borderland. Erosion was, therefore, an effective process along the western side of the PRB, and differential exhumation may have been further enhanced by coupling between erosion and deformation. The western zone of the PRB has been relatively stable since deposition of the youngest dated strata in the Alisitos Fm. at -1 IS Ma. Its lower lithosphere has, thus, probably not experienced major changes since this time other than slow cooling and minor exhumation following arc magmatism. In striking contrast the adjacent transitional and eastern zones experienced extensive orogenesis including voluminous La Posta magmatism and possible formation of a continental plateau. Strong variation Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 238 in Cretaceous shortening coupled with similar degrees of denudation between the transitional and eastern zones strongly suggests that lower lithospheric processes such as delamination were a major factor in the Late Cretaceous evolution of this part of the batholith. Much of the North American Cordillera experienced an episode of exhumation very similar to that recorded in the Peninsular Ranges between 100 and 80 Ma. In the Canadian Coast Ranges and North Cascades, peak metamorphism and initiation of rapid cooling in assemblages that originated at ~9 kbar occurred around 90 to 75 Ma and has been overprinted by a younger metamorphic and cooling episode in most regions (Whitney et al., 1999). In the Sierra Nevada crustal thicknesses exceeding 60 km and associated high topography has been inferred at about 90 Ma (Wernicke et al., 1996). These latter authors have also suggested that a broad region extending eastwards across the Basin and Range province experienced a similar history, including a sizeable continental plateau, as I have inferred for the PRB at approximately the same time. Thus, Late Cretaceous exhumation appears to have been a margin-wide, or larger-scale, phenomenon along the western margin of North America. Jura-Cretaceous tectonic evolution of the Peninsular Ranges batholith This study has a number of implications for the Mesozoic tectonic evolution of the PRB. Many of these remain to be more hilly tested, particularly in the poorly known region of the batholith to the south. However, with our present knowledge of the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. batholith, I can make the following statements that appear applicable to any tectonic model for the Mesozoic PRB: 1) The transitional zone was a major site of deposition through much of the Mesozoic and inferred facies relationships indicate that basins in this zone deepened to the west 2) Jurassic through Early Cretaceous arc magmatism occurred in the transition and eastern zones of the batholith from southern California through northern Baja. This has been inferred by past studies, but only documented in southern California. The southern PRB is presently not known well enough to determine whether the older phases of this plutonic history are shared to the south. 3) This prolonged history of Jurassic-Early Cretaceous magmatism in the transition and eastern zones of the batholith overlapped in time with magmatism in at least the Santiago segment of the western zone. Although commonly inferred to be older, only <120 Ma ages are presently known from the Alisitos segment 4) An early phase of Jurassic-Early Cretaceous (-151-132) contractional deformation occurred along much of the transition zone. In southern California amphibolite grade metamorphism and an episode of early exhumation are also evident at this time. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 240 5) Contractional deformation continued through Early Cretaceous and into Late Cretaceous time (-132-118 to <80 Ma) in at least the northern and central sections of the transition zone. Local extensional deformation may have accompanied contraction but at present the only known region in which this occurred within the transition zone is in the Cuyamaca Laguna Mountains Shear Zone. 6) A dramatic exhumation history, beginning in mid-Cretaceous time (-100-70 Ma), is shared through the length of the transition and eastern zones of the batholith at least as far south as the Puertecitos Volcanic Field. Regions south of there may have experienced a slightly different history with less total exhumation. Future directions Following is a list of specific problems and ways to test them that would significantly improve our understanding of the PRB as well as arc processes in general. Western zone of the Peninsular Ranges batholith Several important features have been noted in the western zone of the PRB in this study that indicate the Alisitos and Santiago Peak segments may have different origins. These include disparate ages of geological ties to the transition zone to the east and evidence for pre-Tertiary sinistral strike slip history on the Agua Blanca fault. The latter issue is a critical constraint on the evolution of the western zone and is Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 241 currently being examined in a structural study of regions adjacent to the Agua Blanca fault by Paul Wetmore at USC. The issue of disparate evolution and geological ties to the transition zone for the Santiago Peak and Alisitos segments of the western zone also deserves more detailed examination. The Alisitos segment is by far the least understood of the two segments. Only a few ages have been published from this region, and all are less than 120 Ma. Moreover, basement to the Alisitos Fm. is unknown beyond general inferences from geochemistry on a few plutons. Any new geochronologic or isotopic data from this region might contribute towards understanding the origin of this segment. The Santiago Peak segment is better understood, but many of the details of its early evolution are unclear. Did this segment collide with the margin early in PRB development or was its basement already adjoined to the eastern zone when the batholith initiated? Potentially detailed examination of the metamorphic and structural evolution of the flysch assemblage rocks on which the Santiago Peak volcanics were deposited would yield clues about this earlier phase of orogeny. The Alisitos segment is particularly interesting because an apparent range of upper-crustal levels are exposed along strike in the assemblage, and in its southern region an extensional history has been proposed for a time when the northern region and transition zone to the east apparently underwent contraction. In the south, large stratovolcanos and associated caldera complexes of approximately 120-130 Ma (C. Busby unpublished data, personal comm., 1999) have been identified between adjacent basins bounded by hypothesized normal faults oriented both normal and Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 242 parallel to the margin (Busby et al., 1998; Fackler-Adams and Busby, 1998). Farther north, adjacent to the northern SSPM, Johnson et al. (1999b) interpreted the Zarza pluton and other nearby plutonic complexes as cone sheet-bearing ring complexes that formed at 5-10 km below the surface and may have fed higher crustal-level caldera systems. They infer that folding of as yet undated Alisitos host rocks during regional margin-perpendicular shortening occurred pre-syn emplacement of these ~115 Ma plutons. These studies imply that a change from extensional to contractional deformation occurred in the batholith between 130 and 115 Ma. Alternatively, regional contractional deformation was occurring at slightly deeper levels in the northern part of the Alisitos segment while near-surface levels experienced extension farther south. The transition zone to the east shows contractional deformation at midcrustal levels for this general time period. An additional problem relates to establishing whether the Alisitos segment is indeed an exotic tectonic element Thus far, the known contact between the Alisitos segment and transition zone is a reverse fault that juxtaposed the two assemblages between ~115 and 108 Ma. This relationship is only well established in the SSPM; shear zones farther south appear to interleave Alisitos and flysch assemblages, but ages of juxtaposition are more poorly known. Also apparent from this study is the longevity of focused deformation in the transition zone, implying that if the Alisitos segment is a collided exotic arc then a pre-Alisitos tectonic element is missing. Addressing this problem would also hold promise for a better understanding of PRB tectonics before possible collision as this element may either presently exist in die Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 243 western and/or transition zones but has not been recognized, or has been subducted or displaced laterally along the margin. Thus, further structural studies to the south may be able to better elucidate relationships between the western and transitional zones in the batholith. Eastern zone o f the Peninsular Ranyes hatholith The eastern zone of the PRB is perhaps the most poorly understood region of the batholith. Historically, its development has been considered young in PRB evolution, probably because a Late Cretaceous plutonic overprint was relatively strong across this region. However, a number of issues identified in this study as well as in previous ones indicate a much more complex and long-lived history than generally recognized. East-west oriented structures that may pre-date orogeny associated with the PRB appear to be prevalent across the miogeoclinal assemblage. If, indeed, they are Permo-Triassic in age and have not been rotated, then this region may hold important information regarding the interaction of the southern and western early Mesozoic margins of North America. The distribution of Paleozoic miogeoclinal facies and presence of these structures up to the eastern thrust of the fan structure indicates that the western margin may have been truncated before development of die batholith as suggested by Magistrate and Sanders (1995). In contrast to miogeoclinal rocks in the eastern zone of the batholith, rocks of the slope-basin assemblage appear to have been strongly deformed during PRB orogenesis. If this is correct then more distributed shortening should be apparent Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 244 across the southern half of the batholith where miogeoclinal rocks are absent as compared with the north. Moreover, deformation in the transition zone of the south may also differ from regions to the north. Studies of the transition zone in the south have described narrow, steeply dipping mylonite zones which interleave western and eastern zone assemblages (e.g. Griffith and Hoobs, 1993). However structures outside of these shear zones have not been examined in detail. Finally, the eastern extent of the PRB is very poorly defined. Reconnaissance studies in western Sonora would permit issues such as the extent of Jurassic-Early Cretaceous magmatism and Late Cretaceous exhumation eastward of Baja California to be addressed. Lithospheric evolution of the Peninsular Ranges batholith Recent passive seismic studies such as Ichinose et al. (1996) and Lewis et al. (2000) have given us a much clearer picture of the overall crustal geometry in the PRB. However, in the eastern as well as transitional zones of the batholith, the crust appears to have been strongly thinned by Tertiary extension in the Gulf of California. Moreover, lower lithosphere may have played a major role in the mid-Cretaceous exhumation history of the batholith, but at present details of this process are unconstrained. There are two possible approaches to understanding Cretaceous lithospheric evolution across the batholith. First, vestiges of Mesozoic structures may exist in the lower crust and upper lithospheric mantle that may be geophysically Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 245 imaged using both reflection and refraction techniques. Combined with a relatively well-constrained structural history at the surface, this information could yield a much clearer picture of deformation at lower crustal levels and constrain tectonic models for the PRB. A second method of addressing this problem involves obtaining P-T-t constraints on the lithosphere below the batholith from mantle xenoliths entrained in fresh volcanics. Pre-Gulf (>14 Ma) volcanics such as Tertiary arc volcanic sequences preserved on the eastern side of the batholith may be ideally suited to such an effort. This method has been successfully employed in the eastern Sierra Nevada using volcanics that sampled different parts of the lithospheric mantle at various times (Ducea and Saleeby, 1996). Such a study would also be valuable in detailing lithospheric dynamics during continental rifting in the Gulf of California. Reproduced with permission of the copyright owner. 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Further reproduction prohibited without permission. 271 Williams, I.S., and Claesson, S., 1987, Isotopic evidence for the Precambrian provenance and Caledonian metamorphism of high grade paragneisses from the Seve Nappes, Scandinavian Caledonides, II, ion microprobe zircon U-Th-Pb: Contributions to Mineralogy and Petrology, v. 97, p. 205-217. Woodford, A.O., and Harriss, T i\, 1938, Geological reconnaissance across Sierra San Pedro Martir, Baja California: Bulletin of the Geological Society of America, v. 49, p. 1297-1336. Zhu, L., and Kanamori, H., 2000, Moho depth variation in southern California from teleseismic receiver functions: Journal of Geophysical Research (in press). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 272 APPENDIX A: ARE PROCESSES OF DEFORMATION AND MAGMATISM COUPLED? INSIGHTS FROM SPATIAL AND GEOMETRICAL ANALYSIS A growing perception within the geological sciences is that deformation and magmatism are related at a variety of scales and crustal levels, from regional faults and eruptive centers at the surface to regional fruits and plutons in the mid-upper crust, to shear zones and melt migration zones in the lower crust (e.g., Wadge and Cross, 1988; Hutton and Reavy, 1992; Brown et al., 1995).These inferences are based on qualitative conclusions about close spatial, geometrical, and temporal relationships and are repeatedly cited as evidence that the ascent, emplacement, or eruption, and sometimes even the generation of magmas are controlled by tectonic structures (e.g., Hutton and Reavy, 1992; Roman-Berdiel et al., 1997). These conclusions, however, are contradicted by results from an increasing number of quantitative studies of spatial and geometrical relationships among volcanic and plutonic bodies, sometimes from the same regions examined in the above studies. For example, de Bremond d'Ars et al. (199S) and Pelletier (1999) examined the spatial distribution of volcanic and plutonic features in arcs, and although they arrived at different conclusions regarding the distribution of these features (random versus clustered, respectively, they found no linear alignments nor other geometries that would be consistent with an association with fruits. Similarly Wadge and Cross (1988) and Lutz and Gutmann (1995) studied vent alignments in volcanic fields and concluded that alignments are weak and rarely parallel to nearby structures. These Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 273 studies used a variety of techniques such as nearest neighbor, cumulative frequencysize, and linear pattern analysis to statistically quantify populations of igneous features, and although they offer insight into relationships between tectonic and igneous features, they do not directly address the distribution of plutonic bodies versus faults. We have taken the first steps to directly compare pluton and fault populations using statistical methods to analyze spatial and geometrical data from regions that have frequently been cited as examples where close spatial relationships exist Our results directly contradict the qualitative conclusions and have implications similar to the statistical studies listed above. Our results indicate that pluton locations are scattered but preferentially occur away from regional faults and have, at best, only a weak geometric relationship to these faults. Thus, any genetic relationship between faults and plutons in the analyzed orogens results in an uncorrelated or antithetical spatial relationship and weak geometrical relationship. These results indicate that magma is not preferentially channeled along fruits, fault related emplacement models are not widely applicable, and any genetic relationship between regional faulting and magmatism is probably a function of coupled stress fields. Below we explore our results in more detail with the hope of emphasizing the need to incorporate both statistical and geologic data sets in future syntheses. We question the most fundamental observation used in support of a genetic relationship between faults and intrusion: the existence of close spatial and geometrical relationships between regional fruits and plutons in orogens. We begin Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 274 with an example of some of the pitfalls that may occur when attempting to determine spatial relationships from visual inspection. Figure A.la, a computer-generated artificial map showing S regional faults and 19 plutons, may lead the observer to conclude that a close spatial relationship exists between these faults and plutons because many faults intersect plutons. This map is similar to actual examples that we analyze below. However, the artificial map was created using a random number generator, and pluton locations are in fact random with respect to faults, as emphasized in the corresponding plot of the distribution of pluton area versus distance to closest faults. An example in which pluton locations are spatially related to faults is shown in Figure A.lb. This simple example illustrates one problem with determining spatial relationships by visual inspection. Invariably some objects (plutons) will occur adjacent to others (faults) even if the processes that produce them are completely independent, which emphasizes the need to statistically compare populations of objects. Even if spatial and geometrical relationships can be established, causative relationships between processes is not necessarily implied. It is further necessary to determine whether or not the spatial or geometrical relationship has a high probability of resulting from two independently operating processes or whether it requires some coupling. We approach this problem using numerical modeling such as shown in Figure A.1 to rigorously define the full range of possible distributions for known boundary conditions. (Figure A .la represents one possible distribution of random plutons.) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure A. 1 Computer generated simulations offault and pluton distributions illustrating problems with visual inspection ofspatial relationships, (a) map showing locations offive faults and 19 plutons in which plutons visually appear closely associated with faults. However, a random number routine selected these pluton locations as reflected in a uniform distribution of pluton area relative to closest faults in the corresponding histogram. Compare with (b), a map showing locations offive faults and 21 plutons for which an algorithm selected only pluton locations statistically near to faults as reflected in the corresponding histogram showing close relationship between pluton area and faults. 276 Other analytical and geologic issues are discussed in more detail in Paterson and Schmidt (1999). Here we discuss the results and implications of spatial and geometrical analyses of coeval faults and plutons in the following five orogens (Figure A.2; references refer to those not cited in Paterson and Schmidt (1999)): the Armorican Massif, France; Alleghenian Orogeny, southern Appalachians; British Caledonides (Hutton and Reavy, 1992); Maine Caledonides (Brown and Solar, 1998); and Borborema Province, Brazil (Vauchez et al., 1997). These regions were selected because they have been frequently cited as examples showing strong spatial and geometrical correlations between faults and plutons. Thus, we do not discuss whether the mapping and ages of plutons and faults are correct; others far more familiar with these regions have used the same data to argue for close spatial, geometrical, and genetic relationships. We have also chosen a scale for our analyses appropriate for the genetic relationships we wish to test: that is, at the scale of individual faults and plutons. The issue of scale is important because spatial relationships may change from one scale to another. For example, at the scale of entire orogens in Figure A.2, groups of faults and plutons might show a stronger spatial overlap. However any inferred genetic relationship would then imply processes operating at the orogen scale (e.g., subduction). Results of Spatial and Geometrical Analyses One of the most useful methods for establishing spatial relationships is to measure the distribution of pluton area relative to the distance from nearest faults. To Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 277 Figure A.2 Columns across the top: (a) maps of regions, (b) results of spatial analyses, (c) results of geometrical analyses for the following five orogens (listed down left side): (1) Armorican Massif; (2) Alleghenian Orogeny; (3) British Caledonides; (4) Maine Caledonides; and (5) Borborema Province. Spatial analysis results (column B) are plots of integrated pluton area versus distance to closest fault. Average fault spacing and standard deviation and average pluton width are also shown. Comparison of average fault spacing with peaks of pluton area distributions shows poor correlation of pluton area with closest faults. Geometrical analysis results (column C) are histograms of angles between pluton long axes and nearest fault Note large standard deviations. These analyses show a weak tendency for pluton long axes to be oriented subparallel to nearest faults. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 278 Brazil (4) Maine Caledonian, (3) British Caledonian, (2) Alleghenian^ (1) Armorican, France ____ Eastern USA Northern Great Britain Eastern USA ___________ ______ , (A) Orogen (B) Spatial (C) Geometrical Prrr i Peak = 10 km, -1/2 Fault Spacing Fault Spacing = 21 km Bin width = 2.5 km Atlantic Ocean Bin width 4.35 km Bin width = 1.2 km Ocean Virginii Peak = 18 km, -1/4 Fault Spacing Tennessee North South arolin Fault Spacing = 74 km Peak = 6 km, -1/5 Fault Spacing fault spacing = 29.4 km Peak = 30 km, -2/5 Fault Spacing Fault Spacing = 73 km Bin width = 7.5 km Peak = 21.75 km, -1/2 Fault Spacing fault spacing = 40.6 km Distance from Nearest Fault (bin widths) 279 do so, we construct grids on each map (Figure A.2a); for each grid element we calculate the area of plutonic rock and distance to the nearest fault Where faults intersect plutons, we project faults through the pluton for our analysis. Data collected is displayed as a histogram of integrated area of plutonic rock versus distance to nearest faults (Figure A.2b). This technique measures the distribution of pluton area within domains between faults and takes into account the size, shape, and orientation of plutons. Figure A.2b shows results horn the analysis of the five regions described above. Included on these graphs is the average and standard deviation of fault spacing as measured across mean fault strike. Also shown is the average pluton diameter measured perpendicular to pluton long axes. Average pluton diameters are significantly less than the peak widths of distributions and of average fault spacing, indicating that pluton size does not control the distributions. Only in the case of the Borborema Province do shear zones have appreciable thickness relative to pluton sizes, and thus, in this example, we also plot the average shear zone width. For all of the analyses, the most important observations are that distributions show statistically significant peaks and pluton area decreases dramatically toward faults. Examining the five examples in Figure A.2b shows that peaks of distributions are broad and are located at approximately the following average fault spacing: 1/S (British Caledonian), 1/4 (Alleghenian), 2/5 (Maine Caledonian), and 1/2 (Armorican and Borborema). We also used several methods to evaluate geometric relationships between plutons and faults. One of these compares the orientation o f plutons relative to closest Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 280 faults by measuring the angle between long axes of plutons and orientation of closest faults. Angles measured clockwise from a fault are designated as positive, counterclockwise measurements are negative, and an angle of zero indicates that the pluton long axis is parallel to the fault Data are plotted as histograms of angles between each pluton and nearest fault Results from these analyses for the regions discussed above are shown in Figure A.2c. Distributions on these plots are approximately Gaussian, and peaks are within 12° of the orientation of nearest faults with large standard deviations (31°-38°). We also compared the ratio of pluton long-to-short axes with the angle between long axes and nearest faults (for example, Figure A.3a and A.3b), as well as the distance to nearest faults (for example, Figure A.3c and 3d). In all of our examples, neither of these correlations are particularly strong. The strongest occurred in the Armorican and Maine examples and we thus focus on these two. The plots in Figure A.3 suggest that plutons with axial ratios greater than 6:1 are oriented at smaller angles to nearest faults and are distributed closer to faults than plutons with smaller axial ratios. However, we suggest caution in interpreting these results since correlation is only apparent for axial ratios greater than 6:1, correlation only occurs in two of our five examples, and the sample size is small, which may make the study statistically invalid. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 281 Fault C) 6 C _ _ w 5 8 _ : i 4 lA ra o ric a t • • • • ----------- « k , “ T 1 0 1 5 2 0 Pluton Ratio 2 5 J O 1 2 Figure A.3 Plots showing relationship between pluton length/width ratio and angle between pluton long axis and nearest fault (a and b), and relationship between pluton length/width ratio and distance to nearest fault (c and d) for the Armorican Massif and Maine Caledonide examples. These two examples were selected because they show die strongest correlation between pluton ratio and distance to nearest faults. Note that correlation only exists for plutons with axial ratios >6:1, a relationship that was not apparent in our other three examples. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 282 Discussion and Conclusions Our spatial analyses indicate that m axim a of pluton area in the orogens examined are statistically located at a significant distance from faults and, in the case of the Armorican and Borborema provinces, are located as far away as possible. If plutons statistically occurred along faults, then peaks should occur at nearest faults, which are at zero on our plots. If plutons statistically occur as far away from faults as possible, then peaks would be expected to occur at half the average fault spacing. Plutons uniformly distributed with respect to faults would have a relatively flat curve from zero to half-fault spacing and then taper off. Interestingly our distributions are remarkably similar to gamma-type distributions of volcanic vent spacing in arcs found by de Bremond d'Ars et al. (1995), which they demonstrated were reproducible using random numerical simulations. Thus, our results suggest that magmatism in these orogens tends to be a relatively distributed process with potentially a weak tendency for plutons to occur away from rather than near faults. In fact, in the five examples that we have analyzed so far less than 10% of pluton margins are bounded by regional faults and less than 30% of pluton area lies within 8 km of faults. Results from geometrical analyses show that pluton long axes have a wide range of orientations, but that plutons are statistically oriented subparallel to nearest faults. This relationship remains consistent even in regions such as the Armorican Massif (Figure A.2), in which fruit orientations change dramatically across the area. However we found poor correlation between pluton shapes, orientations, and distances Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 283 to nearest faults with the possible exception in two regions of plutons with ratios greater than 6:1. This indicates that faults generally do not act as a prominent anisotropy or focusing mechanism during magmatism and that another more regional mechanism such as other host rock structures or the geometry of the active subduction zone(s) controls the orientations of both faults and plutons. We thus conclude from our analysis and other published quantitative studies that proposed space-making and channeling mechanisms for magma by faults are not widely applicable. Also, processes of deformation and magmatism are not completely independent, but rather both are influenced by, and contribute to, mechanisms such as regional stresses that occur at the orogen scale. To date we have examined relationships between faults and plutons at mid- to shallow crustal levels. The methods we have described could be adapted to examine relationships between tectonic and igneous features at a variety of scales and crustal levels from volcanic centers and fault systems at the surface to outcrop-scale features such as individual leucosomes and shear zones in the lower crust We believe that a variety of other geological phenomena would benefit from being explored in more quantitative detail. A wealth of data is available on geologic maps to which techniques of spatial and geometrical analysis can be readily applied. Present computer capabilities permit rapid measurement of map features and sophisticated statistical modeling of spatial relationships. Quantitative analysis can therefore provide creative and rigorous tests of a variety of geologic problems. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 284 APPENDIX B: U-PB ZIRCON DATA Zircon U-Pb analysis of 6 samples from the southern SSPM was performed in three labs: Melissa Girty at San Diego State University, Dr. Mark Fanning at Australia National University, and Dr. Joseph Wooden at Stanford University. Five to 40 kg of material was collected for each sample and processed using conventional zircon separation techniques at San Diego State University facilities. Standard U-Pb isotope dilution methods (San Diego State University) A sample of recrystallized fine grained gabbro that showed mingling relationships with biotite tonalite in the Suerte complex (SP90) and mostly magmatically foliated hornblende tonalite from the central sheeted complex (SPSSS) were analyzed using U-Pb isotope dilution methods by Melissa Girty at the Baylor Brooks Institute of Isotope Geology, San Diego State University. In addition three fractions of an existing sample from a mylonitized granite sheet in the upper part of the western thrust package collected by Chris Goetz (location on Plate 1, C. Goetz written comm. 1996) were analyzed to increase precision. Zircons were subdivided into nonmagnetic and magnetic fractions using a Frantz Laboratory Barrier Separator with a front tilt of 20° and side tilt of 2°. The nonmagnetic fraction was then subdivided into several size fractions using variable mesh-sized sieve cloth. Each fraction was hand picked to ensure >99.9% purity. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 285 Dissolution and extraction of U and Pb was performed and the resulting fractions were analyzed on a VG Sector 54, seven-collector thermal ionization mass spectrometer using laboratory procedures that generally followed those of Krough (1973). Lead blanks during the period that analyses were made were less than 0.05 nG total Pb. Data were corrected for modem blank Pb, and nonradiogenic model Pb according to values given in Stacey and Kramers (1975), and were reduced and evaluated for analytical uncertainties using software of Ludwig (1989). A Model 2 solution that weighs all points equally was used for all regression analyses. Assuming that the initial Pb composition given by Stacey and Kramers (1975) is accurate, radiogenic Pb/Pb values are better than about 0.2% and U-Pb ratios are better than 0.3% at the 95% confidence level. Results from analyses are given in Table B.l and displayed on Tera and Wasserburg (1972) concordia diagrams in Figure 3.5a, b, and c. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 286 Table B.l U-Pb zircon TIMS analyses for La Suerte pluton, sheeted tonalite complex, and granite. Analyses performed at San Diego State University by Melissa Girty. Sample Fracnon Zircon "“ I T " “ K M 1 Measured rattas* Radiogenic ratios (Ma) (mg) (ppm) (ppm) “ ‘f t “ f t “ J f t “ f t “ Pb “ Pb “ Pb “ U ” 5U “ Pb La.Sucrtc_nluton (SP90) 1. >140 4.4 194 42 0.000084 0.04999 0.19919 0.02030 (130) 0.13646 (130) 0.04875 (136) 2. <140, >200 U 696 152 0.000178 0.05135 0.19351 0.02046 (131) 0.13748 (131) 0.04874 (135) 3. <200. >325 52 193 42 a000067 0.04976 0.I8S59 0.02049 (131) 0.13777 (131) 0.04878 (137) 4. <325 0.1 968 18.6 0.000071 0.04967 001975 0.02089 (133) 0.14004 (133) 0.04862 (129) Sheeted mnaKte eonmlex 5. >200. <140 2.3 161 2.6 0.000288 0.05283 0.09505 0.01593 (102) 0.10672 (103) 0.04860 (128) 6. >325. <200 2.8 218 33 0.000075 0.04958 0.08284 0.01575 (101) 0.10529 (102) 0.04848 (123) 7. <325 0.9 112 1.8 0.000081 0.04969 0.08329 0.01601 (102) 0.10711 (103) 0.04851 (124) 8. >140 3.5 163 23 0.000170 005108 0.08996 0.01557 (100) 0.10429 (101) 0.04858 (128) Granite sheet (Goetz^I989 am ple) 9. >200 12 139 33 0.001953 008073 0.18289 0.02114 (135) 0.15189 (144) 0.05211 (290) 10. >325 1.7 163 3.0 0.000252 0.05210 0.11812 0.01837 (117) 0.12255 (117) 0.08386 (118) It. <325 1.5 183 3.0 0.000126 0.0S0I7 0.10900 0.01633 (104) 0.10881 (105) 0.04832 (115) Samples were spiked with a mixed JC*Pb/2UU spike. •Indicates total Pb. A Pb/Pb values were normalized for mass fractionarico of 0.0 ± 0.05% per mass unit Radiogenic lead corrected for blank (“ Ph/“ *Pb = 18.87; “ P b /^ b * 15.66; **Pb/®*Pb a 38.53) and initial lead (Stacey and Kramers. 1975). Decay constants are those recommended by Jaffcy et aL (1971). Initial-Pb values for samples of La Suerte pluton were “ Pb/“ Pb = 18.497 ± 0.1, J0TPb/*4Pb «= 15.619 ± 0.05. and a*PbrD‘Pb = 38-384 ± 0.2. Initial-Pb values for samples of sheeted tonalite complex and granite were “ Pb/**Pb * 18-52 ± 0 .1 .w Pb/“ Pb a 15.62 ± 0.05. and " P b ^ P b * 38.411 ± 0.2. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 287 U-Pb zircon SHRIMP methods (Australian National University and Stanford University) Samples of orthogneiss from a wall rock screen in the central domain (SP835) and Agua Caliente pluton in the eastern footwall of the fan structure (SP849) were analyzed using SHRIMP (sensitive high-resolution ion microprobe) II instrumentation at the Australian National University. Following standard mineral separation techniques representative aliquots of zircon crystals were mounted in epoxy resin with chips of reference zircon and prepared as polished thin sections. Cathodoluminescence images were used to identify internal structures in the mounted grains before analysis. Each U-Pb analysis consists of six scans through the mass range, and data were reduced using methods similar to those of Compston et al. (1992) and Williams and Claesson (1987). Isotopic ratio uncertainties were estimated using software written by T.R. Ireland (see Muir et al., 1996). Pb-U ratios were normalized relative to the AS3 standard zircon, with 206pb/238u value o f0.1859 and age of 1099.1 Ma (Paces and Miller, 1993). Results of 14 analyses for each of the two samples are shown in Table B.2. Individual ratio and age uncertainties are given at the la level, and mean ages are reported at 2a. Figure 3.5d and e show the data plotted on Tera and Wasserburg (1972) concordia diagrams and associated histograms showing the distribution of magmatic ages determined in each unit Only nonmagnetic zircons were analyzed and cathodoluminescence images show strong oscillatory zonadon consistent with a Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 288 magmatic origin for all zircons. Figure 3.5d and e show that most analyses plot near to concordia and must be dominated by radiogenic lead. A sample of hornblende tonalite from the Rinconada pluton (SP820) and recrystallized fine-grained gabbro from a pluton that intrudes the tonalite sheeted complex and shows good mingling textures with the tonalite (SP832) were selected for analysis by the SHRIMP-RG instrument at Stanford University. Procedures and errors generally follow those for the Australian National University facility. Results of 16 and 10 analyses for each sample respectively are shown in Table B.3. The data are plotted on Tera and Wasserburg (1972) concordia diagrams in Figure 3.5f and g. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 289 Table B.2 U-Pb zircon SHRIMP analyses for orthogneiss from sheeted tonalite complex and Agua Caliente pluton. Analyses performed at Australian National University by Dr. Mark Fanning. drain spot (ppm) (ppm) w • w (ppm) " P b Measnred Radiogenic Age (Ma) “ Pb ± “ Pb ± "*u X “*u £ I.I 1896 27 0.01 40 - <0.01 0.0493 0.0005 39.038 0.430 0.02562 0.00028 163.1 1.8 2.1 496 49 0.10 11 0.000202 055 0.0523 0.0010 38516 0.482 0.02567 0.00032 163.4 50 3.1 894 30 0.03 19 0.000043 0.08 0.0501 0.0008 38512 0.434 0.02595 0.00029 16S.I 1.9 3.2 290 185 0.64 7 0.000012 0.41 aQ527 0.0017 38580 0.613 0.02595 0.00042 1655 56 4.1 1722 990 057 43 0.000071 <0.01 0.0494 0.0005 38.468 0.416 0.02600 0.00028 1655 1.8 5.1 346 56 0.16 7 0.000593 054 0.0514 0.0010 40.470 0595 0.02465 0.00036 157.0 53 5.2 404 154 058 64 0.000004 <0.01 0.0719 0.0006 5.945 0.075 0.16907 0.00214 1007 12 6.1 904 45 0.05 20 0.000128 0.13 0.0505 0.0008 38.476 0.445 0.02596 0.00030 1655 1.9 7.1 494 30 0.06 11 0.000105 -0.00 0.0495 0.0008 39.134 0.482 0.02555 0.00032 1657 50 8.1 548 56 0.10 12 0.000023 0.01 0.0495 0.0008 38533 0.470 0.02609 0.00032 166.0 50 9.1 362 66 0.18 8 0.000026 0.07 0.0500 0.0016 38576 0551 0.02590 0.00037 164.9 54 10.1 249 76 050 6 0.000301 0.14 0.0S06 0.0011 38.427 0573 0.02599 0.00039 165.4 54 11.1 680 27 0.04 14 0.000152 0.18 0.0509 0.0008 39.142 0503 0.02550 0.00033 1653 51 I U 89 40 0.45 16 0.000179 <0.01 0.0731 0.0014 5590 0.093 0.18620 0.00324 1101 18 Agua Caliente nluton (SP149t 1.1 446 88 050 10 - 053 0.0513 15 97 47 0.48 2 0.002601 053 0.0537 51 235 52 052 5 0.000010 0.61 0.0543 3.1 425 40 0.09 9 0.000164 0.08 0.0501 3.2 1382 142 0.10 196 0.000004 <0.01 0.0716 4.1 335 58 0.17 8 0.000242 0.11 0.0503 5.1 92 42 0.46 2 0.000140 150 0.0590 5 5 239 113 0.48 35 0.000027 052 0.0744 6.1 327 41 0.12 7 0.000515 0.06 0.0499 7.1 1300 65 005 38 0.000063 0.40 0.0539 8.1 252 38 0.15 6 0.000427 055 0.0539 9.1 628 86 0.14 14 0.000001 0.15 0.0507 10.1 1415 41 0.03 31 0.000149 <0.01 0.0483 l l .l 590 64 0.11 13 . <0.01 0.0490 0.0011 37.830 0.472 0.02637 000033 167.8 21 0.0028 40.623 0.803 0.02449 0.00049 156.0 3.1 0.0012 39560 0530 0.02532 0.00034 1615 25 0.0009 38.686 0.485 0.02583 0.00033 164.4 20 0.0006 6.131 0.073 0.16329 000196 975 11 0.0011 38.744 0.499 0.02578 0.00033 164.1 21 0.0020 39504 0.700 0.02501 0.00045 1595 28 0.0006 6593 0.091 0.15135 0.00208 909 12 0.0010 38.191 0514 0.02617 0.00035 1665 22 0.0007 28.781 0.756 0.03461 000091 2195 5.7 0.0013 38565 0500 0.02592 0.00034 165.0 21 0.0011 39559 0501 0.02524 000032 160.7 20 0.0007 38.195 0.435 0.02622 000030 166.9 1.9 0.0007 37.941 0.457 0.02637 000032 167.8 20 Uncertainties given at tbe one s level fa* % denotes tbe percentage o f “ Pb that is cotnmnn Pb. Correction for common Pb made using the measured “ U /^P b and ^ P b /^ P b ratios following T en and Wasserburg(1972) as outlined in Compstoo et aL (1992). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 9 0 Table B.3 U-Pb zircon SHRIMP analyses for hornblende tonalite from the Rinconada pluton and gabbro from the sheeted tonalite complex. Analyses performed at Stanford University by Dr. Joseph Wooden. Grain spot U (ppm) (ppm) W MW ~ (ppm) " P b Measured Radiogenic Age (Ma) " P b £ " P b ± “ *u a: “ *U ± 1.1 82 37 0.46 2 0.00361 0.0918 0.0071 60333 1339 0.01652 0.00042 99.8 27 2.1 117 44 038 2 0.00277 0.0974 0.0061 60333 1309 0.01652 0.00033 99.1 21 3.1 128 56 0.44 2 0.00189 0.0717 0.0043 60386 1.422 0.01656 0.00039 1028 23 4.1 79 62 0.78 1 0.00034 0.0502 0.0059 63.052 M31 0.01586 0.00036 1013 24 5.1 76 60 0.79 1 0.00001 0.0549 0.0083 60.643 1.140 0.01649 0.00031 1043 22 6.1 108 76 0.70 2 0.00213 0.0442 0.0048 61.463 1322 0.01627 0.00035 104.6 23 7.1 71 44 0.61 1 - 0.0490 0.0038 63.694 1301 0.01570 0.00037 100.3 24 8.1 83 64 0.78 2 - 0.0568 0.0038 62.150 1375 0.01609 0.00033 101.8 21 9.1 IQS 45 0.43 2 0.00001 0.0492 0.0049 62.617 1.412 0.01597 0.00036 1020 24 10.1 76 59 0.78 I - 0.0462 0.0044 61.9% 1.153 0.01613 0.0003 103.4 20 11.1 133 114 0.86 2 0.00025 0.0532 0.0032 65.963 1323 0.01516 0.00035 96.4 22 12.1 118 87 0.74 2 0.00350 0.0635 04)043 63.412 1.608 0.01577 0.0004 98.9 23 13.1 78 61 0.78 1 0.00044 0.0566 04)056 60350 1.639 0.01657 0.00045 104.8 29 14.1 134 86 0.65 2 - 0.0483 0.0032 62.073 1.002 0.01611 0.00026 103.0 1.7 15.1 82 65 0.79 1 0.00014 0.0527 0.0032 63.898 1.143 0.01565 0.00028 993 1.8 16.1 66 47 0.71 1 0.00001 0.0536 0.0062 63.613 1395 0.01572 0.00032 99.8 22 Gabbro from sheeted tonalite complex (SP8321 1.1 91 28 031 2 0.00468 0.0916 21 98 36 037 2 0.00227 0.0816 3.1 138 3S 036 2 0.00293 0.0904 4.1 326 87 037 5 0.00014 0.0494 5.1 746 284 038 12 - 0.0492 6.1 471 194 0.41 8 - 0.0467 7.1 319 63 030 5 0.00028 0.0496 8.1 357 99 038 5 0.00032 0.0505 9.1 95 42 0.44 2 0.00001 0.0528 10.1 542 106 030 8 0.00023 0.0480 Uncertainties given at tbe one s level Correction for common Pb made using tbe as outlined in CompstonetaL (1992). 0.0059 59.809 1.145 0.01672 0.00032 101.1 20 0.0042 61350 1304 0.0163 0.00032 99.8 20 0.0047 60.864 1.185 0.01643 0.00032 993 20 0.0015 64.061 0.985 0.01561 0.00024 99.7 13 0.0012 63311 0.759 0.01582 0.00019 101.0 13 0.0024 63351 1.000 0.01581 0.00025 1013 1.6 0.0023 63331 0.842 0.01579 0.00021 100.8 1.4 0.0031 64.725 0.796 0.01545 0.00019 983 13 0.0047 63.171 1.676 0.01583 0.00042 100.6 27 0.0023 64.185 0.865 0.01558 0.00021 99.7 1.4 and 307Pb/30*Pb ratios following T en and Wasscrburg (1972) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 291 APPENDIX C: "Ar/^Ar BIOTITE AND K-FELDSPAR ANALYSIS Biotite and K-feldspar from samples collected from the southern SSPM transect were analyzed in Dr. Mark Harrison’s lab at the University of California, Los Angeles under the direction of Dr. M arty Grove. Analysis procedures generally followed those detailed in Quidelleur et al. (1997). Samples of biotite, mostly from fresh granitic pluton material, and K-feldspar from pegmatite bodies were crushed and sieved to -0.6-0.3 mm size high purity concentrates using conventional density and magnetic separation techniques. Mineral samples weighing 5-10 mg for biotite and 20-30 mg K-feldspar were packed in Al foil and Sn foil respectively. These were evenly interspersed with samples of Fish Canyon Sanidine in quartz glass vials. Tubes were irradiated in the Ford Reactor at the Phoenix Memorial Laboratory at the University of Michigan. Correction factors were calculated from K2SO4 and CaF2 included in the tubes for determining interfering neutron reactions, and flux monitor results were calculated from the Fish Canyon samples assuming an age of 27.8 Ma (Cebula et al., 1986). Samples were step heated in a Ta crucible within a double vacuum furnace. Errors in temperature step estimates listed in Table C.l and C.2 range from ±5 °C for 400 °C steps to ±15 °C for 1100 °C steps. Gas released from heating steps was gettered for -10 minutes, then analyzed using a VG1200s automated mass spectrometer. Data from analyses axe given in Table C.l and C.2 . Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 292 Table C.l. Biotite *°Ar/39Ar isotopic measurements. SP7- orthogneiss # T(#C) tOnm) “ Arf*Ar "Atf*Ar nAK"Ar *Arf*Ar ■AKinol) %*Ar» «,Ar*/"ArK Age (Ma) ± ft* (Ma) 1 600 12 820 227E-02 3.27E-03 9.18E-03 520E-14 66.8 5S6 68.6 0.6 2 700 13 7.68 229E-02 1.73E03 2.I5&03 8.I1E-I4 91.0 7.02 86.1 OS 3 800 17 8.48 222E-02 6.87E-03 4.62E03 4.31E-14 83S 7.09 87.0 0.6 4 950 14 730 231E-02 123E-02 224E-03 9.72E-14 90S 6.81 83.6 03 5 1150 14 7.89 238E-02 7.66E02 2.76E4J3 5.06E-14 89.1 7.06 86.6 OS Total gas age (Ma) *oAge(Ma) Weighted mean age (Ma) Standard error MSWD 82.8 OS 83S 0 2 171.4 SP9- orthogneiss # T(0O Krnin) “ A i^A r "A j/"A r ” Ai/"Ar “Ar/»Ar "AtOnoO %*Ar* •Ar*/"ArK Age (Ma) 4 ft* (Ma) 1 600 12 9.83 292E-02 323E-Q3 1.49E-02 3.Q2E-I4 54.8 5.40 66.7 1.0 2 700 12 7.13 259E-02 7.45E-04 1.11E-03 1.06E-I3 94.7 6.78 832 02 3 800 13 721 2.63 E-02 1.46E-03 1.I9E-03 6.9SE-I4 94S 6.83 83.8 02 4 950 13 7.03 269E-02 274E-03 126E-03 6.46E-14 92.8 6S4 80.4 02 5 1150 14 725 2.61 E-02 5.42E-03 129E-03 8.92B-14 93.6 6.81 83.7 02 Total gas age (Ma) soAge(Ma) Weighted mean age (Ma) Standard enor MSWD 81.6 0.4 82S 0 2 22.6 SP17- metavolcanic # T(*C) Kmin) "A tf"A r "Arf*Ar W A r “AtC’Ar "AKtnoI) %*Ar* ®Ar*/"ArK Age (Ma) ± ft* (Ma) 1 650 13 7.10 1.87 E-02 6.89E-03 202E-03 7S1B-I4 90S 6.48 79.6 06 2 730 14 721 IS7E-02 42 IE-03 928E-04 6.67E-14 94.9 6.91 84.8 OS 3 800 16 7.90 1.67E-02 227&02 4.18E-03 223E-14 83.0 6.64 81S 0.9 4 950 14 725 1.64E-02 1S7E-Q2 127E-03 5.81E-14 93S 6.85 84.0 08 5 1150 16 725 1.49E-Q2 4.02E-02 1.I8E-03 6.76E-14 94.0 6.88 84.4 0.6 Total gas age (Ma) eoAge(Ma) Weighted mean age (Ma) Standard error MSWD 82.9 0.7 82.9 02 142 SP20- orthogneiss from Agua Caliente pluton # T fC ) tOnia) *A«/*Ar *Ad"Ar W A r “ Ad"Ar ” Ai(niol) %4° K f 40Ar*A»Ari Age (Ma) 4 ft* (Ma) I 600 15 ion 228E-02 5.66E03 128E-02 320B-I4 62.1 620 77.4 OS 2 700 17 724 2.31 E-02 7.88EOS 123E03 124E-13 942 686 84.1 OS 3 800 17 724 221E-02 9S6E03 1S1E-03 6S2E-14 93.1 687 842 04 4 950 14 720 222E-02 1.I2E02 1S6E03 823E-I4 93.0 6.81 83S 0.4 5 1150 18 726 248E-02 286E02 1.61E03 9.12E-14 929 6.86 84.1 OS Total gas age (Ma) :tgAgc (Ma) I Weighted mean age (Ma) Standard error I MSWD 83.4 OS 1832 0 2 | 152 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 293 Table C.l (continued) SP6 8 - orthogneiss # T(°Q Kmin) ®Ar/*Ar »AK"Ar wAK»Ar “ Ai/»Ar "AKmol) * <0Ai» •AfV"Ar* Age (Ma) (Ma) 1 600 13 15.19 432E-02 2.88E-02 391E-02 832E-15 43.0 638 80.7 1.9 2 700 11 8.06 3.91E-02 236E-03 370E-03 3.70E-14 893 7.23 883 0.5 3 700 15 7.85 3.94E02 3.80E-03 133E4J3 632E-14 94.4 7.44 90.9 0.4 4 950 11 7.69 4.02E-02 2.46E-02 8.41E-04 635E-14 96.0 7.42 90.6 0.4 5 1150 15 7.66 535E4K 4.65&01 1.04&03 8.06E-14 95.7 736 90.0 03 Total gas age (Ma) ±oAge(Ma) Weighted mean age (Ma) Standard error MSWD 89.9 0.4 90.1 0 3 10.2 SP73- bi tonalite from La Suerte pluton # T('C) Kmin) *AK"Ar "Ar/*Ar n AK*Ar *Ar/"Ar "AKraoI) %4°As* ®Ar*/"ArK Age (Ma) (Ma) 1 700 18 7.77 3.45E-02 631E-03 3.22E-03 1.63E-13 87.1 6.80 83.2 03 2 780 57 7.96 339E-02 239E-02 3.10EO3 1.03E-13 87.9 7.03 85.9 03 3 850 22 7.83 332E-02 6.03 E-02 4.94E-03 2.11E-I4 803 635 77.8 0.7 4 950 17 735 3.I9E-02 332E02 237E03 4.99E-14 89.8 6.83 83.6 0.7 5 1150 14 7.89 3.45E-02 4.97E02 331E-03 4.43E-I4 86.7 6.89 84.4 0.8 Total gas age (Ma) eoAge(Ma) Weighted mean age (Ma) Standard error 1 MSWD 83.8 0.6 83.6 03 122.0 SP136- hb tonalite near Rinconada pluton # T(°Q Kmin) ®Ar/"Ar *AK"Ar nAK»Ar “ Ar/"Ar "AKmol) %®Ar* ®ArV*ArK Age (Ma) **** (Ma) 1 600 11 8.18 4.78E-02 436E-01 8.60EO3 5.77E-14 68.6 5.64 69.0 0.8 2 700 16 8.12 4.65E-02 2.15EOI 2.47E-03 1.13E-13 90.4 738 89.8 0.6 3 800 IS 834 4.65E02 7.64E-02 3.89&00 8.82E-14 86.0 737 89.6 0.4 4 950 15 7.83 4.62E02 4.47E-02 132E-03 131E-13 93.6 7.36 893 0.4 5 1150 16 7.97 4.69 E-02 5.60E-OI 1.47E-03 1.17E-13 943 735 91.8 03 Total gas age (Ma) soAge (Ma) Weighted mean age (Ma) Standard error MSWD 87.9 03 893 0.2 1913 SP138- hb tonalite from western zone # T(*C) Kmin) •AK»Ar “ Ai/"Ar n Ai/*Ar *AK*Ar "Ar(mol) %*°Ar* *Ar*A"Ars Age (Ma) (Ma) 1 600 11 7.99 I.llE-01 393E-Q2 6.82E-03 7.12E-14 743 5.95 72.6 0.6 2 700 13 9.40 1.12E-0I 3.92E-02 391E-03 1.01E-13 903 832 103.1 0.6 3 800 17 930 1.05E-01 8.42E-02 1.79E-03 4.97E-I4 93.0 8.76 105.9 1.1 4 950 19 9.46 I.llE-01 1.16E-01 2.44E-03 134E-13 91.9 8.72 1053 0.6 5 1150 16 934 1.15E-01 2.06E-OI 1.60E-Q3 I30E-13 943 8.85 107.1 03 Total gas age (Ma) scA ge (Ma) Weighted mean age (Ma) Standard error MSWD 100.9 0.6 199.4 03 15736 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 9 4 Table C.l (continued) SP139- hb tonalite from western zone # TCQ ((mm) «Atf»Ar *Aif»Ar "A rf-A r *Ad"At "AifmoO %*Ar» *Ar*/"ArK Age (Ma) (Ma) I 600 23 10.61 7.61B02 132E-02 1.46E02 337E-14 58.9 637 76.4 1.0 2 700 15 8.81 7.15E-02 834&03 1.I4E4B 7.43E-14 95.5 8.43 102.2 0.4 3 800 30 8.87 6.96E-02 1.60&02 3.90E-04 1.18E-13 98.0 8.73 1053 03 4 930 18 8.83 7.03E-02 2.22&02 4.44&04 6.72E-I4 97.8 8.70 105.1 OS 3 1150 20 8.77 734E412 3.80&02 4.81E-04 1.72E-13 97.8 8.61 104.1 OS Total gas age (Ma) icA ge (Ma) Weighted mean age (Ma) Standard error MSWD 1013 0.5 102.4 0 2 198.7 SP197- flysch # T (°0 Kmin) “ At/"Ar »Arf*Ar nAi/"Ar “ Ar/"Ar "Ai(mol) *"A r* *ArV*Aric Age (Ma) (Ma) 1 600 15 It.03 1.79&02 137&02 138&02 5.00E-14 573 6.32 76.9 0.7 2 700 15 838 1.63&02 6.67E-03 2.88E-03 4.43E-14 89.4 7.71 93.4 03 3 800 13 8.58 1.65E-02 1.76E-Q2 2.88E-03 4.45E-I4 893 7.70 933 03 4 950 11 8.03 135E-02 138E-02 1.7IE-03 6.81E-14 93.0 730 90.9 0.4 5 1150 13 8.16 138&02 1.83&02 1.67E-03 7.64E-14 933 7.65 92.6 0.4 Total gas age (Ma) icA ge (Ma) Weighted mean age (Ma) Standard error MSWD 89.7 03 91.2 02 102.8 SP140a- hb tonalite from western zone # t c q Kmin) *Atf»Ar “ Ar/*Ar nAt/*Ar *Arf»Ar *Ar(tnol) %®Ar* •Ar*A"ArK Age «?aii (Ma) (Ma) 1 600 28 10.85 134E+00 •2.47E-OI 1.18E4K I.92E-14 66.4 733 903 43 2 700 16 10.43 1.93E-01 -2.66E-01 433E03 I.62E-I4 853 9.10 111.4 7.1 3 780 17 9.43 1.98E-01 -1.49E-01 1.48&03 339E-14 93.9 8.96 109.7 3.9 4 900 16 935 1.89E-01 -1.04E-01 2.16E-03 433E-14 923 8.88 108.8 2.0 5 1000 17 933 2.17E-01 -5.65E-02 8.9IE-04 3.97E-14 95.9 8.93 109.4 2.4 6 1150 17 8.89 1.82E-OI -234E-03 -2.10E-04 9.86E-14 100.0 8.93 109.4 1.1 Total gas age (Ma) *oAge(Ma) Weighted mean age (Ma) Standard error 1 MSWD 108.0 2.7 108.6 0.9 | 2.9 SP133a- granite # TCQ Kmin) “ At/"At “ ArC’Ar W A r “ Atf*Ar *Ar(inoI) %*Ar* '°Ar*/"ArK Age (Ma) 19% (Ma) 1 600 24 1239 3.66E-02 -9JX7&Q2 8.89E03 134E-14 76.8 9.93 1213 6.4 2 700 18 7.19 2.13E-02 -4.11E-02 -6.98E-05 7.90E-I4 99.0 7.18 883 13 3 780 21 7.06 2.07E-02 -2.82 E m -I.78E-04 1.11E-I3 99.7 7.09 873 0.8 4 850 16 736 2.08E-02 -138E01 I.17E-03 1.86E-14 913 6.98 86.0 4.8 5 1000 16 738 2.07E-02 -4.16E-02 2.19E04 7.02E-14 97.7 7.18 883 2.0 6 ttso 44 738 235E-02 -139E02 3.08E-04 1.45E-13 97.9 737 893 1.0 Total gas age (Ma) ±oAge(Ma) 1 Weighted meanage(M a) Standard error MSWD 89.6 1.6 |8 8 3 05 4.9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 9 5 Table C.l (continued) SP409- hb tonalite (La Posta?) # T(°Q tOnin) ®A*^*Ar »Ar/"Ar nAd»Ar "Ar/"Ar ” Ar(mol) %40Ax* “ At*/*At* Age (Ma) ±0** (Ma) 1 600 18 8.94 243E-02 -219E-01 631E-03 201E-14 76.9 7.03 86.7 3.8 2 700 18 7.06 137&02 -731E-02 1.05E-03 730B-U 94.4 6.72 82.9 13 3 780 18 7.05 1.73E-02 -737E-02 I.I3E-04 9.10E-14 98.4 6.99 86.2 1.1 4 900 16 736 1.77E-02 -7.76E-02 9.43E-04 5.25E-14 94.7 6.95 85.7 1.8 5 1000 21 7.09 1.89&02 -6.69E-02 5.02E-04 8.6IE-14 96.8 6.91 853 0.9 6 1150 16 7.15 1.99E-02 -203 E-02 295E-0S 833E-I4 98.8 7.11 87.7 13 Tool gas age (Ma) *oAge(Ma) Weighted mean age (Ma) Standard error MSWD 85.7 1.4 |85.5 0 3 | 1.4 SP441- orthogneiss * T(*C) Kmin) “ Ar/*Ar "Atf*Ar nArf*Ar “ Atf*Ar *Ar(niol) **Ar* *Ar*/wArlc Age (Ma) ±°A* (Ma) 1 600 16 8.80 2.18E-02 -I24E-01 1.08 E-02 4.44E-14 62.7 536 68.9 24 2 700 17 7.19 135E-02 -2J6E-02 1.90E-03 135E-13 913 6.61 813 1.1 3 780 15 7.27 1.34E-02 -4.78E-02 1J8E-03 1.03E-13 93.6 6.84 843 1.1 4 900 16 7.19 138E-02 •6.86E-02 1.26E-03 5.42E-14 93.6 6.79 83.7 2 0 5 1000 18 6.87 I.42E-02 •3.91E-02 1.41E03 1.03E-I3 93.1 6.43 79.4 13 6 1130 16 7.01 135E-02 7.66E-03 4.24 E-04 739E-14 973 6.86 84.7 l.l Total gas age (Ma) toA ge (Ma) Weighted mean age (Ma) Standard error 1 MSWD 81.3 1.4 82.1 0 3 1 7.4 SP448- orthogneiss # T(°C) tfmin) *Ai/*Ar "Aj/*At "At/"At “A r^A r "ArOnoO *"A r* «Ar*A»Ar* Age (Ma) (Ma) i 600 19 23.96 I39E-01 -133 3.45E-02 1.87E-15 523 13.62 1643 40.8 2 700 18 734 4.85E02 -1.13E-01 -339E-05 435E-14 983 731 88.9 1.9 3 780 16 6.86 4.49E-02 -6.12E-02 -4.45E-04 8.68E-14 100.8 6.96 85.9 0.9 4 900 21 7.05 432E-02 -130E-OI -3.92E-04 3.86E-I4 99.7 7.13 87.9 27 5 1000 16 6.93 4.62E-02 -7.05E-02 -9.70E-04 7.0SB-14 1029 7.19 88.6 1.4 6 1150 16 6.94 4.71E02 -4.19 E-02 -4.04E-05 1.10E-13 993 6.92 85.4 0.8 Total gas age (Ma) toA ge (Ma) Weighted mean age (Ma) Standard error 1 MSWD 873 1.6 863 0 3 1 1.7 SP453- orthogneiss # T(°C) t(min) ®Ar/"Ar “ A rfA r nAr/»Ar »Arf»Ar *Ar(mo0 %*Ar* "Ar*/” Arlc Age (Ma) (Ma) 1 600 21 6.99 6J5E-01 -I33& 0I 730E-04 333E-14 95.0 6.74 833 3.4 2 700 22 7.02 9.17E-02 -6.42E-02 262E-04 8.76E-I4 973 6.92 853 0.9 3 780 15 7.03 244E-02 -631E-02 -533E-05 9.17E-14 993 7.02 863 03 4 900 16 7.14 240E-02 -8.69 E-02 I.32E04 4.85E-14 98.0 7.07 87.1 21 5 1000 16 6.92 2-57E02 -5.72E-Q2 1.07E-03 831E-14 94.4 638 813 0.9 6 1150 17 7.08 236E-02 -437E-Q2 -275B4K 8.12E-14 KXXI 7.13 87.8 1.7 Total gas age (Ma) ±oAge(Ma) I Weighted mran agr (Ma) Standard error I MSWD 853 24 184.7 0 3 fZi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 9 6 Table C.l (continued) SP519- hb tonalite from La Suerte pluton # TTO Kmm) "A r/"A r ■At/"Ar W A r “At/»Ar "ArOnoO %®Ar» ®Ar«A"ArK Age (Ma) ±t**m (Ma) 1 600 16 733 432&02 6.78E03 6.93 E-03 538E-14 703 5.16 64.0 23 2 700 15 8.08 337E02 -6.12E-02 237E-03 5.86E-I4 89.1 7.29 89.8 1.7 3 780 17 8.04 336&02 -6.82E-02 273E-03 5.77B-I4 88.4 7.20 88.7 1.8 4 900 17 8.03 3.47E-02 -6.90E-02 240E-Q3 533E-14 893 739 89.8 1.7 S 1000 16 8.06 339E-02 -335E02 284E4J3 7.18E-14 883 7.19 88.6 13 6 1130 t7 834 3.83&02 3.76&02 216E-Q3 335E-14 902 7.68 94.4 23 Total gas age (Ma) ioA ge (Ma) Weighted mean age (Ma) Standard error MSWD 85.7 1.9 86.9 0.7 204 SP569- hb tonalite (La Posta) # TCC) Uniin) *Ar/"Ar "At/” Air “ ArZ-Ar wAi(inol) %®Ar« ®Ar»A"ArK Age * > A , (Ma) (Ma) 1 600 18 5.90 2.42E02 -3.10EO2 4.95E-03 7.07E-14 733 4.41 54.8 13 2 700 16 7.19 205EO2 -334E-02 139E-03 7.79E-14 92.8 6.76 83.4 1.1 3 780 13 8.36 204E-02 -5.16E4J2 7.01E44 4.81E-14 953 8.12 99.7 20 4 900 15 8.64 2.02E-02 -8.90E-02 206E-03 334E-14 903 8.00 983 26 5 1000 16 830 2.05E-02 -7.12E-02 6.67E-04 3.78E-14 953 837 1013 24 6 1150 16 9.25 336E-02 2.24E4)! 137E4B 210E-I4 91.6 8.78 107.6 43 Total gas age (Ma) xaAge (Ma) Weighted mean age (Ma) Standard error MSWD 85.2 2.0 803 07 107.9 ZP40- hb tonalite from Zaiza Pluton (from Scott Johnson) # TCO t(mm) "A i^A r ■Atf»Ar nAr/” Ar “ Ai/^Ar *Ar(moI) %®Ar* *Ar»/*,ArK Age (Ma) (Ma) 1 600 17 14.40 3.89E-01 -5.17E-0I 135E4X2 436E-I5 623 9.75 1193 203 2 700 18 9.93 242E-0I •337E-0I -243E-03 1.80E-14 103.9 1060 129.1 4.6 3 780 21 8.96 230EO1 -132E4H -129E4X3 537E-14 1 0 2 .8 931 113.8 1.7 4 900 18 8.69 216E-01 •8.06E-02 -1.15E-03 6.86E-I4 102.7 9.00 1103 13 5 1000 17 8.81 338E-0I 8.04E-03 -I.41&03 6.17E-14 103.6 930 1126 13 6 1150 16 838 162£4)1 •132E-02 -926E4M 206E-13 1023 8.83 1083 0.4 Total gas age (Ma) eoAge (Ma) Weighted mean age (hb) Standard enor 1 MSWD 1109 1.4 109.1 0.4 16-4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 29 7 Table C.2. K-feldspar ^Ar/^Ar isotopic measurements. SP20- Agua Caliente pluton * TCO Kmin) *At/*Ar “ Atf»Ar "A r^A r “A t^A r "AKmoI) %*Ax* *Ar*/"Ar* Age(Ma) ±3*. I 450 17 111.62 9.44E-02 1.49E02 335E-0I 3.01E-I5 5.9 638 813 1X1 2 450 23 43.65 4.17E-02 1.49E02 1.48E-01 1.13E-15 -03 -0.08 0.0 0.0 3 500 16 6739 5.46E-02 1.49E02 X06E-01 X44E-15 9.4 637 78.7 53 4 500 32 54.16 439E-02 1.49E02 1.66E01 3.03E-15 9.4 5.10 63 3 4.0 5 550 15 31.92 333E-02 1.49E02 8.01E-02 438E-15 25.6 832 100.9 1.7 6 550 16 15.03 1.82E-02 1.49E02 3.46E-02 4.I8E-1S 31.6 4.79 593 13 7 600 17 12.07 I.76E-02 1.49E-02 235E-02 138E-I4 443 539 66.9 0-7 8 600 18 7.40 136E-02 1.49E02 9.08E03 1.10E-14 63.0 4.69 583 0.4 9 650 17 7.62 1.40E-02 1.49E-02 8.86E-03 3.19E-14 653 4.98 61.8 0.4 10 650 16 5.84 130E-02 I.49E02 X75E-Q3 X97E-I4 85.4 5.00 62.1 03 11 700 17 6.94 1.61E-02 1.49E02 5.00E-03 8.03E-14 783 5.44 673 03 12 700 17 6.18 1.22E-02 1.49E02 1.75E-03 4.69E-14 91.0 5.63 69.8 03 13 750 16 635 2.06E-02 1.49 E-02 230EO3 1.03E-13 883 5.79 71.6 0.1 14 750 20 6.13 X46E-02 1.49E-02 936E-04 6.46E-14 94.9 5.82 72.0 0.7 IS 800 16 634 2.74E-02 1.49E-02 230E-03 1.19E-I3 89.6 5.87 72.6 03 16 800 21 6.10 3.64E-02 1.49E-02 630E-04 8.13E-14 963 5.89 7X9 0.1 17 825 18 633 7.69 E-02 1.49E-02 6.84E04 7.01E-14 963 6.00 743 03 18 850 17 637 4.12E-Q2 I.49E02 1.43E-03 7.40E-I4 92.9 5.92 733 0.1 19 875 16 638 1.99E-02 1.49E4J2 231E-03 730E-14 893 5.87 7X7 03 20 900 18 635 204E-02 1.49E-02 234E-03 835E-14 89.4 5.86 7X6 03 21 925 16 633 1.48 E-02 1.49E-02 239E-03 838E-14 893 5.83 7X1 0.1 22 950 16 633 1.47E-02 I.49E-02 239E-03 1.06E-13 89.2 5.83 7X2 0.4 23 975 17 6.48 1.28 E-02 1.49E-02 X14E-03 1.18E-13 89.8 5.83 7X1 0.1 24 1000 16 6.70 I.26E-02 1.49E-02 382E-03 1.18E-13 87.1 5.84 723 03 25 1025 16 6.76 I.24E02 I.49E-02 3.I3E03 1.17E-13 85.9 5.81 71.9 a t 26 1050 17 6.99 137E-02 1.49E-02 3.88E-03 I.16E-I3 83 3 5.82 7X0 0.1 27 1075 18 733 138E-02 1.49E-02 4.86E-03 I36E-13 79.7 5.77 713 0.1 28 1100 17 732 I31E-Q2 I.49E-02 530E-03 1.48E-13 78.6 5.76 713 03 29 1100 38 739 130E02 1.49E-02 S.01E-03 308E-13 793 5.79 71.7 0 3 30 700 685 27.81 363 E-02 1.49E-02 735E-02 7.07E-15 21.7 6.06 75.0 XO 31 1100 42 737 1.30E-02 1.49E-02 4.93E03 137E-I3 79.6 5.79 71.7 03 32 1100 72 738 13 IE-02 1.49E-02 5.I7E-03 105E-13 78.9 5.83 7X2 0.4 33 1100 120 733 130E-02 1.49E-02 5.04E-03 235E-13 793 5.82 7X0 03 34 1100 183 737 139 E-02 1.49E-02 5.06E-O3 I.97E-13 79 3 5.85 7X4 03 Total gas age (M«) eqAgc(Ma) Weighted mean age (Ma) StandanlaTor I MSWD 71-7 0 3 71.8 0 0 I 134.0 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 9 8 Table C.2 (continued) SP133a- granite # TCO Kmin) *A«/"Ar "Atf»Ar n Ai/»Ar »/W »Ar *Ar(mol) %"Ai» ®Ar»/*Ar* AgeTMa) ±H a, 1 450 26 63.68 2.22B-01 0 924E-Q2 5.43E-15 555 35*1 400.1 28 2 450 31 17.08 4.21E-Q2 0 3.04E-02 123E-15 44.9 8.08 99.3 72 3 500 20 21.21 726E-Q2 0 228&02 4.75E-15 67.4 14.45 173.8 1.9 4 500 31 9.43 210E-02 0 8-22E-Q3 3.97E-15 71.8 6.98 86.0 12 5 550 15 16.63 5.20E-02 0 127E-02 I. HE-14 69.8 11.98 1452 3.4 6 550 21 7.47 124E-02 0 2.91E-03 8.01E-15 86-5 6-58 81.3 0.7 7 600 15 925 2.40E-02 0 5.14E-Q3 237E-14 83.4 8.00 982 02 8 600 20 7.02 128E-02 0 1.48E-03 1.80E-I4 92.6 6.56 81.0 0.4 9 650 15 7.78 126&02 0 227E-03 4.44E-14 90.4 7.06 87.0 0.2 10 650 23 6.84 t22E-02 0 6.97E-04 4.02E-14 96-2 6.61 81.6 02 11 700 19 7.02 127E-02 0 929&04 7.98E-14 95.5 6.72 83.0 0.1 12 700 21 6.76 1.18&02 0 3.87E-04 5.63E-14 97.7 6.62 81.7 0.1 13 750 20 6.89 124&02 0 5.30E-04 9.96E-14 912 6.71 828 0.1 14 750 23 6.79 1.19E02 0 322E-04 8.06E-14 97.9 6.66 823 0.1 15 800 43 6.83 1.19E-Q2 0 324E-04 1.71E-13 98.0 6.71 828 0.1 16 800 22 6.79 1.18E-02 0 2.65E-04 4.97E-14 98.2 6.69 826 02 17 825 18 6.81 1.19E-02 0 258E-04 5.40E-14 982 6.71 828 0.1 18 850 16 6.81 1.19E-02 0 2-56E-04 6.08E-I4 98.3 6.71 828 0.1 19 875 19 6.83 1.I8E-02 0 3.09E-04 7.85E-14 98.1 6.72 829 0.1 20 900 16 6.88 1.I9E-02 0 4.64E-04 6.62E-14 97.4 6.72 829 0.1 21 925 17 6.89 1.18&02 0 424E-04 8.66E-14 915 6.73 83.1 0.1 22 9S0 16 6.90 1.19B-02 0 4.95E-04 8.13E-14 912 6.73 83.1 02 23 975 16 6.91 121E-02 0 5.12E-04 828E-I4 912 6.74 832 0.1 24 1000 17 6.96 122E-02 0 622E-04 9J9E-14 96.8 6.75 832 0.1 25 1025 17 7.06 125E-02 0 8.83 E-04 9.99E-14 95.8 6.78 83.6 0.1 26 1050 16 7.16 1.27E-02 0 1.10E-03 I.06E-13 95.0 6.81 84.1 02 27 1075 16 725 1.31 E-02 0 125E-03 1.07E-I3 94.0 6.82 842 0.1 28 1100 17 123 1.36 E-02 0 129E-Q3 1.15E-I3 93.0 6.83 842 0.1 29 1100 26 7.19 1J2E-02 0 U1E-03 126E-13 94.5 6.81 84.0 0.1 30 1100 39 7.18 1.32E-02 0 1.14E-03 1.30E-13 94.8 6.81 84.1 0.1 31 1100 72 7.14 122E-02 0 9.61E-04 1J4E-13 952 6.83 84.2 0.1 32 1100 121 7.11 120E-02 0 9.38E-04 1.04E-I3 95.0 6.81 84.0 02 33 1100 233 122 1.35E-02 0 1.08E-03 1.67E-13 95.1 6.88 84.8 0.1 34 1100 936 129 128E-02 0 I.48E-03 3.6IE-13 93.6 6.93 852 0.1 35 1200 18 1S2 121E-02 0 1.40E-03 9.76E-14 94.0 7.08 872 0.1 36 1225 16 121 1.46 E-02 0 9.6IE-04 238B-13 95.6 7.00 86.4 0.1 37 1250 18 122 1.42E-Q2 0 729E-04 6.96E-13 96.4 6.98 86.1 0.1 38 1300 16 7.17 I28B-02 0 6.66E-04 1.94E-12 96.7 6.95 85.7 0.1 39 1350 19 12* 1J7E-02 0 9.12E-04 3.74E-13 952 6.94 85.6 a t 40 1500 24 7.49 120E-02 0 1.47E-03 8.81E-14 93.7 7.03 86.7 0.1 41 1650 15 8.57 I24E-Q2 0 521E-03 3J2E-14 81.4 7.00 86.4 02 42 1650 17 18.75 225E-02 0 3.94E-02 3.06E-I5 37.0 7.07 87.1 24 Total gas age (Ma) ±qAgc(Ma) Weighted mem agr (Ma) Sandardoior MSWD 852 02 84.1 00 5242 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 9 9 Table C.2 (continued) SP138- western zone pluton * T{°Q Ktma) “ Ar/” Ar »Ar/»Ar vAlf”Az “ Atf»Ar *ArtmoI) * * A f *Ar*/” Arr AgefMa) iff** 1 450 75 101.98 261E01 0 3.19E-01 7.77E-15 7.6 7.78 95.7 8.4 2 450 47 69.63 138E-01 0 211E-01 1.90E-15 103 738 90.9 19.7 3 500 22 29.24 1.89E-01 0 7.04E02 439E-15 28.6 8.41 1033 4.8 4 500 28 18.75 6.61E-02 0 3.89E-02 4.90B-15 383 7.23 89.0 43 5 550 16 26.07 297E-01 0 536E-02 139E-14 36.7 9.61 117.4 1.8 6 550 20 13.05 8.04E-02 0 1.88E-02 133E-14 56.9 7.46 91.9 1.6 7 600 17 18.48 1.82E-01 0 3.29 E-02 290E-14 47.1 8.72 106.9 0.7 8 600 21 10.75 5.44E-02 0 1.UEQ2 239E-I4 69.1 7.45 91.7 0.8 9 650 16 14.96 l.QSE-01 0 232E-02 4.74E-I4 53.9 8.06 99.1 03 10 650 21 8.61 237E-02 0 4.30E03 334E-I4 84.7 731 90.0 0.6 11 700 16 1134 4.78E-02 0 134E-02 5.80E-14 653 737 93.1 0.4 12 700 20 8.03 135E-02 0 235E-03 439E-I4 913 734 90.4 0.4 13 750 IS 9.01 232E-Q2 0 538E-03 6.69E-14 833 7.42 913 03 14 750 21 8.08 1.48E02 0 234E-03 4.8 IE-14 913 739 91.0 0.4 15 800 IS 8.42 I.73B02 0 3.18E-03 6.78E-14 88.4 7.46 91.8 0.3 16 800 22 8.29 1.47E-02 0 273E-03 4.91E-14 89.8 7.46 91.8 0.4 17 825 18 7.99 1.42&02 0 I.69E03 439E-I4 933 7.46 91.9 0.4 18 850 15 831 1.48 E-02 0 231E-03 3.76E-14 90.4 7.45 91.7 03 19 875 15 8.19 1.47E02 0 242E-03 3.92E-14 90.7 7.45 91.7 03 20 900 15 8.11 1.53E02 0 236E-03 4.02E-14 913 7.42 913 03 21 925 22 830 1.66E-02 0 3.40E-03 5.65E-14 87.7 7.47 91.9 03 22 950 24 937 203E-02 0 6.16E-03 6.48E-14 80.0 7.42 91.4 03 23 975 16 10.40 249E-02 0 9.90E-03 3.38E-14 71.4 7.44 91.6 0.6 24 1000 26 11.17 2.90E02 0 134 E-02 6.49E-14 66.9 7.48 931 0.4 25 1025 16 1316 3.41 E-02 0 137E02 5.64E-14 61.6 730 *’23 0.4 26 1050 16 1231 3.61E-02 0 1.70E-02 633E-14 593 7.46 91.9 0.7 27 1075 16 1343 3.74E-02 0 1.65E02 8.68E-14 60.6 734 938 0.4 28 1100 16 1234 3.70E-02 0 138E-Q2 1.48E-13 613 734 938 0.6 29 1100 25 1131 3.48 E-02 0 I32E-02 1.09E-13 65.9 739 93.4 03 30 1100 39 10.89 3.35E-02 0 1.10E-02 1.12E-13 69.7 7.60 933 03 31 1100 74 10.12 3.36E02 0 8.48E03 136E-13 74.9 739 93.4 0.4 32 1100 122 9.74 337E-02 0 7.03E03 I.28E-13 783 7.64 94.0 03 33 1100 241 9.85 3.86E-02 0 731E-03 1.95E-13 77.7 7.67 943 03 34 1100 748 10.48 4.22E-02 0 931E-03 232E-13 73.4 7.70 94.7 0.4 35 1200 17 1133 S30E-02 0 1.I4E-02 9.18E-14 69.6 7.82 96.1 03 36 1225 16 11.00 5.75E-02 0 1.07E-02 274E-I3 70.9 7.80 95.9 03 37 1250 18 1132 538E-02 0 I.19E-02 937E-I3 68.7 7.79 95.8 0.4 38 1350 17 1341 4.42E-02 0 137E-02 208E-12 623 7.75 953 03 39 1500 15 14.10 4.43E-02 0 208E-02 7.40E-14 563 7.93 97.4 03 40 1650 24 19.65 4.75E-02 0 3.95E02 235E-14 40.4 7.95 97.7 0.9 T onlgnagclM a) gqAge(Ma) Weighted mean age (Ma) Standard error MSWD 94.7 03 933 0.1 333 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 0 0 APPENDIX D: APATITE FISSION TRACK ANALYSIS Apatite fission-track analysis was performed by Dr. Ann Blythe at the University of Southern California. Standard crushing and magnetic and heavy liquid mineral separation processes were used to concentrate apatite samples from mostly granitoid rock material. Apatites were mounted in epoxy and sample surfaces ground and polished. Apatite mounts were etched in 7% HNO3 at 18°C for 22s. An "external detector" (e.g., Naeser, 1979), consisting of low-U (<5 ppb) Brazil Ruby muscovite, was used for each sample. Samples were irradiated in the Cornell University Triga nuclear reactor. Following irradiation, the muscovites were etched in 48% HF at 18 °C for 30 min. Tracks were counted using a lOOx dry lens and 1250x total magnification in crystals with well-etched, clearly visible tracks and sharp polishing scratches. A Kinitek stage and software written by Dumitru (1993) were employed for analyses. Standard and induced track densities were determined on external detectors (geometry factor - 0.5), and fossil track densities were determined on internal mineral surfaces. Ages were calculated using zeta - 320 ± 9 for dosimeter glass SRM 962a (e.g., Hurford and Green, 1983). Results from analyses are given in Table D .l. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table D. 1 Apatite Fission Track Analyses 301 Is I 1**1 * £— u s * H Ilf I i s I O ' d o s No INr* o oo*1e 8 Mo 3 d — — ^O (7) 0.3S’<od (S) ro i d I d 41 4t 41 41w 41 41C M 41 41 41 41 41 N e trt N r- v© » m 00 r». in m «n in «n * m in in in in m >o+t oo r» o* p* f i in d o» 4f 41 41 41 41 r» m — oo oo -• « O N N « m r - oo \© oo f* o *o +■ •n r* ^ — ~ H H H m O' m ^ n w d *n d r* m 41 O*o 41Oi 3 ^ "v oo ? 5 ? S S 2 S I p8sS!g5g3g^«F!fm«R ; = S a ? ' ' R = 5 - s a S S 2 c $ S“ ?S-S?X=SP5oE= n n m S n S m n n n n n n n n ' m rt rt SPSO--C-2SJjPtgN JjN 8 s — N £1 8 mm 00 9) s § a Q s m m m m m e r** m a. ci eu fiL a. a* a. CL a. a. a. CO c/t cn CO CO CO CO CO CO CO Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (3507) (384) (2050) (100) SP-569 20 2.22 81.88 420.00 94 69.2 ±4.7 13.5 ±0.1 1.22 (3553) (262) (1344) (102) SP-820 20 2.12 234.38 921.01 86 86.3 ±4.2 14.4 ±0.1 0.91 (3380) (540) (2122) (105) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table D.l (continued) T n tk Lengths; Length Measurement Bins (uml: Sample# 5-6 6-7 7-8 8-9 9-10 10-11 11-12 12-13 13-14 14-15 15-16 16-17 17-18 Total SP-20 0 0 0 0 1 3 13 31 31 16 8 1 0 105 SP-73 0 0 1 1 0 2 II 20 16 18 3 0 0 72 SP-l33a 0 0 0 0 1 0 7 18 33 21 6 1 0 87 SP-136 0 0 0 0 1 1 2 3 IS 7 0 0 0 29 SP-138 0 0 0 1 0 3 4 3 12 17 5 1 0 45 SP-139 0 0 0 0 t 1 4 6 26 35 29 9 0 111 SP-140 0 0 0 0 0 0 0 1 4 1 1 0 0 7 SP-l40b 0 0 0 0 0 0 0 5 2 3 0 0 0 10 SP-374 0 0 0 0 0 0 0 3 1 0 1 0 0 5 SP-409 0 0 0 1 0 2 II 25 44 16 8 1 0 108 SP-540 0 0 0 0 0 1 1 13 39 23 8 1 0 86 SP-543 0 0 0 0 0 0 4 19 39 30 8 0 0 KM SP-569 0 0 0 0 0 4 3 29 35 22 6 2 1 102 SP-820 0 0 0 0 0 1 0 3 27 48 22 4 0 105 All samples were processed by K. Schmidt and analyzed by A. Blythe at the University of Southern California. Parentheses show number of tracks counted. Ages were calculated using zeta = 320 ± 9 for dosimeter glass SRM 962a (c.g., Hurford and Green, 1983). All ages are population ages, with the conventional method (Green, 1981) used to determine errors on sample ages. The chi-square test estimated the probability that individual grain ages for each sample belong to a single population with Poissonian distribution (Galbraith, 1981;. ‘ Central ages (Galbraith and Laslett, 1993) were used for samples that failed the chi-square test at <5%. 302 303 APPENDIX E: STRAIN ANALYSIS As part of this project a reconnaissance strain analysis was conducted using Rf/Phi technique (Ramsay and Huber, 1983) on 12 samples of volcaniclastic lithologies from the western side of the fan structure and it’s footwall (Fig. E.l). Samples were oriented in the field and three random, mutually perpendicular cuts were made on each sample to produce three faces, the intersections of which were assigned three coordinate axes. On each of the three surfaces the ratio of long to short axes of 30-50 individual clasts (Rf) was measured and the orientation of the long axes was measured relative to sample coordinate axes. Tensor algebraic methods of Shimamoto and Ikeda (1976) were used to calculate average 2D ellipses for each of the three sample surfaces. Three-dimensional fabric ellipsoids were then calculated using two separate methods, those of Shimamoto and Ikeda (1976) and Miller and Oertel (1979), permitting comparison for internal consistency. Finally the calculated fabric ellipses were reoriented to geographical coordinates, yielding the principal fabric axes from each sample. The resulting fabric ellipsoids represent the total fabric in samples. Deriving strain from this total fabric requires the following assumptions: 1) rocks sampled contained randomly oriented clasts before deformation; 2 ) sample size is less than any scale of deformation heterogeneity; and 3) no volume change occurred in the rock during deformation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure E.1. Map of study area showing sample locations. 304 305 The first assumption is typically problematic because most depositional and diagenetic processes impart a fabric to rocks. However this fabric is commonly minor in magnitude, producing ratios « 2:1 between maximum and minimum principal fabric axes (e.g. Paterson and Yu, 1994). Sparse information on strain in Alisitos rocks in the western zone of the PRB support this contention. Johnson et al. (1999b) conducted a strain study of Alisitos host rocks near the Zarza pluton, and one of their samples, located outside of the structural aureole of the pluton, yielded a XZ ratio of 1.27:1 which they interpreted as representative of primary fabrics in the Alisitos Fm. plus very minor deformation. Thus in highly strained rocks such as along the western side of the fan structure the affect of primary fabrics on total fabric ellipsoids is probably minimal. The second assumption concerns the heterogeneity of rocks during deformation which may strongly affect strain analysis. For example if a spaced cleavage is developed within the matrix of a sample then an analysis of strain from clasts may greatly underestimate the total strain at the sample scale. Thus, ideally, clasts and matrix should have been approximately the same ±eology during deformation to produce results that are representative for a given sample. Problems with clast-matrix viscosity contrasts are considered minimal in this study because in most cases cleavage is developed equally in matrix and clasts and is only weakly deflected across matrix-clast contacts. Moreover samples which do show these effects (noted in Table E.1) vary little from samples analyzed from the same structural position in the fan structure, indicating that this problem is m inim al. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 306 Finally, volume change, particularly preferential volume loss, may affect rocks during deformation which typically imparts an apparent flattening strain. Quantifying volume change in rocks requires procedures such as determining mass balance constraints derived from chemical data. These techniques are beyond the scope of this reconnaissance strain study and were not evaluated. However, features such as dissolution cleavage were not observed in samples suggesting that volume loss may not have been significant Results from strain analyses are shown in Table E.1 and Figure E.2. Strain data show good qualitative agreement with fabric observations made in the field. Higher strain intensities correspond with strong ductile fabrics, and constrictional fabric symmetries are associated with strong lineation fabrics. Strain intensity generally increases steadily from locations west of the Rosarito fault where they range from 0.2- 0.3, to 0.5-0.6 within the fault, ~1.1 in the Alisitos sedimentary-rich unit of the fan structure, and reaches a maximum of 1.4 at the structurally highest sample location in the flysch unit (Fig. E.1; E.2). A discontinuity is apparent between samples with the highest strain intensities and the rest of the samples that corresponds with the approximated location of the thrust that places flysch assemblage rocks that have been multiply deformed over younger Alisitos sedimentary-rich rocks that have experienced a shorter deformation history. For the most part apparent strain symmetry follows the trend in strain intensity up the western side of the fan structure. Samples both west of, and within, the Rosarito fault show flattening to plane strain fabrics (Lode's parameter of 0.5 to 0.8). Similar Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 30 7 Table E. 1 Results from strain analyses in the western domain of the fan structure. Sample Number Axial Ratios Elongations Ellipsoids Orientations Distance from X Y Z X Y Z Strain Sym XY X axis Rosarito Magn. metry Diane fault (m) SP 902 2.833 2.015 I 5851 12.74 -44.04 0.751 0.346 1600 W B 135 1.349 1.259 1 13.06 551 -16.17 0521 0538 198 46 46/278 <10 W SP 317g2 1.505 1.406 1 17.22 9 5 3 -22.10 0.310 0.668 358 84 84/101 <10 W SP I37a 1.950 1-538 1 35.21 6.65 -30.65 0.479 0 589 177 80 72/351 0 SP 137b 2.115 1.83 1 34.71 1657 -3652 0 562 0.614 140 66 68/211 0 SP 317e 2.121 1.978 1 3151 22.63 -3759 0587 0.814 135 76 8/137 0 SP 828 4.438 3.941 1 70.97 51.83 •61.48 1.171 0.841 07131 0/071 1600 E SP 801 6.345 3.753 1 12054 30.45 -65.24 1.346 0.432 31144 43/057 1800 E SP 548b 4.507 2.726 1 95.34 18.12 -56.66 1.084 0.332 39 56 49/169 1800 E SP 806 4.285 2.864 1 85.77 24.17 -56.65 1.063 0.446 306 19 18/046 1900 E SP 100 6.195 2.806 1 139.13 8.33 -61.4 1593 0.132 340 35 24/019 2100 E SP 99 6.871 1.533 1 213.44 -30.06 -5458 1.431 -0556 037 57 32/061 2700 E Strain intensity = 1/3 [(e,-ej) + + (ej-e,) ] , where e„ e^, and e, are the principal natural strains (assuming isotropic primary fabrics). Lodes parameter= [2(e2> - e,- ej]/(e, - %). From Hossack (1968). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 308 i 1 s & « & « e 9 « 0 * Strain intensity ■ Lode's parameter • • . • • • • ■ ■ ■ 1 ■■ ■ • ■ A• ■ ■ West East -1 •2000 •1000 0 1000 Distance from Rosarito fault 2000 3000 Figure E.2. Strain intensity and Lode’s parameter plotted against distance from the Rosarito fault Strain intensity generally increases up the west side of the fan structure and apparent strain symmetry become more constrictional. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 30 9 symmetries are apparent across the Alisitos sedimentary-rich unit with some deviations such as sample SP828 which shows more plane strain symmetry. Moving into the flysch unit a rather dramatic change occurs to constrictional fabrics (Lode’s parameter of—0 .6 ). Results from strain analyses in the Alisitos sedimentary-rich unit are quite typical for shear zones in general. Deformation in orogenic regions is commonly assumed to follow a bulk plane, inhomogeneous simple shear strain deformation path (e.g. Ramsay, 1967). A number of studies have confirmed that shear zone deformation is commonly non-coaxial (e.g. Simpson, 1983; Hanmer and Passier, 1992), however, strain measured in many shear zones show apparent flattening strains (e.g. Coward, 1976; Mitra, 1979; O’Hara, 1990). Ideas that have been proposed to reconcile this problem include attributing volume loss a widespread role in shear zone processes, and thus imposing an apparent flattening strain on plane strain deformation. A study designed to test this hypothesis using chemical mass balance constraints in a partially sheared, lithologically homogeneous pluton found that little volume change was apparent in the deformed region (Bailey et al., 1994). These authors speculated that flattening strains in thrust belt tectonics resulted from loading of footwall rocks by successive stacking of thrust sheets. However, this idea implies that the orogen stretches both perpendicular to the margin, as might be expected in foreland propagating thrust belts, as well as parallel to it Thus the orogen would have to stretch lengthwise, a prediction that is unconfirmed. Alternatively primary fabrics, particularly in volcanic rocks such as those measured in this study, may be a more Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 1 0 important factor than we realize. Significant compaction occurs in many volcanic sequences and may produce a primary flattening fabric that is preserved during subsequent plane strain deformation (e.g. Paterson and Yu, 1994). The strongly constrictional fabrics in the flysch sequence that are apparent in the upper part of the western thrust package may result from a longer deformation history that has been inferred for the Alisitos sedimentary rich unit below. One possible explanation for these fabrics is that nearly orthogonally superposed multiple deformations occurred during the structural evolution of this part of the fan structure. If small scale, tight folding was involved in the early deformation history, as inferred from observations at many stations in the field, then old hinge regions may significantly influence the final fabric measured in these rocks, producing an overall apparent constrictional fabric. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

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Asset Metadata

Creator Schmidt, Keegan Lee (author)

Core Title Investigation of arc processes: Relationships among deformation, magmatism, mountain building, and the role of crustal anisotropy in the evolution of the Peninsular Ranges Batholith, Baja California

Degree Doctor of Philosophy

Degree Program Earth Sciences

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag Geology,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-134963

Unique identifier UC11334637

Identifier 3041518.pdf (filename),usctheses-c16-134963 (legacy record id)

Legacy Identifier etd-Schmidt

Dmrecord 134963

Document Type Dissertation

Rights Schmidt, Keegan Lee

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA