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Contact aureole rheology: New constraints from fieldwork in selected Cordilleran aureoles and from numerical modeling

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Contact aureole rheology: New constraints from fieldwork in selected Cordilleran aureoles and from numerical modeling

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Content INFORMATION TO U SER S 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. ProQuest Information and Learning 300 North Zeeb Road, Ann Arbor, Ml 48106-1346 USA 800-521-0600 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. NOTE TO USERS This reproduction is the best copy available. UMI Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CONTACT AUREOLE RHEOLOGY: NEW CONSTRAINTS FROM FIELDWORK IN SELECTED CORDILLERAN AUREOLES AND FROM NUMERICAL MODELING by Markus Albertz 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) August 2004 Copyright 2004 Markus Albertz Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: 3145153 UMI UMI Microform 3145153 Copyright 2004 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. EPIGRAPH Verstandige Leute kannst du irren sehn, in Sachen namlich, die sie nicht verstehn. Johann Wolfgang von Goethe Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS The work presented in this dissertation was funded by the National Science Foundation (grant EAR-0073943 awarded to Scott Paterson), and the following grants in support of graduate student research: two Sigma Xi grants, two Geological Society of America grants, and two USC Department of Earth Sciences grants. I received additional support from the DAAD (German Academic Exchange Service) to help defray the high cost of living in Los Angeles. I begin by thanking those at USC who were more or less directly involved with my work. I thank my advisor, Scott Paterson, for sharing his expertise in Structural Geology, especially structural and mechanical aspects of magma-host rock systems with me. Greg Davis, Charlie Sammis, Jean Morrison, and Julian Domaradzki served on my oral examination committee, and the latter two on my thesis committee. David Okaya taught me the fundamentals of Fortran90 programming. Helge Alsleben, Dominique Richards, Tim Weber, and Richard Twitchet assisted in the field. The office crew (Vardui, Cindy, Barbara, and John) are gratefully acknowledged for their patience and logistical support. I enjoyed many scientific debates and casual discussions during graduate classes and/or field trips with Lawford Anderson, James Dolan, Anne Blythe, Yehuda Ben-Zion, Tom Henyey, Steve Lund, and Sue Owen. I thank the past (Keegan, Gunilla, Michael, Melissa and Paul) and current members (David, Luke, Rita, Vali, Helge, Geoff, and Claire) of the iii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. structural geology group for reading and editing my manuscripts and for their camaraderie. During the early stages of my project, I much enjoyed trail running and waffle breakfasts with Karen and Keegan. Many others shared my enthusiasm for Volleyball in Manhattan Beach and Ultimate Frisbee games behind the Natural History Museum. I fenced many close bouts with Cormac, Laura and Elisabeth at the Beverly Hills Fencers’ Club. Karl gave me countless demanding, challenging, and priceless (yet free...) fencing lessons. I enjoyed many concerts, mountain- biking, and winter hiking trips with Ross. Thank you all for your friendship! I am most grateful to my “substitute family”, the Pohls (Chris, Dem, Yani, Jordan, the late Fergie, Tiger, Rose, Mister Parsley or Velvet, and the goldfish). They made my life in Los Angeles enjoyable and they provided five years of permanent encouragement, nutritious delicacies, an infinite supply of beer and scotch, and the opportunity to adjust to fat-free milk. Mom and Dad visited me twice in Los Angeles. I thank them for coming long ways and sharing their time with me in the U.S. Their upbringing prepared me to succeed with the challenges I have faced so far. And you, Mecki. You stepped into my life when I was expecting it least. I thank you for your enduring love, passion, humor and friendship (and for regular deliveries of good German chocolate;-). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS EPIGRAPH......................................................................... ... ii ACKNOWLEDGEMENTS................................................................................ iii LIST OF TABLES ............................. ix LIST OF FIGURES............................................................................................. x ABSTRACT xiv CHAPTER 1: INTRODUCTION......................................................................1 Processes in aureoles..................................................................................... 3 Selected Cordilleran aureoles........................................................................8 The Sierra Nevada...................................................................................11 Tuolumne Intrusive Suite........................................................................16 Overview of dissertation 20 CHAPTER 2: PARTITIONED DEXTRAL TRANSPRESSIVE AND CONTRACTIONAL STRAIN FIELDS IN THE CRETACEOUS SIERRA NEVADA BATHOLITH, CALIFORNIA.............23 Introduction....................................................................................................23 Geologic setting..............................................................................................26 Overview..................................................................................................27 Strain fields.............................................................................................. .30 Shear zones...............................................................................................33 Rosy Finch shear zone....................................................................... 36 Gem Lake shear zone........................................................................ 36 Bench Canyon shear zone..................................................................37 Quartz Mountain shear zone.............................................................. .38 Kaiser Peak shear zone.......................................................................38 Courtright shear zone.............................................................. 39 (Proto-) Kern Canyon fault................................................................39 Cryptic shear zones.............................................................................40 Summary............................................................................................42 Field observations......................................................................................... .43 Stratigraphy............................................................................................. .43 Magmatism.............................................................................................. .45 Structure.................................................................................................. .47 Planar fabrics.......................................................................................47 Linear fabrics..................................................................................... 51 Shear zones 51 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Timing Discussion...... Conclusions 54 57 60 CHAPTER 3: NEW CONSTRAINTS ON THE LOCATION, TIMING, AND CUMULATIVE DISPLACEMENT ASSOCIATED WITH THE ENIGMATIC MOJAVE-SNOW LAKE FAULT: IMPLICATIONS FOR MESOZOIC INTRABATHOLITHIC STRIKE-SLIP DISPLACEMENT IN THE SIERRA NEVADA, CALIFORNIA..................................................62 Introduction................................................................................................... 62 Mojave-Snow Lake fault (MSF).....................................................................64 Location......................................................................................................... 68 Timing.............................................................................................................68 Piute Meadow pendant................................................................................... 70 Discussion...................................................................................................... 73 Conclusions 77 CHAPTER 4: MATERIAL TRANSFER, DEFORMATION MECHANISMS AND RHEOLOGY IN PLUTON AUREOLES: IMPLICATIONS FOR MELT-ASSISTED DEFORMATION.......................79 Introduction....................................................................................................79 Material transfer.............................................................................................82 Mount Stuart Batholith (MSB).................................................................82 Tuolumne Intrusive Suite (TIS)................................................................84 Finite strain analysis Microscopic observations Methods.......................................... Mount Stuart batholith (MSB).. Tuolumne Intrusive Suite (TIS) Ductile deformation.................................................. Candelaria Formation, conglomerate of Cooney Results............................................ Mount Stuart batholith (MSB).. Tuolumne Intrusive Suite (TIS) Stoping................... Ductile deformation 85 86 92 93 93 95 98 98 99 .111 112 Lake and rhyolitic ash-flow tuff of Saddlebag Lake. 112 117 118 .119 122 Andesitic metavolcanics... Rhyodacitic ash-flow tuff Horse Canyon Sequence... Brittle deformation Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Interpretation of microstructural observations...............................................123 Discussion...................................................................................................... 126 The effect of melt on deformation........................................................... 126 Material transfer during pluton emplacement.......................................... 133 Conclusions 135 CHAPTER 5: A NUMERICAL TEST FOR RATES OF DISLOCATION CREEP DURING SPHERICAL PLUTON GROWTH: IMPLICATIONS FOR RAPID MATERIAL TRANSFER IN AUREOLES...........................................................................137 Introduction....................................................................................................137 Model description...........................................................................................141 Results............................................................................................................ 148 Discussion...................................................................................................... 149 Conclusions 152 CHAPTER 6: FAST STRAIN RATES DURING PLUTON EMPLACEMENT: MAGMATICALLY FOLDED LEUCOCRATIC DIKES IN AUREOLES OF THE MOUNT STUART BATHOLITH, WASHINGTON AND THE TUOLUMNE INTRUSIVE SUITE, CALIFORNIA............................................ 154 Introduction....................................................................................................154 Displacement in aureoles during emplacement..............................................158 Field observations.......................................................................................... 159 Mount Stuart Batholith (MSB), Washington...........................................159 Tuolumne Intrusive Suite (TIS), California.............................................162 Strain.............................................................................................................. 167 Microstructural observations..........................................................................172 Leucocratic dikes..................................................................................... 174 Host rocks.................................................................................................180 Interpretation of microstructural observations...............................................182 Cooling times..................................................................................................184 Strain rates......................................................................................................186 Discussion...................................................................................................... 191 Strain rates................................................................................................191 Rheological implications..........................................................................196 Conclusions 201 CHAPTER 7: CONCLUSIONS......................................................................... 203 Strain partitioning.......................................................................................... .204 Arc-parallel strike-slip displacement............................................................ .205 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Effect of melt on crustal rheology..................................................................205 Interplay of brittle and ductile material transfer processes during pluton emplacement.................................................................................206 Numerical constraints on dislocation creep rates during pluton emplacement............................................................................................ 207 Field criteria for fast syn-emplacement deformation.....................................207 Growth of crustal magma chambers..............................................................208 BIBLIOGRAPHY . 209 APPENDIX A: RESULTS OF STRAIN ANALYSIS........................................232 Agebraic Shimamoto 3D analysis.................................................................235 Fry analysis.................................................................................................... 237 Primary fabrics- corrected strain ellipsoids................................................... 238 APPENDIX B: COMPUTER CODES............................................................... 239 Dislocation creep modeling........................................................................... 239 Cooling times of leucocratic dikes.................................................................252 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES Table 1.1. Geochronology of the Tuolumne Intrusive Suite.............................. 18 Table 5.1. Modeling parameters..........................................................................144 Table A.l. Results of Shimamoto 3D strain analysis.........................................235 Table A.2. Results of Fry analysis......................................................................237 Table A.3. Results of corrected strain analysis.................................................. 238 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES Figure 1.1. Schematic map showing major Mesozoic batholiths of western North America................................................................................ 7 Figure 1.2. Geologic map of the Mount Stuart batholith, Washington............ 10 Figure 1.3. Updated and highly generalized model for the crustal evolution of the Sierra Nevada.....................................................................14 Figure 1.4. Geologic map of the Tuolumne Intrusive Suite, California............17 Figure 2.1. Schematic map showing localities of host rock pendants in the Tuolumne Intrusive Suite, California..................................................25 Figure 2.2. Geologic map of the Sierra Nevada batholith.................................28 Figure 2.3. Schematic map of the Sierra Nevada batholith showing Cretaceous shear zones associated with dip-slip displacement....................34 Figure 2.4. Schematic map of the Sierra Nevada batholith showing Cretaceous shear zones associated with strike-slip displacement................ .35 Figure 2.5. Geologic map of the Piute Meadow pendant...................................48 Figure 2.6. Geologic map of the Saddlebag Lake pendant................................49 Figure 2.7. Lineations in the Saddlebag Lake pendant..................................... 52 Figure 2.8. Horizontal shear senses at the contact to magmatic rocks in the Saddlebag Lake pendant.....................................................................55 Figure 2.9. “Chocolate tablet” boudinage at the contact between the Tuolumne Intrusive Suite and host rock of the Saddlebag Lake pendant.................................................................................................56 Figure 3.1. Schematic geologic map of the Sierra Nevada showing pinpoints and locations of correlative units..................................................63 Figure 3.2. Geologic map of the Piute Meadow pendant showing key locations in the Lake Harriet and Bond Pass plutons................................... 66 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.3. Magmatically folded dikes of Cathedral Peak granodiorite 72 Figure 3.4. Diagram showing duration of dextral slip versus cumulative displacement for slip rates of 1.5 and 3.5 cm/year.........................................74 Figure 3.5. Schematic geologic map showing arc-parallel shear zones in the Sierra Nevada with a dextral component of displacement.................. 76 Figure 4.1. Geologic map of the southeastern aureole of the Mount Stuart batholith, Washington..............................................................83 Figure 4.2. Photograph showing outcrop-scale truncation of pre-Tuolumne stratigraphy and structure....................................................... 87 Figure 4.3. Schematic map showing hypothetical finite strain ellipsoids at northern and eastern margin of the Tuolumne Intrusive Suite................................................................................................89 Figure 4.4. Stereoplot of host rock stretching lineations in the Piute Meadow and Saddlebag Lake pendants........................................................ 90 Figure 4.5. Geologic map of Saddlebag Lake pendant showing sample locations along two margin-perpendicular transects..................................... .96 Figure 4.6. Schematic geologic map showing the distribution of yz-ellipses of the strain ellipsoids in the Piute Meadow pendant.................100 Figure 4.7. Schematic geologic map showing the distribution of yz-ellipses of the strain ellipsoids in the Saddlebag Lake pendant............... 101 Figure 4.8. Flinn diagram of strain data from the Saddlebag Lake pendant.......................................................................................................... 103 Figure 4.9. Flinn diagram of corrected strain data from the Saddlebag Lake pendant.................................................................................................105 Figure 4.10. Logarithmic Flinn diagram showing possible amounts of volume loss....................................................................................................106 Figure 4.11. Relationship between viscosity contrast and position in the Flinn diagram........................................................................................... 107 Figure 4.12. Spatial distribution of K-values along margin-parallel transects A and B 109 XI Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.13. Spatial distribution of z-axis shortening along margin-perpendicular transects A and B.........................................................110 Figure 4.14. Photomicrographs of host rocks of the Saddlebag Lake pendant.................................................................................................. 114 Figure 4.15. Deformation mechanism map of the Saddlebag Lake pendant............................................................................................................124 Figure 4.16. Diagram showing a temperature-induced transition in deformation mechanisms in the Saddlebag Lake pendant............................. 128 Figure 5.1. Effective viscosity versus strain rate for differential stress values of 0.01 to 100 MPa.................................................................... 140 Figure 5.2. Model geometry of spherical pluton growth.................................... 142 Figure 5.3. Results of dislocation creep modeling showing stress, temperature and strain rate as functions of time and position in the aureole.............................................................................................................147 Figure 6.1. Geologic map of the Mount Stuart batholith, Washington......... 156 Figure 6.2. Geologic map of the Tuolumne Intrusive Suite, California.........157 Figure 6.3. Schematic illustration of finite strain analysis on folded leucocratic dikes............................................................................................. 169 Figure 6.4. Photographs of tightly to isoclinally folded leucocratic dikes showing percent shortening associated with folding.............................171 Figure 6.5. Photomicrographs of leucocratic dikes and host rocks of the Mount Stuart batholith and the Tuolumne Intrusive Suite...................... 176 Figure 6.6. Cooling times for leucocratic dikes.............................................. 188 Figure 6.7. Strain rates for folding of leucocratic dikes associated with ca. 80% aureole shortening....................................................................190 Figure A.l. Map of the Piute Meadow pendant showing sample locations..........................................................................................................233 Figure A.2. Map of the Saddlebag Lake pendant showing sample locations 234 > Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Plate 1. Geologic map of Piute Meadow pendant.............................(in back pocket) Plate 2. Geologic map of Saddlebag Lake pendant..........................(in back pocket) xiii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT Detailed field work in spectacular exposures of the Tuolumne Intrusive Suite and its host rock pendants, including the Piute Meadow and Saddlebag Lake pendants suggests that voluminous Late Cretaceous magmatism occurred contemporaneously with partitioned regional contraction and dextral transpression. Preliminary field work in additional pendants, including the Benson Lake, May Lake pendants, and host rocks of the Soldier Lake and Green Lake plutons support these conclusions. These findings are compatible with previous studies which suggest that increasingly oblique convergence between the Farallon and North American plates resulted in partitioning of the resulting deformation. However, models that suggest decoupling of deformation into parallel reverse and strike-slip faulting cannot be supported. Insufficient magnitudes of arc-parallel strike-slip faulting are further supported by lacking strike-slip structures in the Piute Meadow pendant. This continually exposed pendant occupies a key position between shallow and deep water deposits of the Snow Lake and Saddlebag Lake pendants, respectively, which have been proposed to be tectonically juxtaposed along the speculative Mojave-Snow Lake fault. However, considering the Late Cretaceous plate tectonic framework, differential strike-slip displacement of more than ca. 200 km is unlikely to have occurred along the Continental margin of North America, even if large uncertainties are accounted for. Finite strain analysis and microstructural observations along margin- perpendicular transects in the Saddlebag Lake pendant indicate that magma xiv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. emplacement has a fundamental effect on crustal rheology. The observations show that dramatic rheologic effects are unlikely to occur in the dislocation creep regime. However, the onset of melt-assisted granular flow and diffusion creep coincides with a striking increase in finite regional strain, indicating spectacular rheological weakening. Strain analysis on magmatically folded dikes in the aureoles of the Mount Stuart batholith and the Tuolumne Intrusive suite combined with thermal modeling suggest that strain rates associated with this folding can range from 10'2 to 10'lj s'1 . Numerical modeling of dislocation creep during spherical pluton expansion and associated host rock shortening yields maximum strain rates of 10'1 1 s'1 , implying that melt-assisted granular flow and microfracturing are plausible deformation mechanisms that could accommodate extremely fast strain rates in pluton aureoles. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 1: INTRODUCTION Rheology is generally defined as the mechanical behavior of materials and the science of deformation and flow of matter (Bames et al., 1989). More specifically, the science of rheology addresses the relationship of stress and strain in solids and the relationship between stress and strain rate in fluids in motion (Turcotte and Schubert, 2002). In the geologic context of the dissertation presented here, rheology deals with complex mechanical behavior of rocks during deformation under varying pressure and temperature conditions associated with magma emplacement into the continental crust. Contact aureoles are found in pluton environments and they are those portions of the rocks surrounding an intrusion that are both thermally and structurally altered due to heat transfer and deformation related to magma emplacement, respectively. Aureoles are ideal natural laboratories, because pronounced gradients in stress, temperature, and strain rate are encountered, allowing thorough characterization of Theologically critical parameters under realistic dynamic conditions. Understanding deformation of rocks under varying pressure and temperature conditions is of fundamental importance because the tectonic evolution of the Earth (and other planets) ultimately is controlled by mechanical processes that take place at all scales, from the grain scale to the outcrop scale, and from outer crustal layers to the deep mantle. For instance, the effect of grain scale processes on crustal rheology is emphasized by several contributions which address consequences of lower crustal 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. flow for the development of crustal thickness contrasts (McKenzie et al., 2000), grain size-dependant strain localization into ductile shear zones in mono- and polymineralic rocks (Handy, 1989), the effects of assuming a Newtonian or a power- law rheology on continental deformation (England and McKenzie, 1982) and tectonic surges due to magma emplacement into the crust (Hollister and Crawford, 1986). Investigations into the rheology of rocks are typically carried out under controlled deformation conditions in the laboratory to explore mechanical end- member behavior of rocks. For example, numerous experiments have been conducted which resulted in large amounts of data that describe brittle failure and flow of rocks (Byerlee, 1968; Brace and Kohlstedt, 1980; Etheridge, 1983; Hirth and Tullis, 1989), ductile flow of rocks (Luan and Paterson, 1992; Hirth and Tullis, 1992; Gleason and Tullis, 1995), and transitional regimes in between (Hirth and Tullis, 1994). Stimulated by the notion that the presence or absence of fluids might exert a strong influence on the rheology of the crust, more recent studies focused on fundamental relationships between aqueous fluids and deformation (Tullis and Yund, 1989; Hirth and Kohlstedt, 1996) as well as partial melting and deformation (Rutter and Neumann, 1995; Connolly et al., 1997; Mecklenburgh and Rutter, 2003). Integration of laboratory deformation experiments, theoretical considerations on micro- and macrophysical processes, microstructural and outcrop-scale observations have become more common but at the same time enormous specialization in each discipline and broadening of the field (De Meer et al., 2002) constitutes a great o Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. challenge for modem rheological studies. Hence, in addition to enhancing dialogue between researchers from different disciplines, rheological studies should combine the strengths of different approaches in an interdisciplinary and comprehensive sense. Beside the occurrence of numerous processes during magma emplacement in the crust, aureoles are characterized by pronounced spatial and temporal gradients rendering them ideal locations for studying the rheology of rocks under a range of different pressure and temperature conditions. Consequently, aureoles provide unique opportunities to decipher rheological constraints from the preserved microstructures and the inferred deformation mechanisms. On the following pages of this chapter I introduce typical processes that occur in contact aureoles. Next I briefly summarize some key aspects of two Cordilleran aureoles that were selected for detailed field studies, namely the aureoles of the Mount Stuart batholith, Washington and the Tuolumne Intrusive Suite, California. I conclude this chapter with an overview of my dissertation. Processes in contact aureoles The most obvious process that distinguishes aureoles from other geologic settings is the transfer of heat from intrusive heat sources into the surrounding rocks. Heat transfer occurs mostly by conduction and convection. Powerful numerical solutions are available for multidimensional modeling of heat conduction associated Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. with cooling intrusives (e.g. Furlong et al., 1991, and references therein) and they can account for quite complex effects, such as transient geotherms, intrusive geometry, latent heat, and multiple intrusive events. In highly permeable rocks, fluid convection dominates heat transport. Fluid convection and the nature of fluid flow in aureoles are controlled by fluid pressure and the amount of porosity as well as the degree of permeability (e.g. Spear, 1993). Ultimately, the chemical effects of fluid flow can alter the chemistry of aureoles and induce metasomatism (Furlong et al., 1991, and references therein). An entire group of important process closely related to heat transfer in aureoles, including neocrystallization, recrystallization, and anatexis occur during contact metamorphism. Neocrystallization results from metamorphic reactions driven by changes in Gibbs free energy of reactions (Kerrick, 1991). The significance of neocrystallization lies in the generation of facies-diagnostic mineral assemblages which are utilized to constrain pressure and temperature conditions of a given area of investigation. Recrystallization usually involves a decrease or increase in grain size and it is associated with migration and modification of grain boundaries without chemical changes (e.g. Passchier and Trouw, 1996). Anatexis is the process of partial melting of rocks in high-grade contact aureoles. It can have a critical influence on the physical and chemical evolution of contact aureoles because anatectic melts are transient sinks for aqueous volatiles, resulting in a reduction of water activity in the solid residual material (Kerrick, 1991). 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Aureoles also record important information with regards to unraveling pluton emplacement processes. In a milestone study considering deformation in aureoles, Buddington (1959) provided a comprehensive survey of granitic plutons in North America. He summarized the emplacement characteristics according to a threefold depth zonation, including (with increasing depth) an epizone, mesozone, and catazone. Plutons of the epizone reveal mostly dicordant relationships with their host rocks. The mesozone is marked by more complex characteristics between plutons and host rocks, including both discordant and concordant relationships. Plutons of the catazone generally show concordant relationships with their host rocks. These associations of emplacement depth, temperature, and deformation characteristics indicate that multiple material transfer processes (MTPs) operate in aureoles to accommodate local space for the intruding magma. In an attempt to determine the relative importance of various MTPs in aureoles of different crustal depths, Paterson et al. (1996) supported Buddington’s (1959) study. The most commonly occurring MTPs include: roof doming, stoping, ductile flow, lateral translation, and assimilation. Paterson et al. (1996) concluded that vertically, laterally and temporally changing host rock MTPs operate during the growth of magma chambers, and in particular downward displacement in narrow aureoles constitutes an exchange process at the scale of the Earth’s crust. Ongoing studies in areas not described in this dissertation by researchers involved in some aspects of my research indicate that the simultaneous operation of various grain-scale deformation mechanisms in aureoles is a very common 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. phenomenon during magma emplacement. For instance, Scott Paterson interpreted discordant truncation of stratigraphy and of emplacement-related ductile structures in many aureoles as evidence for stoping. In addition, Scott Johnson found microstructural evidence for fracture-induced development of ductile microshear zones in the San Jose pluton, Mexico (Johnson et al., in press). The North American Cordillera is by marked by substantial magmatism and consequently the formation of contact aureoles, most of which occurred during Mesozoic times. The igneous rocks of intermediate to silicic compositions in the western United States have been interpreted as the result of subduction of several Pacific plates beneath the western margin of North America (Lipman, 1992). Major North American magmatic arcs associated with Mesozoic subduction and magmatism include the Alaska-Aleutian Range, the Coast Range, the Cascades Core, and the Sierra Nevada (Fig. 1.1). I selected two Cordilleran aureoles for detailed geologic mapping and strain analysis. Using as selection criteria excellent exposure conditions of both igneous and host rocks, logistical support through collaborators and the amount of previous work performed, I favored the Mount Stuart batholith, North Cascades, Washington and the Tuolumne Intrusive Suite, Sierra Nevada, California for my field campaigns. In addition, the Tuolumne Intrusive Suite has been viewed as the type intrusive suite of a magmatic arc, and thus some aspects of my work may be applicable to other intrusive suites around the world. In the following sections, I briefly describe the general geologic settings of the North Cascades and the Sierra Nevada. Next, I 6 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 500 km Study A reas CANADA Alaska-Aleutian Range Coast Range UNITED STATES Sierra Nevada Peninsular Ranges MEXICO Figure 1.1. Schematic map showing major Mesozoic batholiths of western North America. Arrows point to study areas in the Cascades, Washington, and the Sierra Nevada, California. 7 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. summarize some key aspects of the Mount Stuart batholith and the Tuolumne Intrusive Suite. Due to the amount and type of funding available I conducted most of my work in the Tuolumne Intrusive Suite and consequently, more material relevant to the Tuolumne Intrusive Suite is presented here. Selected Cordilleran aureoles The North Cascades The North Cascades form the southernmost exposed portion of the Coast Range batholith (Paterson and Miller, 1998) and together they preserve the mid- to deep- crustal structural, magmatic and metamorphic record of Cretaceous and Paleogene continental-margin tectonics in the northwestern Cordillera (e.g. Monger et al., 1982; Tabor et al., 1989). The North Cascades experienced intense Cretaceous deformation and contemporaneous magmatism (McGroder, 1991; Umhoefer and Miller, 1996) associated with a component of orogen-parallel stretching during shortening as indicated by subhorizontal extension lineations (Brown and Talbot, 1989). The Cascades Core is bounded on the west and northeast by major high-angle Tertiary faults and on the south by a middle Cretaceous thrust (Miller and Paterson, 2001, and references therein). Assemblage of tectonostratigraphic terranes occurred before Late Cretaceous plutonism and metamorphism (Tabor, 1989). 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The Mount Stuart batholith is the largest pluton emplaced in the Cascades and the geologic setting is ideal for quantifying the amount of host rock marker deflection during emplacement. Mount Stuart batholith (MSB- ) The MSB can be geometrically subdivided into three parts, including a hook shaped region and a mushroom-shaped part that are connected by a sill-like body (Fig. 1.2). The geometrical subdivision is partly reflected by the composition of the intrusive rocks. Granodiorite forms the bulk of the hook-shaped region, the northern domain of the mushroom-shaped part is composed of diorite, and the remaining areas consist of biotite-homblende tonalite (e.g. Albertz et al., in press). Recent U/Pb geochronology on zircon indicates that the MSB was constructed over a ca. 5.4 million years long period from ca. 96.3 to 90.9 Ma (Matzel et al., 2002). The country rock into which the MSB intruded consists of two main units. The southeastern host rock consists of the Ingalls complex, a Late Jurassic Ophiolite complex (e.g. Southwick, 1974; Metzger et al., 2002; and Miller, 1985) that includes peridotites, serpentinites, gabbros, as well as various metasedimentary rocks. These rocks were juxtaposed along numerous Late Jurassic faults in or adjacent to a fracture zone in an ocean or marginal basin (Miller and Mogk, 1987). The northwestern part of the host rocks is composed of the Chiwaukum Schist which has experienced multiple periods of deformation and metamorphism. The contacts 9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 122°W 10 km Figure 1.2. Geologic map of the Mount Stuart batholith (MSB), Washington. G= granodiorite. D= diorite. T= tonalite. 10 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. between the intrusive rocks and the country rocks are generally steep in the southeastern and central areas and they dip moderately to steeply away from the pluton at its northwestern end (Paterson and Miller, 1994). The MSB occupies a key position in the debate about large-scale regional translation associated with the Baja-BC hypothesis (e.g. Beck and Noson, 1972 and many subsequent studies, e.g. Housen et al., 2002). However, this debate is not of primary importance for this dissertation. Differential translation within the aureole of the MSB by syn-emplacement reverse slip of the Tumwater Mountain shear zone (Miller and Paterson, 1992) is more pertinent but because only a small portion of the aureole was affected, the significance of translation with regards to accommodating emplacement is only minor. The Sierra Nevada The Sierra Nevada earns recognition globally for its spectacular exposures of a Phanerozoic composite batholitic belt (Saleeby, 1999). The Sierra Nevada extends for much of the length of the state of California. It is ca. 720 km long, reaching from the Mojave Desert on the south to the vicinity of Mount Lassen on the north, and is about 60 to 100 km wide. Mount Whitney, in the southern part of the range, attains a height of almost 14,500 feet and is the highest point in the conterminous United States. The arc formed as a product of the prolonged subduction of ocean floor beneath the southwestern edge of the North America plate (Dickinson, 1981). 11 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Flanking it for most of its length, on the west side, is the great Valley Group and the Franciscan Complex representing associated forearc-basin deposits and the subduction complex (Cowan and Bruhn, 1992), respectively. On the east it is bordered by the vast Basin and Range Province, composed of dozens of small north- trending mountain ranges separated by fault-bounded valleys, stretching across Nevada into Utah (Moore, 2000). Bateman and Wahrhaftig’s (1966) classic paper was the first effort to synthesize a broad range of topics, such as the nature and petrogenesis of the batholith, relation of the batholith to its metamorphic host rocks, and the modem physiography of the range. Their concepts are extraordinarily well developed, although their work was published just prior to the inception of the plate tectonic paradigm. In the past three decades, however, a tremendous amount of research has been done on the Sierra Nevada batholith and its surrounding rocks resulting in some modifications of Bateman and Wahrhaftig’s milestone paper. The Paleozoic and Mesozoic strata of the Sierra Nevada have been complexly folded and faulted, and bedding, cleavage, and lineations, including fold axes are commonly steep (Bateman and Wahrhaftig, 1966). Subsequent work (Bateman, 1985; Saleeby et al., 1986; Merguerian and Schweickert, 1987; and Sharp, 1988) established a westward younging sense from the Shoo Fly complex to the Calaveras complex within the Foothills belt and explained this pattern by ensrmatic terrane- accretion processes. Saleeby (1992) and Saleeby et al. (1992) suggest that the Sierra Nevada metamoiphic host rocks have undergone multiple phases of sinistral and 12 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. dextral translations and truncations. In the Western foothills, multiple generations of structures are interpreted as having developed in response to at least three periods of Paleozoic, Early to Middle Jurassic, and Late Jurassic strike-slip displacements, oblique convergence, and transposition along thrust faults (Saleeby and Sharp, 1978; Wolf and Saleeby, 1995). Small remnants of the Roberts Mountains and Golconda allochthons are recognized in pendants of the east-central Sierra (Schweickert and Lahren, 1987), indicating that the mid-Paleozoic and Permian-Triassic Antler and Sonoma orogenies affected the eastern host rock domain of the Sierra Nevada. Saleeby (1999) updated Bateman and Wahrhaftig’s (1966) model for crustal evolution of Sierra Nevada (Fig. 1.3) incorporating new structural, petrological and geochemical data. Late Paleozoic time was characterized by transform truncation of the Cordilleran miogeocline along the axial region of the Sierra Nevada and the initial accretion of ensimatic or Panthalassan crust/mantle sequences to the western wall rock domain. The first-order consequence was the establishment of an ocean- continental lithospheric boundary in what was to become the axial region of the pre- batholithic framework. Early Mesozoic time was characterized by arc magmatism and crustal imbrication. A two-sided orogen is pictured with west-verging structures along the west side of the Sierra and east-verging structures along the east side. Mid-Cretaceous time was marked by voluminous subduction zone-related magmatism that migrated eastward with time whereby progressively more continental crustal components were encountered by the ascending mafic magmas, resulting in melting of the crustal components and the co-mingling and mixing of the 13 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Late Paleozoic Paleozoic ophiolitic R o b c n s M ? 5 . & GolconPa t~ C f melange ) allccnthcns --------------------------y ■ - — rA miooeoclino •‘ .■ .'.•V . o cean ic crust Panthalassa oceanic li^iosphere AO km w n t i m j h / _ V.' V ] i-'rotcrozo7 ' ^ > _ “ • \ 1 I i r > * :c upper tOO km transform truncation zone Early Mesozoic strike-siigiauit tectonic accretion of oceanic terrenes / ' arc very fctte direct 1-0 5 evidence* remaining cfsubcucticn nonets) sialic crust / l t t e continental mantle oceanic crust b acyj^c ^ thiusbng -n r»vanabte h crustal assimiaton subducton Mid-Cretaceous svnbathoiiSiic ductile ^ - h ~,ve.ra . ... Great Valley forearc Sasin shear zone igrunsbntes ocean Av / “ 7 0 V .'A / I unknown oceanic ; crus: subduction r r a n c is c a n Latest Cretaceous ^o cean incision ot major drainages begins m ot ot plutons ^ A ‘ <i ' f S ^ rr^ fo w an C : "t . • , T , / ’ .lifethruss. ' r . - large-scale return ‘ raItjngi „jxjng j ^ y fractionation C u f, © P subeuction zone magmatism magmatism culminates at -8a U a tower N. American crust -25 -50 km equilibrati bathclithic ectogite Franciscan subduction ■.vestcrn Foothills Neogene Great Valiev Sirira crest Basin S Range -hi) kvr. Ceiarmr.otec — founaenng e c io g itc batnolith root duccle lower crust / asthencspltor? Figure 1.3. Updated and highly generalized model for the crustal evolution of the Sierra Nevada (from Saleeby, 1999). 14 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ascending mafic magmas and the crustal melt products. Batholithic magmatism ceased in Late Cretaceous time possibly due to a change to sub-horizontal subduction (Dickinson, 2000). Furthermore, this time period experienced cooling of the crustal column and the equilibration of the batholithic mafic keel or root in the eclogite facies (Ducea and Saleeby, 1998). Interpretation of recent seismic refraction studies and petrological studies on mantle xenoliths indicate that between mid- Miocene and Pliocene-Pleistocene time, the mafic root and its lithospheric underpinnings were replaced by asthenospheric mantle (Ducea and Saleeby, 1996, 1998). During Neogene time, regional extension and presumably the related ascent of asthenospheric mantle are thought to have instigated the delamination of the eclogite facies mafic root and it’s foundering into the deeper mantle. Hamilton (1969) first applied the emerging plate tectonics paradigm to the Sierra Nevada batholith and suggested that subduction-zone magmatism is the prime source of the batholithic magmas. Isotopic variations in Cordilleran-type batholiths has been interpreted as either reflecting the nature of deep-crustal and upper mantle source regimes (Kistler and Peterman, 1973; Silver and Chappel, 1988; Hildreth and Moorbath, 1988; Ague and Brimhall, 1988), or as reflecting higher level assimilation and fractional crystallization (DePaolo, 1981; Saleeby, 1990). Saleeby (1999) interpreted the regional isotopic data to identify deep Proterozoic sialic basement, depleted mantle, and supracrustal components in the source region of the batholith. Two possible mechanisms appear reasonable for the introduction of these supracrustal components into the deep magmatic systems: (1 ) substantial crustal 15 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. thickening by the end of the Jurassic and isostatic equilibrium may have introduced originally shallower rocks into deep crustal melting levels (Saleeby, 1999), or (2) the downward flow and displacement pattern of pendant rocks and their adjacent silicic metavolcanic units is suggested to represent return flow dynamically linked to magma generation and ascent, resulting in a scenario where fertile pelite-psammite units were transported to great depths and partially to completely melted (Saleeby, 1990). The Tuolumne Intrusive Suite is viewed as the type intrusive suite of the Sierra Nevada and many of the aspects addressed in this dissertation, including determining finite strains, strain rates and deformation mechanisms were investigated in this plutonic complex. Tuolumne Intrusive Suite (TISl The ca. 2000 km2 large (map area), horizontally elongate TIS (Fig. 1.4) is considered the type intrusive suite in the Sierra Nevada (e.g. Bateman and Chapell, 1979; Bateman, 1992). Field data suggest that the suite was emplaced in a series of pulses with each successive pulse displacing older units. The TIS reveals normal zoning including an outer mafic phase (the Glen Aulin and Glacier Point tonalites to the west and Kuna Krest granodiorite, Kkc to the east), and inner, more felsic phases (Half Dome granodiorite, Khd, Cathedreal Peak granodiorite, Kcp, and Johnson porphyry, Kjp; see Table 1.1 for acronyms). A geochemical study of mafic enclaves 16 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Kcp TIS LA Kkc Khd 10 km Figure 1.4. Geologic map of the Tuolumne Intrusive Suite (TIS), California. Kkc= Kuna Crest granodiorite. Khd= Half Dome granodiorite. Kcp= Cathedral Peak granodiorite. Kjp= Johnson Granite porphyry. LA= Los Angeles. 17 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (Reid et al., 1983) and a study using Sr and Nd isotopes (Kistler et al., 1986) suggest that different internal phases of the TIS were not derived from a single parent magma by closed system fractionation; instead, intrusion of several separately evolved magma pulses were required to yield the zoning of the TIS. From available radiometric U-Pb zircon ages for all of these pulses a best minimum and best maximum range of ages can be estimated which brackets the time during which intrusion took place (Table 1.1). Table 1.1. Geochronology (* = Kistler and Fleck, 1994; ** = Fleck et al., 1996; *** = Coleman and Glazner, 1997; f = Paterson, cross-cutting relationships, in progress). TIS unit Ages of short range Ages of long range (Ma) (Ma) Kkc (Kuna Crest granodiorite) 92t 9 4 *** Khc (Half Dome granodiorite) 92 -j- j*** 89* Kcp (Cathedral Peak granodiorite) 8 8 .1 *** 8 8 .1 *** Kjp (Johnson Porphyry granite) 85.4*** g9** Duration of TIS construction 6 . 6 m.y. 1 2 m.y. The host rock to the TIS consists of the 102 Ma El Capitan granite and other plutons in the west along with several septae of Jurassic metasedimentary rocks (Bateman, 1992). Host rock is typically truncated along the margins with an absence of strong TIS-related ductile fabrics. Plutons and Triassic and Jurassic metavolcanic and metasedimentary rocks host the TIS to the north where host rock contacts and fabrics strike N to NW and generally show discordant relations on published maps. 18 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Recent mapping shows, however, that margin-parallel TIS-related ductile strain exists locally. Foliation patterns along the southern margin are discordant in numerous places, but locally swing toward parallelism with the contact (Bateman, 1992). The eastern margin exposes a range of metasedimentary and metavolcanic rocks including lower Paleozoic to Triassic eugeoclinal rocks of the Antler and Golconda allochthons, and Triassic to Jurassic arc rocks with associated plutons (e.g. Schweickert and Lahren, 1993,1999; Greene and Schweickert, 1995; Stevens and Greene, 1999). These rocks record a complex history of Paleozoic and Triassic thrusting and folding as well as Late Jurassic to Early Cretaceous contraction resulting in west-dipping thrust faults, folds, cleavages, and down-dip lineations. These structures are locally truncated as well as overprinted by TIS-related strain. Two scenarios of regional deformation during Mid-Cretaceous magmatism have been suggested: Tobisch et al. (1995) proposed a model of asthenospheric comer flow resulting in variations in deformation fields ranging from contractional over neutral to weakly extensional deformation. Greene and Schweickert (1995) and Tikoff and Greene (1997) considered steep syn-crystallization shear zones along the eastern margin of the TIS part of a regional transpressional Sierra Crest shear system. Rates of convergence between the Farallon plate and North America were constrained by Engebretson et al. (1985), who estimated the convergence velocity normal to the trench axis to have been ca. 100 km / m.y. between 100 and 85 Ma. The depths of two crustal boundaries constrain the rheological layering of the Cretaceous Sierran crust. The depth of the Moho indicates the thickness of the crust 19 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and the brittle-ductile transition defines the boundary between the viscoelastic lower and the elastic upper crust. The depth of the Moho can be estimated from pressure determinations of eclogite facies xenoliths in the San Joaquin volcanic field, Central Sierra Nevada batholith (P ~ 25 kbar, Ducea and Saleeby, 1998). Ducea (2001) suggested that the Mesozoic California arc was at least 70 km thick. Structures in the Cretaceous Sierra Nevada arc, which includes the Cretaceous Sierra Nevada, are compatible with a depth of the brittle-ductile transition of ca. 7.5 km. Shallow emplacement depths of no more than ca. 10 km have been inferred from weakly metamorphosed volcanic units (e.g. Bateman, 1992) and Al-in-homblende geobarometry (Ague and Brimhall, 1988). More recent constraints on emplacement depths of the TIS are provided by Webber et al. (2001), who performed hornblende barometry on pegmatite samples from the Kuna Crest granodiorite. Their study yields crystallization pressures ranging from 2.4-2.9 kbar which indicate presently exposed emplacement depths of 7.2-8.7 km. Overview of dissertation My dissertation forms a multidisciplinary approach to provide modem quantitative constraints on aureole rheology. My contribution constitutes a step in combining classic structural geological approaches, such as field mapping and microstructural observations with numerical modeling to integrate these different approaches. The main focus of my work has been on the relationships between finite 20 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. strain, strain rates and deformation mechanisms derived from fieldwork in two representative pluton aureoles of the North American Cordillera, aided by generic and specific numerical modeling to examine strain rates during magma emplacement into the crust and simultaneous deformation. Some elements of my work have been performed in collaboration with researchers at USC and other Universities. Bob Miller (San Jose State University) introduced me to the geology in the Mount Stuart region. Scott Paterson (USC) provided detailed background information and material for scientific discussion on the Tuolumne Intrusive Suite. David Okaya (USC) guided my numerical modeling efforts. Scott Johnson (University of Maine) participated in some aspects of the numerical modeling. The main body of this dissertation consists of manuscripts at various stages of publication and preparation for submittal. Some repetition exists because the individual chapters were written to stand alone. However, the reader will find the reappearance of introductory material helpful, particular when attention is given to specific chapters and not to the dissertation as a whole. Most of my fieldwork has been taken place in the Sierra Nevada of California, leading to new insights into the tectonic evolution of the Sierra Nevada batholith. Hence, I provide an up-to-date synopsis of the regional geology and tectonic setting of the Sierra Nevada in chapter 2. In particular, I will focus on dextral transpression and partitioning of the deformation resulting from oblique convergence between the Farallon and North American plates. Chapter 3 addresses a specific problem in quantifying the magnitude of strike-slip displacement in the Sierra Nevada. In this 21 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. chapter, I present an alternative (and I believe more realistic) scenario on the location, timing and cumulative displacement associated with the enigmatic Mojave- Snow Lake fault, which has been proposed to have accommodated up to 400 km arc- parallel dextral displacement between the Sierra Nevada and the Mojave desert. Chapter 4 is dedicated to material displacement in aureoles of the Mount Stuart batholith and the Tuolumne Intrusive Suite. This chapter provides a detailed description and interpretation of finite strains measured in both aureoles. In chapter 5 I provide constraints on strain rates associated with magma emplacement derived from numerical modeling of dislocation creep to simulate pressure- and temperature dependent behavior of a grain-scale deformation mechanism. Complimentary strain rate constraints derived from fieldwork and potential field criteria for quantifying strain rates during magma emplacement are discussed in chapter 6 . This chapter utilizes magmatically folded dikes in combination with thermal modeling to provide a template for estimating strain rates in natural aureoles. In chapter 7 1 conclude my dissertation with a summary of the main findings and I point out prospective future directions for further research. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 2: PARTITIONED DEXTRAL TRANSPRESSIVE AND CONTRACTIONAL STRAIN FIELDS IN THE CRETACEOUS SIERRA NEVADA BATHOLITH, CALIFORNIA Introduction Since the origin of the first model of strain partitioning in settings of oblique plate convergence (Fitch, 1972), decoupling of deformation into arc-parallel dip-slip reverse and strike-slip faults has been recognized in ancient and modem settings worldwide (e.g. Beck, 1983; Jarrard, 1986; Jackson, 1992; Pinet and Cobbold, 1992 and therein; Dolan et al., 1998; Andronicos et al., 2003). The ubiquity of coexisting reverse and strike-slip faults suggests a common mechanism for strain partitioning at oblique convergent continental margins. Several analog and numerical experiments address the mechanism of strain partitioning. For example, Pinet and Cobbold’s (1992) analog modeling suggests that the angle between mechanically significant internal boundaries and the plate motion vector is critical. Subsequent work using analog materials (Pubellier and Cobbold, 1996) implies that oblique arc-continent collision may result in subduction reversal immediately after the collision, followed by installation of strike-slip faults and partitioning into frontal convergence and strike-slip displacement. Molnar (1992) proposed that partitioning of oblique convergence into pure strike-slip motion and pure convergence is intrinsically related to the rheological structure of the lithosphere and the resulting perpendicular and parallel orientations of the principal 23 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. stresses and strain rates with respect to the Earth’s surface. Braun and Beaumont (1995) employed finite element modeling to demonstrate that the relative amount of convergence versus transcurrent motion determines whether partitioning occurs. Teyssier et al. (1995) provided a quantitative relation between the angle of plate motion, instantaneous strain axes, and strike-slip partitioning. Gutscher (2001) suggested coupling between the upper and lower plate as a mechanism for producing strain portioning in oblique convergent settings. Despite the new insight from theoretical studies detailed field studies are required to further constrain and evaluate these models. We present a Cordilleran example of strain partitioning during Cretaceous transpression in the Sierra Nevada batholith, California. Partitioning of deformation into contemporaneous contractional and strike-slip motion has been suggested previously for the Californian Late Cretaceous Sierra Nevada arc. For instance, Tikoff and de Saint Blanquat (1997) proposed that horizontal displacement along the dextral Rosy Fynch shear zone occurred in combination with concurrent transarc contraction. The authors interpreted this to represent strike-slip and dip-slip motion, respectively, related to oblique convergence between the Farallon and North American plates between ca. 92 and 83 Ma. Despite sub-vertical stretching lineations in the Gem Lake shear zone (Greene and Schweickert, 1995), Tikoff and de Saint Blanquat (1997) suggested that it and the Cascade Lake shear zone (Tikoff and Greene, 1997) are in structural continuity with the Rosy Finch shear zone. The authors further stated that together, these combine to form the dextral Sierra Crest 24 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. N Piute Meadow Pendant Soldier Lake Pendant Saddlebag Lake Pendant Benson Lake Pendant May Lake Pendant 0 2. 10km Figure 2.1. Schematic map showing localities of host rock pedants in the Tuolumne Intrusive Suite, California. Kkc= Kuna Crest granodiorite. Khd= Half Dome granodiorite. Kcp= Cathedral Peak granodiorite. Kjp= Johnson Granite porphyry. 25 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. shear zone. However, our detailed field observations in magmatic rocks and host rock pendants of the Tuolumne Intrusive Suite (TIS, Fig. 2.1) do not support major strike-slip displacement north of the Gem Lake and Rosy Fynch shear zones. Instead, our paper documents two concurrent shear zones systems, arc-parallel reverse faulting along the axis of the TIS and synchronous dextral transpressive shear along the eastern margin of the TIS, suggesting a modified form of strain partitioning. We propose that Late Cretaceous oblique plate convergence resulted in partitioned dextral transpressive and contractional strain fields in the Sierra Nevada batholith with only minor horizontal strike-slip displacement. We begin our contribution with a tectonic overview of the Mesozoic Sierra Nevada and a summary of preexisting work on strain fields, followed by a detailed discussion of our new structural and geochronological data. We elaborate on the implications of our study on frequently proposed large-scale lateral transport of crustal blocks (e.g. Lahren and Schweickert, 1989; Busby-Spera and Saleeby, 1990; Grasse et al., 2001). Finally, we discuss why Cretaceous strain in the Sierra Nevada was not partitioned into pure dip-slip and strike-slip faults, but simultaneous contraction and transpression instead. Geologic Setting A complete summary of the geologic evolution of the area now occupied by the Sierra Nevada batholith is beyond the scope of this paper. The focus of our study is 26 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the Late Cretaceous, and hence we provide a brief geologic overview of the Cretaceous Sierra Nevada, followed by a discussion of strain fields and the role of shear zones relevant to our study. Overview To explain the origin of the Sierra Nevada batholith, several models of subduction of Pacific ocean floor beneath the North American plate and the resulting magmatic arc have been proposed and discussed in the literature (e.g. Hamilton, 1969; Moores, 1970; Dickinson, 1981). Major modifications in recent times (e.g. Saleeby, 1999; Ducea, 2001) led to a more modem view of subduction-related arc magmatism and deformation in the Sierra Nevada. The Sierra Nevada batholith forms the middle section of the Western Cordillera extending from the Coastal batholith in Canada to the Peninsular Ranges batholith in Mexico. The elongate NNW-striking batholith (Fig. 2.2) is composed of numerous individual plutons and intrusive suites, mostly Cretaceous in age, which aerially dominate over the batholitic host rock (e.g. Bateman, 1992; Cowan and Bruhn, 1992; Lipman, 1992). Most plutons are steep sided, preserve internal magmatic fabrics, and progressively young eastwards, the latter correlating with more mafic compositions of the early Cretaceous (Bateman, 1992). During the late Cretaceous (ca. 98-86 Ma) Sierra Crest magmatic event large mounts of granitoids were 27 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 40” San Franciscc Legend Cretaceous Jurassic Triassic Host rocks Figure 2.2. Geologic map of the Sierra Nevada batholith (modified from Bateman, 1992). 28 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. emplaced into the crust which are now exposed in an area of more than 4,000 km2 (Bateman, 1992; Coleman and Glazner, 1997). Although shallow crustal levels are indicated by depths of crystallization ranging from ca. 3-10 km (Bateman, 1992; Cowan and Bruhn, 1992; Webber et al., 2001), to date, few roof exposures have been found. The batholitic host rock comprises various pre-Cretaceous generally NW- to NNW-striking plutonic, metavolcanic, and metasedimentary rocks (Cowan and Bruhn, 1992; Bateman, 1992). The northwestern part of the Sierra Nevada batholith is enveloped by Paleozoic arc-related terranes and subduction complexes. Most of the southeastern part was emplaced in Precambrian and Paleozoic strata of the Cordilleran miogeosyncline. Upper Jurassic and Lower Cretaceous plutons exposed in the western areas cross-cut rocks that were accreted and deformed during the Nevadan orogeny. Paleozoic units occur to the east where satellite plutons are exposed in the White-Inyo Mountains. Several host rock screens and pendants occur between individual plutons and intrusive suites. Figure 2.1 shows the distribution of major host rock pendants of the central Sierra Nevada batholith, those which relate directly to our work we address in more detail below. Exposures include: the Saddlebag Lake pendant (Brook, 1977; Schweickert and Lahren, 1993), the Ritter Range pendant (Schweickert and Lahren, 1993; Greene and Schweickert, 1995), the Log Cabin pendant (Stevens and Greene, 1999), the Mount Morrison pendant (Stevens and Greene, 1999), the Mount Goddard pendant, the Snow Lake pendant (Lahren and Schweickert, 1989; Wahrhaftig, 2000), 29 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the Benson Lake pendant (Wahrhaftig, 2000), the May Lake and Piute Meadows pendants (this study). Strain fields Several workers characterized the Cretaceous regional deformation regimes in the Sierra Nevada as extensional, neutral, contractional, and transpressional. Tobisch et al. (1986, 1993) originally proposed extensional strain fields whereas later they favored neutral to weakly extensional strain fields (Tobisch et al., 1995). In contrast, regional contraction was suggested by Tikoff and Teyssier (1992, and references therein), Tikoff and de Saint Blanquat (1997), and Tobisch et al. (2000). Contraction was followed by transpressive deformation and increasingly more important strike-faulting (Tikoff and Greene, 1997; Sharp et al., 2000; McNulty et al., 2000; Pachell et al., 2003). Based on work in the Mount Goddard and Ritter Range pendants, Tobisch et al. (1986) presented an extensional model for the Cretaceous evolution of the Sierra Nevada. Their model involves tilting of beds along listric normal faults during NE- SW directed extension and subsequent generation of a pronounced regional NW to NNW-striking and steeply dipping metamoiphic fabric through magma emplacement over a period of ca. 40 million years. The authors concluded that arc-intemal dynamics were responsible for generating the structures and observed strain variations, rather than regional contraction due to collision. The fact that the 30 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. structures postdate the youngest rocks in the section yields a maximum age of deformation of 131 ± 6 Ma. In further support of their extensional model, Tobisch et al. (1993) interpreted steep stretching lineations and NNW-striking / SW-dipping regional foliation and kinematic indicators in the Mount Givens pluton to be consistent with west-side-down displacement. The authors provided U/Pb zircon ages and 40Ar/3 9 Ar ages of hornblende and biotite from rocks of the Courtright shear zone with down-dip kinematics (see section on shear zones), suggesting extensional activity between 94 and 90 Ma. We note that widespread extension has also been suggested for the Early Mesozoic arc. For instance, Busby-Spera (1988) proposed that the Late Triassic-Early Jurassic arc of Arizona, California, and western Nevada strongly resembled the present-day extensional arc of Central America. Contraction was relatively widespread during the early and mid Mesozoic (e.g. Schweickert and Lahren, 1987; Paterson et al., 1989a; Cowan and Bruhn, 1992; Saleeby, 1999), but unambiguous evidence for crustal-scale contraction during the late Cretaceous has only rarely been documented. Tikoff and Teyssier (1992) and Tikoff and de Saint Blanquat (1997) noted that regional contraction is indicated by the refolded foliation of the Lamarck granodiorite (originally presented by Bateman, 1992, and therein), which occurred contemporaneously with the emplacement of adjacent, ca. 90-83 Ma old granitoids. Ducea (2001) suggested that Precambrian basement units of western North America were underthrust beneath Mesozoic oceanic or continental margin arc rocks, ultimately resulting in the voluminous Cretaceous magmatism that generated substantial amounts of plutons. However, 31 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. several studies suggest that Cretaceous strain fields were more complicated than represented by pure extensional or contractional models. Ultimately, the complexity appears to be related to increasingly oblique convergence between the Farallon and North American plates, resulting in progressive transition from contraction to transpression to during the Late Cretaceous. Spatially and temporally fluctuating strain fields have been suggested by Tobisch et al. (1995) who proposed that asthenospheric mantle comer flow ultimately may have been responsible for neutral or weakly extension to contraction (transpression). McNulty et al. (2000) utilized structures in ca. 90 Ma old Mount Givens pluton (Central Sierra Nevada) to suggest that increasing obliquity of the plate convergence vector was associated with a transition from contractional to transcurrent tectonics at ca. 90 Ma. Likewise, Sharp et al. (2000) employed 40Ar/3 9 Ar ages of metamorphic hornblende, muscovite, and biotite as well as microstructural and petrological observations to propose that bedding-inclined cleavage in Triassic to Jurassic country rock of the Ritter Range pendant formed during or before ca. 87-82 Ma in a distributed dextral transpressive regime. Tikoff and Greene (1997) interpreted the coexistence of domainal shallowly and steeply plunging stretching lineations within tire Sierra Crest shear zone system as the result of wrench-dominated transpression, whereby different finite strains were recorded by different units. Some studies indicate that the transition from contraction to transpression was followed or accompanied by arc-parallel strike-slip faulting. For instance Tobisch et al. (1995) proposed that significant dextral strike-slip and oblique motion began at 32 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ca. 90 Ma, replacing dominantly transpressional deformation with a minor component of dextral strike-slip between ca. 105 and 90 Ma. In a like manner, waning of dextral transpression at ca. 80 Ma was indicated by Pachell et al. (2003) who indicated that sinistral strike-slip faults in granitic rocks of the Lake Edison area (central Sierra Nevada) were related to kilometer-scale kinking in a dextral transpressional regime. In summary, the Cretaceous Sierra Nevada was subject to a kinematic transition from regional contraction to transpression and strike-slip faulting at approximately 90 Ma. Both contractional and transpressional deformation involved displacement of unites on shear zones. In the following section, we will briefly summarize previous work on shear zones relevant to our study. Shear zones The purpose of this section is to provide a snapshot of shear zone activity during the Mid- Late Cretaceous. Below, we summarize the available constraints on timing, sense of displacement, length, and cumulative slip of major Cretaceous shear zones extracted from key publications. For faults with unconstrained lengths but known cumulative displacement, we suggest that the displacements can be used as a proxy for the minimum lengths of strike-slip faults. Dip-slip shear zones are illustrated in Figure 2.3 whereas shear zones associated with strike-slip displacement are shown in Figure 2.4. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Bench Canyon shear zone TIS Quartz Mountain shear zone Courtright shear zone Kaiser Peak shear zone - 36 100 km 120 ' 118' Figure 2.3. Schematic map of the Sierra Nevada batholith showing Cretaceous shear zones associated with dip-slip displacement. See text for discussion. The Tuolumne Intrusive Suite (TIS) is shown for reference. 34 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Gem Lake shear zone ca. 20 km TIS Bench Canyon shear zone ca. 4.6 km 4 Intrabatholithic Break 3 ca. 100 km Rosy Finch shear zone 20-70 km Mojave-Snow Lake fault 200-400 km - 36' (Proto-) Kern Canyon shear zone (ca. 40) 6.5-13 km Garlock fault 100 km 120 118 Figure 2.4. Schematic map of the Sierra Nevada batholith showing Cretaceous shear zones associated with strike-slip displacement. Dashed lines depict cryptic and hypothetical faults. See text for discussion. The Tuolumne Intrusive Suite (TIS) is shown for reference. 35 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rosy Finch shear zone (Tikoff and de Saint Blanquat, 1997, and therein) The 1-4 km wide and ca. 80 km long Rosy Finch shear zone (RFSZ) extends from the Evolution Basin alaskite northwards through plutons of the Mono Pass Intrusive Suite. Steeply dipping foliation, shallowly plunging lineation and kinematic indicators, including SC-fabrics, folded dikes, imbricated megacrysts, mica fish, and asymmetric poiphyroclasts indicate a dextral sense of shear. Microstructures and AMS analysis in Mono Creek granite indicate that the RFSZ was active between 8 8 and 80 Ma. No constraints are given on cumulative displacement. The RFSZ is in structural continuity with the Gem Lake shear zone to the north. Gem Lake shear zone (Greene and Schweickert, 1995, and therein) According to Greene and Schweickert (1995) the ca. 30 km long and 1 km wide Gem Lake shear zone (GLSZ) provides the first direct evidence for a dextral shear zone in the wall rock pendants of the central Sierra Nevada. The GLZS consists of three segments, the Gem Lake segment, the Kuna Crest segment, and the Ritter Range segment, which are marked by various degrees of partitioning between simple shear and pure shear. All three segments display a NNW-striking, steeply southwest- to east dipping mylonitic foliation and a subvertical stretching lineation. Horizontal kinematic indicators, such as asymmetric poiphyroclasts and minor folds, 36 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. rotated veins and tension gashes, and SC-fabrics indicate dextral displacement. Based on correlation of displaced marble lenses the magnitude of dextral displacement is at least 20 km. Syntectonic granodiorite of Kuna Crest and syndeformational mica indicate that the GLSZ was active from ca. 91 to 80 Ma. Bench Canyon shear zone (McNulty, 1995, therein) The ca. 20 km and 0.25 to 3 km wide Bench Canyon shear zone (BCSZ) is divided into 4 domains: the Red Devil Lake pluton, sills associated with the Red Devil Lake pluton, Cretaceous volcanic rocks, and a northern sill of the Mount Givens pluton. NW-striking and moderately to steeply northeast and southwest dipping mylonitic foliation and steeply plunging lineations with SC-fabrics, shear bands, asymmetric porphyroclasts and folds indicate top-to-the-southwest ductile thrust displacement. McNulty (1995) utilized strain analysis to estimate the net maximum thrust displacement. He integrated the obtained shear strain of 2.3 over 2000 m (i.e., the average shear zone width) to yield a displacement of ca. 4.6 km. However, mesoscopic extensional structures, including meter-scale graben, normal faults, extensional low-angle ductile shears, and asymmetric tension gashes within and outside the same sections of the BCSZ suggest a complex history. An early- phase deformation was constrained to have occurred between ca. 101 and 95 Ma whereas a late-phase deformation took place between ca. 90 and 78 Ma. 37 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Quartz Mountain shear zone (Tobisch et al., 1995, and therein) The ca. 10 km long and 1-2 km wide Quartz Mountain shear zone (QMSZ) is characterized by a generally weak, steeply dipping solid-state foliation locally grading into mylonites and ultramylonites with down-dip stretching lineations. Sense-of-slip indicators, including SC-fabrics, asymmetric poiphyroclasts, and hornblende as well as biotite fish indicate reverse shearing during contractional deformation. 40Ar/3 9 Ar geochronology on hornblende and biotite suggests that this shear zone was active between 98 and 87 Ma. The cumulative displacement is unkown. Kaiser Peak shear zone (Tobisch etal., 1995, and therein) The Kaiser Peak shear zone (KPSZ) is ca. 3 km long and ca. 1 km wide. It displays ca. 1 m wide domains of conjugate ductile shears and a steeply plunging down-dip stretching lineation. The solid-state foliation shows SC-fabrics which indicate reverse movement, consistent with contraction. The U/Pb zircon emplacement age of the Dinkey Creek pluton and 40Ar/3 9 Ar dating of syn- ultramylonitic biotite yield a duration of shear from ca. 102 to 91 Ma. The displacement is unknown. 38 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Courtright shear zone (Tobisch et al., 1993, and therein) The Courtright shear zone (CSZ) is a ca. 0.8-2 km wide and ca. 10 km ductile shear zone which extends through the margins and host rocks of three plutons of the Shaver Intrusive Suite. A steeply southwest-dipping and northwest-striking shear zone foliation is a combination of magmatic and solid-state deformation. A steeply southwest-plunging stretching lineation is developed. Rare kinematic indicators, for example, cr- clasts, steeply dipping extensional shears, more gently dipping conjugate reverse shear zones that occur only close to the margin of the Mount Givens pluton, and steep, down-dip stretching lineations indicate a steep W-dipping normal shear zone. U/Pb geochronology on zircon and 40Ar/3 9 Ar dating of hornblende and biotite suggest that the CSZ was active from ca. 94-90 Ma. The cumulative displacement is unconstrained. (Proto-) Kern Canyon fault (Busby-Spera and Saleeby, 1990, and therein) The north-trending and ca. 140 km long Kem Canyon fault (KCF) is a major topographic feature that bisects the southern part of the Sierra Nevada into the Great Western Divide and the Eastern Crest. The northern part of the KCF branches out into a ca. 6 km wide zone that contains several fault splays. Offsets in Mesozoic plutons indicates that the KCF accommodated ca. 6.5 to 13 km dextral displacement in a ca. 100 m wide brittle zone. Displacement on the KCF was preceded by at least 39 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 40 km ductile displacement associated with the NNW-trending and ca. 130 km long proto-Kem Canyon fault (PKCF). Mylonitic deformation associated with the PKCF produced abundant kinematic indicators, including SC-fabrics, offset feldspar fragments and dikes, asymmetric feldspar phenocrysts and biotite fish, which all consistently indicate a dextral sense of shear. U/Pb geochronology on zircon in plutonic and metavolcanic rocks suggest that the PKCF was active between ca. 105 and 85 Ma. Slip on the KCF is more loosely constrained by previous geochronology and probably occurred sometime between 80 and 3.5 Ma. Cryptic shear zones In addition to the exposed shear zones described above, several speculative fault zones have been proposed, mostly based on correlation of presumably displaced rock units exposed in various places and geochemical arguments. These cryptic faults include: Intrabatholithic break 2 (IBB2), Intrabatholithic break 3 (IBB3), and the Mojave-Snow Lake fault. EBB2 and IBB3 are indicated by an abmpt truncation and northward displacement of the S r,- = 0.706 and sn< iT = -2 isopleths of Mesozoic plutons that accommodated dextral displacement and disrupted the wall rocks and plutons of the Sierra Nevada batholith (Kistler, 1993, and therein). Crosscutting relationships between plutons and the assumed trace of IBB2 indicate that displacement occurred between about 100 and 90 Ma. In addition to ca. 90 km dextral offset of the Sr; = 40 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.706 isopleth, displacement associated with EBB3 includes separation of Triassic plutons on both sides of the trace of the proposed fault as well as correspondence with a structural break between roof pendants. Displaced middle Jurassic metavolcanic rocks and Cretaceous plutons that intrude D3B3 suggest that shear may have occurred between ca. 179 and 117 Ma. The exact magnitude of displacement is difficult to ascertain but combined the two intrabatholithic breaks could have accommodated up to 200 km dextral slip. Note that EBB 2 and IBB3 subsequently have been referred to as the Axial Batholithic break and the Eastern Batholithic break (Saleeby and Busby, 1993). Another potentially important strike-slip shear zone with major displacement is the proposed Mojave-Snow Lake fault (MSF). Strong resemblance of Uppermost Proterozoic and Lower Cambrian miogeoclinal rocks (Lahren et al., 1990; Schweickert and Lahren, 1990) and similar U/Pb ages of detrital zircons (Grasse et al., 2001) between the Snow Lake pendant and the western Mojave Desert suggested that the cryptic Mojave Snow Lake fault accommodated ca. 400 km dextral displacement. However, geochemical provenance studies of volcaniclastic and tuffaceous rocks in the northern Sierra Nevada indicated a source region in western Nevada, yielding a strike-slip displacement of only ca. 200 km associated with the MSF (Lewis and Girty, 2001). Besides these correlative arguments, no compelling structural evidence for a fault of the proposed magnitude has been documented. A comprehensive re-iteration of the existence and significance of the Mojave-Snow Lake fault is provided in chapter 3. 41 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Summary Major arc-parallel dextral strike-slip displacement during the Late Cretaceous does not appear to have been widespread. Several shear zones, particularly those proposed to be associated with large-scale cumulative displacements on the order of hundreds of kilometers, are cryptic and may need further investigation. Some shear zones may connect to form along-strike continuous shear zones. For instance, the Sierra Crest shear zone system (Greene and Schweickert, 1995) has been postulated to link the Cascade Lake, Gem Lake, and Rosy Finch shear zones. Some authors consider these dextral shear zones to coincide with a major structural break in the western United States Cordillera along which emplacement of magma was localized during the Cretaceous (Wyld and Wright, 2001). With regards to measurable total cumulative displacement accommodated by reverse and strike-slip faults, the above compilation shows that generally all displacements were on the order of no more than tens of kilometers. Larger amounts of displacement on the order of hundreds of kilometers have previously been assigned to strike-slip faulting, but these faults and their associated displacements remain speculative. 42 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Field observation Stratigraphy In addition to older Cretaceous and Triassic plutonic rocks (see section on magmatism below), the enclosing rocks of the Tuolumne Intrusive Suite (TIS) are composed of lower Paleozoic to Triassic deep-water deposits (note that we avoid the term ‘eugeoclinal’ because of potential but unintended genetic implications) of the Antler and Golconda allochthons, and Triassic to Jurassic metavolcanic rocks (e.g. Schweickert and Lahren, 1987,1993,1999; Greene and Schweickert, 1995; Stevens and Greene, 1999). These rocks occur along the eastern margin of the TIS and they include sand- and siltstones, quartzites, cherts, shales, and conglomerates. Continuous exposures are found in the Ritter Range pendant (RRP) and the Saddlebag Lake pendant (SLP). Additionally, some quite thin conglomerate beds crop out in the northeastern part of the Piute Meadow pendant (PMP). Chert, shale, siltstone, argillites, quartzites, and various calcareous rocks in the easternmost part of the SLP have been correlated with the Palmetto Formation in Candelaria, Nevada and a shale unite containing graptolites suggests that this Formation is Ordovician in age (Schweickert and Lahren, 1993). The Palmetto Formation is stratigraphically overlain by volcaniclastic sandstone and pillow basalt whose ages are unconstrained. Conglomerates and sandstones of the Diablo Formation, as well as quartzofeldspathic silt- and sandstone of the Candelaria Formation conformably 43 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. overly the pillow basalt. Schweickert and Lahren (1993) reported Permian conodonts in limestone clasts of the Diablo Formation. Direct age constraints for the Candelaria Formation are not available but it is overlain by upper Triassic metavolcanic rocks (see below). The fluvial conglomerate of Cooney Lake represents a transitory stage towards deposition of volcanic rocks at subaerial conditions. The conglomerate contains components that stem from all underlying units and rarely plutonic rocks. In addition to the above metasedimentary rocks exposed mostly along the northeastern and eastern margin of the TIS, isolated outcrops of quartzite and very rarely calc-silicate rocks occur between the western TIS margin and older plutonic rocks. Relatively large and continuous exposures of these rocks constitute the Snow Lake, Benson Lake and the May Lake pendants. Apparent mismatch between the shallow water rocks of these pendants and shelf deposits in the nearby White and Inyo Mountains stimulated the proposition of major dextral displacement associated with the enigmatic Mojave-Snow Lake fault (see above and chapter 3). The mostly metasedimentary units above are overlain by various metavolcanic rocks. The new sequence begins with the 222 +/- 5 Ma (U/Pb zircon, Schweickert and Lahren, 1987) rhyolitic ash-flow tuff of Saddlebag Lake. It is overlain by undated metaandesitic massive lava flows, thin tuff layers, and locally breccias. The andesitic rocks are topped by a rhyodacitic ash-flow tuff that strongly resembles the rhyolitic ash-flow tuff of Saddlebag Lake, but Rb/Sr whole rock geochronology suggests an age of 185 Ma for this unit. It is noticeable that the felsic metavolcanic 44 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. rocks frequently contain lithic fragments of andesitic metavolcanics and vice versa. Interlayered calc-silicate rocks, volcanic sandstones of the Horse Canyon sequence (Schweickert and Lahren, 1993, and therein) as well as thin rhyodacitic and andesitic metavolcanics form the host rock at the margin of the TIS. Diagnostic features indicative of a volcanic source for the metavolcanic rocks exposed at SLP are not present locally, but Schweickert and Lahren (1999) suggested on the base of lithological relationships that a volcanic vent formed a caldera near Tioga Pass. Radiometric ages for the metavolcanic rocks of the PMP are not available but strong correlative arguments suggest that the metavolcanic rocks of the PMP are stratigraphically part of the SLP. Magmatism Amongst the many magmatic rocks exposed in the Sierra Nevada batholith, the Tuolumne Intrusive Suite (TIS) is of primary importance for our study and it thus deserves a brief description. The ca. 2000 km2 large (map area), horizontally elongate TIS (Fig. 1.4) is considered the type intrusive suite in the Sierra Nevada (e.g. Bateman and Chapell, 1979; Bateman, 1992). Field data suggest that the suite was emplaced in a series of pulses with each successive pulse displacing older units (displacement processes that operated during emplacement of the TIS are described in detail in chapter 4). The TIS reveals normal zoning including an outer mafic phase (the Glen Aulin and Glacier Point tonalites to the west and Kkc to the east, and 45 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. inner, more felsic phases (Khd, Kcp, and Kjp, see Table 1.1 for acronyms). A geochemical study of mafic enclaves (Reid et al., 1983) and a study using Sr and Nd isotopes (Kistler et al., 1986) suggest that different internal phases of the TIS were not derived from a single parent magma by closed system fractionation. Instead, intrusion of several separately evolved magma pulses were required to yield the zoning of the TIS. From available radiometric U-Pb zircon ages for several of these pulses we can estimate a best minimum and best maximum range of ages which brackets the time during which intrusion took place (Table 1.1). In addition to Triassic plutons of the Scheelite Intrusive Suite (Bateman, 1992) east of the Saddlebag Lake pendant, the Tuolumne Intrusive Suite (TIS) is enclosed along its southern, western and northwestern end by mostly Early to Mid-Cretaceous plutonic rocks. Granodiorite and granite associated with the nested Intrusive Suite of Washburn Lake (concordant ca. 98 Ma U/Pb age of granodiorite of Red Devil Lake, Stem et al., 1981), the Intrusive Suite of Merced Peak (discordant ca. 98 Ma U/Pb age of granodiorite of Jack Ass Lake) and the Intrusive Suite of Buena Vista Crest (discordant U/Pb age of ca. 100 Ma of granodiorite of Illiloutte Creek, 112 and 107 Ma age of granodiorite of Ostrander Lake, Stem, 1981), host the TIS in the south. The western margin consists of various granites of the Intrusive Suite ofYosemite Valley (e.g. ca. 103 and 102 Ma concordant U/Pb ages of El Capitan Granite). The northwestern margin of the TIS exposes granite and granodiorite (e.g. Granite of Bond Pass and Granodiorite of Lake Harriet) which are probably Early Cretaceous in 46 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. age (field relationships, Wahrhaftig, 2000). U/Pb zircon geochronology on Lake Harriet granodiorite is in currently in progress. Structure The metasedimentary and metavolcanic host rocks east and northeast of the TIS recorded a complex history of Paleozoic and Triassic thrusting and folding and Late Jurassic to Late Cretaceous contraction that formed steeply northeast- and southwest- dipping reverse faults, folds, metamorphic foliation and down-dip stretching lineation (Schweickert and Lahren, 1993; Greene and Schweickert, 1995; Tobisch et al., 2000; this study). Foliation and lineation are best developed near and strike- parallel to the contact with the TIS, but locally, the TIS has intruded discordantly across these structures. Intrusive rocks of the TIS show a well-developed magmatic foliation. Planar Fabrics Metamorphic foliation is defined by a well-developed cleavage in the field and by referred orientation of elongate minerals, such as feldspars, micas, and hornblende in thin sections. Foliation strikes north-northwest and dips steeply to the north-northeast and south-southwest in the Piute Meadow pendant (PMP, Fig. 2.5), Saddlebag Lake (SLP, Fig. 2.6), May Lake pendant (MLP), Green Lake (GLP), and 47 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ~ 'V \V \ Legend j . I I Cover deposits o : ---- < , , P C I'— ' — l Andesitic volcanics tr --- u i | | Aplite I Km I Cathedral Peak granodiorite 2 I .- . km I Half Dome granodiorite < ___ in | : unc | Buckeye Creek monzonite o q | .m e | Granodiorite of Long Canyon 53 < fcgBEl Metaplutonics c c Metavolcanics Metasedimentary rocks Original petrography by Chesterman (1975) and Wahrhaftig (1955-1980) Figure 2.5. Geologic map of the Piute Meadow pendant. Short solid lines indicate foliation trajectories. 48 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Planar fabrics Solid state foliations G iftfiM i Bedding MappatioiS’ s ' ^oliatio.in®5 (n=65) Figure 2.6. Schematic geologic map of the Saddlebag Lake pendant. Symbols indicate strike and dip direction of foliations. The lithology is described in Figure 6.2 . 49 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Soldier Lake pendants (SDLP). Locally, moderate dip angles are encountered along the TIS margin in the SLP. The consistent north-northwest strike is re-oriented in the northeastern half of the PMP (Fig. 2.5). At this location, the foliation wraps from the orientation of the regional grain to parallelism with the pluton margin. This local deflection could have occurred either during emplacement of the TIS, by virtue of the intruding magma transferring stress into the host rock causing it to deform, or subsequent to magma solidification during regional NNE-SSW directed shortening. Granites and granodiorite of the TIS show a well-developed magmatic foliation defined by alignment of K-feldspar megacrysts (if present), plagioclase, and mafic minerals, such as biotite and hornblende. The intensity of the magmatic foliation slightly increases with proximity to the TIS margin. Along with the increasing intensity towards the contact, in many places the magmatic foliation becomes margin-parallel. However, an additional E-W striking orientation of the magmatic foliation occurs scattered throughout the TIS and appears to be unrelated to specific positions in the pluton or with respect to its margin. Besides margin-parallel and E- W orientations, the magmatic foliation mimics the north-northwest striking metamorphic foliation of the host rock. Mutual crosscutting relationships were not found and so an unambiguous sequence with regards to different generations of magmatic foliations could not be established. The Early Cretaceous granite of Bond Pass and granodiorite of Lake Harriet at the northwestern TIS margin show development of a foliation that parallels the regional north-northwest striking grain and the contacts. Preliminary microscopic 50 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. observations indicate that the generation of this foliation occurred under magmatic to sub-magmatic conditions. Linear Fabrics Stretching lineation in metamorphic rocks is defined by quartz aggregates, feldspar, and mica. Sub-vertical down-dip plunges and various trends of the lineation dominates the pattern in all host rock pendants that we examined. However, careful mapping in the SLP showed that locally the lineation orientation changes to moderate plunges with more consistent southeasterly trends (Fig. 2.7). This phenomenon is best observed along the TIS margin in the SLP. Shear zones North-northwest striking, south-southwest vergent reverse shear zones occur in the northwestern part of the PMP (Fig. 2.5). These reverse faults displace Early Cretaceous granitoids as well as metandesitic rocks and aplites. Unambiguous shear sense indicators, including well-developed SC-fabrics, asymmetric porphyroblasts, and offset of dikes and tension gashes confirm reverse shear kinematics. Preliminary thin section observations suggest that most of the displacement occurred under magmatic to sub-magmatic conditions. Similar reverse shear zones have been reported from the enclosing rocks of the Green Lake pluton (Onezime, pers. comm., 51 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 38 00 - - • • ^ • V — tv '- " ..- ' - -: iiiW sli«*i H ost rock 2 km n=233 Figure 2.7. Lineations in the Saddlebag Lake pendant. Arrows on left hand side indicate magmatic lineations. Arrows on right show host rock stretching lineations. 52 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2002) and at the northeastern margin of the TIS (Hutton and Miller, 1994). We note that reverse shear zones near the Green Lake pluton are south-southwest as well as north-northeast vergent and thus resemble a fan structure. Hutton and Miller (1994) found overprinting relationships that suggest near synchroneity of TIS emplacement and deformation. Likewise, our collaborators Robert Miller and Jonathan Miller observed that magmatic foliations in Kuna Crest tonalite at the southwestern and southern margin of the TIS were overprinted with high-T ductile reverse shear zones (Miller and Miller, 2003) and that small, cm-dm scale ductile shear zones are also present in the El Capitan granite and the outer TIS units (e.g. Kuna Crest and Sentinel) but not in the inner TIS units (e.g. Half Dome and Cathedral Peak, Miller et al., 2001). Ongoing field work and in the MLP and SDLP confirms that dextral- horizontal and reverse shear dominated. Previously proposed pluton-down shear in the MLP based on mesoscopic shear sense indicators (Taylor et a., 2001) cannot be supported. The lack of displaced pinpoints hinders minimum estimates of displacement. We recall that the TIS margin in the SLP is characterized by locally moderately southeast plunging stretching lineations whereas the remaining areas of the SLP display sub-vertical lineations and dominantly dextral-horizontal kinematics. In order to test a relationship between moderate lineation plunge (i.e., < 60°) and sense of shear, we performed detailed field studies along strike of the TIS contact. In addition to consistent pluton side-up criteria parallel to lineation, we found both dextral and sinistral sense of shear indicators perpendicular to lineation, indicating 53 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. lateral displacement. In addition, we encountered many places in which any particular sense of shear could not be determined and we inferred that these places underwent vertical stretching only. Comparison of Figure 2.7 and Figure 2.8 shows that a relationship between moderately plunging lineation and dextral or sinistral displacement cannot be stated. However, the accurate match between moderately plunging lineations and the lack of dominate shear senses in the northern half of the SLP suggests that a relationship between lineation orientation and pure shear domains may exist. Additional evidence for pure shear deformation at the contact is provided by the relatively common occurrence of leucocratic dikes that have undergone stretching in two mutually perpendicular directions (i.e., sub-vertical and sub-horizontal). Figure 2.9 illustrates such “chocolate tablet boudinage”. Timing At its northern margin, the TIS truncates the granite of Bond Pass and the granodiorite of Lake Harriet. K/Ar geochronology on hornblende and biotite on the truncated plutons yielded ages of 96-92 and 85 Ma, respectively (Frei et al., 1984). Although these ages most likely represent cooling ages, they imply that reverse faulting must have occurred at or before ca. 96 Ma. U/Pb zircon geochronology on granodiorite of Lake Harriet and sheared metaandesite of the PMP is currently underway at the University of Arizona to constrain the crystallization age and thus 54 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Kinematics • right-lateral • left-lateral • none/flattening Host rock Figure 2.8. Horizontal shear senses at the contact to magmatic rocks in the Saddlebag Lake pendant. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.9. “Chocolate tablet” boudinage at the contact between the Tuolumne Intrusive Suite and host rock of the Saddlebag Lake pendant, indicating sub-vertical and sub-horizontal stretching. Lens cap (diameter ca. 5 cm) for scale. 56 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. help estimate the timing of reverse faulting. In any case, the reverse faulting must have come to an end by ca. 8 8 Ma, because Cathedral Peak granodiorite of this age truncates the reverse faults in the PMP. The timing of contractional deformation can be further constrained utilizing the ubiquitous north-northwest striking magmatic foliation present in the major intrusive units of the TIS. Formation of this foliation under magmatic conditions is consistent with a north-northeast to south-southwest directed contractional strain field between ca. 94 and 8 8 Ma. The waning stages of shortening are represented by magmatically folded dikes of Kuna Crest granodiorite, Half Dome granodiorite and Cathedral Peak granodiorite that occur at the TIS margin in the SLP (see chapter 6 for a thorough discussion of these dikes and their implications) and north of the northern TIS. These dikes suggest that shortening occurred at their crystallization ages of 94 to 8 8 Ma. Because the dikes preserved magmatic conditions, and no pervasive ductile overprint could be documented in thin sections (see chapter 6 ), regional shortening (i.e., any ductile deformation) must have ceased at around 8 8 Ma. Discussion Our data is consistent with simultaneous regional contraction and transpression. Temporally coexisting reverse faults and zones of transpression are consistent with oblique convergence between the Farallon and North American plates during the Late Cretaceous (e.g. Engebretson, 1985; Glazner, 1991; Kelley, 1993). In view of 57 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the very widespread partitioning of oblique plate convergence into parallel reverse and strike-slip faulting (e.g. Fitch, 1972; Beck, 1983; Jarrard, 1986; Jackson, 1992; Pinet and Cobbold, 1992 and therein; Dolan et al., 1998; Andronicos et al., 2003) one would expect this phenomenon to occur in the Late Cretaceous Sierra Nevada batholith as well. Tikoff and de Saint Blanquat (1997) actually proposed transpressional shearing and strike-slip partitioning during the Late Cretaceous. The authors employed ductile strike-slip faulting associated with the Rosy Finch shear zone and reverse sense of shear indicators in the Kaiser Peak, Quartz Mountain, and Bench Canyon together with north-northwest striking foliations in the Mono Lake, Lake Edison, and Turret plutons to support their model. They also suggested that dextral strike-slip displacement extended north into the SLP where they represent it by the Cascade Lake shear zone. However, our field work contradicts sub-horizontal mineral lineations in the SLP, suggesting that dextral strike-slip faulting may have been a local phenomenon and that it occurred along discontinuous or isolated shear zone sections. Moreover, our compilation of Late Cretaceous shear zone activity strongly suggests that most shear zones are characterized by reverse kinematics and those proposed to have accommodated hundreds of kilometers of dextral horizontal displacement are enigmatic. We recognize that steeply plunging mineral lineations can be consistent with horizontal tectonic transport (e.g. Sanderson and Marchini, 1984; Huddleston, 1988). For example, Tikoff and Greene (1997) explained the concurrent occurrence of vertical and horizontal mineral lineation in the same shear zone (i.e., the Sierra Crest 58 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. shear zone system) in terms of a strain gradient. The authors stated that the younger plutons through which the shear zone cuts, have recorded significantly less strain than the older plutons. This would be compatible with strain accumulation effects whereby mineral lineations rotate towards verticality with increasing finite strain. However, our field work along the eastern TIS margin never showed shallow lineations, regardless of the ages. Hence, our findings are incompatible with partitioning of oblique plate convergence into reverse and strike-slip shear zones. Our data are better compatible with partitioned contraction and dextral transpression, implying that complete decoupling of deformation did not occur. This brings us to consider the role of mechanical aspects of strain partitioning in settings of oblique convergence. According to Jarrard (1986), three parameters control variations in the occurrence and the offset direction of strike-slip faults behind trenches: convergence obliquity, strength of the overriding plate, and coupling between subducting and overriding plates. The last two points seem to be particularly important as several modeling studies suggested that the angle between mechanically significant internal boundaries and the plate motion vector is critical (Pinet and Cobbold, 1992). In contrast, Braun and Beaumont (1995) demonstrated that the relative amount of convergence versus transcurrent motion determines whether partitioning occurs. Their finite element models showed that in convergence-dominated systems, oblique slip develops, whereas in transcurrent- dominated systems coexisting thrust faults and strike-slip faults develop. Some degree of coupling between the upper and lower plate seems to be required as a 59 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. mechanism for producing strain pardoning in oblique convergent settings (Gutscher, 2001). However, these theoretical approaches merely represent mechanically simplified snapshots and it is unclear how significant they are with regards to representing strain partitioning at natural plate margins. An interesting concept is that partitioning of oblique convergence into pure strike-slip motion and pure convergence is intrinsically related to the rheological structure of the lithosphere and the resulting perpendicular and parallel orientations of the principal stresses and strain rates with respect to the Earth’s surface Molnar (1992). In this context, more work is required to assess the role of crustal depth on strain partitioning. It is possible, that pluton emplacement-levels of the crust, even if as shallow as ca. 1 0 km, are too deep for principal axes of stress and strain rate to become perpendicular or parallel to the Earth’s surface during oblique convergence. Many studies of active strain partitioning took place in the upper crust, utilizing seismicity for constraints on slip directions. Additional complications are likely to arise from anomalously shallow geotherms due to the emplacement of large volumes of magma into the cmst within short time periods. Conclusions Fieldwork in intrusive rocks of the Tuolumne Intrusive Suite (TIS) and its associated host rock exposures indicates that Late Cretaceous oblique convergence between the Farallon and North American plates resulted in partitioned contraction 60 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and transpression during pluton emplacement. Oblique mineral stretching lineations and dominantly dextral shear sense indicators along the eastern margin of the TIS are best compatible with dextral transpression. Steeply north-northeast dipping reverse faults at the northwestern TIS margin and steeply north-northeast and south- southwest dipping reverse faults at the eastern margin indicate that transpression occurred contemporaneously with contraction. Our results suggest that previous models of decoupling into parallel reverse and strike-slip faulting in the central Sierra Nevada batholith cannot be supported. Ongoing work in the May Lake pendant, Benson Lake pendant, and host rocks of the Soldier and Green Lake plutons will help constrain the geometry and detailed kinematics of partitioned contractional and dextral transpressive strain fields in the Late Cretaceous Sierra Nevadan arc. Pending geochronology will be utilized in combination with fieldwork to determine the longevity of contraction across the Sierra Nevada. 61 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 3: NEW CONSTRAINTS ON THE LOCATION, TIMING, AND CUMULATIVE DISPLACEMENT ASSOCIATED WITH THE ENIGMATIC MOJAVE-SNOW LAKE FAULT: IMPLICATIONS FOR MESOZOIC INTRABATHOLITHIC STRIKE-SLIP DISPLACEMENT IN THE SIERRA NEVADA, CALIFORNIA Introduction An outstanding problem regarding the geology of the Sierra Nevada, California, is estimating magnitudes of Mesozoic strike-slip displacement (e.g. Kistler, 1993). For example, a strong resemblance of Uppermost Proterozoic and Lower Cambrian miogeoclinal rocks (Lahren et al., 1990; Schweickert and Lahren, 1990) and similar U/Pb ages of detrital zircons (Grasse et al., 2001) between the Snow Lake pendant and the western Mojave Desert suggested that the cryptic Mojave Snow Lake fault (MSF) had ca. 400 km dextral displacement (Fig. 3.1). However, geochemical provenance studies of volcaniclastic and tuffaceous rocks in the northern Sierra Nevada indicated a source region in western Nevada, yielding a strike-slip displacement of only ca. 200 km associated with the MSF (Lewis and Girty, 2001). In addition, there is no compelling evidence for the MSF along the southward projected trace of the MSF in the Mineral King pendant in the southern Sierra Nevada (Saleeby and Busby, 1993). These conflicting constraints motivated a re- evaluation of the MSF hypothesis. The purpose of this paper is to examine the available constraints on the location, timing, and maximum possible cumulative displacement of the MSF, and to test 62 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sailor Canyon Formation 38°N Saddlebag Lake pendant Snow Lake pendant Snow Lake block Speculative Mojave-Snow Lake fault 36°N Kern Canyon fault Garlock fault Western Mojave Desert Sierra Nevada San Andreas fault Pacific Ocean 33*w N Legend I • -| Inyo facies Irrtri Death Valley facies fcxgxl Roberts Mountains allachthon I ' . - .---I Paleozoic miogeoclinal belt Shoo Fly Complex 100 km Figure 3.1. Schematic geologic map of the Sierra Nevada (modified from Lahren et al., 1990; and Grasse et al., 2001). Solid half circles represent schematic pinpoints. Solid circles show locations of correlative units. 63 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8445 6^712 whether a proposed structure such as the MSF is a likely hypothesis to explain large- scale strike-slip displacement in the Mesozoic Sierra Nevada. To address this problem, I present constraints on the timing and maximum possible cumulative displacement on the MSF obtained from field work combined with plate reconstruction models that address Farallon-North America interaction during the Cretaceous (Engebretson et al., 1985; Kelley, 1993). These constraints yield a maximum duration of dextral slip associated with the MSF of ca. 12 m.y., and consequently a likely cumulative displacement of ca. 180 km. However, my recent fieldwork in the Piute Meadow pendant (Fig. 3.2) indicates that structural evidence for the MSF in Pre-Cretaceous metamorphic rocks is completely missing which necessitates an alternative explanation of major Mesozoic strike-slip displacement in the Sierra Nevada. Below I will argue that the absence of strike-slip structures in this pendant cannot be explained by emplacement of the Sierra Nevada batholith. Given the lack of direct evidence for strike displacement, I suggest an alternative model involving laterally stepped shear zones with a dextral component that may have collectively accommodated displacement of ca. 151 - 207 km along the axis of the Sierra Nevada during the Cretaceous. Mojave-Snow Lake fault (MSF) The MSF was originally proposed because of similarities between rocks in the Snow Lake pendant, Central Sierra Nevada and rocks in the Mojave Desert. Lahren 64 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.2. Geologic map of the Piute Meadow pendant. Key features include: discordant relationships between intrusive units of the Tuolumne Intrusive Suites and its older host rocks, NNE-SSW regional shortening direction indicated by magmatic foliation trajectories, and steeply plunging stretching lineations in host rock. 65 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. Poles to host rock foliations Host rock lineations Total Data: 517 To al Da a : 348 Poles to magmatic foliations £ £ U l H < 0 3 o uj o £ K O O (0 C 0 3 Legend E3 Cover deposits [ill Andesitlc volcanlcs □ Aplite 1 * * 1 Cathedral Peak granodiorite l _ ! ^ J Half Dome granodiorite | 1 Kbc | Buckeye Creek monzonite | Klc | Granodiorite of Long Canyon \m ? > \ Metaplutonics B a n Metavolcanlcs n Metasedimentary rocks Original petrography by Chesteiman (1975) and Wahrhaftig (1955-1980) O n O n Total Data : 374 et al. (1990) correlated sedimentary units of the Snow Lake pendant with uppermost Proterozoic to Cambrian passive margin sequences of the Death Valley facies exposed in the western Mojave Desert (Fig. 3.1). Correlative formations include: the Stirling quartzite, the Wood Canyon Formation, the Zabriskie Quartzite, and the Carrara Formation. Lahren et al. (1990) based their correlation on lithologic similarities, bedding style, stratigraphic sequence, approximate stratigraphic thickness, the presence of trace fossils (Skolithos), and the lack of volcanic rocks. Additionally, based on similar crystallization ages, petrography, textures, and major element chemistry, Lahren et al. (1990) correlated rhyolitic and basaltic dikes in the Snow Lake pendant with the Independence dike swarm in the western Mojave Desert. Grasse et al. (2001) found that U/Pb detrital zircon ages from the lower and upper quartzites of the Snow Lake pendant (1.0-1.15, ca. 1.42, and 1.69-1.85 Ga, Grasse et al., 2001) are similar to detrital zircon ages from the Zabriskie Quartzite in the Mojave Desert. These observations suggest that strata of the Snow Lake are correlative with the shallow water deposits of the western Mojave Desert, consistent with ca. 400 km dextral displacement along the proposed MSF. However, Lewis and Girty (2001) studied geochemical patterns in volcaniclastic and tuffaceous rocks of the Sailor Canyon formation (Fig. 3.1) in the northern Sierra Nevada. Trace and Rare Earth element patterns indicate possible volcanic source regions in western Nevada, suggesting that displacement associated with the MSF may not have been more than ca. 2 0 0 km. 67 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Location North of the Snow Lake pendant, Lahren and Schweickert (1989) placed the northern continuation of the MSF about 110 km to the east to the position of the Pine Nut fault. This position is consistent with Lewis and Girty’s (2001) correlation of the Sailor Canyon formation with volcanic sources in western Nevada. Constraining the position of the MSF south of the Snow Lake pendant is more problematic. Schweickert and Lahren (1990) inferred that the trace of the MSF lies along the west edge of the Mineral King pendant. However, Saleeby and Busby (1993) critically re examined the stratigraphy and structure in the Mineral King pendant and they did not find any compelling evidence for a structure of the proposed magnitude of the MSF. Schweickert and Lahren (1993) proposed that the MSF lies west of rocks of the Antler and Sonoma orogenic belts of the Saddlebag Lake pendant. Hence, the position of the MSF is confined to a ca. 10-15 km wide corridor between the Snow Lake and Saddlebag Lake pendants in the central Sierra Nevada (Fig. 3.1). Timing Originally, the timing of dextral displacement associated with the MSF has been constrained using 150 Ma old rhyolitic and basaltic dikes in the Snow Lake pendant that predate the MSF and that resemble the Independence dike swarm (Lahren et al., 1990). On the other hand ca. 110 Ma old batholithic rocks that were undeformed by 68 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the MSF were assumed to postdate the MSF (Lahren and Schweickert, 1989). However, considering the plate tectonic framework during the Cretaceous and structural relationships in the Piute Meadow pendant, an alternative scenario for the timing and duration of dextral displacement associated with the MSF is equally permissive. Oblique convergence along many continental margins worldwide resulted in partitioning of deformation into contractional and strike-slip structures, both parallel to the arc (e.g. Beck, 1983; Jackson, 1992; Andronicos et al., 2003). Strain partitioning has also been suggested for the Sierra Nevada (Tikoff and de Saint Blanquat, 1997). Hence, the maximum possible duration of dextral slip on the MSF can be determined if the beginning and the end of dextral-oblique convergence between the Farallon and North American plates are known. Engebretson et al. (1985) reconstructed relative motions between oceanic and continental plates in the Pacific basin for the past 180 million years. An important aspect of their study is a transition from sinistral- to dextral-oblique convergence between the Farallon and North American plates at ca. 100 Ma. In an updated version of Engebretson et al.’s (1985) model, Kelley (1993, unpublished MSc thesis) showed that sinistral-oblique convergence ended at ca. 100-95 Ma. In addition, a recent exchange with Engebretson confirmed the end of the sinistral-oblique period of convergence at ca. 95 ± 10 Ma (2003, personal communication). Despite some uncertainties associated with the fixed hot spot approach (Stock and Molnar, 1988) used by Engebretson et al. (1985) and Kelley (1993), 100 Ma seems to be a 69 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. reasonable estimate and this is supported by sinistral shear on the arc-parallel Pine Nut fault (Oldow et al., 1984) and Glazner’s thoughts on oblique subduction (1991). The end of dextral displacement can best be constrained using cross-cutting relationships between the MSF and batholithic rocks. My fieldwork in the Piute Meadow pendant (Fig. 3.2) revealed that ca. 8 8 Ma old Cathedral Peak granodiorite, a major intrusive unit of the Tuolumne Intrusive Suite, is in direct contact with rocks that are in a key position for the MSF. Because no evidence for dextral strike-slip displacement was found in Cathedral Peak granodiorite, dextral slip on the MSF must have ended before or at ca. 8 8 Ma. Hence the maximum duration of dextral strike slip displacement associated with the MSF is ca. 12 m.y. Piute Meadow pendant The Piute Meadow pendant continually exposes various metaplutonic, metavolcanic and metasedimentary rocks (Fig. 3.2) between the Snow Lake and Saddlebag Lake pendants. Dominant rock types that predate Cathedral Peak granodiorite include a sequence of rhyolitic to dacitic and andesitic metavolcanic rocks interlayered with thin conglomerate beds in the northern part. The western part of the Piute Meadow pendant is marked by Cretaceous granodioritic host rocks (Wahrhaftig, 2000) that are imbricated by reverse faults. Unambiguous kinematic indicators, including SC-fabrics, asymmetric porphyroclasts, and offset of markers, 70 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. as well as steep down-dip stretching lineations are consistent with dip-slip southwest-vergent reverse faulting. Due to multiple episodes of regional deformation and possibly emplacement of pre-Cathedral Peak plutons, the host rocks are strongly foliated (Tobisch et al., 2000). The foliations strike NNW and dip steeply to the NNE and SSW (Fig. 3.2). Some emplacement-related deflection of this foliation occurred locally in the northern aureole of the Cathedral Peak granodiorite. Stretching lineations in the host rocks plunge subvertically (Fig. 3.2). There is no evidence for any structure in metaplutonic, metavolcanic and metasedimentary rocks of the Piute Meadow pendant indicative of strike-slip displacement. The ca. 8 8 Ma old Cathedral Peak granodiorite forms the largest exposure of Late Cretaceous magmatic rocks in the area and it truncates the contacts and reverse faults separating different host rock units. Furthermore, Cathedral Peak granodiorite carries a well-developed magmatic foliation that parallels the regional trend defined by the host rock foliation. In addition, local parallelism to the pluton margin occurs, yielding the circular distribution of the poles to the magmatic foliations in Figure 3.2. Evidence for strike-slip displacement is absent. Tightly folded dikes of Cathedral Peal granodiorite occur in metavolcanic host rocks along the pluton margin. Criteria outlined by Paterson et al. (1998) indicate that the folding occurred under magmatic to sub-magmatic conditions (i.e., without crystal-plastic deformation). The folded dikes define axial planes parallel to the orientation of the regional host rock foliation. Consequently, the folded dikes of 71 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. £ 2 15 cm £2 Figure 3.3. Magmatically folded dikes of Cathedral Peak granodiorite indicating NNE-SSW directed regional shortening at ca. 8 8 Ma. Sj= maximum principal shortening direction. 8 2= intermediate principal shortening direction. 8 3= minimum principal shortening direction. 72 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Cathedral Peak granodiorite recorded NNE-SSW directed regional shortening at ca. 8 8 Ma (Fig. 3.3). Discussion Given that 8 8 Ma old Cathedral Peak granodiorite is unaffected by MSF-related deformation and dextral strike-slip motion began at ca. 100 Ma, a duration of 12 m.y. represents the maximum interval of dextral displacement. Assuming that the Mojave Snow Lake block was transported northward at Jarrard’s (1986) average slip rate for terrane displacement of 1.5 cm/year, we obtain a maximum cumulative displacement of ca. 180 km (Fig. 3.4). I note that 1.5 cm/year is probably on the high end of slip rates on arc-parallel faults, yielding maximum rates. In any case, 180 km dextral displacement amounts to only about half of the proposed 400 km (e.g. Schweickert and Lahren, 1990) but it matches the estimated displacement of 200 km of Lewis and Girty (2001) and Saleeby and Busby (1993). In contrast, assuming a higher slip rate of 3.5 cm/year, about 400 km dextral displacement could be accommodated in 12 m.y. However, 3.5 cm/year is the average slip rate of the San Andreas fault today (SCEC, 2003), which accommodates ca. 70% of the strike-slip displacement resulting from the oblique convergence between the Pacific and North American plates. For this slip rate to be applicable to the MSF, either the MSF would have to have been a major plate boundary fault or 73 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. c © < D > £ a o 3 « E a. 3 .2 O -a 700 600 500 400 Proposed displacement of 400 km 300 200 100 Slip rates: "San Andreas Fault: SCEC (20031 "Terrane displacement: Jarrard (1986) o 20 25 3 C Duration of slip [m.y.] Figure 3.4. Duration of dextral slip versus cumulative displacement for slip rates of 1.5 and 3.5 cm/year. 74 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the plate convergence rates must have been significantly higher than today. These requirements cast doubt on the assumption that 3.5 cm/year is a representative slip rate for the MSF during the Late Cretaceous, and a rate of 1.5 cm/year seems to be more appropriate. The most plausible location for the trace of the MSF in the vicinity of the Snow Lake pendant would be between the Snow Lake and Saddlebag Lake pendants (Schweickert and Lahren, 1993). Furthermore, a strike-slip fault of the proposed magnitude of ca. 200-400 km should leave clear structural evidence in rocks that were affected by the MSF. However, my fieldwork in the Piute Meadow pendant revealed that structures suggestive of the required displacement are missing. Some authors have argued that emplacement of the Sierra Nevada batholith obliterated earlier structures (e.g. Lahren et al., 1990). However, Cathedral Peak granodiorite truncates earlier contacts and structures in rocks of the Piute Meadow pendant discordantly, and there is no evidence for penetrative ductile emplacement-related deformation. In addition, the reverse faults, the folded dikes of Cathedral Peak granodiorite, and the NNW-striking magmatic foliations in Cathedral Peak granodiorite strongly suggest that the Piute Meadow pendant underwent contractional deformation at pre- to syn- 8 8 Ma. Hence, direct evidence for the MSF in a key position is lacking. However, an alternative scenario can be constructed that accounts for ca. 200 km dextral displacement even if a continuous MSF structure did not exist. 75 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PMP Gem Lake shear zone ca. 20 km SNP SLP TIS RRP Bench Canyon shear zone ca. 4.5 km Intrabatholithic Break 3 ca. 100 km Rosy Finch shear zone 20-70 km Hypothetical Mojave-Snow Lake fault max. 255 km (Proto-) Kern Canyon shear zone (ca.40) 6.5-13 km Garlock fault 120' 118' N Legend □ Mesozoic granitoids EZ3 Western metamorphic belt ■ Antler orogenic belt EZ3 Paleozoic miogeoclinal belt B a g Host rock pendants 100 km Figure 3.5. Schematic map showing arc-parallel shear zones in the Sierra Nevada with a dextral component of displacement (modified from Greene and Schweickert, 1995). The combined cumulative slip amounts to ca. 151 - 207 km. 76 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.5 shows the locations and cumulative displacements of other Mesozoic shear zones with a component of dextral displacement, including the Gem Lake (Greene and Schweickert, 1995), the Bench Canyon (McNulty, 1995), the Rosy Finch (Tikoff and de Saint Blanquat), the (Proto-) Kem Canyon shear zones (Busby- Spera and Saleeby, 1990), and Intrabatholithic Break 3 (Stevens et al., 1992). With the exception of Intrabatholithic Break 3, Late Cretaceous activity of these shear zones, matching the newly constrained maximum duration of dextral slip on the MSF, has been documented by the authors. The combined dextral cumulative displacement of these shear zones alone is on the order of 151 - 207 km. This magnitude of dextral displacement is sufficient to explain the ca. 2 0 0 km displacement proposed by Lewis and Girty (2001). However, none of these faults are located in the Piute Meadow pendant, suggesting that either the lithological correlations between the Snow Lake and Saddlebag Lake pendants are erroneous, or that the position of the enigmatic MSF or alternative strike slip structures must lie east of the Saddlebag Lake pendant. Conclusions New fieldwork in the Piute Meadow pendant, a host rock pendant continually exposed in a key area between the Snow Lake and Saddlebag Lake pendants, revealed that structures suggestive of the proposed MSF are completely missing. Therefore, if strike-slip displacement on the order of hundreds of km occurred in the 77 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Late Cretaceous, a structure of the proposed magnitude of displacement must occur east of the Saddlebag Lake pendant. In any case, considering the plate tectonic framework during the Late Cretaceous, differential displacement of a maximum amount of ca. 2 0 0 km is unlikely to have occurred along the margin of North America during the time available. However, this displacement could have been accommodated by a single continuous MSF structure east of Saddlebag Lake or collectively by several adjacent shear zones. 78 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 4: MATERIAL TRANSFER, DEFORMATION MECHANISMS AND RHEOLOGY IN PLUTON AUREOLES: IMPLICATIONS FOR MELT- ASSISTED DEFORMATION Introduction This paper presents direct evidence for melt-induced rheological softening during pluton emplacement into the upper crust. Finite strain analysis in aureoles of the Mount Stuart batholith (MSB) and the Tuolumne Intrusive Suite (TIS) show a dramatic increase of finite strain towards the pluton margins. Microscopic observations in the TIS reveal that a melt-induced transition from dislocation creep to diffusion creep and grain boundary sliding spatially coincides with extreme z-axis shortening values. Our results document the critical importance of magma emplacement on the rheology of pluton aureoles and the lower crust. The role of magmatism on crustal rheology has been investigated in previous field studies (e.g. Hollister and Crawford, 1986; Davidson et al., 1992; Hollister, 1993; Tommasi et al., 1994; Nelson et al., 1996; Neves et al., 1996; Brown and Solar, 1998). From these studies it seems obvious that magma emplacement and/or partial melting is often associated with strain localization. Rock and rock analog deformation experiments confirm that major rheological changes occur during deformation of partially molten rocks (Dell’Angelo et al., 1987; Kohlstedt, 1992; Rutter, 1997). On the other hand, melt-induced weakening can be followed by hardening after crystallization and thus the crust may harden to strengths even 79 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. greater than the pre-melting strength (Handy et al., 2001). Hence, magma emplacement into the crust yields substantial rheological changes, regardless of whether weakening or hardening results from partial melting or solidification, respectively. However, in view of the possibility that melt may trigger tectonic surges (e.g. Hollister and Crawford, 1986) the primary objective of this study is to document amplification of finite strain due to the presence of melt during deformation. Despite the seemingly common association of melt and strain localization at various scales in the field, it is often difficult to ascertain whether emplacement of melt triggers deformation or vice versa. This chicken-and-egg problem, which also has fundamental implications for melt segregation, is currently matter of active debate (e.g. Schmidt and Paterson, 2000; Brown and Rushmer, 1997; Marchildon and Brown, 2002). Any causal relationship between melt and deformation can best be addressed by trying to tie together microstructures, metamorphic processes, and finite strains. For example, the transition from low-to intermediate-temperature melt-absent deformation mechanisms to high-temperature deformation mechanisms which require the presence of melt, would be in favor of melt-induced enhancement of deformation, particularly if such a transition went along with abruptly increasing finite strain. Contact aureoles are ideal geologic settings to examine the effect of melt on deformation for the following reasons: (1) magma emplacement into the crust installs pronounced temperature gradients, (2 ) if suitable rock types are present, 80 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. prograde metamorphic reactions record temperature changes, (3) temperature- induced transitions in deformation mechanisms take place (e.g. Rosenberg and Stuenitz, 2003; Vernon et al., in press), and (4) finite strains commonly increase towards pluton margins (e.g. Paterson and Vernon, 1995; Johnson et al., in press). Furthermore, the majority of exposed plutons reveal steep contacts with their host rocks, rendering investigations along desirable true margin-perpendicular transects a relatively simple task. Additionally, the ubiquitous exposure of aureoles in arcs provides much more opportunity to study rheological changes compared to the lower crust, because some aureoles (i.e., in particular those of the upper crust) are subject to steep temperature gradients similar to those across the entire crust, but condensed into much shorter length scales. We present field and microstructural evidence for a striking spatial coincidence of a melt-induced transition in dominate deformation mechanisms and abruptly increasing finite strain in aureoles of the Mount Stuart batholith (MSB), Washington and the Tuolumne Intrusive Suite (TIS), California. In the TIS, deformation mechanisms change from low-temperature dislocation creep through high- temperature dislocation creep ultimately to melt-assisted diffusion creep and grain boundary sliding at the pluton margin. Metamorphic phase assemblages and microphotographic evidence support that temperatures of at least 650 °C coincide with the onset of melt-assisted superplasticity and an abrupt increase in finite strain at a distance of ca. 100 m to the pluton margin. Our results are consistent with the 81 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. concept of melt-enhanced deformation in that the onset of partial melting and associated superplasticity triggers dramatic rheological changes in contact aureoles. Material transfer Mount Stuart batholith (MSB) The results of recent fieldwork in the Ingalls Complex located to the southeast of the MSB revealed that deflection of regional markers occurred during emplacement of the mushroom-shaped part of the pluton. Regional markers are represented by the dominate metamorphic foliation and contacts between units which were interpreted to be cryptic strike-slip faults (Miller and Mogk, 1987). The markers consistently display pre-intrusive regional E-W striking orientations outside of the aureole of the MSB. Figure (Fig. 4.1) illustrates the geometrical relationships of the marker deflection. A large diabase body and serpentinite are located in the northern and southern halves of the map in figure (4.1), respectively. Deflection of the diabase and the serpentinite occurred by clockwise and counterclockwise rotation, respectively, from a regional E-W strike toward NNE thus paralleling the margin of the mushroom-shaped part of the MSB. In addition, the E-W striking regional metamorphic foliation is similarly rotated toward parallelism with the NNE striking margin of the MSB. Both markers, the contacts and the regional foliation indicate a width of the structural aureole of ca 2.3 km. This is consistent with a ca. 2 km wide 82 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. □ □ □ □ / Inaccessible-not mapped Q uaternary Basalt Tonalite Diabase Gabbro Peridotite Serpentinite Metasediments M SB Ingalls Complex 70° - 89° dip of foliation Sample location 1 km —I A Figure 4.1. Geologic map of the southeastern aureole of the Mount Stuart batholith, Washington. 83 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. thermal aureole defined by mica pseudomorphs after cordierite (Paterson et al., 1994). In addition to deflection of regional markers, the aureole of the MSB is marked by the occurrence of folded dikes which are compositionally and petrographically similar to the tonalite exposed in the southeastern part of the MSB. Criteria outlined by Paterson et al. (1998) suggest that the folding occurred under magmatic to submagmatic conditions (see thorough discussion in chapter 6 ). Folding under magmatic conditions in turn implies that the deformation and emplacement occurred simultaneously. The folding in the aureole and an increase in the intensity of the metamorphic foliation toward the contact in combination with a general absence of brittle structures indicate that the dominate material transfer process (MTP) during emplacement of the southeastern part of the mushroom-shaped of the MSB was ductile flow. Tuolumne Intrusive Suite (TIS) In order to examine material transfer processes (MTPs) associated with emplacement of the TIS we performed detailed fieldwork in two selected host rock pendants around the TIS. Based on general and structural geologic key relationships, the Piute Meadow pendant (PMP), located at the northern end of the TIS, and the Saddlebag Lake pendant (SLP) at the eastern TIS margin were selected for field 84 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. campaigns. The dominate MTPs include stoping and ductile flow, which are described in detail below. Stoping Before presenting evidence for stoping in the Piute Meadow pendant (PMP) and the Saddlebag Lake pendant (SLP), I wish to emphasize that a debate exists in the literature as to what kind of evidence is of highest significance to document stoping in the field. The debate has arisen due to the fact that stoped blocks are usually not volumetrically significant in intrusions, suggesting that stoping cannot be very important in accommodating local space for an intrusion. In contrast, recent views suggest that the lack of compelling evidence (i.e., preservation of stoped blocks in intrusive rocks) is the best evidence for the efficiency of stoping. For example, the rate at which stoped blocks sink is much greater than the rate at which magmas crystallize, and hence only blocks formed during final crystallization should be trapped within a chamber (Paterson and Okaya, 1999). Hence, abundant discordant intrusive contacts may indirectly indicate that stoping is a pervasive and ubiquitous magma emplacement mechanism throughout the crust (Yoshinobu et al., 2003). The structural relationships in the PMP are characterized by discordant contacts. All NNW-striking structures and contacts that pre-date emplacement of TIS magma are truncated at high angle along the stepped but mostly ENE-striking pluton margin and hence this scenario can best be described as a perfect example of what is 85 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. informally referred to as a “cookie-cutter” relation. The truncation can be observed at various scales, ranging from a 1:24,000 map scale (Fig. 2.6) to the outcrop scale (Fig. 4.2). Although stoped blocks occur quite ubiquitously along the margins of intrusive rocks of other Cretaceous plutons in the Sierra Nevada (e.g., Pignotta et al., 2001), stoped blocks are mostly absent in the intrusive rocks of the PMP. Likewise, evidence for stoping is present in the SLP. Although the pre-existing anisotropies and the pluton margin generally show concordant relationships (i.e., both strike NNW), and hence truncation can be difficult to recognize, structures and contacts are locally truncated discordantly. For example, the pluton margin is characterized by distinctive along-strike steps (e.g. the step in the Horse Canyon sequence in the northern third of Fig. 2.6) indicating that material has been removed and that stoping contributed to local space accommodation in the SLP. hi addition, stoped blocks and rafts of various sizes occur close to the pluton margin. Small blocks are a few cm in length and width. The largest block found is approximately 100 m long and shown on the 1:24,000 scale map of Figure 2.6. The long axes of the stoped blocks are typically oriented parallel to the margin of the TIS. Ductile deformation The following statement of Bateman (1992) suggests that ductile flow was the most important material transfer process (MTP) that operated during emplacement of the TIS: “Episodic emplacement of the plutonic magmas, which expanded as they 86 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.2. Photograph showing outcrop-scale truncation of pre-Tuolumne stratigraphy and structure. Lens cap (ca. 5 cm in diameter) for scale. Cathedral Peak granodiorite occurs on the left hand side. Note aligned K-feldspar megacrysts. Rhyodacitic host rock is shown on the right hand side. Note truncation of north- northwest-striking host rock foliation (parallel to long side of photograph). 87 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. were emplaced, undoubtedly accounts for much of the deformation of the wall rocks of the batholith and of the country rock remnants within the batholith”. Consequently, the TIS is an ideal location to study the intensity of ductile aureole deformation related to magma emplacement. Furthermore, the contrasting structural relationships between the PMP and the SLP (i.e., pre-existing anisotropies at high angle to pluton margin in the PMP versus parallelism between anisotropies and pluton margin in the SLP) are particularly interesting with regards to quantifying the intensity of ductile deformation. Assuming that ductile shortening normal to the TIS margin occurred homogeneously along the length of the margin, the angular discrepancies between the PMP and SLP should result in fundamentally different interactions between pre-existing fabric ellipsoids and the superposition of a subsequent emplacement-related fabric ellipsoid. During emplacement-related deformation, the long axes of the pre-existing and the emplacement-related fabric ellipsoid should be at a high angle. In contrast, the two long axes of the pre-existing and emplacement-related fabric ellipsoid should be parallel to each other in the SLP. Hence, the resulting finite strains should be higher in the aureole of the SLP compared to the PMP aureole (Fig. 4.3). This hypothesis is tested below. Ductile structures presumably related to magma emplacement (but see discussion below) in the aureoles of the PMP and the SLP are manifest in subvertically plunging stretching lineations (4.4). However, the field observations also suggest that ductile flow in the PMP was significantly less important than in the SLP. Except for very local deflection of a contact between andesitic and rhyodacitic 88 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 2 10 km Figure 4.3. Schematic map showing hypothetical finite strain ellipsoids at the northern and eastern margin of the Tuolumne Intrusive Suite, California. Black ellipses indicate regional strain, white ellipses depict emplacement-related strain, and grey ellipses indicate resulting finite strain. 89 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Piute Meadow pendant Saddlebag Lake pendant ° O * • • • oD “ o e • . «» »!„ - P • < e « a l ' o " ' n=348 n=233 Figure 4.4. Stereoplots of host rock stretching lineations in the Piute Meadow and Saddlebag Lake pendants. 90 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. metavolcanics by counter-clockwise rotation (NE comer of map in Figure 3.2), reorientation of the pre-emplacement NNW-striking metamorphic foliation occurs only locally and these instances it is concentrated in extremely narrow zones which display maximum widths of 1-2 m. Strain markers represented by lithic fragments in metavolcanic rocks lack a gradient in finite strain across the aureole. In contrast, the recent field observations in the SLP reveal that intense ductile deformation occurred during emplacement of the TIS. The immediate contact zone of the aureole is marked by boudinage of competent layers in two roughly mutually perpendicular directions. The deformation resulted in quite illustrative examples of chocolate tablet boudinage (2.10). Furthermore, the aureole is characterized by the occurrence of magmatically folded leucocratic dikes, similar to those described in the aureole of the Mount Stuart batholith (MSB) above. The fact that folding of these dikes occurred before solidification of the magma was complete strongly suggests that magma emplacement was accompanied by ductile aureole shortening. The magmatically folded dikes in the MSB and the TIS carry important implications for aureole rheology but this issue is discussed in considerably more detail in chapter 6. The SLP contains rock types some of which correlate with the metavolcanic rocks of the PMP. Both, andesitic and rhyolitic to dacitic metavolcanics are present and sometimes they are interlayered. Hence, similar strain markers can be examined to constrain finite ductile strains associated with magma emplacement. It also turns out that strain markers are much more abundant in the SLP. Additional strain 91 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. markers are found in the form of mostly quartzite- and chert-bearing conglomerates. The field observations in the SLP show a pronounced gradient in finite strain that is particularly distinctive within a distance of ca. 1 0 0 m from the pluton margin, indicating that the ductile structural aureole surrounding the TIS in the SLP is relatively narrow. The field observations summarized above yield a semi-quantitative picture of the distribution of ductile strain in the aureoles of the PMP and the SLP. It is obvious that ductile flow during magma emplacement was essentially absent in the PMP and much more important in the SLP. However, in order to more accurately document the difference in intensity of strains between the two pendants, quantitative finite strain analysis is required. Finite strain analysis Based on the different structural relationships in the Mount Stuart batholith (MSB) and the Tuolumne Intrusive Suite (TIS), I applied several methods of finite strain analysis. To derive finite strains from deflection of regional markers in the MSB, I utilized angular changes between host rock contacts and the pluton margin. To perform strain analysis on strain markers in host rocks of the TIS two methods were used: an algebraic equivalent of the R f/(j) technique, and a Fry-type center-to- center technique. I describe the methods in detail below. 92 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Methods M ount Stuart batholith (MSB) Fieldwork was performed in the southeastern part of the aureole of the mushroom-shaped part of the MSB because deflection of host rock markers here is strongest. Deflection by clockwise and counterclockwise rotation of the diabase and the serpentinite, respectively, was utilized to determine the shear strains associated with this deformation. Application of this method requires the assumption that the deflection occurred as rigid rotation under constant volume and plane strain conditions. The angular change between a reference line and another that was originally perpendicular to it can be converted to shear strain y according to 7 = ta n ^ , (1) where IfTs the angle between the reference line and the deflected line (Ramsay and Huber, 1983). The NNE-striking southeastern pluton margin of the mushroom shaped part of the MSB represents the reference line and the rotated contacts of the diabase and serpentinite with adjacent units represent the deflected lines. The resulting shear strains were used to calculate the ellipticity or ratio R of the two- dimensional strain ellipses applying the equation 93 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Subsequently, the ellipticity was used to reconstruct the radius r of the unit circle using r - V R (3) where x and z are the long and short axis of the strain ellipse, respectively. The bulk aureole shortening was then determined according to (4) /o where 1 $ is the undeformed width of the aureole (i.e., the radius of the unit circle) and I] the width of the shortened aureole indicated in the field by the area over which the marker deflection occurs. 94 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Tuolumne Intrusive Suite (TIS) In general, the rocks in the TIS are very well suited for quantitative strain analysis. I utilized abundant lithic fragments in andesitic to rhyodacitic metavolcanics and quartzitic to cherty components of metaconglomerates as strain markers, respectively. The andesitic metavolcanic rocks carry abundant felsic lithic fragments, and conversely, the felsic metavolcanics contain more mafic lithic fragments. This color contrast rendered field-based strain analysis a relatively simple routine. Furthermore, excellent exposure conditions allowed measurement of the long and short axes of strain markers on at least two principal surfaces in the field. Whenever possible, a minimum of 30 measurements was taken per surface. In many places, however, a sufficient amount of strain markers was not present, and in these cases 3-5 measurements were taken to obtain at least a crude estimate of the finite strain. In addition to acquiring strain data where ever possible in the field, data and samples were collected along two roughly E-W striking (i.e., normal to the pluton margin) transects (transects A and B in Fig. 4.5). Samples of conglomerate were collected and strain analysis was performed in the laboratory after the samples had been prepared using in-house rock cutting facilities at USC. Three mutually perpendicular cuts were made on each sample parallel to the principal surfaces of the fabric ellipsoids. The R f/< |) technique (Ramsay and Huber, 1983) and algebraic methods of Shimamoto and Ikeda (1976) were applied to calculate average two- dimensional strain ellipses for each of the three surfaces in rocks that contain strain 95 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 k m Figure 4.5. Schematic geologic map of Saddlebag Lake showing sample locations along two margin-parallel transects. The lithology is shown in Figure 6.2. 96 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. markers. Fry’s (1979) center-to-center technique was used to calculate the two- dimensional strain ellipses in rocks that lack strain markers. Subsequently, the two- dimensional strain ellipses were algebraically combined to determine the three- dimensional ellipsoids (Shimamoto and Ikeda, 1976). A complication in finite strain analysis arises due to depositional and diagenetic processes that impart a fabric to rocks. The effect of primary fabrics needs to be considered because the true tectonic strain can be obscured by the presence of primary fabrics (e.g. Paterson and Yu, 1994; Paterson et al., 1995). Depending on the orientations of the ellipsoids with respect to each other, subsequent superposition of a tectonic fabric onto rocks with primary fabric results in higher or lower intensities of finite strains. Where appropriate, algebraic corrections were made using an average primary fabric ellipsoid derived from appropriate rock types. A basic premise of the correction applied in this study is that the interaction between the primary fabric ellipsoid and the tectonic strain ellipsoid occurred coaxially. This assumption accounts for the maximum effect of primary fabrics but it potentially underestimates the intensity of the tectonic strain. A useful way of representing three-dimensional strain data is plotting them on a Flinn diagram (Flinn, 1962). The Flinn diagram is a graphical representation of the shape of a strain ellipsoid made by plotting the yz- ratio on the x-axis and the xy- ratio on the y-axis. The diagram is divided into an upper field of constrictional strains where prolate strain ellipsoids plot. The lower field represents flattening strains and oblate shapes plot in this field. The line dividing the two fields denotes 97 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. plane strain. The three-dimensional strain data were separated by rock type and plotted on modified Flinn diagrams using the routines of Ramsay and Huber (1983) and Twiss and Moores (1992). Logarithmic K-values were calculated to represent the shape of the strain ellipsoids numerically (Ramsay and Huber, 1983). Results M ount Stuart batholith (MSB) Applying the shear strain approach explained above to the deflection of the diabase body and the serpeninite in the aureole of the MSB (Fig. 4.1) yields a bulk aureole shortening of ca. 48%. Although the shortening was estimated using rigid marker rotation, the deformation obviously must have been accommodated by gray scale ductile deformation in the surrounding rocks. Due to likely gradients in temperature in the aureole, the finite strains should follow a gradient with increasing strain intensities toward the pluton margin. However, the lack of suitable outcrop- and handspecimen-scale strain markers unfortunately renders documentation of strain gradients in the southeastern aureole of the MSB an impossible task. 98 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Tuolumne Intrusive Suite (TIS) Before describing the results of strain analysis in detail I note that, despite the requirement of measuring a sufficient amount of axes lengths (e.g. Paterson, 1983, and references therein), detailed strain measurements (30 measurements per surface) and preliminary measurements (3-5 measurements) in the same outcrop essentially yielded identical results. Hence, in the following section no distinction is made between detailed and preliminary measurements. The strain data include z-axis shortening values derived from magmatically folded dikes that also occur very close to the pluton margin. The dikes and the method used to constrain strains are explained in chapter 6 . All strain data are listed in detail in appendix A. I recall that an important motivation for strain analysis in the TIS was the hypothesis that due to how the pre- emplacement and emplacement-related strain ellipsoids combine, the finite strains should be significantly higher in the Saddlebag Lake pendant (SLP) compared to the Piute Meadow pendant (PMP). In order to test this hypothesis, the yz-ellipses of the finite strain ellipsoid were plotted on maps of the PMP (Fig. 4.6) and SLP (Fig. 4.7). The stretching lineations in both host rock pendants plunge subvertically and hence the representation of the yz-ellipses parallel to the map surfaces closely approximate the true yz-ellipses in the field. Comparing the ratios and spatial distribution of the yz-ellipses of the finite strain ellipsoids in the host rocks of the PMP and SLP reveals two important findings: (1) the magnitudes of finite strains in both the PMP and the SLP are remarkably similar; and (2) a 99 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.6. Schematic geologic map showing the distribution of yz-ellipses of t± strain ellipsoids in the Piute Meadow pendant. The lithology is shown in Figure Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 38'OCT v£ 0 1 2 km Figure 4.7. Schematic geologic map showing the distribution of yz-ellipses of the strain ellipsoids in the Saddlebag Lake pendant. The lithology is shown in Figure 6.2. 101 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. pronounced strain gradient involving increasing strain intensities towards the pluton contact can only be observed in the SLP. These findings strongly suggest that the finite strains in the PMP have no relevance regarding syn-emplacement material transfer processes (MTPs). In contrast, ductile flow associated with magma emplacement was much more important in the SLP and therefore the results presented below strictly apply to the SLP. Displaying all initial strain data on a Flinn diagram shows that they define a linear trend and most plot in the flattening field relatively close to the plane strain line (Fig. 4.8). The data with the lowest strain intensities come to fall exactly on the plane strain line. The line representing the data departs from the plane strain line and progressively shifts into the flattening field with increasing strain intensities. The following observation indicates that further data processing was necessary: the intensities of finite strain retrieved from Fry analysis are systematically lower than those obtained from applying R f/(j) and algebraic techniques to rocks that bear strain markers in the form of lithic fragments (see appendix A). Hence, it is a plausible assumption that the lithic fragments were more sensitive to the formation of primary fabrics by depositional processes than the matrix. Consequently, I assume that the finite strains retrieved from analyzing lithic fragments yield composite strain ellipsoids that consist of a primary fabric and a tectonic fabric component. On the other hand, samples of metavolcanics that lack lithic fragments presumably recorded tectonic and/or emplacement-related strain only. 102 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Saddlebag Lake Pendant E2-E3 Figure 4.8. Flinn diagram of strain data from the Saddlebag Lake pendant Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Plotting the corrected data on a Flinn diagram shows that the data arrangement with respect to the plane strain line changes significantly (Fig. 4.9). The corrected data fall onto a straight line subparallel to the plane strain line. The samples on which Fry analysis was performed were not corrected and hence remain on the plane strain line. The parallelism between the data and the pane strain line can be explained in terms of volume loss. The dashed lines in figure (Fig. 4.10) indicate the position of the plane strain line for various amounts of volume loss. The best fit between the data and the modified plane strain line is obtained at ca. 10-30% volume loss. Despite the well-defined linear pattern of the corrected data, a few conglomerate samples yield finite strains that plot in the field of constriction (Fig. 4.10). At first view, this misfit seems to constitute an inconsistency with the main data set. However, during deformation under certain circumstances, viscosity contrasts between inclusions and the surrounding matrix in conglomerates can result in the generation of constrictional fabric ellipsoids, although predominantly plane strain deformation may have taken place. Freeman (1987) investigated this effect using a theoretical approach. He found that, during progressive plane strain deformation of matrix-supported conglomerates, the higher the viscosity contrast the more the resulting strains shift into the constrictional field on a Flinn diagram, resulting in apparent prolate fabric ellipsoids (Fig. 4.11). The conglomerates sampled in the SLP reveal matrix-supported characteristics. Most components are not in contact with each other and they are entirely surrounded by a fine-grained matrix. Hence, a likely 104 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Saddlebag Lake Pendant (corrected) 0 12 3 E2-E3 Figure 4.9. Flinn diagram of corrected strain data from the Saddlebag Lake pendant 105 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Saddlebag Lake Pendant (corrected) k = i E2-E3 Figure 4.10. Flinn diagram showing possible amounts of volume loss. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. E1-E2 N' E2-E3 Figure 4.11. Relationship between viscosity contrast and position in the Flinn diagram (modified from Freeman, 1987). 107 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. explanation for the observed prolate fabrics is that plane strain deformation of conglomerates with viscosity contrasts between inclusions and matrix progressively shifted the data into the constrictional field. Although plotting the data on a Flinn diagram is helpful in evaluating the quality of the data and obtain general insights into the nature of the finite strain, it reveals no information about the spatial distribution of the types of strain and the magnitude of shortening. To explore the relationship between the strain types and shortening as a function of position in the aureole, I represent the K-values and values of z-axis shortening along transects A and B (Fig. 4.13). The spatial distribution of K-values shows a remarkably homogenous pattern. The data fall on a well-defined straight line consistent with average K-values of ca. 0.7 to 0.9. A gradient or change in type of strain towards the pluton margin could not be observed. Hence, in terms of strain type, the host rock is marked by homogeneous plane strain. Plotting the strain intensities as a function of distance to the pluton margin reveals a different pattern. Despite a large amount of scatter in the data, both transects, A and B show an increase in z-axis shortening towards the margin (Fig. 4.13). The magnitude of finite z-axis shortening dramatically increase from an average amount of ca. 43% to ca. 70% and ca. 85% in transect A and B, respectively. The increase in z-axis shortening occurs in a ca. 100 m wide portion of the host rock. 108 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Northern T ransect (A ) (original data) 0.8 Distance from contact [m] Southern T ransect (B) (original data) to 1.0 X 0.8 Distance from contact [ml Figure 4.12. Spatial distribution of K-values along margin-parallel transects A and B. Solid line represents plane strain. 109 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. % z-axis shortening % z-axis shortening Northern Transect (A ) (corrected for primary fabrics) 0 200 400 600 800 1000 1200 1400 1600 Distance from contact [m] Southern Transect (B) (corrected for primary fabrics) -*r.r - - - f - — - — t 100- 90- 80- 70 I 60- 50- 40- 301 2 0 - 1 °- 0 I " i " T"111 i I " i r- - r 1 1 1 i 0 200 400 600 800 1000 1200 1400 1600 1800 Distance from contact [m] IT Figure 4.13. Spatial distribution of z-axis shortening values along margin-parallel transects A and B. 110 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Microscopic observations Bob Miller kindly provided thin sections prepared from samples collected throughout the Ingalls Ophiolite complex in the southeastern part of the mushroom shaped part of the Mount Stuart batholith (MSB). In the Tuolumne Intrusive Suite (TIS), oriented samples were collected along two transects normal to the pluton margin in the Saddlebag Lake pendant (SLP) (Fig. 4.5). Additional samples were collected off the transects in the SLP. 2x3 inch large thin sections were prepared parallel to the lineation and perpendicular to the foliation. Care was taken to avoid samples marked by strong hydrothermal alteration. Where appropriate, I describe the microstructures together with metamoiphic phase changes. The great majority of the thin sections from the MSB show strong evidence for static recrystallization. The thin sections show strain-free and equidimensional grains of plagioclase, hornblende, biotite and opaque oxides with well-developed 120 0 triple junctions between them. The homogeneous nature of static recrystallization throughout the aureole suggests that a thermal event lasted long enough to affect the aureole rocks of the MSB equally. Hence, any microstructures formed during emplacement of the mushroom-shaped part most likely were obliterated by subsequent static recrystallization. However, a few thin sections prepared from host rocks at the contact to Mount Stuart tonalite reveal the following rheologically significant features: several triple junctions between grains of the same or different mineralogy contain both anhedral quartz and/or feldspar (Fig. 6.5L). The 111 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. enclosed minerals strongly resemble interstitial feldspar observed in statically recrystallized granite generated by crystallization of partial melt between grains (Rosenberg and Riller, 2000). Hence, a plausible interpretation is that partial melting took place in the MSB aureole right at the pluton margin. Strong evidence for static recrystallization could not be found in the aureoles of the TIS, and hence samples collected in the SLP provide much more information about changes in deformation mechanisms and metamoiphic phase transitions. Below, I document both microstructural and metamorphic changes in the SLP as a function of distance to the pluton margin. I present data from selected thin sections representative of different rock types that are exposed in the SLP from east to west. Microstructures diagnostic of ductile deformation are documented first, followed by a description of evidence for brittle deformation. Ductile deformation Candelaria Formation, conglomerate o f Cooney Lake and rhyolitic ash-flow tu ff o f Saddlebag Lake The lithology in the easternmost part of the SLP is defined by fine-grained quartzo-feldspatic silt- and sandstones of the Candelaria formation that in some places show preserved cross-bedding as well as bands and lenses of Cooney Lake conglomerate. The occurrence of chiastolite (Fig. 4.14A), a version of andalusite 112 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.14. Photomicrographs of host rocks of the Saddlebag Lake pendant. Abbreviations: And= andalusite, Qz= quartz, Plg= plagioclase, Bio= biotite, Mus= muscovite, Sill= sillimanite, Ep= epidote, Hbl= hornblende, Opx= orthopyroxene, Cpx= clinopyroxene. A. Conglomerate of Cooney Lake with inter- or syn-tectonic andalusite. Foliation wraps around andalusite crystal. Matrix shows rare inhomogeneously flattened quartz grains. B. Rhyolitic ash-flow tuff of Saddlebag Lake. Quartz phenocrysts are marked by patchy to sweeping undulose extinction and deformation bands. C. Metaandesite. Plagioclase shows evidence for brittle fracturing. Note the bookshelf-type separation of a phenocryst in the lower half of the photograph. Biotite grows in plagioclase cracks and forms coalescing bands. D. Rhyodacitc ash-flow tuff. Matrix grains are markedly larger than those in Fig. 4.14B. Quartz phenocryst show blocky, typical “chessboard” extinction patterns which indicate simultaneous basal and prismatic slip in quartz. E. Horse Canyon sequence. Neocrystallization of fibrous sillimanite and K-feldspar indicate the second sillimanite isograd. F. Horse Canyon sequence. Coexistence of hornblende, clinopyroxene and orthopyroxene, suggesting that the intersection of the enstatite- and diopside-producing reaction lines is preserved. G. Horse Canyon sequence. Triple-grain interstices are filled with quartz which is characterized by weak undulose extinction. H. Horse Canyon sequence. Quartz-filled triple-grain interstices. Note that all minerals appear strain-free. I. Horse Canyon sequence. Four-grain junction depicted in box suggests that grain boundary sliding may have accommodated large strains. 113 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 114 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.14. Continued. 115 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. characteristic for contact aureoles (Deer et al., 1992), is indicative of temperatures of ca. 350-700 °C (Bohlen et al., 1991) in the pressure range of interest (2-3 kbar, Ague and Brimhall, 1988; Webber et al., 2001). Andalusite preserved inclusions associated with crystal growth, yielding the characteristic chiastolite pattern (Deer et al., 1992). The lack of additional inclusion patterns that might mimick the external foliation render determining timing relationships between andalusite growth and deformation difficult. However, the observation that the foliation wraps around andalusite crystals (Fig. 4.14A) is consistent with inter- or syn-tectonic growth of andalusite. Within the generally fine-grained quartz- and subordinately feldspar- bearing rocks, some variation in grain size in alternating bands can be observed. Some domains of slightly larger grain size likely represent recrystallized conglomerate components. The observed small grain size renders recognizing diagnostic micro structures difficult. Nonetheless, some grains with various degrees of weak elongation and very rarely bulging grain boundaries can be observed (Fig. 4.14A). Such inhomogeneously flattened grains indicate dislocation creep regime 1 of Hirth and Tullis (1992). Stipp et al. (1999) observed similar bulging recrystallization in quartz at temperatures of up to 350 °C. The rhyolitic ash-flow tuff of Saddlebag Lake marks the host rock west of the sedimentary units. Thin sections show a well-preserved poiphyritic texture. Roughly equidimensional quartz phenocrysts are embedded in a fine-grained matrix consisting mostly of quartz, plagioclase and subordinately biotite and muscovite. 116 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Quartz grains in the fine-grained matrix locally show very weakly lobate grain boundaries that strongly resemble those observed in the metasedimentary units described above. Additionally, the larger quartz phenocrysts show patchy to sweeping undulose extinction and the development of deformation bands (Fig. 4.14B). Despite the lack of evidence for metamorphic phase transitions in the rhyolitic ash-flow tuff of Saddlebag Lake, the microstructure indicates that low- temperature dislocation creep at ca. 300-400 °C was the dominate deformation mechanism of crystal-plastic creep in metasedimentary rocks of the Candelaria formation and the Cooney Lake conglomerate as well as in the rhyolitic ash-flow tuff of Saddlebag Lake. Andesitic metavolcanics To the west of the rocks in the SLP described above andesitic metavolcanics are exposed. These rocks show well-preserved porphyritic textures. Plagioclase phenocrysts are embedded in a fine-grained matrix containing plagioclase, biotite and minor amounts of quartz and opaque minerals. Biotite occurs as coalescing bands that define the foliation (Fig. 4.14C) together with aligned plagioclase phenocrysts. Sub- to euhedral shapes of plagioclase and the lack of recrystallized grains along the margins indicate that plagioclase did not undergo crystal-plastic deformation. Instead, some plagioclase phenocrysts show clear evidence for cracking and boudinage (Fig. 4.14C). In addition to the occurrence of biotite in 117 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. shear bands, it is noticeable that the inter-boudin regions of brittlely deformed plagioclase are marked by neocrystallization of biotite, suggesting that its growth took place during deformation. Biotite shows evidence of solid-state deformation. Kinking is absent, indicating that distribution of biotite in shear bands was largely achieved by crystal-plastic flow. However, due to the small grain size the possibility exists that evidence for brittle behavior of biotite and subsequent reorientation and/or recrystallization was overseen. In any case, the exclusively brittle behavior of plagioclase and the foregoing evidence for dislocation creep in quartz-dominated rocks are consistent with deformation temperatures of ca. 350-450 °C in the metaandesitic unit. Rhyodacitic ash-flow tu ff Rhyodacitic ash-flow tuff marks the host rock west of the andesitic metavolcanics. The preserved textures strongly resemble those of the rhyolitic ash- flow tuff of Saddlebag Lake further east (see above). In fact, the rocks are so similar in terms of composition and texture that they can be viewed as the equivalent to the rhyolitic ash-flow tuff of Saddlebag Lake closer to the pluton margin. A fine grained matrix dominated by quartz and feldspar contains larger quartz phenocysts. Due to the relatively simple mineralogy, metamorphic phase transitions did not occur. The matrix grains are considerably larger than those in the rhyolitic ash-flow tuff of Saddlebag Lake. Despite the larger grain size, they reveal lobate grain 118 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. boundaries that bulge into adjacent grains indicating grain boundary migration accommodating recrystallization (Fig 4.14D). Grain boundary migration recrystallization is diagnostic of Hirth and Tullis’ (1992) dislocation creep regime 3 observed at high laboratory temperatures. Stipp et al. (1999) suggested that grain boundary migration recrystallization in naturally deformed quartz occurs at temperatures above 500 °C. Quartz phenocrysts are progressively more flattened than those in the rhyolitic ash-flow tuff of Saddlebag Lake. The quartz phenocrysts reveal characteristic chessboard patterns defined by alternating rectangular extinction (Fig. 4.14D). Such quartz chessboard patterns indicate simultaneous operation of the prismatic and basal slip systems in quartz. Kruhl (1996) systematically examined chessboard patterns in quartz-bearing rocks in various geologic settings and estimated temperatures typically in the range of 600-750 °C. Though diagnostic metamorphic mineral assemblages are absent, the microstructures described are compatible with high-temperature dislocation creep at ca. 500-750 °C. H orse Canyon Sequence The thin sections described next were prepared from samples of volcanic sandstones and calc-silicate rocks of the Horse Canyon sequence (Schweickert and Lahren, 1993). These rocks occur in the northern half of the contact zone in the SLP. The most significant observation with regards to constraining temperature is the breakdown of muscovite and the neocrystallization of both fibrous sillimanite and K- 119 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. feldspar (Fig. 4.14E). The coexistence of these metamorphic reactants and products indicates that metamorphic reactions proceeded at the second sillimanite isograd at which presumably most sillimanite is produced during prograde metamorphism (Spear, 1993). The P-T diagram commonly used for constraining temperatures during metamorphism of politic rocks indicates that, at pressures of 2-3 kbar, the corresponding temperature lie in the range of 610 to 630 °C. Thin sections of the same rock type but right at the pluton margin reveal mineralogical relationships that imply similarly high temperatures. Figure 4.14F shows that epidote has disappeared and that hornblende, orthopyroxene and clinopyroxene coexist. A T-Xco2 diagram for calc-silicate rocks indicates that the coexistence of orthopyroxene and clinopyroxene is consistent with the intersection of the two reaction curves, which in turn define a unique point in P-T space. As a matter of fact, using pyroxenes to constrain temperature requires detailed electron microprobe analyses to determine the mineral compositions. Furthermore, the chemistry of the Horse Canyon sequence is considerably more complicated than represented in the T-Xco2 diagram of Spear (1993). However, within these limitations, temperatures of around 610 °C can be crudely estimated. Besides providing constraints on temperature, rocks of the Horse Canyon sequence reveal key information with regards to deformation mechanisms. In view of the intensely shortened rocks at the contact (see results of finite strain analysis above), microstructures indicative of grain-scale ductile deformation should be expected. However, the thin sections show a lack of lattice-preferred orientation 120 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and small, equant and essentially strain-free grains of mostly feldspar, quartz, and biotite are present. The roughly equidimensional grain shapes prevented formation of a strong shape-preferred orientation. In many places, 120° triple junctions are developed between grains and the interstices are filled with quartz. In some cases interstitial quartz shows weak undulose extinction (Fig. 4.14G) but in other cases quartz-filled interstices appear to be completely void of crystal-plastic strain (Fig. 4.14H). Locally, feldspar grains have rounded edges at grain triple junctions. The foregoing body of observations strongly suggests that the dominate deformation mechanism close to the pluton margin was superplasticity by melt-assisted grain boundary sliding. Despite the lack of direct evidence, superplasticity is the most plausible deformation mechanism to explain high strain intensities of ca. 80% shortening at the outcrop scale (Fig. 4.7) and the absence of associated intracrystalline plasticity with high strains. Grain boundary sliding is supported by the rare occurrence of four-grain junctions which may represent the intermediate stage of grain switching during superplasticity (Fig. 4.141). On the basis of these observations, the temperatures during deformation were at least as high as ca. 650 °C. In view of the proximity to the intrusive heat source, the efficiency of superplasticity might have been significantly enhanced by high temperatures and the presence of melt. In addition to higher diffusion rates, melt residing in interstices and along grain boundaries may have resulted in lubrication and an effective increase of the deformation rates (see chapter 6 ). The presence of a 121 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. free fluid during deformation is further supported by abundant fluid inclusions (Fig. 4.14H, I). Brittle deformation In addition to the ductile deformation mechanisms inferred above, the host rocks of the TIS are marked by grain-scale fracturing. All rock types are effected by fracturing but an increase of the intensity is especially noticeable toward the west, resulting in pervasive fracturing of host rocks at the pluton margin. The distinctive increase of the relative importance of grain-scale fracturing is illustrated in comparing Figure 6.50 and Figure 6.5P. The former shows quartz phenocrysts in rhyodacitic ash-flow tuff very close to the margin. The latter shows alike quartz phenocrysts in rhyolitic ash-flow tuff of Saddlebag Lake at a distance of ca. 1 km to the pluton margin. Both samples show clear evidence for grain-scale fracturing, but the fracturing is much more intense in the former sample. Quartz phenocrysts literally are mechanically abraded and small grain fragments axe smeared out along the grains. An important question is when did this fracturing occur and can it be related to deformation during magma emplacement? Microphotographic evidence shows that the cracking contributed to a qualitatively more intense foliation toward the pluton margin, indicating syn-emplacement fracturing. 122 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Interpretation of microscopic observations The microstructures and metamorphic mineral assemblages are consistent with transitions in the dominant deformation mechanism along a temperature gradient. The observations indicate progressively increasing temperatures from regional background temperatures of ca. 300-400 °C to melting temperatures of at least ca. 650 °C at the pluton margin. In its easternmost part, the rheology of the Saddlebag Lake pendant (SLP) was dominated by low-temperature dislocation creep (Fig. 4.15). With increasing proximity toward the pluton margin, high-temperature creep became the controlling deformation mechanism. Closest to the pluton margin, where the temperature was highest, deformation was dominated by melt-assisted superplasticity. Grain boundary sliding and diffusive mass transfer were most likely the main grain-scale mechanisms that accommodated superplasticity. With grain boundary sliding the mechanism for accommodating large amounts of strain is relative movement between grains. The microscopically perceived lack of intracrystalline plasticity in rocks close to the pluton margin may be interpreted as an indication for static recrystallization. However, tightly folded dikes necessitate large amounts of strain, strongly supporting grain boundary sliding or granular flow. This mechanism is presumably particularly efficient at high temperatures because diffusion rates are strongly temperature-dependant and partial melting provides melt that may have a lubricating effect on the grain boundaries. 123 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.15. Deformation mechanism map of the Saddlebag Lake pendant. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The change in deformation mechanisms toward the pluton margin was accompanied by metamorphic phase transitions. For example, neocrystallization of biotite generated coalescing shear bands in metaandesite that accommodated more strain than the fine-grained quartz and feldspar matrix. The immediate contact zone is characterized by reduced grain size. Originally poiphyritic textures in metavolcanic rocks were obliterated. Muscovite broke down on the expanse of metamorphic growth of fibrolite and K-feldpar. The resulting small grain size may have aided superplasticity as is commonly observed in lower crustal mylonites (Behrmann and Mainprice, 1987). Grain-scale fractures are observed through out the SLP aureole whereby the amount of intra-grain fractures increases towards the pluton margin. A plausible explanation for how these fractures formed is positive volume change during dehydration melting. Rushmer (2001) performed experiments on volume change associated with dehydration melting of muscovite + biotite-bearing pelites and biotite + plagioclase + quartz gneiss samples. She observed that the samples containing muscovite developed melt-filled cracks (see also Conolly et al., 1997) while the biotite-bearing gneiss did not. An interesting implication of Rushmer’s (2 0 0 1 ) study is that external deformation may be required to segregate melt from the lower crust during partial melting. The importance of deformation regarding distributing and draining of melt is further illustrated by Rutter and Neuman (1995). The authors showed that high-strain rate deformation of Westerly granite with ca. 10% melt present resulted in distributed cataclasis. Hence, positive volume change 125 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. associated with dehydration melting of deforming muscovite might ultimately be responsible for an increasing amount of intra-grain fractures closer to the margin of the TIS. Discussion The effect of melt on deformation Our study strongly suggests that partial melting at the margin of the Tuolumne Intrusive Suite (TIS) had a dramatic weakening effect on regional deformation. Intensities of regional strain remarkably increase at a distance of ca. 100 m to the pluton contact. The increase in finite strain is accompanied by increasing temperatures and progressive transitions in the dominate deformation mechanisms. From lower to intermediate temperatures, low-temperature dislocation creep gave way to high-temperature dislocation creep, respectively. The scattered nature and lacking gradient in finite strain within these areas of intracrystalline plasticity indicate that the transition from low- to high-temperature dislocation creep was not associated with increasing finite strain. However, an abrupt increase in finite strain can be observed with further temperature increase. At the highest temperatures (at least ca. 650 °C) inferred from phase assemblages and microphotographic evidence for the presence of melt at the pluton margin, melt- and diffusion-accommodated grain boundary sliding dominated the deformation resulting in superplasticity. 126 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Superimposing temperature, finite strain and dominate deformation mechanism as a function of distance to the pluton margin reveals a striking spatial coincidence between the onset of melt-assisted superplasticity and a dramatic increase in finite strain (Fig. 4.16). Highest values of z-axis shortening of 70-85% correspond exactly to microstructural evidence of melt-assisted deformation by superplasticity. We recall that the finite strain recorded in the Saddlebag Lake pendant (SLP) most likely resulted from regional deformation alone (see foregoing section on finite strain analysis). This is supported by a lacking ductile aureole in the Piute Meadow pendant (PMP; Fig. 4.6) and homogeneously distributed plane strain in the SLP (Fig. 4.12). Although we do not attribute the finite strain in the SLP to pluton emplacement by virtue of submitting stress, we use the term ‘emplacement-related deformation’ in the sense that heat supplied with emplacement of magma served to trigger rheological softening. Several previous field-based studies support that the presence of melt may have a significant effect on crustal rheology worldwide (e.g. Hollister and Crawford, 1986; Davidson et al., 1992; Hollister, 1993; Tommasi et al., 1994; Nelson et al., 1996; Neves et al., 1996; Brown and Solar, 1998). Hollister and Crawford (1986) studied the relationships between the formation of melt and deformation in the Coast Mountains of British Columbia. They proposed that melt derived from anatexis or intruded weakens the crust significantly, resulting in tectonic surges associated with large-scale displacements. Davidson et al. (1992) documented the emplacement of melt into an active deep-crustal shear zone in the Maclaren metamorphic belt, 127 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. /'SUre^. „ ° Cl,an'srn ■ ° iagtan, S a r f < 0 ^ ; ° Q J 'S ' a " ^ b „ Pe^ t ^ c ed sni0° « M °^3, *000 ° ^ 6 c°Pyi r'Qh t °^ner ' U rther l2g ePr° ^ o tion Pr°hib, Alaska. Aided by modeling of crystallization times they estimated the finite displacement associated with this shear zone may have been up to 1 0 0 km in ca. 1 m.y. Hollister (1993) examined melt-deformation-relations in several orogenic belts, including the Central Gneiss Complex, British Columbia, the Maclaren Glacier metamorphic belt, Alaska, the Bhutan Himalaya (also see Nelson et al., 1996), and the Hercynian orogenic belt, Sardinia. Based on the common occurrence of melt- filled shear bands, Hollister (1993) argued that thrusting of anatectic migmatite from the lower crust to the middle crust may play a crucial role in localizing strain into shear zones. Eventually, the presence of melt in the middle portions of overthickened crust would trigger extension. Tommasi et al. (1994) studied the relationship between magmatism and deformation in a lithospheric-scale shear zone of the Dom Feliciano belt, Brazil. Based on the occurrence of magmatic fabrics within the shear zone, the authors suggested that the presence of magma in the shear zone triggered localization of deformation whereby shear and mylonitic strain localization continued under solid-state conditions. Likewise, Neves et al. (1996) found evidence for magma-related strain localization and nucleation of shear zones in the Borborema province, Brasil. In view of the foregoing globally observed magma-related strain localization, an important question is what are the underlying mechanical processes that accommodate such focusing of strain? A common characteristic in the above settings that are marked by strain localization may be melt-induced rheological weakening (Dijksrta et al., 2002). To put it in a mechanical context, an increase in temperature brings about partial melting 129 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. which in turn presumably triggers a transition in the dominate deformation mechanisms from grain-size-insensitive intracrystalline plasticity, mostly by dislocation creep, to grain-size-sensitive diffusion creep and/or granular flow (or grain boundary sliding; note that the terms are used interchangeably in the literature). This is demonstrated by numerous experimental studies on rocks (e.g. Dell’Angelo et al., 1987; Kohlstedt, 1992; Mei et al., 2002, note the nicely illustrated melt pockets at triple junctions and wetted grain boundaries in their Figure 6 ; Mecklenburgh and Rutter, 2003) and rock analogs (e.g. Ree, 1994; Park and Means, 1996; Rosenberg and Handy, 2000). Field-based studies indicate that grain boundary migration is a very common deformation mechanism in nature, particularly in mylonitic shear zones (e.g. Behrmann, 1985; Behrmann and Mainprice, 1987; Fliervoet et al., 1997; Herwegh and Jenni, 2001; Whitmeyer and Simpson, 2003). However, many of the above case studies exemplify superplasticity in the lower crust, suggesting that this deformation mechanism predominately occurs at deep crustal levels. Our results imply that superplasticity need not be limited to the lower crust but that it can operate at much shallower levels, particularly in intrusive environments (see also Rosenberg and Stiinitz, 2003), provided the required temperature conditions are met. A critical minimum melt percentage may be required to induce a transition in deformation mechanisms. Arzi (1978) first suggested that a threshold melt content of ca. 20% represents a rheological critical melt percentage (RCMP) for partially melted rocks to experience enhanced deformation. In a comprehensive review paper, 130 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rosenberg (2001) provided an updated view of the concept of the RCMP. He concluded that, despite a strong dependency on experimental conditions, a RCMP occurs in the laboratory in partially molten rocks with low-viscosity. In nature melt viscosities are inferred to be low and therefore a RCMP probably occurs there as well. An additional rheological threshold appears to occur at melt fractions of 0.4 (Rosenberg, 2001). However, syntectonic melt segregation may prevent melt fractions of even 0.1 (Brown and Rushmer, 1997), particularly if shear zones are involved in draining melt out of the system. On the other hand, melts can concentrate in shear zones, and in this instance a RCMP could result with regards to bulk deformation the rock (Rosenberg and Handy, 2000). In addition to grain boundary sliding and diffusion creep, fracturing may also occur during superplasticity. Several experiments produced grain-scale fractures in partially molten rocks, attesting to the significance of melt overpressure during deformation and/or grain-to-grain stress transmission before the melting temperature is reached (e.g. Conolly et al., 1997; Rutter and Neumann, 1995; Rushmer, 2001). An important question arising from the foregoing discussion is how fracturing interacts with melt generation and between-grain distribution of fluids during progressive deformation. Cataclasis necessarily involves dilatency and so fracturing under melt-present conditions may in fact drain more melt into rocks with fracture- enhanced permeability. For instance, Brown and Solar (1998) proposed that build up of melt pressure and thus fracturing during waning deformation in orogenic systems causes channelized melt transfer through shear zones. They authors also 131 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. stated that during active deformation, continuous redistribution of melt by percolative flow would prevent build-up of melt pressure. Hence, a mechanism of self-enhancing positive feedback is likely to result from simultaneous melting and deformation whereby progressive deformation serves to distribute melt along grain boundaries. Besides melt, aqueous fluids may strongly influence the interplay between melt and deformation. The consequences of fluid distribution on deformation mechanisms and rheology have been investigated by Tullis et al. (1996). Using an aqueous fluid in their experiments, the authors demonstrated that deformation- enhanced fluid distribution can cause a transition from dislocation creep to diffusion creep. Hall and Parmentier (2000) showed numerically that fractionation of water upon melting of deforming peridotite results in decreasing effective viscosity and thus localization of melt. We assume that water weakening in partially molten granitic rocks may have a similar effect of focusing of melt and thus strain localization. Water is an abundant product of prograde dehydration reactions, particularly along temperature gradients in contact aureoles. The importance of water weakening is also supported by Mecklenburgh and Rutter’s (2003) experiments that showed that rounded grains were best developed in wet samples of partially molten synthetic granite. Despite common association of melt and deformation in contact aureoles and the lower crust, none of the relevant contributions cited above provides direct estimates of the magnitude of melt-assisted deformation. Our study indicates that an increase 132 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. in finite strain from average values of ca. 43% z-axis shortening to 70-85% z-axis shortening is genetically related to a melt-induced transition from intracrystalline plasticity to superplasticity close to the margin of the TIS. It follows that actively contracting regions marked by emplacement-induced partial melting are profoundly affected by rheological changes and they might undergo up to twice the amount of shortening that would normally develop without the influence of melt. Material transfer during pluton emplacement Paterson et al. (1996) supported Buddington’s (1959) notion regarding the complexity of magmatic systems and in particular the perception that the operation of material transfer processes (MTPs) change vertically, laterally and temporally. As a consequence of temporally changing MTPs, the evidence of deformation that occurred earlier in a pluton aureole might be removed by subsequent processes. This hypothesis is consistent with the common observation that ductile flow accounts for only part of the local space provided for an intrusion. For example, a compilation of strain data collected from fieldwork in several plutons around the world shows that only ca. 30% or less of the required material transfer was accommodated by ductile wall rock flow (Paterson and Fowler, 1993; Paterson and Vemon, 1995). In view of the occurrence of stoped blocks in some aureoles, two obvious possibilities exist to explain the insufficient amount of ductile wall rock flow: either stoping completely accommodated the remaining amount of material transfer, or evidence of earlier high 133 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ductile strain was removed by subsequent stoping. Evidence of loss of previously ductily deformed portions of aureoles have been reported from magmatic systems in the Sierra Nevada (e.g. Albertz et al., 2000) and the Peninsular Ranges batholiths (Wetmore et al., 2001). Therefore, removal of early ductile aureole strain by stoping seems a viable means of effectively reducing finite strain in aureoles. However, determining the exact contribution of stoping to space accommodation is problematic unless pluton margins are stepped, so that portions of the original aureole are exposed along with sections where finite strain has been reduced by stoping. Our field observations clearly reveal a stepped pluton margin (Fig. 2.7) and they show that earlier formed ductile strain, though most likely attributable to regional deformation, was lost by stoping. Along the margin-perpendicular northern and southern transects the preserved maximum finite strains amount to ca. 70 and 85% z- axis shortening, respectively (Fig. 4.13). Projecting the position of maximum strain from the southern transect along strike onto the equivalent position in the northern transect shows that this space is now occupied by Cathedral Peak granodiorite (Fig. 4.13). Based on highly discordant relationships between plutonic and host rocks in the step, and the occurrence of host rock blocks entirely surrounded by granodiorite, we suggest that stoping is a plausible mechanism of brittle truncation and replacement of host rock by magma. Thus, a difference of ca. 15% finite strain between the two strain transects can be accounted for by stoping. We note that this value is a minimum estimate for stoping because we have no means of ascertaining whether additional ductile strains have been lost along the southern strain transect. 134 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The preserved ductile aureole strains cannot be employed to determine the amount of space accommodated by the intrusion because our strain analysis strongly suggests that regional deformation ultimately is responsible for the strains measured in the SLP. In any case, our study presents direct field evidence for removal of emplacement-related (i.e., temperature-induced rather than by submitting stress) strain in aureoles, supporting the conclusion that high strain in inner aureoles can be lost to stoping. Conclusions The main conclusions resulting from our study are as follows: spatial coincidence between a melt-induced transition in deformation mechanisms and abruptly increasing finite strains in the Saddlebag Lake pendant support that magma emplacement has a fundamental effect on rheology. A lacking correlation between the transition from low-to high-temperature dislocation creep and finite strain suggests that dramatic rheological changes do not take place in the dislocation creep regime. The onset of melt-assisted granular flow and diffusion creep match exactly the abrupt increase in finite strain, indicating that the initiation of superplasticity results in spectacular rheological weakening. Regarding host rock material transfer during pluton emplacement, our study confirms the ubiquity of two common observations: (1) multiple processes operate during chamber construction, and (2 ) brittle processes are capable of removing 135 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. earlier ductile strain. The aureole of the Saddlebag Lake pendant is marked by discordantly truncated stratigraphy and intense ductile structures. The truncation is due to stoping whereas high temperatures at the pluton margin enhanced regional deformation. In addition, a step in the margin reveals a difference in finite strain, depending on how far the magma stepped into the host rock. Therfore, earlier formed ductile structures most likely have been lost to stoping. 136 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 5: A NUMERICAL TEST FOR RATES OF DISLOCATION CREEP DURING SPHERICAL PLUTON GROWTH: IMPLICATIONS FOR RAPID MATERIAL TRANSFER IN AUREOLES Introduction Vast a mounts o f m agma w ere e mplaced o ver r elati vely s hort t ime i ntervals i n magmatic arcs around the world. For example, in the Sierra Nevada, California, the Sierra Crest magmatic event (ca. 98-86 Ma) resulted in generation of more than 4,000 km2 of granitoids now exposed in map view (Bateman, 1992; Coleman and Glazner, 1997). Given that displacement of an equal amount of host rock is required, an obvious question arising from the occurrence of such large magma volumes is: by what mechanism is this host rock displaced? Buddington (1959) and Paterson et al. (1996) suggest that the construction of large crustal magma chambers involves multiple material transfer processes (MTPs, e.g. doming, stoping, ductile deformation, translation, volume loss, assimilation). However, the contribution of any particular MTP to the overall space accommodation ultimately is controlled by which underlying grain-scale deformation mechanisms (e.g. dislocation or diffusion creep, cataclastic deformation) are active and how fast they operate at the local pressure and temperature conditions. Strain rates in pluton aureoles are currently a subject of considerable interest. For example, several field and microstructural studies of individual plutons suggest 137 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. that strain rates associated with displacement in aureoles lie in a range of 1 0 '8 to 1 0 ‘lj s' 1 (e.g. Karlstrom et al., 1993; John and Blundy, 1993; Miller and Paterson, 1994; Nyman et al., 1995; McCaffrey et al., 1999; Fernandez and Castro, 1999; Johnson et al., 2004). Some of these rates are several orders of magnitude higher than regional strain rates, generally considered to vary between 10' 13 and 10‘ 15 s' 1 (e.g. Price, 1975; Pfiffner and Ramsey, 1982). However, deformation mechanisms, particularly those that can accommodate fast strain rates, such as fracturing, can be hard to recognize in aureoles and other geologic settings because subsequent events (e.g. annealing, regional deformation) can modify or completely obliterate earlier microstructures (e.g. Knipe, 1990). In this paper we use a numerical model to examine rates of dislocation creep during spherical pluton growth. We note that spherical pluton expansion is likely to have occurred in some cases (e.g. San Jose pluton, Johnson et al., 2003) but we make no attempt to generically apply our model to all crustal plutons. We utilize our simplified model to examine strain rates associated with ductile host rock shortening because realistic transient and spatially variable changes in pressure and temperature can easily be simulated. Our model builds on two recently published purely kinematic models of dike-fed spherical pluton expansion (Johnson et al., 2001; Gerbi et al., 2004). The two models involve magma transport into the chamber through feeder dikes with constant fill rates. Johnson et al. (2001) assume that wall rock strain is accommodated by ductile flow. Gerbi et al. (2004) allow additional aureole shortening through stoping. 138 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Characteristic of kinematic models, the rocks have no mechanical behavior. Consequently, the rates of aureole shortening depend strictly on the chosen pluton fill rates. Both studies result in wall rock strain rates that are extremely rapid (up to 1 0 '7 s' 1 for most of the emplacement) and temporally variable (fastest strain rates occur during the early stages of chamber growth). These fast rates imply extremely short pluton growth times. For example, calculated times for a model pluton to grow to a final size of 5 km are 1846 to 0.13 years for minimum and maximum fill rates, respectively (Johnson et al., 2001). In addition, such fast rates of ductile flow have dramatic implications for the rheology of pluton aureoles. The effective viscosity of rocks deforming at a strain rate of 1 0 '7 s' 1 and differential stresses of 1 0 - 100 MPa would be on the order of ca. 108 Pa s, that is approximately the effective viscosity of bitumen at room temperature (Fig. 5.1; Bames et al., 1989) and much lower than 1016 Pa s, the lowest experimentally constrained viscosity of crystalline granites (Talbot, 1999, Fig. 5.1). We focus on dislocation creep and apply a flow law for wet quartzite (Hirth et al., 2 0 0 1 ) to simulate ductile shortening of quartz-rich aureoles typical for the mid to upper crust. Transient temperature variations are simulated by finite difference heat conduction modeling. Our results indicate that rates of ductile flow by dislocation creep in pluton aureoles are unlikely to be higher than 10' 11 s'1 . These results fall within t he r ange o f s train r ates i ndicated b y field a nd m icrostructural s tudies ( see references above). Compared to extremely fast rates of ca. 10' 7 s' 1 indicated by 139 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 16 12 ,8 ,4 100 Mpa 10 MPa 1 MPa 0.1 MPa 0.01 MPa Crustal rocks Bitumen i* 4 •2 ,-4 ■ 6 ■ 8 10 12 •14 i [s-1] Figure 5.1. Effective viscosity versus strain rate for differential stress values of 0.01 to 100 MPa. Horizontal lines indicate reference viscosities for bitumen at room temperature and the lowest published viscosity of crustal rocks. Vertical lines show strain rates of dike-fed expansion derived from kinematic modeling (a., Johnson et al., 2001; Gerbi et al., 2004), rates obtained in this study (b.), and an average rate of regional deformation (c., Price, 1975; Pfiffher and Ramsey, 1982). Effective viscosity was calculated using the routine described in Poirier (1985). 140 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. kinematic models of pluton expansion, the strain rates obtained in this study are much lower. Therefore, rates of dislocation creep are sufficiently fast to accommodate aureole shortening around some plutons, but extremely rapidly expanding plutons require additional deformation mechanisms to allow for very fast material transfer. Model description For direct comparison with Johnson et al.’s (2001) and Gerbi et al.’s (2004) kinematic models, we use a similar spherical geometry but we include stress- and temperature-dependent behavior of the aureole. The sphere grows to a final radius of 5 km (Fig. 5.2A). A sphere is a useful end-member because it provides the most conservative growth rates. Although magma supply through feeder dikes is a popular model for the growth of magma chambers (e.g. Clemens et al., 1997), we note that there is some difficulty with this approach utilized by Johnson et al. (2001) and Gerbi et al. (2004) because minimum magma flow rates are required to avoid freezing and other mechanisms of magma chamber growth, such as in situ melting or pervasive magma migration in hot country rock (Weinberg, 1999), are also plausible. Hence, for the purpose of our study, we assume that magma is supplied into the chamber by any viable means to maintain constant magma pressure. Because o f t he a bundance o f q uartz-bearing rocks i n t he c ontinental c rust, w e assume that the aureole is composed of homogeneous wet quartzite. We note that a 141 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A. Host rock aureole B. Host rock aureole Magma cham ber Magma cham ber Figure 5.2. Model geometry. A. r is distance from pluton center. Pluton radius rp and aureole width x increase as a function of time. < j t and < jt depict radial and tangential stress, respectively. B. Maximum stretching direction represents direction of maximum strain rates. 142 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. more realistic scenario is one where several minerals collectively control the rheology of rocks of the continental crust (e.g. Handy, 1990), but because our study is primarily concerned with first-order rheological effects during emplacement, our mono-mineralic representation of the aureole is adequate. We define the aureole as the zone of dislocation creep, bounded by the magma-host rock interface on one side, and the outer aureole edge, xw , on the other side (Fig. 5.2A). xw is set where the temperature exceeds 300 °C in the host rock, a value at which dislocation creep in quartz-bearing rocks typically dominates (Stockert et al., 1999; Hirth and Tullis, 1994, and references therein). We apply a dislocation creep flow law of the form: e(x) = A f m crnest ( 1) where s is the strain rate at a given place in the aureole, x = position within the aureole, A = pre-exponential constant,/ = water fugacity, m = fugacity exponent, a - differential stress, n = stress exponent, O = activation energy, R = universal gas constant, and T = temperature. P arameters for this flow law were experimentally determined by Luan and Paterson (1992) and Gleason and Tullis (1995). Subsequent work by Hirth et al. (2001) tested the flow law against natural constraints and produced the physical parameters that we use (Table 5.1). Similar to rock deformation experiments, we assume that a given rock volume undergoing deformation is shortening parallel to the direction of maximum principal 143 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 5.1. Modeling parameters. Parameters A , f m and n from Hirth et al. (2001). Parameter Value Unit Pre-exponent constant A 6.3 x 10'‘“ MPa'V" Water fugacity f 37 MPa Fugacity exponent m 1 Stress exponent n 4 Jmol^K' i Universal gas constant R 8.3144 Chamber-internal pressure P 1 0 0 MPa Specific heat C (magma, aureole, host rock) 1 0 0 0 Jkg-'K-' Density p Magma 2550 kgm' 3 p Aureole 2650 kgm' 3 p Host rock 2750 kgm' 3 W m 'V Coefficient of heat conduction k (magma, aureole, host rock) 1 Ambient host rock temperature T h o st 300 °C Magma emplacement temperature T m a g m a 850 °C stress and extending in the direction(s) of minimum principal stress (Fig. 5.2A). The associated length change as a function of time represents the strain rate (Fig. 5.2B). We regulate the two variables of the flow law, stress and temperature (Equation 1), which depend on the position in the aureole and time. We define the differential stress at any given time that enters the flow law as the maximum shear stress (Equation 2-60 in Turcotte and Schubert, 2002): 144 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. cW = ^ (o -r -C 7 t), (2) where < r r and c rt are the radial and tangential stress components (Fig. 2B), respectively, given by: < J r = P- 3 r p (rp+ x) and < j , = - P - 3 2 (rP + x) (3) (4) where P is the magma pressure in the chamber, rv is the pluton radius, and x is position within the aureole. Because to date, direct measurements of magma pressures in natural plutons are not possible, we utilize indirect constraints to define the magma pressure in our study. Utilizing the relationships for magma pressure first stated in Baer and Reches (1991, note that these authors refer to ‘ magma driving pressure % Hogan et al. (1998) determined that at depths shallower than the brittle-ductile transition magma pressure can range from ca. 60 to 150 MPa. Field and microstructural observations in other geologic settings (e.g. mylonites in the Eastern Alps, Italy, Stockhert et al., 1999; Ruby Gap duplex, Australia, Hirth et al., 2001) show that differential stresses of 60-160 MPa are consistent with dislocation creep, showing that our choice of P = 100 MPa is within plausible limits for the 145 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. purpose of this study. Magma pressure in real plutons may vary through time, but this issue is beyond the scope of our study. Our boundary condition of temporally constant magma pressure yields maximum strain rates. We apply an explicit implementation of the one-dimensional finite difference method (Croft and Lilley, 1997) to solve the one-dimensional heat conduction equation (Ranalli, 1987) ^ d T d 2T CP-^7 = k ^ T ’ (5) ot o r ‘ where C= specific heat, p - density, T = temperature, t — time, k = coefficient of heat conduction, and r is distance from the pluton center. Reduction to one dimension is permissible due to the radial symmetry of the model. For a geothermal gradient of ca. 30 °C / km in arcs, an ambient far-field temperature of 300 °C represents a crustal depth of ca. 10 km. A liquidus temperature of 850 °C is maintained in the pluton center, simulating continuous magma supply from a hypothetical magma generation site. Transient cooling occurs from the center outward. The simulations were calculated for 1 million years over a total radial distance of 1 0 km at 1 0 m node spacing; Figure 5.3 shows results for the inner 7 km. In order to couple strain with aureole shortening in our modeling, at each node point the strain rate s is converted to incremental strain musing 146 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A. Differential stress Distance lm] B. Temperature Distance [m] C. Strain rate Distance Im] Figure 5.3. Results of dislocation creep modeling. Horizontal axis shows distance from the center of the magma chamber. Vertical axis indicates modeling time. Solid black lines depict the interface between magma chamber and host rock. Dashed lines represent outer aureole edge, xw > 300 °C. The following panels show the results computed along the aureole width, x as a function of time: (A.) Differential stress. (B.) Temperature. (C.) Dislocation creep strain rate. 147 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. where A t = time step increment. The incremental strains are then numerically integrated over the aureole width, xw, to obtain the bulk aureole strain for a given time step: The bulk aureole strain directly feeds back into pluton growth because the aureole shortening c orresponds to r adial p luton growth. W e r eset t he n ode p arameters t o account for growth and the time loop begins anew. With time both the pluton radius, as well as the aureole width, increase (Fig. 5.3). We illustrate changes in differential stress (Fig. 5.3A), temperature (Fig. 5.3B), and dislocation creep strain rates (Fig. 5 .3C) as a function o f time and aureole width. The stress and temperature values are highest close to the magma chamber margin and fall off over short distances. For example, during the early stages of chamber growth (ca. 10,000 years), temperatures of over 800 °C at the contact decrease to ambient host rock temperatures of 300 °C over a distance of only 1 km. The thermal (7) 0 Results 148 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. aureole b ecomes considerably wider with time. A t half the chamber construction time (500,000 years), the thermal aureole is ca. 4 km wide. Variations in strain rates follow a similar pattern. The highest strain rates are on the order of 1 0 'n s' 1 and occur very close to the pluton margin where stress and temperature are also at their maximum. The rates decay to typical rates of regional deformation of 10' 14 s' 1 at ca. 1 to 1.5 km distance from the pluton margin for much of the time and at greater distance they become geologically insignificant ( < KT’V ’)• Discussion Our results show that maximum rates of dislocation creep (10'n s"1 ) in aureoles of spherically expanding plutons are higher than published rates of regional deformation (10' 13 to 10' 15 s'1 ). However, our rates are much lower than average strain rates predicted in the kinematic models of dike-fed expansion (ca. 1 0 '7 s'1 , Johnson et al., 2001; Gerbi et al., 2004). The main aspect accounting for the different results between this model and the kinematic models is the implementation of constant magma pressure and heat transfer versus constant fill rate through a feeder dike, respectively. Our model is sensitive to temporal and spatial variation in stress and temperature whereas the kinematic models are sensitive to fill rate only and they do not include any form of mechanical host rock response. 149 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Our results have implications for dike-fed spherically expanding pluton models and pluton emplacement in general. The kinematic models of Johnson et al. (2001) and Gerbi et al. (2004) as well as recent work on the San Jose pluton (Johnson et al., 2003) imply that dike-fed plutons must fill rapidly (in order to avoid freezing of the feeder dike), which in turn requires fast strain rates in the aureoles to accommodate space for the growing pluton. Although we have now begun to recognize potentially fast deformation mechanisms in aureoles, including cataclastic flow (Johnson et al., 2003) and melt-assisted granular flow (Albertz et al., in press), we doubt that extremely fast strain rates occur throughout the duration of pluton growth or that rapid construction of crustal plutons can occur in times as short as 1846 to 0.13 years (Johnson et al., 2001). Our newly determined dislocation creep rates of 1 0 ~ n s' 1 in aureoles of spherically expanding plutons translate into a growth period of ca. 1 million years for a pluton with a radius of 5 km, which seems more realistic. A question that remains to be answered is whether early fast strain rates are typically accommodated by brittle fracturing and cataclasis whereas the microstructural evidence is erased by subsequent stoping, ductile deformation, or annealing. Our results fit very well with estimated strain rates from field-related studies (Karlstrom et al., 1993; John and Blundy, 1993; Miller and Paterson, 1994; Nyman et al., 1995; McCaffrey et al., 1999; Fernandez and Castro, 1999; Johnson et al., 2004). For example, Nyman et al. (1995) utilized 90% ductile thinning of the aureole rocks and thermal modeling to estimate aureole strain rates on the order of 150 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 0 ' 12 s'1 , indicating that the dislocation creep rates of 1 0 ' 11 s' 1 obtained in our study may be operating fast enough to accommodate aureole shortening in some natural systems (although we do not agree with the emplacement model of Nyman et al., 1995; our view is represented in Paterson and Vernon, 1995). Furthermore, our study implies that ductile strain is highly concentrated in a narrow zone near the pluton margin and this result, which was found by previous theoretical studies (Weinberg and Podladchikov, 1994) is confirmed by field-based work (Paterson and Fowler, 1995) which shows that ductile aureoles are as narrow as 0.0-0.3 of the pluton radius. Despite the match of our model results with some field studies, we note that ductile aureole shortening, regardless of which ductile deformation mechanism(s) dominate(s) during deformation, cannot be the only process to accommodate material transfer associated with chamber growth of many natural plutons. Marker deflections and measurements of ductile strain in many natural aureoles indicate that most aureoles recorded enough bulk shortening to account for only ca. 30-40 % of the material displacement necessary for magma emplacement (Paterson and Vemon, 1995). The fact that the remaining 40-70% of the material displacement need to be accommodated by some means indicates that in addition to ductile shortening other processes must operate. For example, highly irregular and discordant pluton margins, often associated with host rock blocks and rafts entirely enclosed by solidified magma, suggest that thermal cracking and stoping also occur during pluton emplacement. Such brittle mechanisms could easily remove evidence of earlier high 151 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. strain in the inner aureoles (Paterson and Vernon, 1995), resulting in an incomplete record of directly measurable ductile aureole strain. Notwithstanding the fact that other deformation mechanisms likely contribute to aureole deformation, our work supports Hirth et al.’s (2001) extrapolation of the dislocation creep flow law (Equation 1) to natural deformation. This alone is an interesting aspect because published flow laws have been derived on the basis of low-strain deformation experiments and hence the extrapolation to high-strain deformation i n n ature m ay b e p roblematic (De Meer e t al., 2 002). H owever, o ur results, which were obtained for a wide range of stress and temperature conditions, are consistent with field-based studies of moderately to highly strained aureoles that experienced ductile shortening at strain rates of 10' 8 to 10' 13 s' 1 (e.g. Karlstrom et al., 1993; John and Blundy, 1993; Miller and Paterson, 1994; Nyman et al., 1995; McCaffrey et al., 1999; Fernandez and Castro, 1999; Johnson et al., 2004), suggesting that the application of Hirth et al.’s (2001) calibrated flow law to natural deformation is suitable. Conclusions Rates of ductile shortening in pluton aureoles by dislocation creep alone are unlikely to be higher than 1 0 'n s’1 , suggesting that aureoles shortening at rates faster than 10' 11 s' 1 must occur by other deformation mechanisms. Likely deformation mechanism which could operate at fast-enough rates to exceed rates of dislocation 152 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. creep by several orders of magnitude include cataclastic flow and melt-assisted granular flow. However, ductile material transfer in aureoles shortening at rates of 10' 11 s' 1 or slower can be accommodated by dislocation creep alone. Further complimentary theoretical and field-based work is necessary to examine the rates and relative contributions of alternative deformation mechanisms (e.g. diffusion creep, cataclastic flow, melt-assisted granular flow) over variable stress and temperature ranges. 153 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 6: FAST STRAIN RATES DURING PLUTON EMPLACEMENT: MAGMATICALLY FOLDED LEUCOCRATIC DIKES IN AUREOLES OF THE MOUNT STUART BATHOLITH, WASHINGTON AND THE TUOLUMNE INTRUSIVE SUITE, CALIFORNIA Introduction Orogenic belts in the Northwest-American Cordillera are characterized by geologically fast input of large amounts of Jura-Cretaceous intrusive material (e.g. Bateman, 1992; Coleman and Glazner, 1997; Ducea, 2001). For example, magmatism in the Sierra Nevada arc occurred in two major episodes, one in the Jurassic (160-150 Ma) and a volumetrically more important one in the Late Cretaceous (100-85 Ma). The addition of such enormous quantities of new material needs to be accommodated by displaced host rock most likely under a variety of processes. Host rock displacement can occur by various mechanisms, including ductile flow, brittle processes, and a combination of the two, operating at different rates. Several previous studies suggested that strain rates in pluton aureoles are moderately to significantly higher than rates of regional deformation (e.g. Pavlis, 1996; Johnson et al., 2001). Estimates of strain rates accommodating material transfer in aureoles lie in the range of 10'1 0 to 10'1 2 s'1 (e.g. Karlstrom et al., 1993; McCaffrey et al., 1999; Fernandez and Castro, 1999; John and Blundy, 1993; Miller and P aterson, 1994). O n t he o ther h and, estimated t ectonic s train r ates are m uch slower and have been suggested to vary between 10"1 3 and 10'1 5 s'1 (Price, 1975; 154 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Pfiffher and Ramsay, 1982; Paterson and Tobisch, 1992; Dunlap et al., 1997; Foster and Gray, 1999; Muller et al., 2000; Hirth et al., 2001). Pluton growth has a significant impact on host rock strain rates, whether or not a specific emplacement model is chosen. Emplacement models use fast strain rates to allow a given magma volume to be emplaced more rapidly than slower strain rates. Johnson et al. (2001) suggested that, at very fast strain rates, growth of dike-fed plutons can outpace conductive heating of aureole rocks, resulting in complete emplacement o f such p lutons b efore the aureole rocks undergo significant contact metamorphism. Although several studies proposed faster-than-tectonic strain rates in aureoles (Johnson et al., 2004), to date there exists a lack of convincing field evidence for such fast rates. In this paper we present field evidence for fast strain rates during emplacement- related deformation of aureole rocks. We describe tightly folded leucocratic dikes in aureoles of the Mount Stuart Batholith, Washington (Fig. 6.1), and the Tuolumne Intrusive Suite, California (Fig. 6.2). Excellent three-dimensional exposures in both aureoles allowed accurate finite strain estimates by restoring the original lengths of the now folded dikes. Timing constraints were obtained from analytical cooling models for leucocratic dikes. Our results are intriguing because classical models of dislocation creep, the most common mechanism of high temperature ductile deformation of quartz-bearing rocks (e.g. Hirth et al., 2001; and references therein), are unlikely to provide very fast strain rates at typical temperature conditions in 155 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. |~ | Inaccessible-not mapped | • ' -| Quaternary m Basalt | | Tonalite M SB Diabase m Gabbro j—H Peridotite Ingalls Complex IPfiSI Serpentinite [ ■ [ Metasediments 70° - 89° dip of foliation Sample location Figure 6.1. Geologic map of the Mount Stuart Batholith. (A.) Location map. MSB= Mount Stuart Batholith, Washington. G= granodiorite; T= tonalite; D= diorite. (B.) Location of field studies (rectangle in Fig. 6.1 A). 156 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 2 4 * W R hyodaotic ash-flow tu ff: ca. 185 M a (R & -S r w hole ro ck : B atem an et al.. 19 83 ). Sample location R h y o litte ash-flow tu ff of Saddlebag L ake: 2 2 2 * /> 5 M a < U * tb zircon: S chw eiekert and L ahren. 1987) 60° * 89° dip of foliation C andelaria F orm ation: Q uanzofeidspathic sittstones and finegrained sandstones Figure 6.2. Geologic map of the Tuolumne Intrusive Suite. (A.) Location map. TIS= Tuolumne Intrusive Suite, California. Kkc= Kuna Crest granodiorite (92-94 Ma, Coleman and Glazner, 1997); Khd= Half Dome granodiorite (89-91 Ma, Kistler and Fleck, 1994; Coleman and Glazner, 1997); Kcp= Cathedral Peak granodiorite (88.1 Ma, Coleman and Glazner, 1997); Kjp= Johnson granite porphyry (82-85.4 Ma, Fleck et al., 1996; Coleman and Glazer, 1997). (B.) Location of field studies (rectangle in Fig. 6.2A). 157 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. contact aureoles. Instead we believe it is likely that multiple mechanisms operate but that fast processes, such as melt-assisted granular flow of host rock associated with cracking, are required to facilitate extremely rapid aureole shortening. Displacement in aureoles during emplacement Large exposures of plutons require that substantial amounts of pre-existing material have been assimilated (thermally and chemically limited) or transferred through the crust. Paterson et al. (1996) re-emphasized Buddington’s (1959) view that the construction of large crustal magma chambers involves complex internal processes as well as multiple host rock material transfer processes (MTPs). Both studies indicate that multiple material transfer processes operate in pluton aureoles, all at different rates, and that these processes are subject to both temporal and spatial changes. Aureoles are complex settings because besides being marked by pronounced gradients of rheology and strain, they are places in which all three components of deformation, i.e., rigid body translation and rotation, and strain are typically required to accommodate the necessary material transfer during emplacement. Although the main purpose of this study was to quantify strain in the aureoles of the Mount Stuart Batholith and the Tuolumne Intrusive Suite and derive strain rates, we also examined translation and rotation in a qualitative sense for the two plutons. Below, we introduce the geologic settings and then briefly summarize the relevant evidence on 158 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. possible rigid translation and rotation of aureole rocks associated with the emplacement of the two plutons. In the remainder of the paper we focus on strain rates associated with aureole shortening. Field observations Mount Stuart Batholith (MSB! Washington The Cretaceous ca. 700 km2 large mid-crustal composite MSB consists of biotite- homblende tonalite and subordinate phases, including granodiorite and diorite (Fig. 6.1 A). New U/Pb zircon geochronology indicates that the MSB was constructed over a ca. 5.4 million years period from ca. 96.3 to ca. 90.9 Ma (Matzel et al., 2002). The MSB is hosted in the southeast by the Ingalls complex (Fig. 6. IB) which has been interpreted to represent a Late Jurassic ophiolite complex (e.g. Southwick, 1974; Metzger, et al. 2002). The Ingalls complex includes peridotites, serpentinites, gabbros, and various metasedimentary rocks which were juxtaposed along numerous Late Jurassic faults in or adjacent to a fracture zone within a large ocean or marginal basin (Miller and Mogk, 1987). The multiply deformed and metamorphosed Chiwaukum Schist hosts the MSB in the northwestern part of the pluton. Contacts with the country rock are generally steep in the southeastern and central regions (Fig. 159 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6.IB) and dip moderately to steeply away from the pluton at its northwestern end (Paterson et al., 1994). There is much debate about large-scale regional translation of the MSB (Baja-BC hypothesis, Beck and Noson (1972) and many subsequent studies, e.g. Housen et al., 2002) but this discussion addresses translation separate from any caused by emplacement. Of greater relevance to our study, differential translation within the aureole during emplacement has been reported by Miller and Paterson (1992), who documented syn-emplacement reverse slip of the Tumwater Mountain shear zone. However, because this translation only affects a small portion of the aureole, it is of minor importance with regard to accommodating emplacement (Paterson et al., 1994). The width of the southeastern thermal aureole (where our studies of folded dikes was performed, Fig. 6.IB) can be constrained by mica pseudomorphs after cordierite occurring at least as far as 2 km from the batholith (Paterson et al., 1994). This estimate largely corresponds with an average width of the structural aureole of ca. 2.3 km. Along with emplacement-related penetrative transposition of pre-existing fabrics within < 500 m of the pluton margin, an intense steeply plunging sub-solidus stretching lineation developed. Kinematic indicators (e.g. asymmetric porphyroblasts, drag of foliation, and rare SC-fabrics) in the contact zone are consistent with a pluton-side-up sense of displacement relative to the host rock. Finite strain can be measured at two scales. At the scale of the entire aureole strain estimates using marker deflections yield bulk estimates of aureole shortening, 160 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. including rotations, strain, and volume change. At the scale of individual samples, however, strain and volume change may be recorded, but any information on rotation is not preserved. Strain markers at the sample scale are absent in the MSB and hence we utilized marker deflections to constrain bulk aureole shortening. Host rocks of the TIS contain a sufficient amount of strain markers (see below) and we can also measure marker rotation. Significant rotation from regional orientations of host rock markers in the ca. 2.3 km wide southeastern aureole of the MSB is indicated by rotation of steep Late Jurassic shear zones, geologic contacts, and metamorphic foliations (Paterson et al., 1994; Albertz et al., 2001a). Outside of the aureole, host rock markers remained unstrained and undeflected during emplacement from regional east-west striking orientations. Quantitative strain analysis utilizing this deflection (see Ramsay and Huber, 1983, session 1 for equation) in the southeastern aureole yields a shear strain of y = 1.65, implying that the aureole underwent a bulk shortening of ca. 48% (Albertz et al., 2001a). We note that this estimate of bulk shortening is independent of deformation mechanisms and includes both penetrative strain and rigid rotation. More competent units were subject to rotation (e.g. diabase and peridotite in Fig. 6 .IB) while penetrative strain occurred in less competent layers (e.g. metasediments in Fig. 6 .IB). Moreover, the southeastern part of the aureole is characterized by the occurrence of ca. 2 0 cm wide tightly folded dikes of granodioritic composition that are connected to the main intrusion and confined to the immediate high strain zone near 161 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the contact. Because these dikes are compositionally and texturally related to the MSB, and microstructural evidence indicates syn-magmatic folding (see microstructural observations), we are confident that this folding was related to emplacement of the MSB. Tuolumne Intrusive Suite (TIS). California The Cretaceous ca. 2000 km2 large TIS (Fig. 6.2A) is one of the largest magmatic bodies in the Sierra Nevada Batholith. The TIS is normally zoned with progressively younger and more felsic pulses towards the center, including: an outer mafic phase (Kuna Crest granodiorite, 94-92 Ma, Coleman and Glazner, 1997), and inner, more felsic phases (Half Dome granodiorite, 91-89 Ma, Cathedral Peak granodiorite, 88.1 Ma, and Johnson porphyry, 85.4-82 Ma, Kistler and Fleck, 1994; Fleck et al., 1996; Coleman and Glazner, 1997). The host rock consists of the 102 Ma El Capitan granite and older plutons in the west along with several septae of presumably Jurassic metasedimentary rocks (Bateman, 1992). Plutons, Triassic to Jurassic metavolcanic and metasedimentary rocks host the pluton to the north. The eastern margin exposes a range of metasedimentary and metavolcanic rocks including Lower Paleozoic to Triassic eugeoclinal rocks (Fig. 6.2B) and associated plutons. 162 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Several studies suggest that significant translation within the aureole of the TIS may have occurred during emplacement (e.g. Schweickert and Lahren, 1990; Grasse et al., 2001; Tikoff and Teyssier, 1992; Tikoff and Greene, 1997), but translation will not affect finite strain estimates for the following reasons: the above translations affect all rocks (both within and outside aureoles) and thus do not contribute to strain within the aureoles; and relative to the rates of ductile flow obtained in this study, rigid translation along shear zones (few mm to cm per year) is slow, and therefore will not play a major role over the time scales we consider in this paper. The question remains, however, whether emplacement-related deformation in the TIS aureole involved rigid rotation of pre-existing rocks. Paleomagnetic results from the fine-grained and coarse-grained Mount Gibson plutons (85.3-93.5 Ma, K/Ar hornblende, 83.6 Ma, K/Ar biotite) and the Grace Meadow pluton (92-96 Ma, K/Ar hornblende, 85 Ma, K/Ar biotite, Frei et al., 1984) that occur west of the shear zones discussed above indicate that translation and rotation about a vertical axis of the Sierra Nevada relative to the North American craton were not of significant magnitude. However, there has been considerable discussion in the literature about the significance of horizontal rotation. Steeply dipping beds of ca. 160-130 Ma (U/Pb zircon; Tobisch et al., 1986) metavolcanic rocks in the Ritter Range and Mount Goddard areas are truncated by the relatively undeformed Lamarck pluton (92 +/- 1 Ma, U/Pb zircon, Coleman et al., 1995) and the Mount Givens pluton (92.8- 87.9 Ma, U/Pb zircon, McNulty et al., 2000). Tobisch et al. (1986) originally proposed an extensional model involving rotation of beds to high tilts as a result of 163 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. listric normal faulting between ca. 130-90 Ma. However, in a later study, Tobisch et al. (2 0 0 0 ) concluded that steep tilting of the metavolcanic rocks for about 2 0 0 km along the eastern Sierra Nevada occurred by multiple mechanisms between ca. 164- 80 Ma. The authors performed detailed fieldwork in the Ritter Range and concluded that rotation of bedding to high tilts (ca. 80° southwest) involved three principal mechanisms: thrusting, rotation during regional ductile strain, and local rotations during downward flow in aureoles. They quantified the amount of rotation from thrusting to be ca. 45° between ca. 164-105 Ma, and suggested that the remaining 35° rotation was distributed between the other two mechanisms between ca. 98-80 Ma and was associated with emplacement of voluminous plutons of the Sierra Nevada Batholith, including the TIS. We note, however, that two recent findings strongly suggest that host rock units and structures were already oriented steeply prior to emplacement of at least the Cathedral Peak phase of the TIS: (1) steeply dipping beds and sub-parallel metamorphic foliations in the Green Lake area are truncated by ca. 168 Ma plutons (U/Pb zircon age from Mundil and Nomande, pers. comm., 2003); and (2) along its northern margin, the Cathedral Peak granodiorite of the TIS truncates steeply dipping metavolcanic rocks, older plutons, metamorphic foliations in both, and reverse faults at very high angle (Paterson and Vemon, 1995; this study). Therefore, it is unlikely that emplacement of the Cathedral Peak pulse (the source of some of the dikes considered in this study, Fig. 6.2B) was associated with significant rigid rotation of host rocks. But there may have been significant rotation, perhaps up to 35°, during emplacement of the earlier pulses. 164 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. At the meso-scale, host rock bedding and foliations along the eastern margin of the TIS are concordant to the pluton margin. Contacts between various host rock units, metamorphic foliations in the host rock, and the pluton margin strike north- northwest and dip ca. 60-89° east-northeast throughout the area of investigation (Fig. 6.2B). In the ca. 0.8 to 1 km wide structural aureole, however, the dip angle changes from an average regional dip of ca. 80° to an average angle of ca. 60° close to the pluton margin. We can use this deflection to estimate emplacement-related bulk shortening. Applying the same equations we used to constrain bulk shortening of the MSB aureole (Ramsay and Huber, 1983, session 1), we obtained a shear strain of y = 0.364 and an average bulk aureole shortening of ca. 37%. In addition, locally, strongly discordant relationships between the pluton and its host rock occur where the Cathedral Peak contact follows 90° bends, cuts out stratigraphy, and truncates individual rock units at high angles, suggesting that stoping partly accommodated the emplacement. The intensity of emplacement-related ductile strain increases markedly in the immediate vicinity of the contact (ca. 10m). Strain markers yield shortening estimates of ca. 54% for this portion of the aureole (see Paterson et al., 1989a, for equations). Furthermore, careful observations in the contact zone reveal that locally migmatitic leucosomes (mostly injection migmatites but some may indicate in-situ melting) form veins and pockets that both truncate and intrude along pre-existing host rock anisotropies. Migmatization typically occurs only in stoped blocks that are completely surrounded by solidified magma and it does not occur pervasively in the 165 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. aureole, suggesting that aureole temperatures during emplacement of the TIS may have been very close to the host rock solidus. In the region examined the intrusive rocks and the aureole are characterized by moderately to steeply plunging magmatic lineations in the pluton and sub-solidus stretching lineations in the host rocks. The stretching lineations are marked by shallower orientations approaching the pluton contact. Kinematic indicators in the aureole, including asymmetric porphyroblasts, drag of lithologic contacts and structures, and displacement of leucocratic dikes, consistently indicate a pluton-side- up sense of displacement as well as mostly regional dextral shearing consistent with the transpression model of Tikoff and Greene (1997). The eastern margin of the TIS is marked by the presence of leucocratic dikes, aplitic to granodioritic in composition. These dikes occur within tens to hundreds of meters of the contact, and many can be traced directly into the pluton. Dike widths typically decrease with increasing distance from the pluton margin. Because the dikes are connected to the pluton and occur only close to the contact, we feel confident in relating these dikes to the three main units of the TIS that we mapped in this area (Cathedral Peak, Half Dome, and Kuna Crest, Fig. 6.2B). The dike widths range from ca. 5 mm to ca. lm. Some dikes have pegmatitic cores and aplitic margins. Based on crosscutting relationships and the degree of folding, several generations of dikes can be recognized. Typically, older dikes are tightly to isoclinally folded and boudinaged, whereas younger dikes show straight margins and cut previously folded dikes. Rarely, several generations can be observed in the same 166 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. outcrop, which strongly supports our interpretation that folding occurred during emplacement of the TIS. Strong hydrothermal alteration of the aureole occurs in local zones of leaching and of epidote-chlorite-quartz veins. However, we have carefully collected samples outside these hydrothermally altered zones in our studies. Because the leucocratic dikes of the MSB and TIS share many common structural characteristics, we discuss how finite strains were derived from these dikes in both aureoles below. Strain We have mapped tightly folded leucocratic dikes with inter-limb angles of 10-40° in aureoles of the MSB and TIS. Field exposures and hand specimens of these dikes show strong mineral alignment parallel to the orientation of the axial planes of the folded surfaces, which in turn, are parallel to the metamorphic foliations in the host rocks of the two plutons. The axial planar foliations are defined by quartz, feldspar, hornblende (MSB), or biotite (TIS). A brief description of our finite strain analysis is given below. We applied a passive folding model to the dikes. Intuitively, a buckling model may appear to be more appropriate, but we question whether, regardless of the effective viscosity ratio between the dikes and the host rocks, these thin dikes played a major role in mechanically controlling the folding. Due to the volumetrically small proportion of the leucocratic dikes in the aureoles, a more likely scenario is that the 167 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. dikes underwent passive folding, while the bulk deformation was accommodated by the surrounding host rocks. Furthermore, our passive folding model accounts for a case of general shear. We do not distinguish between pure and simple shear during dike folding because both scenarios are possible (which then is a case of general shear), and the use of a particular model does not significantly affect our strain estimates obtained from the folded dikes. Although we considered only dikes with true three-dimensional exposure, we need to take into account potential errors in estimating finite strains from the folded dikes. For example, some amount of lengthening and shortening of the dikes could be attributed to folding and volume loss during crystallization. We note, however, that for the purpose of our study any potential errors in estimating finite strains from dikes can be neglected. We show in the discussion section that for any particular folded dike of our study, moderately large to huge errors in strain estimates are required to yield an order-of-magnitude increase or decrease in strain rate, respectively. Assuming that the dikes intruded in an originally planar shape, we restored the original dike lengths by retro-deforming them. An original dike length, k , was measured along the centers of the dikes and a present dike length, l\, was determined by measuring a straight line length from the l0 endpoints, perpendicular to the axial surfaces (Fig. 6.3). The results show that finite strains range from 70 to 85% shortening (Fig. 6.4). In addition to estimating finite strains, we used our field observations on folded leucocratic dikes to track a sequence of intrusive events that took place during 168 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Before folding After folding > 0 l« -----------H h /0-/i % Shortening = --------- * 100 ' 0 Figure 6.3. Schematic illustration of finite strain analysis on folded leucocratic dikes. D ikes with 3 -D exposure were used for estimating finite strains. O riginal dike lengths were determined by retro-deforming the folded dikes. An original dike length, /o, was measured along the centers of the dikes and a present dike length, l\, was determined by measuring a straight line length from the Iq endpoints, perpendicular to the axial surfaces. Finite strains range from 70 to 85% shortening (Fig. 6.4). Errors in strain estimates can be neglected for the purpose of this study (Fig. 6.7B). 169 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6.4. Photographs of tightly to isoclinally folded leucocratic dikes showing percent shortening associated with folding. Circles show sample locations of Figure 6.5. (A.) Mount Stuart Batholith. (B.)-(F.) Tuolumne Intrusive Suite. 170 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 171 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. progressive folding. For example, particularly well-exposed crosscutting relationships in one outcrop in the TIS aureole (Fig. 6.4E) show a folded leucocratic dike and several smaller, much less intensely folded to relatively undeformed dikes that originated from the larger folded dike. One thin dike crosscuts a boudin on the upper limb of the folded dike. Hence, the following sequence of events can be reconstructed for this particular example: (1) intrusion of a planar leucocratic dike; (2) folding of this dike; (3) boudinage of the upper limb of this dike; and (4) re intrusion of felsic melt into a boudin of the leucocratic dike (line drawing in Fig. 6.4F). The latter is particularly interesting because the re-intruded felsic dike that also originated in the larger leucocratic dike suggests that, even after folding and boudinage melt was still present in the leucocratic dike. Below we describe our microstructural analyses to determine whether these folds and the associated axial surface foliations formed under magmatic conditions and to constrain which deformation mechanism(s) operated in the dikes and host rocks to accommodate this folding. Microstructural observations We collected oriented samples in the axial planar and limb regions of the folded leucocratic dikes in the aureoles of the MSB and the TIS (Fig. 6.4). In the TIS aureole, we retrieved samples from areas that were not characterized by strong hydrothermal alteration. Three mutually perpendicular 2x3 inch large thin sections 172 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. were prepared from each sample: one parallel to the lineation and foliation, one parallel to the lineation and perpendicular to the foliation, and one perpendicular to the lineation and foliation. Sampling was only performed in dikes that could clearly be related to the MSB and the TIS and where the fold geometry could be constrained due to excellent three-dimensional exposure. Furthermore, we ensured that all dikes sampled showed criteria outlined by Paterson et al. (1998) to document that folding largely occurred under magmatic to sub-magmatic conditions. In this paper, we define ‘magmatic conditions’ as deformation by rigid body rotation of crystals in a melt (suspension-like behavior) and ‘sub-magmatic’ as deformation conditions with less than the critical amount of melt for suspension-like behavior, but without sufficient interference between crystals to cause plastic deformation (Paterson et al., 1989b). We begin this section with a brief description of the mineralogy for the two case studies and then, because of largely similar findings, combine the summary of our microstructural observations for the dike and host rock samples in both the MSB and the TIS aureoles. We will point out differences where they exist. Leucocratic dikes of the MSB aureole contain the following minerals: plagioclase, quartz, hornblende, biotite, chlorite, sphene, opaque oxides, apatite, and zircon. The metasedimentary host rock consists of plagioclase, hornblende, biotite, and opaque oxides. Despite a higher amount of quartz and K-feldspar, dikes of the TIS aureole show a similar mineralogy. In addition, epidote occurs in one sample that was collected close to a regional brittle fault (Fig. 6.2B). Metavolcanic host rocks of the TIS consist of plagioclase, biotite, quartz, and opaque oxides. Our 173 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. microstructural findings clearly support our field observations: alignment of plagioclase, K-feldspar, quartz, biotite and hornblende (MSB), and plagioclase, K- feldspar, quartz, and biotite (TIS) defines the axial planar foliation measured in the field (Fig. 6.5A). The question we wish to address now is: what were the temperature conditions under which folding of the leucocratic dikes and formation of the observed axial planar foliation occurred? Leucocratic dikes Typically, aligned feldspars display euhedral grain shapes (Fig. 6.5A). Plagioclase commonly preserves primary igneous zoning and is characterized by intra-granular cracks (Fig. 6.5B) that strongly resemble the sub-magmatic microfractures described by Bouchez et al. (1992). Bouchez et al. (1992) suggested that such cracks form as the result of stress concentration at contacts between grains in the presence of a residual melt. Among the criteria for recognizing sub-magmatic fractures are: (1 ) fractures transecting single grains; (2 ) a crack-filling mineral that is compositionally and crystallographically continuous with a groundmass grain; and (3) the minerals filling the cracks being consistent with a residual melt. We found ample evidence for all three criteria in our thin sections. We commonly observed wedge-shaped cracks reaching into single plagioclase grains filled with quartz, optically continuous with quartz between plagioclase groundmass grains. Commonly, small plagioclase fragments appear to have migrated into the cracks or 174 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6.5. Photomicrographs of leucocratic dikes and host rocks of the MSB (A.,I.,L.) and the TIS (B.-H.,J.,K.M.-P.). Abbreviations: Qz= quartz, Plg= plagioclase, Ksp= K-feldspar, Fsp= feldspar, Hbl= hornblende; Bio= biotite, Myr= myrmekite, Op= opaque oxide. (A.) Leucocratic dike of the MSB showing aligned euhedral feldspar grains with primary igneous zoning and equidimensional quartz pockets. (B.) Leucocratic dike of the TIS. Sub-magmatic microfracture in plagioclase. Quartz filling the crack is optically continuous with quartz outside of the crack. Note that small plagioclase fragments reside in the crack and near the crack entrance. (C.) Leucocratic dike of the TIS. Destructed plagioclase. Note that several sub-magmatic microfractures mechanically disintegrated the plagioclase crystal. Quartz is optically continuous inside and outside the cracks. Single plagioclase fragments appear to be floating in the quartz matrix. (D.), (E.) Leucocratic d ikes o f the T IS. S ub-magmatic m icrofractures c ontaining c omposite fillings. Feldspar fills the crack tips while the remaining quartz volumes are filled with optically continuous quartz, suggesting that feldspar crystallization in the cracks was followed by crystallization of quartz. (F.) Leucocratic dike of the TIS. Sub- magmatic microfracture filled with quartz and an opaque mineral. (G.) Leucocratic dike of the TIS. “ Chessboard” subgrain pattern in equidimensional quartz pocket indicating operation of subgrain formation parallel to both, the prismatic and basal planes of quartz at relatively high temperature conditions. (H.) Leucocratic dike of the TIS. Myrmekite fringes the margin of a larger K-feldspar grain. Note that the exsolution lamellae are fanning towards the K-feldspar phenocryst and the euhedral grain shapes of myrmekite. (I.) Metasedimentary host rock of the MSB. Static recrystallization of plagioclase and hornblende. (J.) Metavolcanic host rock of the TIS. Zoned plagioclase with irregular margin. (K.) Metavolcanic host rock of the TIS. Plagioclase and K-feldspar boundaries are fringed by myrmekite and quartz. (L.) Metasedimentary host rock of the MSB. Arrows indicate triple junctions between grains that may be filled with interstitial melt. (M.) Metavolcanic host rock of the TIS. Arrows indicate triple junctions between grains that may be filled with interstitial melt. (N.) Metavolcanic host rock of the TIS displaying intra-grain cracks. (O.) Metavolcanic host rock of the TIS at the pluton margin. Quartz grains are mechanically abraded. Matrix includes K-feldspar and plagioclase. (P.) Metavolcanic host rock o f the TIS at distance to the pluton margin. The original crystal shape of quartz is roughly preserved. Matrix includes K-feldspar and plagioclase. 175 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 176 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6.5. Continued. I l l Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6.5. Continued. 178 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. terminated near the crack openings (e.g. Fig. 6.5B). Other plagioclase grains show much more intense cracking, for example parallel crack walls and heavy fragmentation of plagioclase grains (Fig. 6.5C). Such cracks occur more commonly in TIS samples than in MSB samples. Quartz is much more common in the MSB samples, separating plagioclase grains. Furthermore, some microfractures are filled with quartz and feldspar (Figs. 6.5D and 6.5E), suggesting that melt intraded these microcracks, and subsequent crystallization of feldspar and quartz occurred. In one example, we found an opaque mineral in a crack filling (Fig. 6.5F). We observed some hydrothermal alteration of mainly muscovite (sericitization, not shown) and feldspar (saussuritization, Fig. 6.5E), but no evidence was found for substantial secondary deposition of quartz or other typical vein minerals at the thin section scale. These observations suggest that many of the cracks formed at sub-magmatic condition, but we cannot rule out the possibility that some formed later in the solid state. Regionally, ample evidence of hydrothermal activity in these aureoles exists, but the fact that in our samples inter-grain hydrofractures and alteration are relatively rare, compared to intra-grain sub-magmatic cracks supports our identification of these features as sub-magmatic microfractures. Quartz typically occurs interstitially between feldspar grains. Interstitial quartz is roughly equidimensional in shape and typically shows minor evidence for subgrain development. Rarely, two mutually perpendicular subgrain boundaries (e.g. Fig. 6.5G) can be observed. Kruhl (1996) suggested that such rectangular “chessboard” patterns indicate formation of subgrains parallel to the prismatic and basal planes of 179 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. quartz which is compatible with temperatures of ca. 600-750 °C. Some samples show minor evidence for dynamic recrystallization of quartz. Locally, quartz grain boundaries are irregular and bulge into adjacent quartz grains. However, all quartz aggregates and grains are roughly equidimensional, and strongly elongate grains with highly irregular grain boundaries were not found. Hence, our microstructural analyses show that quartz has experienced only minor solid-state deformation. Figure 6.5H shows relatively small aggregates of myrmekite that occur between a K-feldspar grain of a leucocratic dike and metamorphic host rock consisting mainly of quartz, plagioclase, and biotite. There has been considerable debate over the formation of myrmekite, and a magmatic origin (e.g. Hibbard, 1987), as well as formation during solid state deformation (e.g. Vernon, 1991), have been suggested. In our samples, myrmekite occurs only along the margins of larger K-feldspar grains, and it preserves its primary features (e.g. fanning lamellae, mostly straight grain boundaries). Regardless of how and when it formed, major solid-state deformation following myrmekite formation is unlikely because the delicate vermicular intergrowths of myrmekite would not survive much strain (Hanmer, 1982). Host rocks Thin sections of the host rocks (metasediments in the MSB and metavolcanics in the TIS) show reduced grain size of all minerals and strong alignment of minerals with unequal aspect ratios (e.g. all minerals in Fig. 6.51, MSB; and biotite in Fig. 180 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6.5 J, TIS). 120° triple junctions between grains of the same type are common in samples of the MSB host rocks (e.g. plagioclase in Fig. 6.51), pointing to static recrystallization. TIS host rock samples reveal similar 120° triple junctions between grains in some cases, but more commonly, the grain boundaries are quite irregular (e.g. Fig. 6.5J) and do not appear to be consistent with static recrystallization. However, the quartz and feldspar grains are not strongly elongated by crystal-plastic processes. For example, lobate grain boundaries indicative of grain boundary migration are mostly lacking, largely excluding pervasive dynamic recrystallization. Small myrmekite and subgrains of quartz reside in interstices and coat feldspar grain boundaries (Fig. 6.5K). In addition, some MSB and TIS samples (Fig. 6.5L and 6.5M, respectively) show interstitial quartz and/or feldspar which may represent partial melts. These microstructures look remarkably similar to interstitial feldspar in statically and dynamically recrystallized granite described by Rosenberg and Riller (2000). However, we also conducted cathodoluminescence to identify partial melts, but evidence for large amounts of partial melts residing in interstices or along grain boundaries could not be found. The TIS samples exhibit evidence for pervasive brittle deformation. Fig. 6.5N depicts a representative example showing intra-grain fractures which typically are limited to individual grains. Some of the cracks are filled with biotite at the grain margins. Penetrative intra-grain cracking is confined to the host rock near the pluton margin. Figure 6.50 (collected near the pluton margin) illustrates that originally roughly equidimensional quartz grains (Fig. 6.5P; rhyolitic ash-flow tuff of 181 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Saddlebag Lake collected well beyond the pluton margin; see Fig. 6.2B) are marked by intense intra-grain cracking. Cracking of larger grains appears to occur more frequently along the grain margins, eventually leading to mechanical abrasion of larger grains and generation of smaller grain fragments. Figure 6.50 nicely illustrates that small grain fragments were smeared out and transported from the grain margins to the tips. We note that despite our efforts to retrieve samples from hydrothermally unaltered areas, all samples show evidence for various degrees of fluid circulation through the aureoles. For example, fluid inclusions are common in MSB and TIS samples. In addition, muscovite appears in proximity to the TIS margin. However, because we did not observe any inter-grain fractures in our host rock samples it is unlikely that the intra-grain cracks in our host rock samples formed as a result of hydro-fracturing. Interpretation of microstructural observations Our microstructural observations suggest the presence of melt during folding of the leucocratic dikes. Feldspar grains in the dikes became aligned parallel to the axial surfaces of the dike folds and the metamorphic host rock foliation outside of the dikes by rigid body rotation in a melt. Minor evidence for recrystallization of quartz was found and is inferred here to be insignificant with regard to the amount of strain needed to accommodate the folding for the following three reasons: (1) 182 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. individual quartz grains and subgrains with lobate grain boundaries are not strongly elongate; (2 ) deformation is not localized into high strain shear bands as occurs in many other deformed quartz-rich rocks; and (3) preservation of rather delicate lobate grains of myrmekite along the margins of larger euhedral K-feldspars suggests an absence of significant sub-solidus deformation subsequent to the formation of myrmekite. In summary, the leucocratic dikes deformed mainly in the magmatic to sub-magmatic state, where igneous minerals crystallized from a melt, rotated into alignment and fractured upon grain to grain contact. Melt filled the cracks and led to in-situ crystallization of feldspar, quartz, and less commonly opaque minerals. Upon solidification, folding was mostly completed, and only a minor amount of deformation (no more than a few percent strain) continued in the solid state. The above scenario raises the question of how the folding of the leucocratic dikes was accommodated in the host rocks. Obviously, both the dikes and the host rocks of the MSB and the TIS were subject to shortening associated with folding, and both materials must have been subject to approximately the same strain. Volumetrically, however, the dikes constitute only a small fraction of the material that underwent folding, and therefore we argue that the bulk deformation must have been controlled and accommodated by the host rock. Our microstructural observations do not show much evidence for crystal-plastic creep and hence we suspect that intra-grain cracking and the presence of melt played an important role during host rock shortening close to the pluton-host rock interfaces. We suggest that intra-grain cracking mechanically abraded grains and generated smaller grains with highly 183 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. irregular grain boundaries. Furthermore, we speculate that melt may have resided in interstices and along grain boundaries and that quartz and feldspar crystallized from the melt. The presence of melt could have triggered melt-assisted granular flow. During subsequent solid-state deformation of the host rocks, myrmekite developed and may have been further smeared out between grain boundaries. This interpretation would explain the lack of strong crystal-plastic deformation and grain elongation. Our microstructural observations, together with field evidence for simultaneous ductile flow, truncation of pre-existing rock units by brittle fracturing, and possibly rigid rotation indicate that multiple processes operated to accommodate the bulk strain associated with emplacement-related aureole shortening in the MSB and TIS. In addition, our microstructural interpretation favoring intra-grain cracking and melt- assisted granular flow as the dominant deformation mechanisms in combination with only minor annealing and presumably rapid cooling by circulating fluids are consistent with fast strain rates during emplacement of the MSB and the TIS. Cooling times We argued above that the finite strains during folding of the dikes were produced during magmatic conditions, the maximum duration of which would be the time between the emplacement and cooling to the solidus temperatures of the dikes. To constrain this maximum duration we conducted a series of analytical computations to 184 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. determine the cooling times for the leucocratic dikes. Delay of cooling by additional heat due to the effect of latent heat during crystallization was taken into account (Furlong et al., 1992). We assumed that crystallization was completed when the solidus temperature (caption to Figure 6 .6 ) was reached at the center of a dike. The temperature of a dike center was calculated using: where Tcem - temperature in dike center, 7h0 S t = ambient host rock temperature, Tem p = emplacement temperature of dike, x = dike width, t = time, K = thermal conductivity, p = density, and C = specific heat capacity (Furlong, 1991). The effect of latent heat was considered by replacing the specific heat of the magma with an equivalent value that accounts for latent heat according to: where L = latent heat of fusion, 7i,q = dike liquidus, and Tso \ = dike solidus. The modeling parameters are listed in the caption to Figure 6 .6 . We calculated cooling r \ / \ - * • T _ r j, , T em p T h o s t L r - ___2 1 c e n t 1 h o s t ' (i) 185 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. times for ambient host rock temperatures r h o S t of 600 and 650 °C. Granodiorite solidus temperatures Tso\ were varied from to 650 to 700 °C. Figure 6 . 6 shows the resulting cooling times for a range of dike widths, ambient host rock temperatures, and dike solidus temperatures. At an ambient host rock temperature of 600 °C (Fig. 6 .6 A), cooling occurs quite rapidly and crystallization in the dike centers is complete within minutes to ca. 1 week. At a host rock temperature of 650 °C, however, cooling times are considerably longer (Fig. 6 .6 B). For granodiorite r s o i values between 660-700 °C, crystallization is complete after days to ca. 1 year, while a Ts0\ of 650 °C results in cooling times that are much longer. In this case, crystallization of the dike centers would be complete after 100 to 1 ,0 0 0 , 0 0 0 years, depending on dike width. Strain rates We used a standard strain rate equation where the stretch s = 1 < JI\ was determined from our strain calculations (s = 3.36 to 6.89, Fig. 6.4) and t = time derived from the maximum possible crystallization times (Fig. 6.7A). We recall that the lack of evidence for real sub-solidus deformation (3) of dikes in the thermal models (t = 5 x 102 s to 6 x 1012 s). Our calculations yield strain rates associated with folding of leucocratic dikes in the range of 1 0 ‘2 to 1 0 ‘ 13 s' 1 186 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6 .6 . Cooling times for leucocratic dikes. (A.) 7h0 S t = 600 °C. (B.) Jhost = 650 °C. Lines represent different temperature differences between the magmatic dike solidus and the solid host rock temperatures. Boxes indicate cooling times derived from actual dike widths relating to our field observations. Modeling parameters: dike width x = 0.001 - lm; thermal conductivity K = 2.65 W nf’K"1 (Furlong et al., 1991); density p = 2500 kg/m3 (calculated after McBimey, 1984, using data from Bateman and Chapell, 1979); specific heat C = 1142 J/kg/K (Turcotte and Schubert, 2002); latent heat L = 150,000 J/kg (Hanson and Glazner, 1995); Time t = 1 - 1014 s; dike (granodiorite) emplacement temperature = liquidus 2 1 ,- q = 850 °C; dike solidus r so i = 650 - 700°C at ca. 2-3 kbar (Wyllie, 1971; Cullers et al., 1993; Scaillet et al., 1995). 187 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A. 7host = 600»C « o o a » M * U O © E Dike width B . 10» 10® u 10® itr t m m max . 1.000,000 yoan _________— 650 660 — — — 670 680 ------------- 690 ------------- T O O 188 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6.7. (A.) Strain rates for folding of leucocratic dikes associated with ca. 80% aureole shortening. Horizontal reference lines represent rates of deformation experiments (e.g. Dell’Angelo et al., 1987), wall rock strain rates associated with dike-fed expansion (Johnson et al., 2001), maximum rates of dislocation creep during p luton growth (Albertz et al., 2 0 0 1 b) and r egional deformation ( e.g. Price, 1975; Pfiffiier and Ramsay, 1982; Paterson and Tobisch, 1992; Dunlap et al., 1992; Foster and Gray, 1999; Muller et al., 2000; Hirth et al., 2000). (B.) Theoretical errors required to cause an order-of-magnitude change in strain rate. Stretch estimates, variation in parameters, strain rates obtained in our study, and equation (3) were used to determine the error in finite stretch required to increase or decrease the strain rate to the next higher or lower order of magnitude, respectively. An increase in strain rate would require errors of hundreds of percents whereas a decrease in strain rate would require errors of tens of percents. 189 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Strain rate A. Fie 4C Fig. 4£ Fig. 4A.D Fig. 48 1 I C / > 10" ' > ‘10 ,•12 ,-13 ,-14 ,-15 0 005 0.15 0.1 0.2 0.25 0.3 0.35 04 rs o r 700°c 7sol = 650 ’ ’ C . = 650 °C = 700 °C ■ 650 °C Reference strain rates Deformation experiments — Dike-fed expansion — - Dislocation creep — Regional deformation Dike width [m] B . Fig. 4C Fig.4E Fig.4A.D I ! I Fig. 48 1000 100 : (j o c © m 0.25 0.3 0 0.05 0.1 0.15 0.2 0.35 0.4 increase in strain rate Decrease in strain rate = 600*C host - 7sol=700 °C - 7sol=700 °C - 7sol=650 °C - 7sol=650 ® C 7host = 650#C ■ 7sol=700°C ■ 7sol=700 °C ■ 7sol=650°C ■ 7sol=650 ® C Dike width [m] 190 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. indicate that the deformation was restricted to the melt-present conditions and the strain rates obtained likely represent the last stages of pluton emplacement. The rates obtained for dike solidus temperatures in the range of 660-700 °C are extremely fast and the significance of which is discussed in detail below. Discussion Strain rates Our strain rates strongly depend on host rock and dike solidus temperatures. Whereas host rock temperatures of 600 and 650 °C and dike solidus temperatures of 660-700 °C result in strain rates of 10"2 to 10'5 s'1 for all dike widths, a host rock temperature of 650 °C and a dike solidus temperature of 650 °C yields strain rates of 10'1 0 to 10'1 3 s'1 . In addition to variation in dike solidus temperature, errors in estimating finite strains may affect our strain rates. Using the stretch estimates, variation in parameters, strain rates obtained in our study, and equation (3), we determined the error in finite stretch that would be required to increase or decrease the strain rate to the next higher or lower order of magnitude, respectively (Fig. 6.7B). For all dikes considered, increase in strain rates would require errors of hundreds of percents whereas a decrease in strain rate would require errors of tens of percents. We re-emphasize that we had true 3-D exposure for examining the dikes considered in this study and hence we argue that any error associated with estimating 191 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. strains inthefieldislesst han t he t heoretical e rror r equired to e ause a n o rder-of- magnitude change in strain rate. Assuming the most conservative scenario (dike solidus temperature equals host rock temperature) the strain rate associated with folding of the thickest dike in our i "5 i study (ca. 10" s") falls within regional strain rates, but folding of the thinnest dike could have occurred ca. 4 orders of magnitude faster. “Normal” rates of 10"1 3 to 10" 1 5 s"1 for regional deformation have been indicated by several studies (e.g. Price, 1975; Pfiffher and Ramsay, 1982; Paterson and Tobisch, 1992; Dunlap et al., 1997; Foster and Gray, 1999; Muller et al., 2000; Hirth et al., 2001). Other studies have modeled strain rates associated with emplacement-related deformation, and here we briefly summarize two relevant works (Fig. 6.7). Johnson et al. (2001) constructed a kinematic model to test whether fast strain rates in pluton aureoles can be used to identify natural examples of dike-fed chamber expansion. They evaluated pluton-filling rates and associated wall-rock strain rates for several end-member dike widths, flow rates, and pluton shapes (spherical and ellipsoidal). Under the assumption that all host rock deformation during pluton emplacement occurs by ductile strain, the results indicated very rapid wall rock strain rates over the entire growth period (ca. 10'7 s"1 for most of the time). These rates, however, vary dramatically with time and distance from ca. 10‘2 to 10"4 s"1 early and © 1 * 5 1 near the pluton margin to ca. 10" to 10" s" at the maximum distance (5 km) and the last growth stages. 192 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A subsequent study elaborated on mechanical implications of a dike-fed ballooning model utilizing a similar geometry, pluton filling rates, and dimensions for the purpose of direct comparison (Albertz et al., 2001b). Because creep is the most widely assumed process of ductile deformation in pluton aureoles, Albertz et al. (2001b) examined rates of dislocation creep associated with aureole shortening. The work used a finite difference heat conduction model to simulate the limited temperature and stress variation around a growing natural aureole and computed associated rates of dislocation creep in the surrounding host rock. Similar to the kinematic model (Johnson et al., 2001), a spherical geometry was applied. The results suggested that dislocation creep occurs at a maximum rate of 10'u s'1 in a narrow zone (ca. 10-50 m) very close to the contact, and that strain rates decay to “normal” regional background rates over a relatively short distance (ca. 500 m). Comparing the strain rates obtained from the dislocation creep model (10'n s'1 ) and the dike-fed ballooning model (10'7 s'1 ) to the strain rates determined in this study •y 1 ^ 1 (10'" to 10’ s"), it is obvious that dislocation creep is not fast enough to accommodate aureole shortening at the higher strain rates determined in this study. A fundamental difference exists in the durations considered in the above dislocation creep model and the short durations of dike folding we calculated in this study. Though the above mentioned studies considered relatively long durations of magma chamber construction (e.g. millions of years), the results of the present study apply to comparatively short durations for the fast strain rates obtained (e.g. days to hundreds of years to under rare circumstances one million years). It remains highly 193 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. unlikely, that the extraordinarily fast rates calculated from the dike folding should be extrapolated to the entire durations of construction of the MSB or TIS (e.g. several million years). In order to test this, we calculated rates of bulk aureole shortening using finite strain estimates from host rock markers in each aureole and estimates for the d uration o f m agma c hamber c onstruction. Estimates f or a verage b ulk a ureole shortening are 48% and 37% for the MSB and the TIS, respectively (see field observations above). Based on new geochronological data by Matzel et al. (2002), the MSB was emplaced between ca. 96.3 and 90.9 Ma, yielding a maximum duration of pluton construction of ca. 5.4 million years, only part of which affected the aureole we examined. Individual units of the TIS that are present in our area of interest show crystallization ages of ca. 91-89 Ma (HalfDome granodiorite), 88.1 Ma (Cathedral Peak granodiorite), and 92-94 Ma (Kuna Crest granodiorite). Hence, each magma pulse of the TIS was probably emplaced over a maximum duration of no more than ca. 2 million years. Using equation (3) of this paper, we obtained strain rates associated with bulk aureole shortening of 1.13 x 10'1 4 s'1 and 2.53 x 10'1 4 s'1 for the MSB and the TIS, respectively, assuming that the rates were constant over the duration of emplacement and throughout the aureoles. Our estimates for the duration of construction of the MSB are maximum estimates, and it is likely that the mushroom-shaped part of the MSB was constructed in less time. However, we note, that even if the mushroom-shaped part was constructed anywhere within 100,000 to one million years, our strain rate estimates would vary only by one order of magnitude, i.e., 10'1 3 to 10'1 4 s'1 . These rates are significantly slower than our 194 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. calculated fast strain rates associated with dike folding, but they match published rates of regional deformation (Fig. 6.7A). Our c ooling m odels p rovide a m uch finer r esolution for t he 1 ikely d uration o f shortening at one location in the aureoles, as they yield cooling times that are spread over many orders of magnitude (several days to one million years). Numerous studies show that pluton construction is commonly associated with sequential magma pulses (e.g. Paterson and Vernon, 1995; Morgan and Law, 1996; Schaltegger, 1997; Brown and McClelland, 2000; Miller et al., 2001; Coulson et al., 2002). Therefore, we speculate that one possible explanation is that extremely fast aureole shortening may occur in a pulsatory fashion. Episodic magma emplacement may be facilitated by magma pulses emplaced at short-lived strain rates of 10'1 0 s'1 and faster, resulting in average aureole shortening rates of ca. 10'1 3 to 10'1 4 s'1 , when the entire duration of pluton construction is considered. Likewise, volcanic eruptions are typically of a periodic nature (e.g. Shaw, 1985), and Huppert and Sparks (1988) suggested that multiple inputs of basalt into continental crust trigger rapid cyclic formation of silicic magma in the source region. Chamber construction in the mid to lower crust could occur by means of distinct short-lived surges, necessarily associated with fast strain rates in the surrounding rocks, and resulting in a highly dynamic rheology in pluton aureoles. 195 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rheological implications Our field and microstructural observations showed that multiple processes, including crystal-plastic creep, cracking, rigid rotation, stoping and possibly melt- assisted grain boundary slip, operated in the host rocks to accommodate material transfer and shortening. However, one of the fundamental questions arising from this study is: what deformation mechanism(s) can operate alone or in combination with other processes to allow “faster-than-normal” rates of aureole shortening of 10'2 to 10'1 3 s'1 ? Johnson et al. (2001) presented several potential indicators of fast deformation rates in aureoles, including: (1) preservation of intrusive brecciation around some plutons, (2) radiating fracture sets oriented perpendicular to the pluton margin and direction of minimum compressive stress, and (3) fracturing of minerals that were present prior to emplacement. Our microstructural observations revealed that major crystal-plastic creep did not occur in the host rocks but we found evidence for intense intra-grain cracking and possibly melt-assisted granular flow. We are particularly intrigued about the roles of cracking and partial melts fringing grain boundaries potentially leading to accelerated rates of ductile flow by cataclastic flow and melt-assisted granular flow, respectively. Laboratory deformation experiments have shown that cracking and melt-assisted granular flow may occur quite commonly, in particular at fast strain rates. However, evidence for melt-assisted granular flow may be difficult to recognize because draining of melt during progressive deformation may remove direct evidence for this process. 196 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Melt-assisted granular flow has been inferred by several workers in experiments as well as in microstructural studies of migmatites in other pluton aureoles and in lower crustal regions. For example, Dell’Angelo et al. (1987) investigated the effect of small amounts of melt on constant strain rate (10'6 s'1 ) deformation in granitic rocks in the dislocation creep regime, and found a transition from dislocation creep to melt-enhanced diffusion creep associated with a controlled reduction in grain size from 10-50 pm to 2-10 pm, with 3-5% melt present. Rushmer (1995) conducted an experimental deformation study of partially molten natural amphibolite using a hydrous phase breakdown reaction. The experiments documented a shift in deformation from dominantly crystal-plastic to fracture to viscous flow with increasing melt fraction. Rutter and Newman (1995) studied experimental deformation of Westerly granite to determine the mechanical properties of the combined melt and crystalline matrix of grains. The results showed that high-strain rate deformation with ca. 10% melt present led to brittle faulting, and that distributed cataclasis with shear-enhanced compaction between 10% and 45 % melt fraction gave way to viscous flow of melt at higher melt fractions. In addition, Rutter and Newman (1995) observed strain softening with increasing strain during experiments, evidenced by a drop of the applied stress from 500 to less than 1 MPa. Rosenberg and Handy (2000) used norcampher-benzamide aggregates as analogs for partially molten quartzofeldspathic rocks and performed deformation experiments considering draining, constant displacement rate, and simple shearing. They found that extensional shear surfaces developed on the granular and sub-granular scale which 197 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. served as melt conduits. However, they noted that the experiments indicated that once the melt crystallizes, only locally do microstructural traces betray former melt migration, rendering it difficult to recognize diagnostic microstructural evidence for deformation under melt-present conditions. Rosenberg (2001) added that grain boundary sliding is difficult to identify due to the lack of unambiguous microstructures, but the effect of melt on the onset of grain boundary sliding can clearly be documented during “see-through” deformation experiments of norcampher in the presence of melt. Rushmer (2001) carried out experiments on volume change associated with dehydration melting of muscovite + biotite-bearing pelites and biotite + plagioclase + quartz gneiss samples. The study showed that muscovite- bearing samples developed melt-filled cracks (as in Conolly et al., 1997) while biotite gneiss did not, suggesting that volume change alone is not an important driving force for melt segregation in biotite-only-bearing assemblages and that external deformation may be required to segregate melt from the lower crust during partial melting. M.S. Paterson (2001) developed a flow law for deformation of partially molten rock in which the primary mechanism is granular flow. The results show that at 10pm grain size, the predicted strain rate is on the order of 1 0 ~ 6 s'1 , which matches the laboratory range and also the intermediate strain rates associated with dike folding in our study. Note that all of the above experimental studies showed that high enough pore pressures can develop due to melting reactions and applied stress, so that inter-granular and less commonly, intra-granular cracks can develop and facilitate draining of melt. 198 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In addition to the above evidence of melt-assisted cracking and grain boundary slip from experimental studies, recent field observations indicate that this mechanism is likely to occur in nature as well. Nicolas and Ildefonse (1996) studied microstructures in gabbros of the Oman ophiolite and explained that suspensions with melt fractions of 20% and less could flow without signs of plastic deformation by means of inter-granular slip and contact melting when mutual impingement occurs. Rosenberg and Riller (2000) compared partial-melt topology in statically and dynamically recrystallized granite of the Murray granite pluton (Canada). They observed that in the dynamically recrystallized partially melted granites feldspar seams are preferentially aligned along quartz grain boundaries sub-perpendicular to the foliation, and occasionally transect individual quartz grains. Rosenberg and Riller (2000) suggested that inter-granular and rarely intra-granular fractures may have had a strong effect on the rheology by facilitating dynamic wetting of grain boundaries favoring grain boundary sliding. Marchildon and Brown (2001, and references therein) studied migmatites in the Onawa contact aureole (Maine, USA) and presented a body of microstructural evidence in support of an anatectic origin for the Onawa migmatites, including the presence of albite-rich overgrowths on plagioclase, cuspate grains of plagioclase, K-feldspar, and locally cordierite, and rounded quartz grains. These observations indicated that feldspars crystallized from a melt which occupied interstices between original or previously crystallized quartz. Berger and Rosenberg (2002) examined microstructures (weak shape-preferred orientation of all minerals and inter-granular K-feldspar film) of deformed anatexites 199 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of the aureole of the Bergell Pluton (Central Alps, Europe) and suggested that granular flow was the dominant deformation mechanism where melt was present in the system. Brown et al. (1995) noted that melt escape may be a cyclic process, with matrix- dominated deformation of drained systems evolving to melt-dominated deformation of undrained systems as melting progresses, whereas the time intervals for melt segregation and escape are short. Hence, the cycle of melt accumulation and melt loss would repeat itself, and the system would alternate between closed and open (Brown, 2001, and references therein). This may be relevant to our suggestion that aureole shortening in the MSB and TIS may have occurred in a pulsatory fashion, indicating that cycles of very rapid aureole shortening may have alternated with shortening at strain rates typical of those of regional deformation. We suggest that the rates of melt generation and escape might influence the amount of cracking and control the rate and degree at which grain boundaries are coated with partial melt, thus ultimately controlling the rate at which the rocks can deform at melt-present (or melt-absent) conditions. With increasing melt fraction and hence increasing degree of grain boundary melt-coating, strain rates may progressively increase due to increasingly higher diffusion rates, until all melt is expelled rapidly due to compaction or filter pressing (Brown et al., 1995) and through pathways provided by crack networks, and the cycle begins anew. 200 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Conclusions The major conclusions resulting from this study are the following: finite strain analyses on magmatically folded leucocratic dikes in aureoles of the Mount Stuart Batholith and the Tuolumne Intrusive Suite, combined with thermal modeling, yield strain rates associated with emplacement-related aureole shortening that span a wide range of orders of magnitude and can be much higher than rates of regional deformation. We suggest that tightly folded magmatic dikes in these and other pluton aureoles can be used as a criterion for “faster-than-normal” host rock strain rates, provided that field and microstructural observations are compatible with folding under magmatic conditions and that relatively thin dikes (< 20 cm) are used. One important implication for the growth of crustal magma chambers is that, rather than extremely rapid continuous deformation, chamber construction may occur in a pulsatory fashion, whereby incremental chamber growth and ascent might be accommodated in the host rock by cyclic high strain rate surges. Ultimately, the high strain rate surges might be causally linked to a periodicity in melt generation, segregation, and escape from grain boundary regions in aureoles. Rapid material transfer in the aureoles of the Mount Stuart Batholith and the Tuolumne Intrusive Suite (and possibly many other magma-host rock systems around the world) was accommodated by multiple displacement processes, all operating at different rates. The most likely processes that can accommodate fast rates are rigid rotation, brittle cracking, stoping and melt-assisted granular flow, 201 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. whereby one process might overprint another during progressive deformation, supporting our notion that aureoles are highly dynamic systems. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 7: CONCLUSIONS In this dissertation I have described field and microstructural work in selected North American Cordilleran contact aureoles combined with numerical modeling to address aureole rheology during magma emplacement into the crust. In addition to providing new insight into the effect of melt on crustal rheology, the results of this study place up-to-date constraints on the tectonic evolution of the Sierra Nevada batholith, California. Specifically, my work has implications for the following: (1) the nature and mechanics of strain partitioning related to oblique convergence between the Farallon and North American plates during the Late Cretaceous, (2) previously proposed large-scale arc-parallel dextral displacement on cryptic strike- slip faults, (3) the rheological effect of the presence of melt on regional deformation, (4) the interplay of brittle and ductile host rock material transfer processes during pluton emplacement, (5) numerical constraints on rates of ductile shortening in pluton aureoles by dislocation creep, (6) field criteria potentially indicative of fast strain rates during pluton emplacement, and (7) growth of crustal magma chambers. Below I summarize these implications. 203 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Strain partitioning Recent fieldwork in intrusive unit of the Tuolumne Intrusive Suite (TIS) and its associated host rocks indicates that Late Cretaceous oblique convergence between the Farallon and North American plates resulted in partitioned contraction and transpression contemporaneous with emplacement of large volumes of magma into the crust. Oblique mineral stretching lineations and dominantly dextral shear sense indicators along the eastern margin of the TIS suggest a strain regime of dextral transpression. Steeply north-northeast dipping reverse faults at the northwestern TIS margin and steeply north-northeast and south-southwest dipping reverse faults at the eastern margin indicate that contraction occurred at the same time. These results contradict previous models that propose decoupling of deformation into parallel reverse and strike-slip faulting in the central Sierra Nevada batholith. The geometry and the detailed kinematics of partitioned contractional and dextral transpressive strain fields in the Late Cretaceous Sierra Nevadan arc are currently under investigation through field work in the May Lake pendant, Benson Lake pendant, and host rocks of the Soldier and Green Lake plutons. In addition, U/Pb zircon geochronology is currently in progress to place better constraints on the longevity of contraction across the Sierra Nevada arc. 204 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Arc-parallel strike-slip displacement Fieldwork in the Piute Meadow pendant, a host rock pendant continually exposed in a key area between shallow and deep water deposits of the Snow Lake and Saddlebag Lake pendants, respectively, failed to produce convincing evidence for a structure of the proposed magnitude of the Mojave-Snow Lake fault. The lack of strike-slip structures in this area suggest that arc-parallel displacement, if required at all, must have been accommodated east of the Saddlebag Lake pendant. However, considering the Late Cretaceous plate tectonic framework, differential strike-slip displacement of more than ca. 200 km is unlikely to have occurred along the Continental margin of North America, even if large uncertainties are accounted for. Effect of melt on crustal rheology Finite strain analysis and microstructural observations along margin- perpendicular transects in the Saddlebag Lake pendant indicate a striking coincidence between an increase in temperature, the onset of partial melting and abruptly increasing finite strain. The transition from low-to high-temperature dislocation creep farther away the pluton margin is not associated with a change in finite strain. These observation support that magma emplacement has a fundamental effect on rheology. In a mechanical context, the results suggests that temperature- induced rheologic effects are unlikely to occur in the dislocation creep regime. 205 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. However, the onset of melt-assisted granular flow and diffusion creep match a dramatic increase in finite regional strain and thus indicate that the initiation of superplasticity in contact aureoles requires the presence of melt and it results in spectacular rheological weakening. Interplay of brittle and ductile material transfer processes during pluton emplacement The results of this study confirm the ubiquity of two common observations with regards to host rock material transfer during pluton emplacement: (1) multiple processes operate during chamber construction, and (2) brittle processes are capable of removing earlier ductile strain potentially obliterating in part the earlier emplacement history. The metamorphosed volcanic and sedimentary host rocks in the aureole of the Saddlebag Lake pendant are marked by discordantly truncated stratigraphy and structures along the pluton margin. Steps in the margin and the occurrence of isolated, ductilely deformed host rock xenoliths in magmatic units suggest that the truncation is due to stoping of a ductile aureole. In addition, one step in the margin reveals a significant difference in finite strain along strike, illustrating that earlier formed ductile structures have been lost to stoping. 206 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Numerical constraints on dislocation creep rates during pluton emplacement A numerical model of spherical pluton expansion and associated ductile shortening of a hypothetical aureole consisting of pure quartzite indicates that rates of dislocation creep are unlikely to be higher than 10"1 1 s'1 . Therefore, aureoles shortening at rates faster than 1 O'1 1 s'1 must occur by different deformation mechanisms. The best candidates of deformation mechanism that could exceed rates of dislocation creep by several orders of magnitude include cataclastic flow and melt-assisted granular flow. On the other hand, ductile material transfer in aureoles shortening at rates of 10'1 1 s'1 or slower can be accommodated by dislocation creep alone. This study strengthens the importance of more theoretical and field-based work to examine the role and rates of other deformation mechanism, including diffusion creep, cataclastic flow, and melt-assisted granular flow (see below) over transient stress and temperature domains. Field criteria for fast syn-emplacement deformation Finite strain analysis and thermal modeling of magmatically folded leucocratic dikes in the aureoles of the Mount Stuart batholith, Washington and the Tuolumne Intrusive Suite, California yield strain rates associated with emplacement-related aureole shortening that span a wide range of orders of magnitude and can be much higher than rates of regional deformation. The microstructural observations suggest 207 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. that the most likely processes able to accommodate fast rates include brittle cracking and melt-assisted granular flow, whereby one process might overprint another one during progressive deformation. Consequently, the results suggest that tightly folded magmatic dikes in these and other pluton aureoles can be used as a criterion for “faster-than-normal” host rock strain rates. It is required though that field and microstructural observations are compatible with folding under magmatic conditions and that relatively thin dikes (< 20 cm) are considered because they would ensure rapid cooling. Growth of crustal magma chambers An important implication for the growth of crustal magma chambers is that, rather than extremely rapid continuous deformation occurring throughout the emplacement and growth history, chamber construction may occur in an episodic manner. This implication arises from comparing bulk strain rates and rates derived from magmatically folded dikes. Extrapolating the latter to the entire emplacement duration would yield unrealistically large amounts of material displacement. Hence, incremental chamber growth and magma ascent might be accommodated in the host rock by cyclic high strain rate surges. In fact, the high strain rate surges might be causally linked to a periodicity in melt generation, segregation, and escape from grain boundary regions in aureoles. 208 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. BIBLIOGRAPHY Ague, JJ, and Brimhall, G.H., 1988, Magmatic arc symmetry and distribution of anomalous plutonic belts in the batholiths of California: Effects of assimilation, crustal thickness, and depth of crystallization: Geological Society of America Bulletin, v. 100, no. 6, p. 912-927. Albertz, M., and Paterson, S.R., 2002, Three-dimensional pure shear during transpression: The effect of Cretaceous plutonism on regional strain fields in the Sierra Nevada Batholith: Geological Society of America 2002 annual meeting, Abstracts with Programs, v. 34, no. 6, p. 328. 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Wyld, S.J., and Wright, J.E., 2001, New evidence for Cretaceous strike-slip faulting in the United States Cordillera and implications for terrane-displacement, deformation patterns, and plutonism: American Journal of Science, v. 301, no. 2, p. 150-181. Wyllie, J., 1971, The dynamic earth: textbook in geosciences, John Wiley & Sons, Inc., New York, 416 p. 230 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Yoshinobu, A., Marko, W., Dumond, G., Wolak, J., Bames, C., and Nordgulen, O., 2003, Stoping happens! Geological Society of America Abstracts with Programs, v. 35, no. 6 , p. 93. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX A: RESULTS OF STRAIN ANALYSIS As part of this dissertation quantitative strain analysis was performed on metasedimentary and metavolcanic rocks of the Piute Meadow and Saddlebag Lake pendants. The methods used for strain analysis are described in chapters 4 and 6 . The purpose of appendix A is to show the sample and field station locations from which strain data were retrieved. In addition, all data are listed in tables A.1 to A.3. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. \ 2 km Figure A. 1. Map of the Piute Meadow pendant showing sample locations. 233 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 1 0 9 * 0110 >m w © js I #T © *0187 i m m • * 0 1 8 6 01 7 4 - • oi75»*Sa 0173*) 0 1 8 8 ’0 1 2 9 0 1 1 4 0 11 3 0112 0111 0 1 2 5 / 0 1 2 6 / 0 1 2 7 0 1 8 9 / 0 1 9 0 0 1 2 4 0 11 8 0 1 3 4 ,0 1 3 2 0 1 3 3 •rt irt — o ~ - ^ / \ 5 * 2 5 \ . \ o \ o o 1 ?0157o\' J 0 11 9 0121/0122 01S8 0120 0 131 0 1 3 0 0 12 3 0 1 2 8 2 km Figure A.2. Map of the Saddlebag Lake pendant showing sample locations. 234 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Algebraic Shimamoto 3D analysis Table A. 1. Results of Shimamoto 3D strain analysis. K(R) = Logarithmic K-value (Ramsay, 1983). Station Number Orientations Foliations Strike Dip Lineations Plunge Trend Axial Ratios x y z Extensions x y z K(R) MA151 337 74 73 105 2.80 1.76 1 64.6 3.3 -41.19 0.83 MAI 53 157 88 87 306 4.84 2.51 1 110.4 9.28 -56.51 0.71 MAI 54 157 88 87 306 4.07 2.15 1 97.51 4.43 -51.52 0.83 MAI 55 345 68 66 62 4.67 2.33 1 110.98 4.99 -54.86 0.83 MA157 345 70 - - 3.34 1.94 1 79.22 3.93 -46.31 0.83 MAI 70 157 85 58 155 4.02 2.18 1 95.06 5.73 -51.51 0.79 MA171 170 87 70 243 4.27 2.28 1 100 6.74 -53.16 0.76 MAI 72 172 78 - - 1.83 137 1 34.64 0.95 -26.43 0.91 MA173 156 64 57 291 4.30 236 1 101.3 6.05 -53.16 0.78 MAI 74 141 64 52 282 3.06 1.82 1 72.64 2.56 -43.52 0.87 MAI 75 161 64 62 263 2.78 1.73 1 64.68 2.54 -40.78 0.86 MA213 321 78 72 90 4.55 2.36 1 106.08 7.11 -54.70 0.76 MA22S 340 62 - - 3.46 1.95 1 82.97 337 -47.08 0.86 MA237 315 55 52 42 4.51 238 1 107.41 4.81 -54.00 0.83 MA239 334 69 62 0 2.64 1.72 1 59.24 3.99 -39.61 0.78 MA245 320 80 80 45 5.00 2.40 1 1183 4.94 -56.33 0.84 MA264 339 81 73 10 4.65 2.30 1 111.05 4.46 -54.64 0.84 MA265 161 87 - - 5.22 2.47 1 122.73 5.27 -57.35 0.83 MA280 152 87 66 26 4.70 2.58 1 104.65 12.22 -56.46 0.64 MA286 325 84 - - 3.43 1.94 1 82.33 3.02 -46.76 0.87 MA299 166 77 70 309 3.90 2.09 1 93.79 3.9 -50.33 0.85 MA300 352 85 85 103 10.96 4.10 1 208.24 15.37 -71.88 0.70 MA305 152 88 70 167 14.99 4.71 1 262.71 13.93 -75.80 0.75 MA-314 164 86 - - 3.03 1.85 1 70.73 4.1 -43.74 0.80 MA315 172 86 78 159 5.52 2.71 1 124.03 9.97 -59.41 0.71 MA317a 171 81 - - 3.76 2.13 1 88.05 6.38 -50.01 0.75 MA317b 171 81 - - 16.54 4.81 1 284.51 11.89 -76.76 0.79 MA319 346 74 60 40 4.27 231 1 101.95 4.57 -52.65 0.83 MA323 155 90 88 115 5.19 2.46 1 122.18 5.17 -57.20 0.83 MA-324 336 74 - - 3.79 2.10 1 89.76 5.17 -49.89 0.80 MA-337 154 80 79 208 6.51 3.08 1 139.47 13.45 -63.19 0.66 MA-338 176 85 82 187 4.72 2.49 1 107.7 9.51 -56.04 0.70 MA-339 145 80 63 162 52.78 7.99 1 603.94 6.52 -86.66 0.91 MA-362 336 80 51 354 5.56 2.54 1 129.83 5.15 -58.62 0.84 MA-363 332 77 77 104 7.41 3.03 1 162.45 7.52 -64.56 0.80 MA-364 332 76 72 73 6.52 2.84 1 146.47 738 -62.18 0.80 MA-365 343 61 61 70 5.41 2.62 1 123.58 832 -58.67 0.75 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table A l. Continued. MA367 344 84 83 24 6.98 3.10 1 150.54 11.15 -64.09 0.72 MA368 346 75 52 147 7.20 3.00 1 158.42 7.75 -64.09 0.80 MA385 353 72 63 28 8.45 3.38 1 176.48 10.58 -67.29 0.75 MA386 151 75 67 42 6.65 3.05 1 143.85 11.82 -63.33 0.70 MA395 125 67 55 251 2.34 1.65 1 49.3 5.06 -36.24 0.70 MA396 128 69 1 1 142 7.96 3.33 1 166.94 11.76 -66.48 0.72 MA406 132 82 57 144 4.92 2.50 ' 113.19 8.22 -56.65 0.74 MA446 269 77 89 32 5.53 3.10 1 114.59 20.14 -6121 0.51 MA-448 304 88 74 61 7.00 322 1 147.74 14.03 -64.60 0.66 MA449 278 84 12 98 10.26 4.56 1 184.82 26.51 -72.25 0.54 MA451 309 85 64 315 11.02 3.76 1 218.57 8.63 -71.10 0.81 MA462 304 86 72 303 623 2.79 1 140.43 7.71 -61.38 0.78 MA470 301 75 74 46 2.30 1.56 1 50.14 1.8 -34.57 0.88 MA486 175 81 76 39 13.45 5.95 1 212.03 38.14 -76.80 0.46 MA487 330 81 75 130 5.86 2.92 1 127.43 13.21 -61.16 0.65 MAS 12 25 80 66 48 5.99 2.95 1 129.99 13.18 -61.58 0.66 MA536 135 84 - - 4.56 2.55 1 101.36 12.55 -55.88 0.62 MA-594 319 85 78 349 6.42 2.80 1 144.84 7 -61.83 0.80 MA600 134 85 - - 4.79 2.44 1 111.11 7.45 -55.92 0.76 MA636 285 82 80 1 5.53 2.68 1 124.98 9.23 -59.31 0.73 MA-638 279 84 82 83 3.55 1.98 1 85.55 3.17 -47.76 0.86 MA-640 114 85 84 224 2.92 1.79 1 68.47 2.97 -42.35 0.85 MA804 137 69 69 251 5.63 2.90 1 121.86 14.39 -60.60 0.62 MA822 338 71 - - 4.07 2.17 ' 97.09 4.88 -51.62 0.82 MA882 327 85 - - 1.90 1.43 1 36.04 2.62 -28.37 0.78 MAI 076 312 75 - - 5.09 2.99 1 105.39 20.57 -59.62 0.49 MAI 093 99 90 75 99 1.97 1.50 1 37.46 4.44 -30.34 0.68 MAI 096 180 69 64 97 2.08 1.55 1 40.81 4.9 -32.30 0.67 MAI 104 274 78 78 336 3.23 1.95 1 75.06 5.44 -45.82 0.76 MA-1213 98 87 66 273 2.63 1.67 1 60.49 1.97 -38.90 0.89 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fry analysis Table A.2. Results of Fry analysis. K(R) = Logarithmic K-value (Ramsay, 1983). Sample Number Orientations Foliation Strike Dip Lineation Plunge Trend Axial Ratios x y z Extensions x y z K(R) MA0120 - - - - 1.85 1.38 1 35.02 1.23 -26.83 0.89 MAO 136 166 84 63 160 3.37 1.95 1 80.02 4.04 -46.60 0.82 MAO 138 173 88 - - 1.88 1.39 1 36.55 0.84 -27.38 0.92 M AO 140 - - 76 104 2.01 1.46 1 40.68 1.78 -30.16 0.86 M AO 146 358 73 70 121 2.02 1.44 1 41.54 0.95 -30.01 0.92 M AO 153 153 87 85 224 1.92 1.42 1 37.62 1.58 -28.47 0.87 M AO 155 154 80 79 208 1.55 1.25 1 24.24 0.41 -19.83 0.95 M AO 157 145 80 63 162 7.12 3.14 1 152.75 11.49 -64.51 0.72 M AO 159 136 60 60 349 1.67 1.31 1 28.79 0.72 -22.91 0.92 MAO 163 345 74 70 32 2.34 1.57 1 51.59 1.59 -35.07 0.89 MA0164 329 77 64 359 2.25 1.53 1 48.97 1.39 -33.80 0.90 MAO 166 323 56 - - 2.01 1.44 1 40.90 1.03 -29.75 0.92 MAO 177 336 80 51 354 1.54 1.25 1 24.02 0.42 -19.70 0.94 MAO 178 332 77 77 104 1.46 1.22 1 20.74 0.44 -17.55 0.93 M AO 180 343 61 61 70 1.77 1.35 1 32.62 0.87 -25.25 0.91 MAO 187 - - - - 1.69 1.32 1 29.32 1.20 -23.59 0.87 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Primary fabrics-corrected strain ellipsoids Table A.3. Results of corrected strain analysis. An average primary fabrics ellipsoid with XJY/Z ratios of 1.7/1.3/1 was used to algebraically take into account the effect of primary fabrics. In order to maximize the effect of primary fabrics, co-axial interaction between primary and tectonic ellipsoids was assumed. Thus, the corrected tectonic strain ellipsoids represent minimum strains. Station Number Orientations Foliation Strike Dip Lineation Plunge Trend Axial Ratios x y z Extensions x y z K(R) MAI 53 157 88 87 306 2.85 1.93 1 61.21 9.50 -43.35 0.59 MAI 55 345 68 66 62 2.75 1.79 1 61.64 5.21 -4120 0.74 MAI 57 345 70 - - 1.96 1.49 1 37.31 4.14 -30.07 0.69 MAI 70 157 85 58 155 2.37 1.68 1 49.47 5.92 -36.84 0.67 MA171 170 87 70 243 2.51 1.75 1 53.24 6.96 -38.99 0.64 MAI 72 172 78 - - 1.08 1.06 1 3.16 1.14 -4.16 0.37 MA173 156 64 57 291 2.53 1.74 1 54.23 6.26 -38.98 0.67 MA213 321 78 72 90 2.68 1.82 1 57.91 7.31 -40.99 0.65 MA228 340 62 - - 2.03 1.50 1 40.20 3.49 -31.08 0.75 MA237 315 55 52 42 2.65 1.75 1 58.93 5.00 -40.08 0.74 MA245 320 80 80 45 2.94 1.85 1 67.18 5.15 -43.11 0.75 MA299 166 77 70 309 2.30 1.61 1 48.49 4.10 -35.31 0.75 MA300 352 85 85 103 6.45 3.16 1 136.16 15.60 -63.37 0.62 MA305 152 88 70 167 8.82 3.62 1 177.92 14.15 -68.48 0.69 MA317a 171 81 - - 2.21 1.64 1 44.09 6.58 -34.89 0.61 MA317b 171 81 - - 9.73 3.70 1 194.62 12.11 -69.72 0.74 MA319 346 74 60 40 2.51 1.70 1 54.75 4.76 -38.32 0.74 MA323 155 90 88 115 3.05 1.89 1 70.26 5.36 -44.25 0.75 MA-324 336 74 - - 2.23 1.61 1 45.39 5.38 -34.73 0.67 MA-337 154 80 79 208 3.83 237 1 83.49 13.67 -52.05 0.55 MA-338 176 85 82 187 2.78 1.92 1 59.13 9.73 -42.73 0.57 MA-339 145 80 63 162 31.05 6.14 1 439.36 6.73 -82.63 0.89 MA-362 336 80 51 354 3.27 1.95 1 76.12 5.35 -46.10 0.77 MA-363 332 77 77 104 436 2.33 1 101.09 7.73 -53.84 0.74 MA-364 332 76 72 73 3.83 2.18 1 88.84 7.50 -50.74 0.72 MA-365 343 61 61 70 3.18 2.01 1 71.30 8.45 -46.17 0.65 MA385 353 72 63 28 4.97 2.60 1 111.84 10.80 -57.40 0.68 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX B: COMPUTER CODES Dislocation creep modeling (Fortran90; Chapter 5) PROGRAM growth9 C with externally driven volume supply, min fill rate IMPLICIT NONE REAL:: dt,dV,dR REAL:: dx,maxdistance REAL, PARAMETER:: Pi = 3.141593 REAL, PARAMETER:: sec = 365. * 24. * 60. * 60. REAL, PARAMETER:: cubmpersec = 1000.**3/(365.*24.*60.*60.) REAL, PARAMETER:: gravity=9.81 REAL :: Cspec_heat_p, Cspec_heat_a, Cspec_heat_c REAL :: rhojp, rho_a, rho_c REAL :: Kconduct_p, Kconduct_a, Kconduct_c REAL :: Tjp, T_a, T_c REAL:: time_out,V_x,R_x,dr_dt REAL:: P,dP,A_exp,n_strs,Q,R_gas,s_normal,s_tang REAL:: fix,mfu,bulk,V0 INTEGER:: a, b,G, H, I, J, K, L, M, N, nx, ix, numbersteps, dR_I INTEGER:: IFILE,KTEMP,ICHECK,CCREAT,IEDOT,ISHEAR INTEGER:: INEFF,IEPSILON INTEGER:: CCLOSE, CSEEK, COPEN, CWRITE INTEGER:: step_check,n_outj_out,id_out,i_out INTEGER:: nt_out,iloc_radius,iloc_edge,loc_edge character*60 cname REAL :: time, volume, radius, R,T_edge,dr_t, dV_relax REAL :: dV_dike, dP_relax, dP_dike, fillrate, dV_strain C establish arrays c REAL, DIMENSION (0:600000):: time, volume, width, radius REAL, DIMENSION (600000):: distance, rho, Kconduct REAL, DIMENSION (600000) :: Cspec_heat, T, Twork,dR_x,dR_X_work REAL state(600000) REAL T_out (600000), state_out(600000),edot_out(600000) REAL epsilon_out (600000),shear_out(600000) REAL xtranslated(600000), y2(600000) REAL edot_x(600000),s_shear(600000), epsilon_x(600000) REAL epsilon_t(600000) integer ntime Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. T_edge=300+273.15 C set modeling parameters: C pressure=10 MPa C activation energy Q for melt-free quartzite=223 kJmolA -l C A=6.3e-12 MPaA -n*sA -2 C n=4 C R=8.3144 molA -lK A -l C Flow law calibration after Hirth et al. (2000) b u lk = l.llell fu=37. mfu=l. A_exp=6.3e-12 n_strs=4. Q=135000 R_gas=8.3144 C determine number of steps dt = (1.) * 365. * 60.*60.*24. maxdistance = 7000 dx= 10 fillrate = 4036 fillrate = fillrate * cubmpersec C initial radius & volume C fence post problem -> add one node point nx = EFIX (maxdistance / dx) + 1 PRINT*,"#nodes= ",nx C convert timesteps from years to seconds numbersteps =1000000. c time_out for now in 365 days time_out = (100.* 365.) * 24.*60.*60. nt_out = ifix(time_out/dt) ! print *, 'output every #time steps:',nt_out ! step_check = IFIX (fvolume / dV) ! PRINT *, fvolume, dV, step_check c---------------------------------------------------- C set initial physical parameters C pluton Cspec_heat_p = 1000. rhojp = 2550. Kconduct_p = 1. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. T_p = 850+ 273.15 C aureole Cspec_heat_a = 1000. rho_a = 2650. Kconduct_a = 1. T_a = 650 + 273.15 C country rock Cspec_heat_c = 1000. rho_c = 2750. Kconduct_c = 1. T_c = 300+ 273.15 C set initial values for country rock & calculate dR_x V_x=0. DO H = 1, nx distance(H) = (H-l) * dx T(H) = T_c rho(H) = rho_c Kconduct(H) = Kconduct_c Cspec_heat(H) = Cspec_heat_c state(H) = 3 END DO C create output file using C-commands cname = "growth9.out" EFILE = CCREAT (cname, 6*64+6*8+4) ICHECK = CCLOSE (IFILE) EFILE = COPEN (cname, 2) ICHECK = CSEEK (IFILE, 0, 0) cname = "temp9.out" KTEMP = CCREAT (cname, 6*64+6*8+4) ICHECK = CCLOSE (KTEMP) KTEMP = COPEN (cname, 2) ICHECK = CSEEK (KTEMP, 0, 0) cname = "edot9.out" EEDOT = CCREAT (cname, 6*64+6*8+4) ICHECK = CCLOSE (EEDOT) EEDOT = COPEN (cname, 2) ICHECK = CSEEK (EEDOT, 0, 0) cname = "shear9.out" ISHEAR = CCREAT (cname, 6*64+6*8+4) ICHECK = CCLOSE (ISHEAR) ISHEAR = COPEN (cname, 2) 241 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ICHECK = CSEEK (ISHEAR, 0, 0) ! Cname = "epsilon9.out" ! IEPSILON = CCREAT (cname, 6*64+6*8+4) ! ICHECK = CCLOSE (IEPSILON) ! INEFF = COPEN (cname, 2) ! ICHECK = CSEEK (IEPSILON, 0,0) C check stability for each rock type CALL rockstability(Kconductjp,cspec_heat_p,rho_p,dx,dt) CALLrockstability(Kconduct_a,cspec_heat_a,rho_a,dx,dt) CALLrockstability(Kconduct_c,cspec_heat_c,rho_c,dx,dt) C define index number time = 0. epsilon_t=0. radius=100. V0=(4./3.)*pi*(radius**3) P=100. ntime = 0 DO 1= 1, numbersteps C time steps time = time + dt C note that 'fillrate * dt' accounts for number of steps C determine # of node points for incoming melt and set T pluton dR_I = F IX (((3 * dV ) / (4 * Pi)) ** (1./3.) / dx) +1 C ... and print the results ! PRINT *,Time step:', I C check for location of boundary and set every node beyond boundary C to value before boundary iloc_radius=IFIX(radius/dx)+1 C create work array for dR_x ! DO K=l,iloc_radius ! dR_x_work(K)=dR_x(K) C PRINT*,K,dR_x(K),dR_x(K) ! END DO 242 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. C PRINT*, "contact at node ”, iloc_radius DO ix=iloc_radius+1 ,nx dR_x_work(ix)=dR_x(iloc_radius) END DO C translate node points DO ix=l,nx xtranslated(ix)=distance(ix)+dR_x_work(ix) END DO C now evaluate each point for location of interfaces at each timestep C and assign state variable C 1 = pluton; 2 = aureole; 3 = host rock C initially, all material is country rock, hence state 3 C set maximum distance and node spacing in meters C define three sets of values for density, conductivity, specific heat DO K = l,iloc_radius IF (K <= iloc_radius) THEN state(K) = 1. rho(K) = rho_p Kconduct(K) = Kconduct_p Cspec_heat(K) = Cspec_heat_p END IF END DO DO K=iloc_radius,nx IF (T(K) <= T_edge) THEN loc_edge=distance(K) iloc edee=IFIX(loc edge/dx) GOTO 500 END IF END DO 500 CONTINUE DO K=iloc_radius+l,iloc_edge state(K) = 2. rho(K) = rho_a Kconduct(K) = Kconduct_a Cspec_heat(K) = Cspec_heat_a END DO C PRINT*, "T_edge= ", T_edge," iloc_edge= ",iloc_edge," d= ",distance(K) ! print*, "radius,iloc,edge,iloc_edge:", ! + radius, iloc_radius,loc_edge,iloc_edge 200 FORMAT (3F5.0," ", $) C set up equations for stress DO ix=l,iloc_radius s_shear(ix)=P Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. END DO DO ix=iloc_radius+l,nx s_normal=(P*radius**3)/((distance(ix))**3) s_tang=(-P*radius**3)/(2*(distance(ix))**3) s_shear(ix)=( 1 ./2.)*(s_normal-s_tang) END DO C apply "Gerber Scale" CALL Interpolate_T(distance,xtranslated,T,Twork,nx,y2) DO ix=l,nx T(ix)=T work(ix) END DO C reset input nodes to melt temperature DOb = l,dR_I T(b) =T_p END DO C heat conduction CALLconductld(T,Kconduct,cspec_heat,rho,nx,dx,dt,Twork) C compute strain rate do ix=l,iloc_radius edot_x(ix)=0. epsilon_x(ix)=0. enddo DO ix=iloc_radius+1 ,nx edot_x(ix)=A_exp*(fu**mfu)*(s_shear(ix)**n_strs)*exp(-Q/(R_gas*T(ix))) ENDDO C compute strain distribution across aureole and cumulative strain do ix=iloc_radius+1 ,nx epsilon_x(ix)=edot_x(ix)*dt epsilon_t(I)=epsilon_t(I)+epsilon_x(ix) end do dr_t=epsilon_t(I) C compute volumetric growth radius=radius+dr_t V0=(4./3.)*pi*radius**3 dV_strain=(4./3.)*pi*(dr_tJ |c *3) dV_dike=fillrate*dt 244 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. volume=VO+dV_dike dP=(dV_strain/voluxne)/bulk P=P-dP C calculate pressure change in response to volume change ! PRINT*,I,radius,volume,P if(MOD(i,nt_out) .EQ. 0)then n_out=0 j_out=l id_out=5 DO i_out=j_out,nx,id_out n_out=n_out+l T_out(n_out)=T(i_out) - 273.15 state_out(n_out)=state(i_out) shear_out(n_out)=s_shear(i_out) epsilon_out(n_out)=epsilon_x(i_out) ! PRINT*, "shearstress= ",shear_out(n_out) if(edot_x(i_out) .GT. 0.)then edot_out(n_out)=loglO(edot_x(i_out)) else edot_out(n_out) = 0. endif ENDDO ! write(*,*)'N_out,istep = ’ ,n_out, i do i_out= l,n_out,10 ! WRITE(6,250) (T_out(i_out+a), a=0,9) 1250 FORMAT(10F6.1) enddo ! WRITE (*,*) do i_out = l,n_out,10 WRITE(6,260) (edot_out(i_out+a), a=0,9) 260 FORMAT(5ell.3) enddo WRITE (*, *) do i_out = l,n_out,10 WRITE(6,270) (shear_out(i_out+a), a=0,9) 270 FORMAT(5el 1.3) enddo WRITE (*,*) do i_out= l,n_out,10 WRITE(6,280) (neff_out(i_out+a), a=0,9) 280 FORMAT(5el 1.3) enddo WRITE (*,*) do i_out=l,n_out,10 245 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ! WRITE(6,290) (epsilon_out(i_out+a), a=0,9) !290 FORMAT(5ell.3) ! enddo ! WRITE (*,*) ICHECK = CWRITE (EFILE, state_out, n_out * 4) ICHECK = CWRITE (KTEMP, T_out, n_out * 4) ICHECK = CWRITE (IEDOT, edot out, n_out * 4) ICHECK = CWRITE (ISHEAR, shear_out, n_out * 4) ! ICHECK = CWRITE (IEPSILON, epsilonout, n_out *4) ntime = ntime + 1 endif C end loop over time ENDDO ICHECK = CCLOSE (OTLE) ICHECK = CCLOSE (KTEMP) ICHECK = CCLOSE (EEDOT) ICHECK = CCLOSE (ISHEAR) ! ICHECK = CCLOSE (IEPSILON) ! WRITE (6,300) (nx, numbersteps) 1300 FORMAT (2120," ") write(6,301)ntime,n_out 301 format( Number of output time steps:1 ,i8,/, + Number of output X-nodes :',i8) END PROGRAM growth9 C--------------- ; " :==■•= ■ - ' = c conductld: do ID thermal conduction for one time step: c conductl d(q,Kconduct,cspec_heat,rho,nx,dx,dt,qnew) c NOTE: incoming and outgoing temperatures are in Kelvin c solve Heat conduction equation from Ranalli's Rheology c of the Earth, 1987, pg 149, eq. 7.11 c c cspec_heat*dens*dT/dt = Kconduct dA 2T/dxA 2 c c where Claerbout’ s a is c a= 2*alpha= Kconduct/(cspec_heat*dens) * delta_t/delta_xA 2 c c Kconduct= Watts/(m-degK) ~l-5. c specific heat= J/(kg-degK) ~ 1000. c density = kg/mA 3 ~2670. c thermal diffris= mA 2/s ~10A -6 c 20oct00 modify from fd_latentHeat6.f Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. subroutine conductl d(T,Kconduct,cspec_heat,rho,nx,dx,dt,Twork) real T(nx),Kconduct(nx),cspec_heat(nx),rho(nx),Twork(nx) real dx,dt integer nx c------------------------------------------------------------------------- c Explicit implementation of 1-D finite difference method, c 1-D star is based on (pg 62, Croft and Lilly): c c Ti-1 + Ti+1 - (2 - l/Fo)Ti = 1/Fo T'i c c T'i = Fo [ Ti-1 + Ti+1 - (2 - l/Fo)Ti ] c c Boundary conditions: c Left side: will be reset as new magma, so use next neightbor approach, c Right side: use next neighbor approach, c At edges, use simply a 'next neighbor' approach. This is not as c correct as using convection or fixed edges, but is simpler to c implement. With sufficient number of node points, it's OK. do ix=l,nx Twork(ix) = 0. enddo c loop over spatial points do ix = 2,nx -1 Fo = (Kconduct(ix)/(cspec_heat(ix)*rho(ix))) * dt/(dx*dx) Twork(ix) = ( T(ix-l) + T(ix+1) - (2. -l./Fo)*T(ix)) * Fo enddo c now do simple 'adjacent neighbor' boundary condition ix=l Twork(ix) = Twork(ix+l) ix=nx Twork(ix) = Twork(ix-l) c now transfer new time step results to original array do ix=l,nx T(ix) = Twork(ix) enddo RETURN end subroutine stability 1 d(Kconduct,cspec_heat,rho,nx,dx,dt) real Kconduct(nx),cspec_heat(nx),rho(nx) integer nx, ix xmin = (Kconduct( 1 )/(cspec_heat( 1 )*rho( 1))) * dt/(dx*dx) xmax = (Kconduct(l)/(cspec_heat(l)*rho(l))) * dt/(dx*dx) 247 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. do ix=l,nx Fo = (Kconduct(ix)/(cspec_heat(ix)*rho(ix))) * dt/(dx*dx) if(Fo .LT. xmin)xinin=Fo if(Fo .GT. xmax)xmax=Fo if(Fo .GT. 0.5)then write(6,100)ix,Fo stop elseif(Fo .EQ. 0.)then write(6,200)ix endif enddo write(6,300)xmin,xmax return 100 format('THERMAL STABILITY FAILED> Node’ ,i8,’ Fo=’ ,fl0.4,/, + ’ THERMAL STABILITY > Fo should be < 0.5’ ,/, + 'THERMAL STABILITY > decrease dt or increase dx') 200 format(THERMAL STABILITY FAILED> Fo = 0. at node ’,i8) 300 format(’ THERMAL STABILITY> Fo ranges between’,fl0.4,' and',fl0.4) end subroutine rockstability(Kconduct,cspec_heat,rho,dx,dt) real Kconduct,cspec_heat,rho,dx,dt,dtmax Fo = (Kconduct/(cspec_heat*rho)) * dt/(dx*dx) dtmax = cspec_heat*rho*dx*dx/(2.*Kconduct) if(Fo .GT. 0.5)then write(6,50)Kconduct,cspec_heat, rho,dt,dx write(6,100)Fo write(6,400)dtmax,dt stop elseif(Fo .EQ. 0.)then write(6,50)Kconduct,cspec_heat,rho,dt,dx write(6,200) write(6,400)dtmax,dt stop else write(6,300)Kconduct,cspec_heat,rho,dt,dx write(6,400)dtmax,dt endif return 248 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50 format('THERMAL STABILITY FAILED> K,c,rho,dt,dx:’ ,5el2.5) 100 format('THERMAL STABILITY FAJLED> Fo=’ ,fl0.4,/, + ’ THERMAL STABILITY > Fo should be < 0.5 V, + ’ THERMAL STABILITY > decrease dt or increase dx’ ) 200 format(THERMAL STABILITY FAILED> Fo = 0. at node ’ ,i8) 300 fonnat(’ THERMAL STABILITY OK FOR> K,c,rho,dt,dx> ’ ,5el2.5) 400 format(THERMAL STABILITY MAX DT in sec, orig dt>’ ,2e!2.5) end c c c incoming: c xtranslated() is distance after dR() translation c distance() is desired uniform distance locations c Told() is original Temperature profile c y2(nx) is a work array, c outgoing: c Tnew() is reinterpolated Temperature profile c c c subroutine Interpolate_T(distance,xtranslated,Told,Tnew,nx,y2) real distance(nx),xtranslated(nx),Told(nx),Tnew(nx),y2(nx) integer nx,ix real xloc,Tinteip call spline(xtranslated,Told,nx,0.,0.,y2) do ix=l,nx xloc=distance(ix) if(xloc .LT. xtranslated(l))then Tnew(ix)=Told(l) elseif(xloc .GT. xtranslated(nx))then Tnew(ix)=Told(nx) else Tinterp=0. call splint(xtranslated,Told,y2,nx,xloc,Tinterp) Tnew(ix) = Tinterp endif enddo RETURN end c------------------------------------------------------- c Cubic spline c c From Numerical Recipies c c Given arrays X(i) and Y(i) of length N containing a tabulated function, c i.e., Y(i)=f(X(i)), and given values YP1 and YPN for the first derivative Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. c of the interpolating function at points 1 and N, this routine returns c an array Y2(i) o f length N which contains the second derivatives of the c interpolating function at the tabulated points X(i). If YP1 and/or YPN c are equal to .99E30 or larger, the routine is signalled to set the c corresponding boundary condition for a natural spline, with zero c second derivative on that boundary, c subroutine spline (x,y,n,ypl,ypn,y2) parameter (nmax=10000) dimension x(n),y(n),y2(n),u(nmax) c if(n .GT. nmax)then c write(* c write(*j*)'FATAL ERROR> RAISE DIMENSION IN SPUNEO' c write(* ******************************************’ c stop c endif if(yp 1 .gt .99e30)then y2(l)=0. u(l)=0. else y2(l)=-0.5 u( 1 )=(3./(x(2)-x( 1 )))*((y(2)-y( 1))/(x(2)-x( 1 ))-yp 1) endif do 100 i=2,n-l sig=(x(i)-x(i-1))/(x(i+1 )-x(i)) p=sig*y2(i-l)+2. y2(i)=(sig-l.)/p u(i)=(6.*((y(i+l)-y(i))/(x(i+l)-x(i))-(y(i)-y(i-l)) * /(x(i)-x(i-1 )))/(x(i+1 )-x(i-1 ))-sig*u(i-1 ))/p 100 continue if(ypn.gt..99e30)then qn=0. un=0. else qn=0.5 un=(3./(x(n)-x(n-l)))*(ypn-(y(n)-y(n-l))/(x(n)-x(n-l))) endif y2(n)=(un-qn*u(n-1))/(qn*y2(n-1)+1.) do 200 kr=n-l,1,-1 y2(k)=y2(k)*y2(k+l)+u(k) 200 continue return end c subroutine splint to apply results of spline c c Given the arrays XA(i) and YA(i) of length N, which tabulate a function, c and given the array Y2A(i), which is the output from SPLINE above, c and given a value o f X, this routine returns a cubic spline interpolation c value Y. 250 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. subroutine splint(xa,ya,y2a,n,x,y) dimension xa(n),ya(n),y2a(n) k!o=l khi=n 1 if((khi-klo).gt.l)then k=(khi+klo)/2 if(xa(k).gt.x)then khi=k else klo=k endif gotol endif h=xa(khi)-xa(klo) c if(h.eq.O.) pause 'bad xa input.' a=(xa(khi)-x)/h b=(x-xa(ldo))/h y=a*ya(klo)+b*ya(khi)+ * ((a**3-a)*y2a(klo)+(b**3-b)*y2a(khi))*(h**2)/6. return end Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Cooling times of leucocratic dikes (Matlab; chapter 6) % program cool % cooling times % define constants % rho = density % C = specific heat capacity % K = thermal conductivity % kappa = thermal diffusivity clear Tdike Tliq Tcrit Thost halfthick K rho L C Clat kappa N time temp Tdike = 1123 Tliq =1123 T soi= 973 Thost = 873 halfthick = 0.0005 K = 2.65 rho = 2500 C = 1142 L =150000 Clat = C + (L/(Tliq-Tsol)) kappa = K / (rho * C) N =10000000000000000 % loop over time for time = 1 : 1:N, kappa temp = Thost + ((Tdike - Thost) / 2) * (2 * (erf(halfthick / ((4 * kappa * time) A (1 / 2))))); if temp <= Tliq kappa= K / (rho * Clat) ('Tliq reached') end if temp <= Tsoi temp = (Tsoi in dike center reached’ ) break end time temp end 252 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. NOTE TO USERS Oversize m aps and charts are microfilmed in sectio n s in the following manner: LEFT TO RIGHT, TOP TO BOTTOM, W ITH SMALL OVERLAPS This reproduction is the best copy available. UMf Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Legend Lithology C Cover Quartemary deposits and dense vegetation. Tuffs and tuff breccias of Hanna Mountain: Massive, light- to dark- grey and locally buff-colored, usually well-indurated, poorly layered tuff and tuff breccia, which contain much andesite and some basaltic debris set in a fine-grained matrix of volcanic ash and mineral and lithic fragments (Chesterman, 1S75). Andesite of Walker River Light- to medium grey and pinkish-grey, platy, and well-banded porphyritic andesite ranging from homblende-rich to biotite-rich with phenocrysts of hornblende and biotite set in a fine-grained groundmassof andesine, hornblende, and biotite (Chesterman, 1975). Cathedral Peak granodiorite: 88.1 Ma (Fleck et al„ 1996). Medium-grained biotite- hornblende grano- diorite with conspicuous sphene. Locally porphyritic with abundant large phenocrysts of potassium feldspar. Buckeye Creek monzonite, porphyritic: Massive, porphyritic, light- to medium-grey and locally pinkish quartz monzonite composed of quartz, orthodase, calcic oligodase, biotite, and hornblende; porphyritic with phenocrysts of alkali feldspar in a medium-grained groundmass (Chesterman, 1975). Buckeye Creek monzonite, nonporphyritic: Massive, porphyritic, light-to medium-grey and locally pinkish quartz monzonite composed of quartz, orthodase, caldc oligodase, biotite, and hornblende (Chesterman, 1975). Granodiorite of Lake Harriet: Dark-grey, moderately foliated granodiorite characterized by shreddy dots of biotite and hornblende (Wahrhaftig, 2000). Contains mafic endaves. Granodiorite of Bond Pass: Grey, homblende-biotite granite and granodiorite with moderately large, slightly pinkish K-feldspar phenocrysts (Wahrhaftig, 2000). Contains mafic endaves. MetaandesKe: Medium- to dark grey andesitic metavolcanics with various lithic fragments. Alaskite of Grace Meadow: Sugary, medium- to fine-grained, light-pink to white alaskite with rare dark minerals (Wahrhaftig, 2000). Granite of Stubblefield Canyon: Massive, grey, fine-grained granite Aplite: White- to light-grey, fine- medium grained granitic rock with orange to pink weathering feldspar grains. Diorite: Fine-grained, massive, well-jointed, dark greenish-grey diorite composed of dark-green hornblende, pale greenish-brown biotite, caldc oligodase, pale greenish diopside augite, and minor quartz (Chesterman, 1975). Metadacite tuff: Light-grey, yellow- to rust-brown weathering metamorphic dadte. Locally shiny white mica on foliation surfaces. Contains deformed more mafic lithic fragments in some plapes. Metaandesite: Massive, dark-grey metamorphic andesite with conspicuous feldspar dasts and sometimes deformed more felsic lithic fragments. Compositional layering with more felsic bands in some places. Metaconglomerate: Grey, yellow- to brown-weathering metamorphic conglomerate with deformed components. M ap symbols Mapped geological contact ^ s s Solid state foliation >^8 5 77 Solid state lineation /T 7 7 Maamatic foliation Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Geologic Map of Piute Reproduced with permission of the copyright owner. Further reproduction prohibited without permission Lite Meadow pendant Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. M ap symbols Mapped geological contact V 5 Solid state foliation V 5 Solid state lineation Assumed geological contact V 5 Magmatic foliation V s Magmatic lineation ^ 7 7 ❖ Vertical lineation O Thrust fault Markus Albertz (2001-2003) Clyde W ahrhaftig (1955-1981) s i Normal fault Strike slip fault 0 I 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. West Lineations Poles to solid state foliations Total Data: 135 Equal Area Total Data: 200 Equal Area Poles to magmatic foliations Total Data: 60 Equal Area Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. i i i East Lineations Poles t0 state foliations Total Data:69 Total Data: 138 Equal Area EqualArea Poles to magmatic foliations Total Data: 10 EqualArea Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. NOTE TO USERS Oversize m aps and charts are microfilmed in sectio n s in the following manner: LEFT TO RIGHT, TOP TO BOTTOM, W ITH SMALL OVERLAPS This reproduction is the best copy available. u m T Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Geologic map of Saddlebag Lake pendant m /// u p .... ... Reproduced with permission of the copyright owner. Further reproduction prohibited without permission Lake pendant ( A ib e r t z , 2 0 0 2) 38*00' Legend Lithology 1 g] Cathedral Peak granodiorite: 88.1 Ma (Fli El diorite with conspicuous sphene. Locally poi Khd o (0 a. o iC Halfdome granodiorite: 89 Ma (Kistler and Medium- grained biotite- hornblende granodi large phenocrysts of potassium feldspar. Kuna Crest granodiorite: ca. 92 Ma (Pater colored, fine- to medium- grained, biotite- hoi Sheeted dike complex: Zone of strong inte and host rocks of the Horse Canyon and K oi Quartz-feldspar-gneiss: Distinctively brow probably representing thin rhyolitic to dacitic Horse Canyon Sequence: Grey to green v rock (calcareous lake beds?) with thin ash-fl Rhyodacitic ash-flow tuff: ca. 185 Ma (Rt strongly stretched lithic (pumice) fragments. Andestib'c metavolcanics: Contains tuff bi bedded sedimentary rocks. Massive, fine-gi Rhyolitic ash-flow tuff of Saddlebag Lake Light-grey tuff with conspicuous shiny, broke Flattened lithic (pumice) fragments are comr Conglomerate of Cooney Lake: Lenticulai quartzite, chert, argillite, and rare silicic vole Candelaria Formation: Thinly bedded, qua chert and shale dasts. Locally well-develop Symbols V 7 2 Bedding; dip angle X7 2 Magmatic foliation; dip angle * Magmatic foliation; vertical dip Solid state foliation; dip angle' X Solid state foliation; vertical dip in Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Legend Lithology 0 ) e > © c £ 3 Khd Cathedral Peak granodiorite: 88.1 Ma (Fleck et al., 1996). Medium-grained biotite- hornblende grano diorite with conspicuous sphene. Locally porphyritic withabundant large phenocrysts of potassium feldspar. Halfdome granodiorite: 89 Ma (Kistler and Fleck, 1994) to 91+/-1 Ma (Coleman and Glazner, 1997). Medium- grained biotite- hornblende granodiorite with conspicuous sphene. Locally porphyritic with abundant large phenocrysts of potassium feldspar. . Kuna Crest granodiorite: ca. 92 Ma (Paterson, in progress) to 94 Ma (Coleman and Glazner, 1997). Dark- colored, fine- to medium- grained, biotite- hornblende granodiorite with minor tonalite. Sheeted dike complex: Zone of strong interiayering between TIS-related aplites, pegmatites, granitoids, and host rocks of the Horse Canyon and Koip Sequence. Quartz-feldspar-gneiss: Distinctively brown-wheathering, thin beds of quartz- and feldspar-bearing rock, probably representing thin rhyolitic to dacilic ash flow tuffs. Horse Canyon Sequence: Grey to green wheathering volcanic sandstone and thinly bedded calc-silicate rock (calcareous lake beds?) with thin ash-flow tuff interlayers. Rhyodacitic ash-flow tuff: ca. 185 Ma (Rb-Sr whole rock; Bateman et al., 1983). Welded tuff containing strongly stretched lithic (pumice) fragments. © o c e 3 O ’ © (0 Q . O J d Andestitic metavolcanics: Contains tuff breccias, volcanic conglomerates, lava flows, and minor inter- bedded sedimentary rocks. Massive, fine-grained layers form macro-boudins. Rhyolitic ash-flow tuff of Saddlebag Lake: 222+1- 5 Ma (U-Lb zircon; Schweickert and Lahren, 1987). Light-grey tuff with conspicuous shiny, broken and embayed quartz phenocrysts in a very light matrix. Flattened lithic (pumice) fragments are common. Conglomerate of Cooney Lake: Lenticular, laterally discontinuous conglomerate containing clasts of quartzite, chert, argillite, and rare silicic volcanic and granitic rocks. Candelaria Formation: Thinly bedded, quartzofeldspathic siltstone and fine-grained sandstone containing chert and shale clasts. Locally well-developed Bouma sequences. Symbols y 72 Bedding; dip angle ^ 72 Magmatic foliation; dip angle \ Magmatic foliation; vertical dip A 7 2 Solid state foliation; dip angle X Solid state foliation; vertical dip 2km 0 0 0 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I o u c © 3 E T © C O Q . o 5 £ Rhyodacitic ash-flc strongly stretched lit Andestitic metavoli bedded sedimentary Rhyolitic ash-flow 1 HU Light-grey tuff with o Flattened lithic (purri Conglomerate of C quartzite, chert, argil Candelaria Formati chert and shale das) Symbols V 7 2 Bedding; dip angle x 7 2 Magmatic foliation; dip * Magmatic foliation; vert Solid state foliation; dip X Solid state foliation; vet Magmatic foliations (n=65) View from N border of mapping 119*17'30' Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. K oip Sequence Rhyodacitic ash-flow tuff: ca. 185 Ma (Rb-Sr whole rock; Bateman etal., 1983). Welded tuff containing strongly stretched lithic (pumice) fragments. Andestific metavolcanics: Contains tuff breccias, volcanic conglomerates, lava flows, and minor inter- bedded sedimentary rocks. Massive, fine-grained layers form macro-boudins. _ Rhyolitic ash-flow tuff of Saddlebag Lake: 222+1- 5 Ma (U-Lb zircon; Schweickert and lahren, 1987). Itfiltltl Light-grey tuff with conspicuous shiny, broken and embayed quartz phenocrysts in a very light matrix. Flattened lithic (pumice) fragments are common. Conglomerate of Cooney Lake: Lenticular, laterally discontinuous conglomerate containing clasts of quartzite, chert, argillite, and rare silicic volcanic and granitic rocks. Candelaria Formation: Thinly bedded, quartzofeldspathic siltstone and fine-grained sandstone containing chert and shale dasts. Locally well-developed Bouma sequences. Symbols V 7 2 Bedding; dip angle X7 2 Magmatic foliation; dip angle * Magmatic foliation; vertical dip Solid state foliation; dip angle X Solid state foliation; vertical dip o L 2km -! Magmatic foliations (n=65) °®. Solid state foliations (n=344) Bedding (n=24) i/iew from N border of mapping area (looking S): View from S border of mapping area (looking N): Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

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Asset Metadata

Creator Albertz, Markus (author)

Core Title Contact aureole rheology: New constraints from fieldwork in selected Cordilleran aureoles and from numerical modeling

School Graduate School

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)

Advisor Paterson, Scott (committee chair), Domaradzki, Julian (committee member), Morrison, Jean (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-660047

Unique identifier UC11335752

Identifier 3145153.pdf (filename),usctheses-c16-660047 (legacy record id)

Legacy Identifier 3145153.pdf

Dmrecord 660047

Document Type Dissertation

Rights Albertz, Markus

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