Seismic sequence stratigraphy and structural development of the southern outer portion of the California Continental Borderland

PDF

Download

Open document

Flip pages

Download a page range

Download transcript

Copy asset link

Request this asset

Request accessible transcript

Transcript (if available)

Content SEISMIC SEQUENCE STRATIGRAPHY AND STRUCTURAL DEVELOPMENT OF THE SOUTHERN OUTER PORTION OF THE CALIFORNIA CONTINENTAL BORDERLAND By Calvin Fong Lee 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 (Geological Sciences) May 1992 ' ) Copyright 1992 Calvin Fong Lee UMI Number: DP28602 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. Dissertation Publishing UMI DP28602 Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 UNIVERSITY OF SOUTHERN CALIFORNIA THE GRADUATE SCHOOL UNIVERSITY PARK LOS ANGELES, CALIFORNIA 90007 This dissertation, , written by Calvin Fong Lee under the direction of /z..J..s... Dissertation Committee, and approved by all its members, has been presented to and accepted by The Graduate School, in partial fulfillment of re quirements for the degree of DOCTOR OF PHILOSOPHY Dean of Graduate Studies D ate LI..®.!..!?.?.?. DISSERTATION COMMITTEE P h . D . G e. 1 9 2. L_ 4 7 7 - 3,4 \»\ o eJceT" 3 ACKNOWLEDGMENT I would like to thank the many people and organizations who provided guidance, information, and/or j encouragement during this research: Dr. Thomas L. Henyey proposed the project and advised me during the research, writing, and defense phases of the research? Drs. David A. Okaya and Vincent W. Lee advised on the theories and mathematics of data processing? and Drs. Donn S. Gorsline, Alfred G. Fischer, and Steve P. Lund provided ! help on sedimentological and stratigraphic processes ! within the study area. Additional thanks are extended to Dr. Lund for providing many hours of his time discussing the various theories of borderland formation (i.e., the j question of allochthoniety). ! A major portion of my research benefited from the I help provided by two persons: Dr. Jack G. Vedder identified and provided access to the late 197Os-vintage U.S. Geological Survey seismic reflection profiles used in this dissertation and offered invaluable advice on my interpretation of borderland lithologies and structures? and Dr. Mark R. Legg provided many hours of useful discussions on borderland formation. I gratefully acknowledge their assistance. Thanks are also extended to the following members of the U.S. Geological Survey: Drs. Robert G. Bohannan and Steve Etrim, for inviting me along on the 1990 California Continental Borderland seismic survey and allowing access to the latest U.S. Geological Survey multifold data for use in my dissertation? Messrs. Dennis M. Mann and Jon Childs for advising, providing access to, and assisting in processing the multifold seismic reflection data from two U.S. Geological Survey cruises in 1978 and 1979; and i ] Capt. Alan McChenaghan and the crew of the R/V Samuel P. i Lee, who made the 1990 borderland cruise an enjoyable experience. Included in my acknowledgment to the members of the U.S. Geological Survey is a special note of thanks j to Mr. Eric L. Geist, who processed the 1990 seismic reflection data set. Detailed stratigraphic interpretation would not be possible without the help provided by Messrs. Harold E. Syms, Don Krotzer, Bob MacDonald, and other members of the Minerals Management Service: They provided information on non-proprietary well-log data and advised on industrial sources of additional seismic reflection records within my study area. iii My dissertation was considerably improved by the helpful proofreading and advice on thesis format provided by Messrs. Kevin Mayeda and Stuart Koyanagi, and the drafting of illustrations by Ms. Janet Dodds. A major part of campus life at USC involves maneuvering through the morass of regulations and 'red tapes' within the academic community. This endeavor would be difficult without the assistance of the office staff. They include: Mr. John McRaney, Ms. Rene Kirby, i ; Ms. Virgina Kelley, Ms. Desser Moton, Ms. Cindy Waite, Ms. Sue Turnbow, and Ms. Denise Steiner. My tenure at USC would not be complete without the friendship and help provided by many of my fellow graduate students. In addition to Kevin Mayeda, his wife Grace, and Stuart Koyanagi, there are: Eric Bender, Sandy Stacy, Joe Jackson, Reese Barrick, Peter Bentham, Michelle Robertson, Valerie Ferrazzini, and Geoff Saldivar. Over a 20-year academic and professional career, I have had the pleasure of working on a variety of oceanographic, geologic, and/or geophysical projects, such as an underwater survey for the U.S. Navy, minerals explorations in California and Alaska, and geohazards surveys in California and the Gulf of Mexico; along the way, I had the chance to work with the noted scientist ! and pioneer in marine geology, the late Dr. Francis P. Shepard. The person who made all of this possible by providing my initial training and introducing me to many of the projects mentioned above was Dr. Peter J. Fischer of California State University (Northridge); he instilled in me the joys of oceanography and marine geophysics, and his encouragement and faith in my abilities inspired me to continue my studies towards this doctorate. j In concluding, I thank my wife Janet, for her patience, love and understanding during my tenure at USC, and for assisting me in putting together the final copy of my dissertation? my mom Wing Fong for providing i | encouragement; and Dr. Thomas P. Treichler of Treichler Corporation for generously providing office space and equipment for my study needs whenever I am in the Bay I < Area. Partial funding for this research came from NSF grant no. 6665. i CONTENTS ACKNOWLEDGMENT.........................................ii LIST OF FIGURES...................................... viii LIST OF TABLES ................................................................................................................................................X V LIST OF PLATES....................................... xvi ABSTRACT............................................xviii Chapter pacre I. INTRODUCTION......................................1 Location......................................1 Previous studies............................. 5 Purpose of study............................ 12 Dissertation format......................... 13 II. PACIFIC-NORTH AMERICAN-PLATE INTERACTION .........15 Convergent margin: Late Jurassic to Late Paleogene . ..................... 16 Transform margin: Neogene...................19 Transpressional strike-slip: Quaternary . . .23 III. LITHOLOGIC DESCRIPTIONS........................... 25 General......................................28 Patton Terrane.................... 30 Nicolas Terrane............................. 37 Basement............................... 37 Late Cretaceous and Paleogene.........41 Neogene and Quaternary.................47 Volcanic Rocks.......... 53 IV. DATA ACQUISITION AND PROCESSING...................54 Navigation................................. 58 Acquisition................................. 58 Processing parameters....................... 59 Trace editing............................... 67 Velocity analysis........................... 72 Multiple suppression....................... 89 Migration................................... 95 V. STRATIGRAPHY OF THE SOUTHERN OUTER BORDERLAND. . 101 Concepts of seismic sequence analysis . . . 102 Seismic stratigraphy: southern outer borderland........................ 110 vi Patton Terrane ....................... Ill Western z o n e .................... Ill Central z o n e .................... 114 Eastern zone ..................119 Nicolas Terrane ..................... 120 Western z o n e .................... 129 Northern province ........ 129 Southern province ........ 135 Central z o n e .................... 147 Eastern z o n e .................... 155 VI. STRUCTURAL FRAMEWORK OF THE SOUTHERN OUTER BORDERLAND...................... 167 Methods.....................................169 Pre-Neogene structures .................. 171 Neogene and Quaternary structures ....... 199 Patton-West Nicolas fault trend. . . . 200 East Nicolas fault trend ............ 206 Santa Cruz-San Clemente fault trend. . 211 Nicolas-Catalina Extensional fault trend. .................... 215 VII. DISCUSSION........................................221 Convergent margin ......................... 222 Accretionary wedge ................... 227 Forearc basin.......................... 232 Patton-Nicolas Terrane boundary. . . . 232 Model of convergent margin formation..................235 Transform margin ......................... 241 Remnant slab problem..............242 Subducted ridge controversy . . . 246 Collision-related uplift...........248 Development of Neogene fault trends..................257 Development of modern borderland basins . . .262 Transpressive margin ..................... 265 VIII. SUMMARY.......................................... 267 REFERENCES...........................................275 APPENDIX............. ..............................290 Depth-time conversion of sonic logs to correlate seismic data to borehole descriptions ......................... 290 vii LIST OF FIGURES Ficrure Title Page 1.1 Map of the California Continental Borderland, including location of study area...........................................4 1.2 Division of the southern half of the California Continental Borderland into four segments defined on the basis of basement and early Tertiary lithologies . . .10 2.1 Prior to 30 Ma, translation of allochthonous terranes was a consequence of oblique subduction....................... 18 2.2 History of plate interaction after 30 Ma. . .21 3.1 Location map showing exploratory wells. . . .27 3.2 Distribution of seafloor lithologies within the borderland....................... 33 3.3 Lithologic description of DSDP composite well sections . ........................... 36 3.4 Stratigraphic columns from 10 exploratory wells along the Santa Rosa-Cortes Ridge and offshore Santa Barbara Island...........40 3.5 Thickness of Eocene strata along the Santa Rosa-Cortes Ridge and offshore Santa Barbara Island......................... 45 3.6 Oligocene depositional environments determined from exploratory wells...........49 4.1 Trackline map of U.S. Geological Survey multifold seismic reflection data...........57 4.2 More than half of the 1979 data set contains zones where noise dominate the record........................................61 viii 4.3 A major problem with the 1978 and 1979 data sets was strong water-bottom multiples ................................. 4.4 Processing flow charts; A) CALCRUST, B) U.S. Geological Survey ................ 4.5 An example of an irregular time shift in the shot gathers which must be time- corrected ................................. 4.6 Two examples of noise in shot gathers; A) contains usuable traces, B) contains unusable traces which must be edited out. . .71 4.7 By viewing common-offset gathers, one can visually determine whether any information can be gained from a low signal-to-noise trace ..................... 4.8 Two methods of editing out low signal- to-noise traces; zero out the whole trace, or zero out just the noisy portion........ 4.9 A common midpoint gather after velocity correction for offset distance............ .79 4.10 Horizontal root mean square stacking velocity profile derived from velocity analysis................................... 4.11 Constant velocity panels at every 100 m/sec, beginning at 1950 m/sec........ 4.12 Velocity semblance plots are graphs of coherent energy versus velocities for specific traveltimes....................... 4.13 Partial normal moveout and frequency- domain dip-filtering to attenuate seafloor multiples......................... 4.14 Comparison of a stacked and a multiple- attenuated stack along a portion of line 21 ................................... > 4.15 Smoothed interval-velocity profile used for migration ............................. 5.1 Characteristic external geometries of seismic facies units....................... 106 ix 5.2 Characteristic internal seismic reflection configurations of seismic facies units............................. 5.3 Geomorphological subdivisions of the Patton Terrane........................... . 113 5.4A Flat-lying basin fill of the upper sequence in the central zone, line 903. . . 117 5.4B Mound-shaped fill of the upper sequence, characteristic of turbidite fans, line 110................................. 5.5A Change in reflectivity across the Ferrelo fault zone suggest juxtaposition of differing rock types, line 123 . . . . . 122 5.5B Forearc strata appear to continue over into the Patton Terrane in the southern end of the borderland, line 908 ........ . 123 5.6 Uplift and erosion of the southern end of the borderland resulted in toplap truncation of strata and formation of flat-topped banks ....................... 5.7 The Nicolas Terrane can be subdivided into three zones; western, central, and eastern ................................. . 128 5.8 Change in internal seismic character between reflective UKP strata of the southern, western Nicolas Terrane from chaotic reflections of the northern portion of the terrane................... 5.9 At least three sequences can be identified within the northern West Nicolas Terrane ......................... 5.10 Three seismic sequences can be identified within the UKP megasequence in the southern West Nicolas Terrane. . . . 137 5.11 Uplift and truncation of the UKP megasequence along the southeast edge of the western part of the Nicolas Terrane . . 142 X 5.12A Apparently northwest-prograding reflections overlying an unconformity at the top of the Paleogene, line 904 ....................................... 145 5.12B Apparently northeast-prograding Neogene reflections, line 21.............. 146 5.13 Seismic sequences are correlated to subsurface geology by means of sonic well logs along the Santa Rosa-Cortes Ridge ..................................... 150 5.14 Correlation of sonic velocities with age of strata along the Santa Rosa- Cortes Ridge ............................. 5.15 In the eastern portion of the Nicolas Terrane, the UKP megasequence thins to the north and east up to a major fault at the northern edge of the San Nicolas Ridge, then thickens again on the north side of the fault ......................... 157 5.16 Although the UKP megasequence appears to lap out towards the eastern edge of the Nicolas Terrane, appearance of toplap truncation indicates that there has been uplift and erosion along the Santa Cruz- San Clemente Ridge......................... 159 5.17A Truncation of UKP megasequence at Blake Knoll ............................... 161 5.17B Truncation of UKP megasequence at Shepard Knoll ............................. 162 5.18 Changeover from progradational to flat- lying, onlapping reflections may indicate timing of local basin formation in East Cortes Basin....................... 165 6.1A Line 904 showing remnant subducted slab beneath the Patton accretionary prism . . . 174 6. IB Line 110 showing remnant subducted slab beneath the Patton accretionary prism . . . 175 6.2 Line 902 showing remnant subducted slab beneath the Patton accretionary prism . . . 177 xi 6.3 Remnant subducted slab can be followed eastwards on line 124 up to a zone of diffractions which may be related to volcanic rocks dredged from the overlying seafloor......................... 179 6.4 Upward arch in the downgoing slap at the toe of the accretionary wedge on seismic reflection data is an artifact due to the data being recorded in time .......... 183 6.5A Conversion of seismic sections from time to depth section substantially flattens the dip of the remnant downgoing slab. Line 110 is an example of a depth- con verted seismic section ................ 185 6.5B Conversion of selected seismic sections 124, 904, and 110 to depth................ 186 6.6A-B Forearc strata appear to map well west of the Ferrelo fault zone on line 110........ 190 6.6C-D Forearc strata appear to map well west of the Ferrelo fault zone on line 908........ 192 6.7 Erosion surface at the base of Neogene (?), and toplap truncation of the UKP megasequence, indicate rapid uplift and erosion of the outer borderland at the end of Paleogene or beginning of Neogene time ................ 195 6.8 Suggested thinning of forearc strata against the Patton Terrane................ 198 6.9 Histograms of fault orientation within specific regions of the outer borderland. . 202 6.10 Apparent offset of faults within the 'Patton Ridge fault zone' generally appears to show normal displacement. However, structures similar to the flower structures of Harding and others (1985) suggest that the Patton Ridge fault zone is dominantly strike-slip. . . . 205 6.11 Many faults associated with the Ferrelo fault zone show apparent reverse dip- slip....................................... 208 xii 6.12 Faults classified within the Santa Rosa- Cortes fault zone show apparent vertical or normal throw............................ 210 6.13A Reverse faults and folds associated with east-west-trending structures of the eastern Nicolas Terrane; line 109 is a dip line.....................................213 6.13B Reverse faults and folds associated with east-west-trending structures of the eastern Nicolas Terrane; line 116 is a strike line................................ 214 6.14 Apparent extensional faulting associated with the Nicolas-Catalina terrane boundary.....................................218 6.15 Low-dipping reflections beneath UKP megasequence (?) are shallow-dipping extensional faults.......................... 220 7.1 Diagram of the California Continental Borderland during the period of active subduct ion...................................224 7.2 A composite interpretation of lines 904, 110, and 908 showing possible accretionary prism-forearc relationship preserved within the southern outer borderland...................................226 7.3 High-pressure metamorphic rocks can be exhumed within an accretionary wedge by underplating subducted sediments............231 7.4 Schematic model of modern subduction complexes, showing the similarities in accretionary prism-forearc relationships to that of the southern outer borderland. . 237 7.5 Schematic diagrams of the development of the accretionary wedge and forearc in the outer borderland. .................... 239 7.6 Piecemeal collision of ridge segments, rather than a single collision event, may explain the preservation of the remnant subducted slab beneath the accretionary wedge in the southern outer borderland...................................245 xiii 7.7 Regional uplift at 20 to 24 Ma as a result of ridge-trench collision............250 7.8 7.9 7.10 7.11 Cross-sections AA', BB', and CC' across the extinct Guadalupe spreading center. . Ridge-trench collision and subsequent ridge subduction at 18 Ma may be responsible for uplift and truncation of the Upper Cretaceous and Paleogene section in the southern end of the borderland............................... By 16 Ma, volcanic activity has proceeded along the Santa Cruz-San Clemente Ridge, and rifting has opened up the southern end of the borderland . . Formation of the modern borderland basins appears to be young, possibly late middle Miocene or younger ........ . 253 255 . 260 264 xiv LIST OF TABLES Table Title Page 4.1 Comparison of interval velocities from seismic reflection records to interval velocities in exploratory wells.............. 88 4.2 Comparison of the strengths and weaknesses of the three migration methods utilized in industry today .................100 xv LIST OF PLATES Plate Title Location 1 Location and seafloor isochron map, California.Continental Borderland....................... In pocket 2 Isopach map, Upper Cretaceous through Paleogene .............. In pocket 3 Facies map, Lower Neogene . . In pocket 4 Structure map . . . . . In pocket 5A Line drawing of seismic line 116. In pocket 5B Seismic line 116. . . . In pocket 6A Line drawing of seismic line 124 . In pocket 6B Seismic line 124. . . . In pocket 7A Line drawing of seismic line 123. In pocket 7B Seismic line 123. . . . In pocket 8A Line drawing of seismic line 120. In pocket 8B Seismic line 120. . . . In pocket 9A Line drawing of seismic line 114. In pocket 9B Seismic line 114. . . . In pocket 10A Line drawing of seismic line 122. In pocket 10B Seismic line 122. . . . In pocket 11A Line drawing of seismic line 911. • In pocket 11B Seismic line 911. . . . In pocket 12A Line drawing of seismic line 112 . • In pocket 12B Seismic line 112. . . . In pocket 13A Line drawing of seismic line 904. • In pocket 13B Seismic line 904. . . . In pocket 14A Line drawing of seismic line 110. • In pocket 14B Seismic line 110. ... In pocket 15A Line drawing of seismic line 908. • In pocket 15B Seismic line 908. . . . In pocket xvi 16A Line drawing of seismic line 109. . In pocket 16B Seismic line 109. . . . . In pocket 17A Line drawing of seismic line 115. . In pocket 17B Seismic line 115. . . . . In pocket 18A Line drawing of seismic line 16 . . In pocket 18B Seismic line 16 . . . . . In pocket 19A Line drawing of seismic line 21 . . In pocket 19B Seismic line 21 . . . . . In pocket I ABSTRACT ! During the last several decades, the plate tectonics history of western North America has been defined based : on magnetic anomaly patterns, onland mapping, and limited i I seismic reflection data. However, the knowledge of i : structural and stratigraphic relationships for a large j j portion of offshore California was still limited. Within the southern borderland, pre-1990 models of borderland i formation ranged from allochthonous juxtaposition of four 1 i or more separate terranes to pairing of two accretionary I l | wedge-forearc complexes along a major transform fault; ; late-1980s models invoked some form of detachment I | faulting in association with transform motion. By integrating CALCRUST-reprocessed and newly- | 1 I acquired U.S. Geological Survey multichannel seismic reflection data with surface and subsurface lithologic , information, a new geologic model is proposed for the j southern, outer borderland which refines accepted ! 1 j I theories of borderland formation. This model suggests that prior to 3 0 Ma, the southern outer borderland was undergoing active subduction. The area is composed of an ! outer accretionary wedge complex floored by a remnant i I j xviii -------------------- subducted slab and consisting of greywacke, greenschist, I . and blueschist, and an inner forearc zone of basin-fill i ' i I ; sedimentary rocks. These zones do not appear to be separated by a sharp tectonostratigraphic boundary: I 1) Subsurface well data suggest that rocks of the accretionary complex may extend beneath the forearc zone; : 2) seismic reflection data suggest that the forearc zone i i can be mapped well into the accretionary-wedge complex; j I and 3) a deformation zone similar in character to those i , of active subduction margins was identified immediately j i i < | east of the Patton Terrane. Structural patterns also i ; suggest that northwest-trending fault patterns of the i ! accretionary wedge extends halfway into the forearc zone, j j whereas the other half of the forearc is dominated by | | west-northwest-trending faults, perhaps of the ophiolitic basement. From the available data, the interpreted geologic model would be similar to recent models proposed for the Coast Range-Great Valley area of central 1 I ■ California and the Barbados Islands, in which the j i i accretionary prism is tectonically wedged beneath the j forearc. j Beginning about late Oligocene-early Miocene time, a j : I major period of uplift heralded the impingement of the I I Pacific-Farallon mid-oceanic ridge on the North American j i trench. Major uplift and erosion occurred within the ■ accretionary wedge as well as within the southeastern edge of the outer borderland; the Upper Cretaceous through Paleogene section was uplifted and truncated off ! I i j along a zone extending from Blake to Shepard Knolls, and sedimentation occurred in the form of northeast- ■ i prograding facies within the lower sequence of the I < l j # i Neogene and Quaternary megasequence. Evidence of major i i ! ! strike-slip along the west-northwest-trending faults within the eastern zone of the Nicolas Terrane suggests 1 i | that initial translation of segments of the forearc zone ! of the outer borderland with respect to the accretionary- I wedge zone of the inner borderland may have been along ! these west-northwest-trending structures, j The change from subduction to transform boundary ! occurred in the early or middle Miocene; however, the i | formation of the modern borderland basins probably occurred in late middle Miocene time, with the first ( i evidence of localized prograding and basin-fill seismic ! facies within the upper sequence of the NQ megasequence. I The prograding facies appear to be derived from areas of present-day highs, and prograde towards basinal areas. The discovery of approximately north-south-trending i j extensional faults within the southeastern end of the i 1 | outer borderland suggests that the transform faulting was ; xx 1 accommodated by rifting, perhaps related to the final I phases of ridge-trench interaction. ! i t i i i xxi Chapter I INTRODUCTION The southern California Continental Borderland represents the offshore region of southern and Baja California, extending from the coastline seaward to the base of the Patton Escarpment (250 km at its widest part) and from Point Conception at the northern end southward to the Vizcaino Peninsula (about 700 km; Plate 1). Detailed geologic studies are very important here, because the multitectonic history of this region created geologic complexities which are still not resolved, in terms of geologic relationships across terrane boundaries, earthquake seismological studies, and minerals exploitation. This study will provide new information on the nature and timing of structural and stratigraphic features within the southern outer borderland, and integrate this information into a revised geologic history of the borderland. 1.1 Location The study area lies within the southern, outer half of the borderland, from about latitude 32°30/N southwards 1 to about latitude 31°N, and from the San Clemente fault trend seaward to the Patton Escarpment (Fig. 1.1). Topographically the borderland consists of a series of ridges and intervening basins, the latter filled dominantly with sediments from the adjoining highlands (Gorsline and Emery, 1959; Gorsline, 1981; Teng and Gorsline, 1985). Seafloor relief is quite variable, ranging from above sea-level on the islands within the borderland, to greater than 4 km subsea at the base of the Patton Escarpment. According to Emery (1954) and Doyle and Gorsline (1977), the depth of seafloor also decreases in a southward direction (from 34°N to 30°N), towards a synclinorium in the vicinity of offshore northern Baja. From there, the seafloor rises towards the Vizcaino Peninsula. Although its physiography led early investigators to suggest that the borderland was related to the Basin and Range Province (Emery, 1954; Shepard and Emery, 1941; Yeats, 1976), Jahns (1954) included it as part of the Peninsular Ranges province on the basis of lithology. However, its distinct structural signature led Moore (1969) to classify the borderland as a distinct province different from the other provinces of California. Fig. 1.1. Map of the southern California Continental Borderland, including location of study area. 3 Santa Rosa Is .'^ ^ D Los Angeles Santa Cruz Is. SOUTHERN CALIFORNIA CONTINENTAL BORDERLAND o o Santas. ^ _ Barbara Is. £ Qb Santa Catalina Is 50 100km i _____I San f \ Clemente Is. ‘ -fj °o contours in meters Sari Diego .U.S.A- Mexico i Knolls § Shepard 8 v.Ensenada ft STUDY AREA Location 1.2 Previous Studies Prior to the 1970s, published information about the offshore geology of California was limited to the shallow subsurface via dredge and dart-core samples and high-resolution echograms (i.e., Moore, 1969; Shepard and Emery, 1941); thus knowledge of deep structural and stratigraphic relationships were limited. Geologic and tectonic models were often derived by extrapolating onshore geologic data offshore (i.e. Emery, 1954). By the late 1970s, renewed interest in refining the geology of offshore California was fueled partly by the petroleum industry, which saw the offshore as a potential location for hydrocarbon exploration, and the nuclear-power community, which was worried about active offshore faults which might adversely affect onshore nuclear power plant sites. Geologic and geophysical information from the siting of the Diablo Canyon nuclear power plant were available to the academic community which were used to constrain the tectonics of central California (i.e. Crouch and others, 1984; McIntosh and others, 1991; Sedlock and Hamilton, 1991). Knowledge about the geology of the California Continental Borderland increased enormously during this time, due mainly to the interest of the minerals exploration industry. For the most part, the geologic 5 information which was publicly available was limited to shallow-penetration, high-resolution seismic reflection data in which subsurface events were tied to surface dredge or dart-core information (i.e., Doyle and Gorsline, 1977; Lee, 1977; Nardin and others, 1979); the deep-penetration seismic reflection data needed for resolving detailed stratigraphic and structural relationships were not publicly available until the U.S. Geological Survey began to collect and process their own 24-channel, multifold seismic data in the late 1970s (i.e., Mann and others, 1979). Although numerous studies have been published on various aspects of borderland geology, the following studies are identified as milestones leading to the current understanding of borderland evolution. The earliest knowledge of the geology of the borderland dates back to the work of Shepard and Emery (1941) and Emery and Shepard (1945), who produced preliminary geologic maps showing structures and distribution of rock types throughout a major portion of the borderland. Their studies were followed by a 20 year hiatus when very few research projects were conducted; those that were made were primarily inner borderland surface-sediment studies (i.e. Gorsline and Emery, 1959; Haner, 1969), as well as several geophysical 6 investigations, including seismic refraction (Shor and Raitt, 1956), and gravity and magnetics (Harrison and others, 1966). The next major study was Moore's (1969) high-resolution seismic reflection interpretation of the borderland, which provided the first detailed description of structural deformation styles and correlated shallow subsurface events to surface geology based on dredge or dart-core information. His study also identified turbidity currents as the major depositional mechanism within the borderland. By the 1970s, heightened interest in the borderland as a potential source of petroleum prompted the U.S. Geological Survey to begin a series of fact-finding studies. A meeting was held on Santa Cruz Island in 1975, and attended by personnel from the U.S. Geological Survey, several universities, and the petroleum industry. This meeting was the basis for the publication of American Association of Petroleum Geologists' Miscellaneous Publication 24, which reviewed the current knowledge of the borderland at that time. The U.S. Geological Survey also conducted a series of cruises to collect surface and subsurface information using dart-cores and high-resolution seismic reflection data respectively (Vedder and others, 1974, 1977, 1979, 1981). 7 A deep-stratigraphic-test well drilled to a depth of 2430 m provided information about the stratigraphy of the outer borderland (Paul and others, 1976). By the late 1970s, several facts had become clear about the the borderland: 1) The southern half of the borderland can be divided into four zones, an outer, seaward zone with rocks typical of a subduction complex, followed landward in succeeding order by a forearc zone, a subduction complex, and another forearc zone (Fig. 1.2); 2) the accretionary wedge-forearc pair of the inner borderland is repeated in the outer borderland, except the difference appears to be a higher-grade metamorphism of the accretionary wedge in the inner borderland (Crouch, 1979; Howell and Vedder, 1981; Teng, 1985; Vedder, 1987); and 3) the relationship between the subduction complex and the forearc was unknown, since paleomagnetic data suggest that portions of the borderland may be allochthonous (Gibson and others, 1985; Howell and Vedder, 1981) The intense investigative efforts of the 1970s continued into the mid-1980s, culminating in studies on Transverse-Range rotations (i.e. Crouch, 1979; Hornafius and others, 1986; Luyendyk and Hornafius, 1987) as well as detailed stratigraphic and structural studies of specific segments of the borderland. Crouch, (1981) 8 Fig. 1.2. Division of the southern half of the California Continental Borderland into four segments defined on the basis of basement and early Tertiary lithologies? the segments may be allochthonous (modified from Howell and Vedder, 1981). Los Angeles Santa Rosa Is. SOUTHERN CALIFORNIA CONTINENTAL BORDERLAND | \ Qs ^ San Nicolas Is.! 50 100km contours in meters -O Ensenada STUDY AREA utilized seafloor rock samples collected by the U.S. Geological Survey, complemented with high-resolution seismic reflection data, within the northern Patton Ridge area to determine the timing of basin formation and demonstrate the similarity in the style of tectonic deformation to that along the San Andreas fault. Within the inner borderland, Legg (1985) and Legg and others (1989) used high-resolution seismic reflection and Sea Beam data to determine depositional history and define extensional versus convergent segments along the San Clemente fault system. Finally, numerous studies of surficial stratigraphy and structures have been done by the Geosciences Department at the University of Southern California (i.e., Nardin, 1981; Nardin and others, 1979; Reynolds and Gorsline, 1987; Schwalbach, 1982); however, in a major study of the central borderland, Teng (1985) utilized deep-penetrating multichannel seismic reflection data, supplemented by surficial sediment cores and high-resolution seismic reflection data, to show that subsurface depositional patterns (i.e., Pliocene age) may differ from those of the surface and near-surface (i.e., Plio-Pleistocene age). Furthermore, Teng (1985) was able to: 1) identify deeper seismic reflection intervals of Late Cretaceous through Recent age and determine that the modern borderland basins did not form until possibly 11 early Pliocene time? and 2) map structural patterns, from which a classification of primary and associated secondary trends were recognized. The present study will complement the investigations of Crouch (1981) and Teng (1985) in the borderland north of 32°N, and Legg (1985) in the inner borderland east of the San Clemente fault system. 1.3 Purpose of Study The tectonic history of offshore California is complex, which has led to much controversy. A large part of the controversy can be attributed to tectonic models developed to explain geologic phenomena observed within specific segments or tectonic blocks of California which are then extrapolated to include other segments or blocks outside specific study areas; examples would include arguments over the amount of translation of the numerous terranes making up California (i.e. Howell and Vedder, 1981; Lund and Bottjer, 1991? Sedlock and Hamilton, 1991? Yeats, 1976) and the nature of major block rotations (i.e. Crouch, 1979? Hornafius and others, 1986? Sedlock and Hamilton, 1991). Since many of these phenomena extended offshore, the tectonics of the offshore regions were interpreted in terms of onland observations. 12 The purpose of this study is to use multifold seismic reflection data which have been tied to publicly-released, exploratory-well data as well as published lithologic information to: 1) Image and study subbasement reflections such as shallow-dipping fault planes, sill bodies, and other types of anomalous reflections; 2) study the character of and map the seismic sequences in the sedimentary basins to determine source areas, the nature of depositional environments, and discontinuities as well as disturbances within the sedimentary record? and 3) synthesize the subbasement and basinal reflection data to provide new information concerning the nature and timing of tectonic and sedimentary events within the borderland, as well as provide insight into some of the controversial issues of west-coast tectonics, such as the problems associated with the concept of far-traveled terranes. 1.4 Dissertation Format This dissertation is subdivided into seven chapters. Chapters 1 and 2 will introduce the study area and provide a brief overview of the tectonic history of western North America over the last 60 Ma. Chapter 3 reviews the known geology of the study area from surface and subsurface lithologic sampling 13 programs. The emphasis will be on proprietary well information that has recently been made public. Chapter 4 presents the seismic reflection data and their acquisition and preparation, which will be used in the subsequent interpretation and conclusions. Chapter 4 also describes the seismic data processing techniques used to remove noise and multiples, improve resolution and enhance deep reflections within the study area. Chapters 5 and 6 examine the stratigraphy and structures within the study area as interpreted from seismic reflection records. The interpreted changes in structural and stratigraphic development over time within the southern outer borderland will be discussed in chapter 7 in terms of current theories of borderland tectonics. Chapter 8 will conclude with a brief summary of the dissertation, emphasizing the importance of key points from each chapter to the final interpretation. Chapter II PACIFIC-NORTH AMERICA PLATE INTERACTIONS A discussion of the geology of the California Continental Borderland would be meaningless unless placed in the context of the overall plate-interaction history of the Pacific-North America boundary. At least three different tectonic episodes, which are reflected in the lithology, stratigraphy, and structures within the borderland, have been superimposed onto this plate boundary; the major events include subduction from Late Jurassic through the Paleogene, translation through most of the Neogene, and transpression since 3 to 5 Ma ago. Although the major tectonic events are known, questions remained and debates continued as to the timing of borderland formation and the number and amount of terrane translations since Late Cretaceous time. This chapter will discuss these problems. Present knowledge of the tectonic history is based largely from onland field mapping (i.e., Bottjer and Link, 1984; Carey and Colburn, 1978; Colburn, 1973), paleomagnetic reconstructions (i.e., Hornafius and others, 1986; Lund and Bottjer, 1991; Lund and others, 1991; Luyendyk and others, 1987), seafloor magnetic anomaly interpretations (i.e., Atwater, 1970; Atwater and Molnar, 1973), and seismic reflection records, the latter supplemented with some publicly-available borehole information (i.e. Crouch, 1981; Legg, 1985; Teng, 1985). 2.1 Convergent Margin: Late Jurassic to Late Paleogene Stratigraphic and structural analyses of rock outcrops identified along the coastal margin of California and Baja California, as far inland as the Sierra Nevada, indicate that a convergent has margin existed along the Pacific-North American boundary since the Late Jurassic Period (Bottjer and Link, 1984). In northern Baja California, older arc- and ophiolitic-type rocks identified within the Vizcaino Peninsula and Cedros Island suggest that a convergent margin may date back as far as the Late Triassic Period (Rangin and others, 1983; Minch and others, 1976). Concomitant with subduction was translation of numerous allochthonous terranes as a consequence of oblique convergence; oblique convergence was thought to be particularly pronounced during the time the Kula Plate was still present, which was about 85 to 44 Ma ago (Fig. 2.1; Atwater, 1989; Carlson, 1982). Far-traveled terranes, such as the Peninsular and Sur-Obispo Terranes, 16 Fig. 2.1. Prior to 30 Ma, translation of allochthonous terranes was a consequence of oblique subduction. Terrane motion was believed to be particularly pronounced during the time the Kula plate was present (from Engebretson and others, 1976). Symbols: NA, North American Plate; FA, Farallon Plate? PA, Pacific Plate? HS, hot spot tracks. KU (206) FA(141) \ \ \ PA (64) \ x 03 whose paleomagnetic signatures indicate an origin as far as 2300 km to the south, were believed to have been amalgamated to the California continental margin as recently as Miocene time (Lund and Bottjer, 1991; Lund and others, 1991), although plate-motion reconstructions would indicate that the period from about 42 to 30 Ma was a period in which there was nearly normal subduction of the Farallon plate beneath North America (Carlson, 1982; Engebretson and others, 1985). 2.2 Transform Margins Neoqene By 30 Ma, as the Pacific-Farallon ridge approached North America, the Farallon Plate began to break up (Atwater, 1989). Magmatic activity, which had moved eastward during the Laramide Orogeny, began to slowly drift west again, perhaps due to slowing of subduction and the increasingly younger age of the plate being subducted (Atwater, 1989; Severinghaus and Atwater, 1989) . The initial collision of the Pacific-Farallon ridge with the trench occurred around southern California or northern Baja California (Fig. 2.2A), between chrons 7 and 9 (26 to 28 Ma). At the point of impingement, the continental margin changed from a convergent to a 19 Fig. 2.2. History of plate interaction after 30 Ma (from Engebretson and others, 1976): A) Initial ridge-trench collision was thought to have occurred between 28 to 26 Ma; B) Over the last 28 Ma, dextral strike-slip translated the Pacific plate about 1000 km with respect to the North American plate; C) Relative change in plate motion between 5 to 3 Ma resulted in transpressive versus pure strike-slip along the San Andreas fault system. 20 oo o » 5 * o CD 21 transform margin; the length of the new transform margin increased with time as two triple-junctions, formed at the point of collision, moved west and southeast respectively along the transform margin (Atwater, 1970). From sea-floor magnetic anomaly patterns, the northern junction (called the Mendocino), a trench- transform-transform boundary, has translated the Pacific plate by as much as 1000 km northwest with respect to the North American plate during the past 29 Ma (Fig. 2.2B). Since total strike-slip along the San Andreas Fault proper is only about 300 km, the remaining movement is believed to have occurred along offshore faults (Atwater, 1970; Howell, 1976). Offshore transform motion along a series of faults slivered the continental margin into a series of blocks, or terranes, whose relationship to adjacent blocks may be uncertain depending on the degree of allochthoniety, or strike-slip transport, of a terrane in relationship to adjoining terranes. This relationship may be resolved by paleomagnetic analysis (Coney and others, 1980; Lund and Bottjer, 1991; Lund and others, 1991). Gibson and others (1985) describe coastal southern California as a composite of 13 allochthonous terranes, outboard of autochthonous and parautochthonous rocks of the North American craton. Howell and others (1987) describe the Patton and Catalina Terranes in terms of 22 post-Eocene emplacement at their present location, the Patton along strike-slip faults, the Catalina by uplift below a tensional detachment surface. According to Sedlock and Hamilton (1991), there could be up to about 700 km of strike-slip on the Ferrelo fault system. The suggestion by these and other authors (i.e. Atwater, 1970; Gibson and others, 1985; Howell and others, 1987) is that although the Patton Terrane is an accretionary wedge complex and the Nicolas Terrane is a forearc zone, the two terranes were formed apart from each other, and subsequently have been juxtaposed along a major transform boundary. 2.3 Transpressional Strike-Slip: Quaternary A relative change in motion between the Pacific and North American plates is believed to have occurred about 5 to 3 Ma ago, resulting in transpressive rather than pure strike-slip motion along the San Andreas fault system (Fig. 2.2C; Engebretson and others, 1985; Harbert, 1991; Harbert and Cox, 1987). Investigation of faults adjoining the San Andreas fault suggests that portions of the Mojave block may be undergoing compression (Henyey; pers. commun., 1989; Henyey and others, 1989; Li and Henyey, 1989), and Zoback and others (1987) demonstrated that the orientation of compressive structures along the 23 San Andreas does not agree with that predicted from pure strike-slip tectonics. Their conclusions would agree with other recent investigations which suggest that compression in the form of thrust faulting is the major deformational style in southern California. Interpretation of earthquake focal mechanisms for the Los Angeles Basin area by Hauksson (1989), Hauksson and Jones (1987), and Hauksson and Saldivar (1989) indicates that a major source of large earthquakes may be the buried, east-west-trending thrust faults (blind thrusts) proposed by Davis and others (1989). Recent studies in the Transverse Ranges by Humphreys and others (1984) and Namson and Davis (1988) indicate that young (2 to 3 Ma) compressional features may be due to the presence of a subduction zone beneath the ranges, and Zoback and others (1987) and Mount and Suppe (1987) showed that the direction of maximum horizontal compression in California is almost perpendicular to the San Andreas fault system. 24 Chapter III LITHOLOGIC DESCRIPTIONS Until recently, two methods have been used to determine the subsurface geology within the California Continental Borderland: Extrapolation of lithologies exposed onland and on the islands to the offshore; and lithologic information derived from dredge hauls, mainly off the Patton Escarpment (Emery, 1954; Emery and Shepard, 1946). More data on seafloor lithology became available during the 1970s, when the U.S. Geological Survey collected dart-core samples of the seafloor on several borderland cruises (Vedder and others, 1974, 1977, 1979, 1981) and participated in a COST (Consortium for Offshore Stratigraphic Test) well (Paul and others, 1976). Unfortunately, the drawback with seafloor sampling programs is that bedrock outcrops are usually exposed along bathymetric highs; thus seafloor sampling programs tend to be spotty. Recently, data from exploratory wells along the Santa Rosa-Cortes Ridge and offshore Santa Barbara Island have also become available to the author (Fig. 3.1); the r 25 Fig. 3.1. Location map showing exploratory wells. 26 ?5°^0 VJ Blake Knolls J 30 Miles 4 0 Kilometers Contours in meters CHEVRON OCS P -0 2 4 5 No. I MOBIL OCS P -0 2 8 9 No.l GULF OCS P -0 2 5 8 No.l TEXACO OCS P -0 2 5 7 No.l GULF OCS P -0 2 6 2 No.l EXXON OCS P -0 2 7 2 No.l MARATHON OCS P -0 2 7 6 Nal SHELL OCS P -0 2 7 7 No.l TEXACO OCS P - 0 2 8 6 No.l O C S -C A L 7 5 -7 0 No.l data provide new information on the lithostratigraphy within the Nicolas Terrane. This chapter will describe the lithologies within the southern outer borderland, beginning with a general description of the overall lithology of the borderland, followed by a description of the lithology within the southern Patton and Nicolas Terranes. Surface descriptions are based dominantly on the dart-coring data of the U.S. Geological Survey, whereas subsurface descriptions are based on information from exploratory wells within the Nicolas Terrane and three DSDP wells drilled at one location (designation number 468) in the Patton Terrane. The lithologic data will be tied to seismic reflection data in chapter 5. Aided by the technique of seismic sequence stratigraphy, the nature of the depositional environment can be interpreted for a large portion of the study area. 3•1 General The geologic history of the California Continental Borderland is unknown prior to the Late Jurassic period. By this time, the borderland was already a convergent margin (Bottjer and Link, 1984). The oldest rocks are Late Jurassic greenstones, gabbros, diorites and leucotonalites on Santa Cruz Island; together with 28 patches of sedimentary and volcanic rocks exposed onshore within the Transverse and Peninsular Ranges, they represent an accretionary wedge-forearc-island arc complex (Bottjer and Link, 1984? Hill, 1976; Mattinson and Hill, 1976). Adjacent to the borderland, similar but older rock types can be found along coastal North America, from the Klamath-Sierran Terrane and Great Valley to Cedros Island and the Vizcaino Peninsula. The geologic data suggested that a convergent margin setting may have existed along the eastern Pacific as far back as Late Triassic time (Barnes, 1988? Minch and others, 1976? Rangin and others, 1983). However, this margin was not static; evidently there were several episodes of back-arc basin formation and closure, as well as oblique translation of allochthonous terranes or slivers of the subduction complex (Lund and Bottjer, 1991; Rangin and others, 1983) . By Oligocene time, the Pacific-Farallon mid-oceanic ridge was in close proximity to the North American margin. Tectonic uplift or eustatic lowering of sea level resulted in nonmarine onland deposition throughout much of southern California as well as at locations on the Channel Islands (Howell and Vedder, 1981? Howell and others, 1987? Vedder, 1987). 29 Volcanic activity began at about 26 Ma in southern California, followed by a rapid rise in sea level. Marine sedimentary rocks occurred throughout coastal southern California and the Channel Islands, and marked the beginning of a regional marine transgression (Vedder, 1987), possibly signifying the onset of transform faulting (Atwater, 1970; Howell and others, 1987). 3.2 Patton Terrane The main reason for the division of the outer borderland into the Patton and Nicolas Terranes was the belief that the two belts represent tectonostratigraphic terranes separated by distinct stratigraphic and structural histories as defined by Coney and others (1980). The outer belt, referred to as the Patton Terrane, is composed of melange-type basement, whereas the Nicolas Terrane is believed to be floored by ophiolites (Howell and Vedder, 1981; Vedder, 1987, personal commun., 1991). The Patton Terrane is described as an accretionary greywacke terrane characterized by greywacke complexes with no known crystalline basement (Gibson and others, 1985). It is composed of meta-sedimentary and meta-ultramafic rocks, including altered basalt, laumontitic graywacke and argillite, serpentinite, and 30 ultramafic rocks (Howell and Vedder, 1981). According to Crouch (1981), blueschist-facies rocks (i.e. glaucophane schist) tend to occur on the east side of the Patton Terrane? he suggests that this may be an indication of landward increase in metamorphic grade similar to what is observed in the Franciscan complex of northern and central California. The metasedimentary rocks are also comparable to the Franciscan complex of northern and central California on the basis of similarities in chemistry, appearance, and metamorphic mineralogy (Crouch, 1981). The oldest fossiliferous sedimentary rocks recovered from the seafloor in the Patton Terrane are Santonian or Campanian (87 to 73 Ma) claystone and fine-grained sandstone from Garrett Ridge (Fig. 3.2). Massive, fractured, hard siltstone recovered from the Patton Ridge south of San Miguel Gap contained foraminifers whose age zonation ranged from Late Cretaceous to Oligocene. Eocene sandstone recovered on a ridge slope west of West Cortes Basin is the only known exposures of Early Tertiary strata in the Patton Terrane (Vedder and others, 1979, Kennedy and others, 1987). Throughout most of the Patton Terrane, upper Oligocene to Recent sedimentary rocks are believed to overlie Franciscan Basement (Crouch, 1981? Gibson and 31 Fig. 3.2. Distribution of seafloor lithologies within the borderland (From Vedder, 1987). 32 u LJ ITsanZ Miguel Is. Santa Sa n ta < ^ /? V .Cruz Is. ^ Rosa Is. EXPLANATION Los Angeles Pliocene and Quaternary sedimentary rocks Miocene and Pliocene volcanic rocks, local Miocene schist breccia u / A'- ’ ;v.\* r -1 . Late Oligocene and Miocene sedimentary rocks Paleocene, Eocene and Oligocene sedimentary rocks Cre taceous sedimentary rocks *1 Catalina Is. fWVT-'p • . ; • > w X - v : < s ? •\ • • C3 mSI ’ 0 ‘ sHJVn<LC. < , \* colas i s '•* **•''* Basement rocks San Nicolas No data Fault, generalized J Svedrup . Bank * • • 0 Dali Bank. • « Clemente Is. Son Diego Tanner Bank USA Mexico / . xx 'Blake* • • * . ' J Knolla • * vcy^' ■ * * * * . * * • r f ’viiy—^ Cortes Bank / , •'. ; o V Bank^o*. ••n /t- o Shepard K n o l l . . . M ■ Slxtymlle . ' ! • t rBank '\ ° * ; A , 4 0 km others, 1985? Vedder, 1987). The Oligocene strata are believed to represent shallow-water shelf and slope deposits, formed during a period of eustatic sealevel lowstand, whereas Miocene and younger sediments tend to be finer-grained, basin-fill and drape deposits (Crouch, 1981; Vail and others, 1977; Vedder, 1987). Oligocene sandstone and siltstone were recovered only at one location, Sverdrup Bank (Vedder and others, 1977), whereas Miocene and younger deposits occur extensively throughout the terrane in such diverse locations as bank tops, depressions, as well as the Patton Escarpment. Subsurface lithologic information is available for the outer Patton Terrane from DSDP wells no. 468, 468a, and 468b, drilled on a valley slope adjacent to the Patton Escarpment (Fig. 3.3). The drilling program recovered hemipelagic, terrigenous, and volcanogenic sediments. Pliocene-Quaternary sediments were dominantly nannofossil and foraminiferal oozes. Upper Miocene sediments consist mainly of nannofossil and diatom- nannofossil oozes and were found only in wells 468a and 468b; an unconformity separates Pliocene from middle Miocene sediments in well 468. Middle Miocene sediments consist of terrigenous fine-grained sediments (clay, silt, and minor sand), claystone, and volcanogenic ash, glass, and breccia. The breccia is composed of vesicular 34 Fig. 3.3. Lithologic description of DSDP composite well section (From Yeats and others, 1981). Subbottom Depth (m ) 0- 100- 200- 3 0 0 - 4 0 0 - i i H a ( i » n # » £ A *7 / . V * £~7 I , 1 t ? - O .p - .O ; 0 .0 f l - . O: 0- » • ' * » • * ' O .Q .Q .v 7 v 74 v Plio+Qt 0 ) ■O ■O a > o _l ■O ■O Olive gray glauconitic to yellowish gray nannofossil-foraminifera ooze: Glauconitic silt sand and sandy clay Interbedded pale olive nannofossil ooze and olive brown diatom- nannofossil ooze. Lenses and patches of greenish black glauconitic sand and sandy silt Minor ash layers Moderately w ell-indurated, upper portion contains common to abundant diatoms and coccoliths. Lower portion is well-indurated and consists chiefly ofdolomitic silty claystone Litholoaic Svmbols |-i— ij Nanno-foram ooze <= h= > Calcareous chalk I - 1 - - 1 - ! Nannofossil ooze v vl Volcanic breccia -2-£J and La pi 1 li |------ 1 Clayey silt /silty clay Volcanic rock fragments V/] Sand o\ basaltic and andesitic rock fragments along with tuffaceous and volcaniclastic sandstone? the breccia is believed to have been derived from local source areas (Yeats and others, 1981). 3.3 Nicolas Terrane The Nicolas Terrane represents the second of four terranes within the borderland (Howell and Vedder, 1981? Teng, 1985? Vedder, 1987). It is interpreted to be a forearc on the basis of similarities to the lithologies of the Great Valley Sequence in central California and the Valle Fonnation in the Vizcaino Peninsula (Barnes, 1984? Minch and others, 1976). For this study, the lithologic descriptions have been subdivided into basement, Upper Cretaceous and Paleogene, and Neogene to present, because this study has noted that from exploratory well data these intervals are lithologically distinct, and that they are separated by distinct sequence boundaries recognized on the seismic reflection records (seismic stratigraphic analysis will be discussed in chapter 5). 3.3.1 Basement Basement lithology within the Nicolas Terrane is uncertain because there are only four locations where 37 basement can be identified, and at those locations, two suggest ophiolite basement, whereas two suggest "Franciscan" basement. Crystalline basement of schistose volcanic rocks, hornblende-rich diorite, and leucotonalite on Santa Cruz Island is interpreted to represent a weakly-metamorphosed segment of late Jurassic oceanic crust, possibly of ophiolitic affinity (Hill, 1976; Mattinson and Hill, 1976). Along a narrow, unnamed ridge about 10 km southwest of Santa Rosa Island, quartz-crossite schist, actinolite schist, and saussuritized gabbro (?) mixed in with glaucophane schist may also indicate an ophiolite basement (Howell and Vedder, 1981? Vedder, 1987). However, an Exxon exploratory well on Tanner Bank apparently identified metamorphic basement of "Franciscan" affinity, and a Texaco well on Dali Bank may have encountered similar "Franciscan" basement (Fig. 3.4; also, Webster et al, 1985). It should be noted that J.G. Vedder (personal commun., 1991) has cautioned that "Franciscan" basement may be wrong because there may be problems distinguishing rocks of "Franciscan" affinity from ophiolitic rocks. Within the southeastern edge of the study area, dart-coring has recovered metamorphic fragments and grains from middle Miocene and younger sedimentary rocks in the vicinity of Blake Knolls-Sixtymile Bank and along 38 Figure 3.4. Stratigraphic columns from 10 exploratory wells along the Santa Rosa-Cortes Ridge and offshore Santa Barbara Island. 39 V _ V V V V o' 'o'o 'o' O O o o e»0 ° o P-262 P-276 P-272 P-277 P-286 75-70-1 6coq/ ' l i i i m i i i i i i I I tiiininium iii I MI| Umil f l I I IHflllllllllHHIII iiiuiiiliiiiiniiii O O O O O 0 6 0 0 0 0 O O 0 O O 0 0 O O _^-z_ | shale T _ — | siltstone | volcanics metamorphics tuffaceous siltstone sandy siltstone o o o o o sandstone conglomerate (3 fossiliferous i limestone unconformity NR C X 3 40 the Santa Cruz-San Clemente Ridge (Vedder and others, 1981). The schistose rocks include quartz-mica schist, plagioclase-epidote-chlorite schist, and one undated sample (no. 145) which contained glaucophane-epidote schist. On the other hand, altered and unaltered gabbroic rock fragments and pebbles in the central part of Blake Knolls (sample nos. 153 and 155) also suggest that ophiolitic basement may be present nearby (Vedder and others, 1981). It should be noted that the oldest strata in which basement-indicating clasts are found appears to be Pliocene age. Although transform motion and exhumation of the Catalina Terrane was established by the middle Miocene, there does not appear to be any evidence of this event in the lithologic nor the stratigraphic records. 3.3.2 Late Cretaceous and Paleogene The oldest sedimentary rocks exposed within the Nicolas Terrane are found on San Miguel Island, where a 3000 m section of Upper Cretaceous coarse-grained turbidites, including granitic- and volcanic-cobble conglomerate, are exposed on the west face of the island (Howell and Vedder, 1981; Vedder, 1987). According to Vedder and others (1981), the oldest sedimentary exposure on the seafloor within the Nicolas Terrane is Late 41 Cretaceous sandstone which was recovered near the northwest slope of Cortes Bank (Fig. 3.2). From information provided by wells along the Santa Rosa-Cortes Ridge, strata of Late Cretaceous age are extensively present throughout the subsurface of the Nicolas Terrane? maximum thickness of recovered strata overlying metamorphic basement (?) is 1700 m. In general, the strata consist of siltstone with sandstone interbeds, although sections whose grain-size are as coarse as conglomerates were recovered in three of the wells (Fig. 3.4)? however, a depositional trend cannot be determined from the limited data. An abrupt transition in sonic velocity in the upper Cretaceous apparently marks an unconformity separating a slightly-metamorphosed, lithified basal section from overlying latest Cretaceous-Paleogene strata. The latter is characterized by sonic velocities in the low to high 3000 m/sec range, while the late Cretaceous and older strata exhibit velocities in excess of 5000 m/sec. Eocene rocks are exposed on San Nicolas Island (Cole, 1975)? Paleocene and Eocene rocks outcrop on San Miguel and Santa Cruz Islands and have been collected from subsea dart cores along the Santa Rosa-Cortes Ridge (Fig. 3.2). Shallow-marine, medium- to fine-grained sandstone and siltstone of Paleocene age underlie 42 coarser-grained Eocene sandstone and conglomeratic lenses of an interpreted west-facing submarine fan system fanning out from the Santa Cruz Island area towards the south, west, and northwest (Howell, 1975; Howell and Vedder, 1981; Vedder, 1987). The Paleocene section in the exploratory wells appears to be dominantly composed of dark grey, hard siltstone and soft to firm claystone, with interbedded fine- to medium-grained sandstone intervals (in well 75-70-1, these can make up to 30 percent of the section). However, two of the wells, P-257 and P-277, contained a coarser Paleocene section; the 480-m section in P-257 appears to grade from siltstone at the top of the section to conglomerate at the base, and medium-grained sandstone appears to be the dominant lithology in P-277. The top of the Paleocene section in well 75-70-1 is capped by a 3-m limestone interval and underlain by a glauconite-rich layer (Paul and others, 1976). The thickest section of Eocene strata encountered in the wells occurs at Tanner Bank, where approximately 1580 m of mainly grayish siltstone is encountered (Fig. 3.5). With the exception of well P-272, the middle Eocene section tends to show a consistently sandier composition compared to upper or lower Eocene sections. In well 75-70-1, internal features in the sandstones such as cut 43 Figure 3.5. Thickness of Eocene strata along the Santa Rosa-Cortes Ridge and offshore Santa Barbara Island as determined from exploratory wells. 44 I I 34° ,450' LONG BEACH NEWPORT BEACH 1070 33 SAN >IEGC 7 6 0 ^°v ^ o ^ 7 / J § 32 30 Miles • 6 0 0 = THICKNESS OF EOCENE STRATA 40Kilometers Contours in meters 118° 120° 119° i i ] I i i i 45 and fill structures, rip-up clasts, flute casts, small scale crossbeds and convolute bedding are interpreted to indicate turbidite deposition (Paul and others, 1976). The Eocene section appears to thin towards the north, south, and east. Towards the north and east, nonmarine Oligocene strata overlying a northward-thinning upper Eocene section may indicate erosion of the upper Eocene as the cause of the northward thinning; however, the middle as well as lower Eocene sections also thin northward, indicating that thinning may be depositional as well (Fig. 3.4). Towards the south, this thinning may be attributed to depositional wedge-out, and is not an erosional effect, since bathyal water depths apparently existed throughout the Eocene and Oligocene in the southernmost area. The direction of transport of Paleocene and Eocene sediments appears to be towards the southwest, as inferred from sedimentary structures in cores recovered from well 75-70-1 (Paul and others, 1976). This direction is in agreement with paleocurrent directions from Eocene strata on San Nicolas Island (Cole, 1975). Oligocene strata are apparently missing on San Miguel, Santa Cruz, and San Nicolas Island (Howell and Vedder, 1981; Cole, 1975), but a 150-m section of coarse-grained Oligocene strata which occurs on Santa 46 Rosa Island is identified as nonmarine (Weaver and Doerner, 1969). East of the Nicolas Terrane, a thin sequence of nonmarine mudstone, sandstone, and conglomerate is sandwiched between Miocene breccia and Late Cretaceous (?) strata at the southeast end of Santa Catalina Island (Howell and Vedder, 1981). Seafloor exposures of Oligocene sedimentary rocks appear to be limited to outcrops found at Dali, Tanner, and Cortes Banks as well as on the shelf north of San Nicolas Island (Fig. 3.5; Howell and Vedder, 1981). In the exploratory wells, the Oligocene interval is dominantly a medium- to coarse-grained sandstone which tends to become finer-grained towards the south (Fig. 3.4). Wells as far south as Dali Bank recovered nonmarine strata. South of Dali bank, five of the wells appear to show a southward progression from neritic to bathyal environments (see appendix 1). From the distribution of nonmarine and marine strata, it is inferred that the transition zone lies south of San Nicolas Island, probably between Dali and Tanner Bank (Fig. 3.6). 3.3.3 Neoqene and Quaternary Strata Lower Miocene shallow-marine, transgressive strata unconformably overlie upper Eocene rocks on San Miguel and Santa Cruz Islands and conformably overlie nonmarine 47 Figure 3.6. Oligocene depositional environments determined from the exploratory wells. 48 34° LONG BEACH HE 33 SAN > IE G G 32 30 Miles 40Kilometers Contours in meters 120' 49 strata of Oligocene age on Santa Rosa Island (Howell and Vedder, 1981). These deposits are typically coarse grained; clast composition in conglomeratic sandstone and breccia beds indicate erosion of underlying Paleogene and Mesozoic strata (Howell and Vedder, 1981). Elsewhere within the borderland, the widespread occurrence of lower as well as middle Miocene breccia beds is interpreted to indicate rapid differential uplift and erosion of an area extending from the Channel Islands area southwards to the area of Santo Tomas-Blake Knolls, and inland along the southern California coast to as far south as near Tijuana (Howell and Vedder, 1981). Lower and middle Miocene strata overlying the transgressive breccia deposits are sandstone and mudstone beds that record sucessively deeper sublittoral and bathyal environments. Late Miocene outcrops at most places within the borderland were subsequently eroded off or were never deposited? the few outcrops that were found implies that there was substantial sea-floor relief in the borderland by that time (Vedder, 1987). Subsea exposures of Miocene strata occur extensively along the Santa Rosa-Cortes Ridge (Fig. 3.2); these exposures for the most part are claystone and siltstone ranging in age from Early to Late Miocene; coarser- grained conglomeratic or breccia deposits were not 50 observed (Vedder and others, 1977, 1979, 1981). Rock samples recovered in the Santo Tomas-Blake Knolls area are mostly silty claystone of middle Miocene age, although one sample on Santo Tomas Knoll was identified as Early Miocene (Vedder and others, 1981). Along the northern portion of San Clemente Ridge, sedimentary rocks recovered from the seafloor are dominantly siltstone and claystone of middle Miocene to late Pliocene age. Subsurface exposures of early Miocene age can be identified in most of the wells along the Santa Rosa-Cortes Ridge, overlying an apparently extensive early Miocene basalt layer (Fig. 3.4). In the well adjacent to Santa Rosa Island, the lower Miocene section is a very sandy siltstone and coarse sandstone interval. In the wells further south, this section is a sandy siltstone or fine-grained sandstone. Middle Miocene strata were identified in only three of the wells within the Nicolas Terrane; at the southern end of the Cortes Bank, the section consists of an interbedded fine-grained sandstone, siltstone, and claystone basal interval overlain by a fine-grained sandstone unit. The well northwest of Santa Barbara Island encountered a basal volcanic tuffs, detrital volcanics, and dolomite interval, overlain by a siliceous 51 shale and siltstone with chert interval lithologically similar to the Monterey Formation. By Pliocene time, the seafloor probably resembles that of the modern borderland (Blake and others, 1978; Howell and Vedder, 1981). Strata of Pliocene age are exposed on Santa Cruz and Santa Rosa Islands as eolian and shallow-marine sandstone units. East of the Nicolas Terrane, small patches of Pliocene are exposed on San Clemente and Santa Catalina Islands as sandstones of a shallow marine and a bathyal environment respectively (Vedder, 1987) . Quaternary sediments can be found as terrace deposits around the islands and along mainland coastal areas and as nonmarine and shallow-marine basin deposits in onland basins (Vedder, 1987). Rapid deposition of nonmarine and marine sediments occur in coastal basins, deposition of marine sediment decreases with increasing distance offshore (Howell and Vedder, 1981). In the outer borderland, subsea occurrences of Pliocene to Holocene deposits range in composition from siltstone and mudstone/claystone to fine-grained sand and silt, and tend to be localized within the modern basins. Total thicknesses of these sediments are uncertain since none are found in the exploratory wells. 52 3.4 Volcanic Rocks Basaltic rocks can be found throughout the borderland, but more has been observed concentrated along the southern edge of the study area (Fig. 3.2; Vedder, 1987). Basalts on Northeast Bank have been dated at about 5 Ma (Hawkins and others, 1971), although Vedder and others (1981) reported older volcanics possibly underlying the Pliocene basalts. Along the Santa Rosa-Cortes Ridge, the Paleogene section is capped by a thick section of Oligocene or early Miocene basalt flows whose maximum, measured total thickness is about 300 m (Fig. 3.4), and volcanic rocks on San Clemente Island have been dated at 15 to 16 Ma (Hawkins and Divis, 1975). The youngest basalts occur offshore of northern Baja California, where several investigators reported ages of 2 Ma and younger (Doyle and Gorsline, 1977? Krause, 1965). 53 Chapter IV DATA ACQUISITION AND PROCESSING Much of the geology of the California Continental Borderland has been inferred based on extrapolating lithologies and structures exposed onland and within the islands to the deep basins, and from seafloor dart-core and dredge samples. However, the borderland is a complex tectonostratigraphic province; several different tectonic episodes have resulted in the formation of an accretionary complex-forearc pair which has been disrupted by translational faults. Abrupt lateral changes in the geology can be expected. Thus the best method of correlating available geologic information to the subsurface is with the use of seismic reflection records; however, outside of shallow-penetrating single-channel seismic records and echograms, deep-penetration-type seismic records which can image at least to basement have not been available to the public until the U.S. Geological Survey began acquiring multifold seismic reflection data in the late 1970s aboard their survey ship, the R/V Samuel P. Lee. 54 The U.S. Geological Survey collected three sets of multifold seismic reflection data, in 1978, 1979, and 1990, and one set of single-fold seismic data collected in conjunction with the 1990 multifold seismic survey (Fig. 4.1). Of these data sets, the 1978 data set was either never processed or the processed data was lost (D.M. Mann, 1989, personal commun.), the 1979 data set was processed, but the data was contaminated with noise and strong water-bottom multiples, and the 1990 data set is currently being processed in-house by the U.S. Geological Survey. Through the auspices of CALCRUST, this author was able to gain access to the 1978 and 1979 seismic reflection data for his research on the stratigraphy and tectonic development of the borderland. Portions of the data sets were reprocessed in order to? 1) decrease the amount of noise in the original data set, 2) increase data quality via trace editing, better velocity analysis, and migration, and 3) determine whether crustal features could be imaged, especially for the generally seismically-transparent accretionary wedge of the Patton Terrane. 55 Fig. 4.1. Trackline map of U.S. Geological Survey multifold seismic reflection data. 56 34° LONG BEACH NE CH -1 3 3 SAN )IEGC 3 2 ° SURVEYS 1990 1979 1978 3 0 Miles 4 0 Kilometers Conto urs in m eters 120° 119° 118° 57 4.1 Navigation Navigation for the three surveys was accomplished with Loran C, complemented with satellite positioning and sonar. This method is the accepted industry standard for seismic acquisition; accuracy is generally within 200 m, depending on location of transponders and zenith angle of satellites. In general, navigational accuracy for the outer portion of the borderland appears to degrade; the recording instruments tended to show large fluctuations in distance traveled versus time. This condition may be due in part to stronger currents in the outer borderland, and the time of day of the survey. Apparently the navigation signal becomes more erratic after 2100 hrs. 4.2• Acquisition The three surveys include a total of about 5400 km of 24-fold seismic reflection data and 3000 km of single-fold data. The streamer length was 2.4 km long, with 24 hydrophone groups spaced at 100-m interval during 1978 and 1979, and 48 hydrophone groups spaced at 50-m interval in 1990. A 1300 cubic-inch, tuned airgun group firing at 50-m intervals was used in 1978 and 1979 to produce 24-fold CDP records; a 2300 cubic-inch system firing at 50-m intervals was used in 1990 to produce 24-fold records. 58 4,3 Processing Parameters Of the older U.S. Geological Survey seismic data sets, the 1979 data set was apparently the only one that has been processed, as part of an open-file report in preparation for offshore lease sales (Mann and others, 1980). The processed 1979 data contained very degraded sections; the original data exhibited portions of records where noise dominated and data quality was extremely marginal (i.e., Fig. 4.2), and many sections in which the base of the sedimentary fill cannot be determined (i.e., Fig. 4.3) In 1986, the author requested permission from the U.S. Geological Survey to process/reprocess the early-vintage data sets. Permission was granted, and the author received shot-gather tapes of the seismic lines from the 1978 and 1979 surveys. For the most part, a standard processing stream was used on the data sets (Yilmaz, 1987) that was similar to that of the U.S. Geological Survey (compare Figs. 4.4A and B). The major differences that appear to enhance data quality in the reprocessed data are: 1) Detailed trace-editing and velocity analysis; 2) multiple- attenuation using an f-k dip-filtering method which depended on a partial moveout of cdp gathers (Yilmaz, 59 Fig. 4.2. More than half of the 1979 data set contains zones where noise dominates the record and the data is uninterpretable. Line 908 is an example. 60 SECONDS oor signal-to-noise P H H f l M U Fig. 4.3. A major problem with the 1978 and 1979 data set was strong water-bottom multiples, which masked deeper primary reflections representing strata, structures, or more importantly, the base-of-fill. SECONDS SW 1500 1600 1700 Sea f H H E P B H k i i i i MJtS s S S r E ! S » m H B H H I i l W f M M U U i l i i i . m m M j&Water-bottom muI s m & f m 4-SS^ ft ^ ry iV e i 2 ,^ 1 U L Base of fill »VW « «sii iLINE 9 w z w , M Fig. 4.4. Processing flow chart: A) CALCRUST-reprocessing of 1978 and 1979 data sets; B) U.S. Geological Survey processing stream. CALCRUST used a partial NMO followed by dip-filtering (FKMULT in the Merlin processing software) to attenuate multiples? this process was deemed to be more effective than predictive deconvolution. Optional Processing Processing Flow Output Trace Edit Geometry CMP Sort Trace Balance Gain (spherical Divergence) Pred. Decon (Short gap, multiple design windows) Fk-Dip Filter (multiple --------► suppression) Filter (broadband) NMO Post-NMO Mute (80% stretch) Stack ■ Migration — (finite difference) ^ Tape: CDP Gathers Trace Balance Pre. Decon (short gap, multiple design windows) Filter (broadband) AGC Pre-NMO Mute Discrete Vei. Analysis (Every 20 CDPs nominally depending on structture) - Tape: CMP Stack Plot: Section Tape: Migration Plot: Section (A) 65 Optional Processing Processing Flow Dmux Resample 4 ms Trace Edit Shot Sort Geometry CMP Sort Output Tape: Shot Gathers Plot: Every 100 shots, near trace monitor SCAG Sonobuoy Tape: CDP Gathers Plot: Every 100 CMPs, near trace monitor Pred ./Spike Decon — Trace Balance Pred. Decon (Short gap, multiple design windows) (Long operator for multiple suppresion) NMO Post-NMO Mute (40% stretch) Tapered - or Median Stack (for multiple suppresion) Stack Filter (broadband) Resample 8 ms Trace Balance Pred. Decon (short gap, multiple design windows) Filter (broadband) AGC Pre-NMO Mute Discrete Vel. Analysis (Every 200 CDPs nominally depending on structure) Time-Varying Filter Plot:Every 200 NMO gathers Tape: CMP Stack Plot: Section Miration ----- (finite difference) Tape: Migration Plot: Section (B) 66 1987), and muting of the multiples in the near traces; and 3) migration using a detailed velocity field. 4.4 Trace Editing Data quality varied considerably between the 1978 and 1979 U.S. Geological Survey seismic reflection data sets. The 1978 data set is relatively noise-free; trace editing consisted of time-shift correcting gathers with irregular zero-time (Fig. 4.5). Conversely, the 1979 data set is dominantly poor quality; the signal-to-noise was extremely low along portions of most of the seismic reflection lines, and very high noise over signal was observed for many traces in the shot gathers, even after the gathers have been filtered to remove the effect of low-frequency DC shifts (Fig. 4.6A). By line 908, signal was lower than noise in more than half of the traces/gather (Fig. 4.6B), and line 910 could not be processed. Preliminary filtering of the shot gathers oftentimes does not eliminate the problem, and despiking routines tend to mute high-amplitude signals as well as the noise. To improve seismic resolution, reprocessing of the 1979 data set by CALCRUST required trace-editing of all of the lines. Two methods were utilized; inspection of individual shot gathers to identify unusable traces, or 67 Fig. 4.5. An example of an irregular time shift in the shot gathers which must be time-corrected. 68 Multipl LINE 20 Shots 1026-1029 rvix trrcwsnr t s L M Fig. 4.6. Two examples of noise in filtered, 1979 shot gathers: A) Low signal-to-noise, but usuable? B) most of the gather contained unusable traces that must be eliminated. 70 H 1. 000 1. 200 1. 400 1. 600 1. 800 2. 000 2. 200 2. 400 2. 600 2. 800 3. 000 3. 200 3. 400 ^LINE 906 Shot 129 3. 600 3. 800 1. 0 30 1. 230 1. 430 1. 6 30 1. 830 2. 030 2. 230 2. 430 i 2. 630 2. 830 3. 030 3. 230 3. 43 3 3. 6 3 3 3. 8 3 3 inspection of common-offset gathers (Fig. 4.7). The latter method is preferred because visual inspection of the trace as a part of the seismic reflection line is a good indication of the signal-to-noise, and thus the resolution, of that trace. Trace-editing usually involves preliminary filtering of the gathers to eliminate the problem with DC shifts, followed by zeroing out the trace/s that are still unusable? however, if poor quality is limited to the portion of a trace that is below acoustic basement, a mute was employed whereby just the noisy portion is zeroed (Fig.4.8). 4.5 Velocity Analysis Resolution of geologic events on seismic reflection records is highly dependent on the ability of the seismic reflection acquisition system to detect the events and enhance their correlative seismic reflections. Thus the purpose of multifold acquisition systems is to vertically sample subsurface geologic interfaces a number of times from different surface offset distances from these interfaces, thereby producing a series of vertical profiles (seismic traces) versus offsets. These profiles are then summed (stacked); the energy from reflecting boundaries representing real events is magnified, whereas 72 Fig. 4.7. Viewing traces within a shot gather may not tell how they will look in a stacked section. By viewing common-offset gathers, one can visually determine whether any information can be gained from a low signal-to-noise trace. 73 18 Traces unusable JK£a kjctwl __ SyTraces unusable LINE 908 Trace l9ffi»SSK 1 . O O C j 1 20 C 1 i 40 C . 1 60 C 1 80 C 2 O O C 2 20 C 2 , 40 C 2,60 C 2 , 80 C 3. O O C 3 , 20 C 3,40 C 3 , 60 C 3 , 80 C Fig. 4.8. Two methods of editing out low signal-to-noise traces: 1) If total trace is noisy, zero out the trace; 2) if a portion of a trace is noisy, zero out that portion. 75 S m T T T H f ^Zeroed 77 S5- tracesS Partially edited LINE 904 CDP34Q LINE 904 CDP 360t random noise is canceled (Fig. 4.9). However, enhancing the energy of reflecting events depends on the ability to accurately sum, or stack, a reflecting event from a series of seismic traces, which in turn is dependent on accurately determining the different offset distances in the subsurface. In turn, determining the different offset distances is strongly dependent on knowing the velocity structure of the subsurface materials; velocity analysis of a collection of traces is necessary to determine subsurface velocity structures. Accurate velocity structures are important in data processing, both to correctly image the seismic data and to provide optimum resolution of structures and stratigraphies. Laterally, velocity is affected by the dip of stratal boundaries and other subsurface events . Therefore, the data were reprocessed with detailed velocity profiles in which velocities were picked at 1000-m intervals; in areas of complex stratigraphy or structure, velocities were picked at 500-m intervals (Fig. 4.10). Finally, the seismic data were migrated using a smoothed velocity field. In general, the velocities derived from optimum stacking of the data agree with velocities derived from other sources. Several velocity analysis methods were employed, not only to improve seismic resolution, but to eliminate the 77 Fig. 4.9. A common midpoint (CMP) gather after velocity correlation for offset distance. Coherent energy from primary reflections will sum, whereas random noise and multiples should cancel due to destructive interference. 1 .5 1 .6 1 .7 1 .8 1 .9 2.0 2 . 1 2 .2 2.3 2.4 2.5 2-6 2-7 2.8 2.9 3.0 3 .1 3.2 3.3 3.4 3.5 3.6 3-7 3.8 3.9 4.0 4.1 4.2 4-3 4.4 4.5 4.6 4 .7 4 .8 4.9 5.0 i Seafloor Multiple £ LINE 21 CDP 540e VMO velocities(m/sec) -1470 Primaries — 1490 1500 1520 1600 1650 1750 1825 -2000 -2150 ■2350 ■2475 i_ _ _ _ 79 Fig. 4.10. Horizontal root mean square (RMS) stacking velocity profile derived from velocity analysis, nominally at every 1000-m interval, at every 500-m where geology appears complex. LINE 16 masking effect of seafloor multiples within the sedimentary section. Seafloor multiples were especially troublesome within the Nicolas Terrane, where the base of the Late Cretaceous and Paleogene sedimentary section sits very near the elevation of the primary seafloor multiple. Constant velocity stack (CVS) sections were analyzed to determine coherent events with depth within the seismic reflection line (Fig. 4.11); however, for analyzing an entire seismic reflection record, the CVS method was computer intensive (thus unacceptably slow). Therefore, a quicker method was used in which coarse velocity-sampling intervals were used in the CVS plots, which were then compared to velocity semblance plots (Fig. 4.12) Velocity semblance plots are graphs of coherent events versus velocities for a given common depth point (CDP) gather. The velocities picked from the semblance plots were cross-checked with velocities from CVS records, and interval velocities derived from the stacking velocities were laterally smoothed and then compared to known borderland subsurface velocities derived from borehole and seismic refraction information to ensure that they were reasonable (Tab. 4.1). Generally the rms interval velocities fell within 20% of velocities 82 Fig. 4.11. The constant-velocity panels at every 100 m/sec, beginning at 1950 m/sec. At increasing velocities, successively deeper parts of the seismic reflection record are imaged. 1950 02050 ;2 5 0 - ri:2250-t 2350;i-i2450 r2550r :2650m2750; [2850 Fig. 4.12. Velocity semblance plots are graphs of coherent energy versus velocities. The greater the peak, the stronger the event on the seismic reflection record. 85 o o o o o C D C \J O O o C O o o o o o o L O SCALED RMS SEMBLANCE AMPLITUDE Interbedded < , 3 multiples 'b Primary Seafloor multiples is & LINE 911 CDP 1640 86 Tab. 4.1. Comparison of interval velocities from seismic reflection records to interval velocities in exploratory wells. 87 Table 4.1 Horizon Base Neogene Base Olig.(?) Base U.Cre. Exploratory Wells 1800-2500 m/sec 2500-3000 m/sec 4000-5000 m/sec W.Cortes Basin 1500-2500 m/sec 2900-4000 m/sec 4300-5700 m/sec E.Cortes Basin 1800-2300 m/sec 2600-3600 m/sec 4200-5800 m/sec determined from exploratory wells? however, fluctuations of as great as 40% can be found. 4.6 Multiple Suppression Seafloor multiples are a serious problem in marine seismic reflection data, oftentimes masking true subsurface stratigraphic and/or structural details. Multiples proved to be especially troublesome in the borderland datasets, especially in two areas, shallow-water areas, and deep subsurface within the Nicolas Terrane. Shallow-water multiples are often the events with the highest amplitudes, effectively masking weaker subsurface geologic interfaces. Several methods have been utilized to attenuate multiples: 1) Moveout difference between primaries and multiples in a CDP gather? 2) dip difference between primaries and multiples in a CDP gather? 3) difference in frequency content between primaries and multiples? and 4) periodicity of multiples (Yilmaz, 1982). The two methods used in this dissertation are based on the moveout difference between primary reflections and multiples within a CDP gather? these methods were utilized because they were readily-available processors within the Merlin processing package, easy to implement, and appear to be very effective. 89 A primary reflection has less moveout than a multiple. Figure 4.13B is a velocity spectrum showing the velocity trends of the primaries and the multiples. By NMO-correcting the gathers using a velocity function between the velocity function of the primaries and that of the multiples, the primaries and multiples migrate to different quadrants on a frequency-wavenumber (f-k) plot (Fig. 4.13C). Multiples can be suppressed by zeroing out the quadrant containing the multiple energy. Inverse NMO-correcting the gather restores the original moveout of the primaries, with the multiples suppressed (Fig. 4.13E). Near-offset energies from multiples are not affected by f-k multiple suppression techniques. At the near offsets, there is hardly any significant moveout offsets; the multiples and primaries are essentially flat. The simplest method of suppressing near-offset energies is to apply an inside mute to CDP gathers. Finally, by applying NMO correction to the gathers using the velocity function for primaries and stacking, a considerable improvement is observed in terms of resolving deep reflections below the zone previously masked by multiples (Fig. 4.14). 90 Fig. 4.13. Partial normal moveout (PNMO) and frequency domain dip filtering to attenuate seafloor multiples (from Yilmaz, 1982): A) Common-depth-point (CDP) gather and corresponding frequency-wavenumber plot; B) determining a PNMO velocity halfways between the primary and multiple reflections; C) CDP gather with PNMO applied; D) dip filtered CDP gather where multiples are attenuated; and E) CDP gather with PNMO removed. 91 IllllllllllllilllllllllllW.^V^-illlPV^Witt i i i i i i i i i i i i i i i i i i i i i i i »v^«! i i i ! - «: ti i i i i i i i i llllllllllllllllllllW:»V.:Y:::lli';?'^iillllllllllll llllllllllilWlilllUi^;iiK«m'V'.(<(«llll iimmiMiwwmwmmmii llll'-^ww illllw llllllll'^ltlllllllllllllll M V U /W\ 1 1 UOn ft IIII Down\\1111IIIIIIIIU 1 1 I ll 0 .0 0.2 0-6 0.6 1.0 1.2 1.6 1.6 2-0 V M V B V P I ^ I *° L__ Hz -10 0 10 Wavenumber (cycle/km) (a) (b) (e) Fig. 4.14. Comparison of a portion of line 21; A) stacked, B) multiple-attenuated stack. 93 Multiple Multiples Primary C34$+0$/1$852:+^/.+707$^.+^^ 274885^120^189323018923008929^984226^^^24^ 4.7 Migration Unless they are flat-lying, reflections on seismic reflection records are not in their true spatial positions directly above the reflectors, but beneath the midway point between shot and the center of the geophone spread. Migration repositions these data elements to their correct locations with respect to their associated reflectors and collapses diffractions, thereby delineating detailed subsurface features such as fault planes (Sheriff and Geldhard, 1982? Yilmaz, 1987). The objectives of migration are to improve lateral resolution, thereby resolving structural and stratigraphic entities, and true amplitudes (Berkhout, 1982). Migration requires a knowledge of the velocity distribution; changes in velocity bend raypaths and thus affect migration (Sheriff and Geldhard, 1982). The interval velocity field used for the migration was derived from detailed analysis of rms velocities as determined from semblance plots and cvs plots, correlated to known velocities from refraction surveys and/or sonic well-logs in the vicinity, and laterally smoothed between adjacent CDP points (Fig. 4.15). Lateral spacing between velocity control points was approximately 1000 m, and at every 500 m in areas where the bathymetry or geology was complex. 95 Fig. 4.15. Smoothed interval-velocity profile used for migration. 96 \ I I I * i 97 J The three migration methods used in industry today are: 1) Integral methods based on Kirchhoff's equation (i.e., diffraction-stack methods), 2) wave-equation migration (finite-difference migration), and 3) Fourier transform solution of the wave equation (frequency-domain migration). A discussion of the mathematics is beyond the scope of this chapter? the formulations have been described in several good texts on data processing, such as Berkhout (1982), Claerbout (1985), and volume 2 of Sheriff and Geldart (1985). Suffice to say that each method has its strengths and weaknesses (Tab. 4.2). The processing softwares available for this study, Merlin and Sierra-Seis, can migrate with a finite-difference or frequency-domain algorithm. Although frequency-domain migration tends to be much faster to complete, horizontal velocity changes due to lateral structural discontinuities within the borderland, plus the low signal-to-noise ratio of much of the late 1970s' seismic data sets, indicated that finite-difference would be more effective. 98 Tab. 4.2. Comparison of the strengths and weaknesses of the three migration methods utilized in industry today (modified from Espey, 1982). 99 Table 4.2 COMPARATIVE ATTRIBUTES OF FULL WAVE MIGRATION METHODS Kirchoff Finite Frequency Summation Difference Domain Advantages: Concept easily grasped Good performance with steep dip Disadvantages: Relatively expensive Advantages: Good performance with low S/N Adaptable to horizontal velocity gradients Disavantages: Relatively expensive Advantages: Least expensive Good performance with steep dip Disavantages: Exaggerates errors due to poor velocity control Poor performance Has difficulty Poor performance with low S/N with steep dip with severe vertical gradients Doesn't handle horizontal velocity gradient correctly Chapter V SEISMIC STRATIGRAPHY OF THE SOUTHERN OUTER BORDERLAND Examination of the stratigraphic relationships within the southern outer borderland is crucial to the understanding of the development of the accretionary wedge-forearc pair, as well as the changes in depositional style as the borderland changed from a convergent margin to a transform margin. In terms of accretionary wedge development, what can the stratigraphic information reveal about the nature of the forearc?. Is the forearc basin a continuous feature once contiguous with that of the Great Valley and the Vizcaino Peninsula, or is the forearc a series of small basins similar to those along the Aleutian Trench (i.e. Bruns and von Huene, 1986; Scholl and others, 1986)? Also, as discussed in the previous chapter, many geologists believe that western North America is composed of a series of allochthonous terranes that have accreted to the North American margin; how have strike-slip motion and docking of these allochthonous terranes affected the contiguity of the accretionary wedge-forearc pair? In terms of the change in tectonic environment, from 101 subduction to translation, what effect did this change have on the depositional environment in terms of deformation of the accretionary wedge-forearc and changes in depositional style? Examination and identification of key seismic stratigraphic events will provide some insight on; 1) the lateral continuity of strata within the forearc and across terrane boundaries, 2) the timing of a major change in depositional patterns, which probably signifies the initiation of change from subduction to translation, and 3) the timing of formation of the modern borderland. This section is divided into two parts, a brief review of seismic stratigraphic principles, followed by a description of the seismic stratigraphy of the southern outer borderland. 5.1 Concepts of Seismic Sequence Analysis Rapid development in seismic acquisition and processing during the 1950s created a revolution in subsurface geologic mapping (Sheriff and Geldart, 1982); it did not take long for earth scientists to realize that unlike other geophysical techniques, a series of closely-spaced reflection seismograms provide a good image of subsurface stratigraphy and structure. By correlating reflective features to geologic features 102 found at the corresponding depth in boreholes, major oil companies began to develop methods of identifying and classifying these reflective features in terms of their geologic meaning. This research culminated in the American Association of Petroleum Geologists' landmark publication on seismic stratigraphic techniques, Memoir 26, in 1977. Since this chapter describes sequence stratigraphy interpreted from seismic reflection records, a brief review of seismic stratigraphic principles is provided in this section, mainly to define terminology; most of the terms are those used in stratigraphy, with perhaps the adjective 'seismic' placed before the term (Payton, 1977) . Seismic stratigraphy is defined as the study of stratigraphy and depositional facies as interpreted from seismic data (Mitchum and others, 1977). A three-step method is generally used to develop a depositional history of the study area: 1) Seismic sequence analysis subdivides the seismic section into packages of concordant reflections, termed depositional sequences, separated by surfaces of discontinuity defined by systematic reflection terminations. These discontinuous surfaces have been interpreted to be widespread unconformities formed during 103 eustatic sea level changes (Vail and others, 1977a, 1977b? Haq and others, 1987) or as a consequence of global and/or regional tectonic processes (Hubbard and others, 1985? Sloss, 1991). The sequences can be grouped into related sequences (megasequences), or subdivided into depositional subenvironments, by means of seismic facies analysis (Hubbard and others, 1985). 2) Seismic facies analysis involves the delineation and interpretation of reflection geometry, continuity, amplitude, frequency, and interval velocity as well as the external form and three-dimensional association of groups of reflections (Sangree and Widmier, 1977). The basic forms of seismic facies units are shown in figure 5.1. These units can be further classified and/or described on the basis of their internal seismic reflection configurations (Fig. 5.2). For instance, a mounded form might be identified as a fan complex. Based on internal seismic reflection characteristics such as reflection continuity or shape, the fan complex might be further classified as simple or compound, and perhaps as inner or outer fan. 3) Seismic waveform analysis provides information on the types and character of lithologies within a depositional environment and the resolvable as well as 104 Fig. 5.1. The external geometry of seismic facies units or sequences (From Mitchum and others, 1977). 105 MOUND TYPES FAN FILL TYPES TROUGH FILL CHANNEL FILL BASIN FILL SLOPE FRONT FILL SHEET SHEET DRAPE WEDGE BANK LENS 106 j Fig. 5.2. Types of internal seismic reflection configurations within seismic facies units (From Mitchum and others, 1977). 107 108 p a r a l l e l and subparallel reflections PARALLEL S U B P A R A L L E L D I V E R G E N T B A S IN -F IL L REFLECTIONS ONLAP FILL ^ C H A O T I C flll'^U S --: O N L A P F I L L C O M P L E X F I L L D I V E R G E N T F I L L PROGRADING REFLECTIONS S I G M O I O C O M P L E X S I G M O I D - O B U O U E O B U Q U E S H I N G L E D H U M M O C K Y C U N O F O R M S REFLECTIONS IN MOUNDED FORMS F A N C 0 M P 1 E X S I M P L E F A N C O M P L E X C O M P O U N O S L U M P M I G R A T I N G W A V E C O N T O U R r T E M O U N D V O L C A N I C M O U N O C A R B O N A T E M O U N D S P I N N A C L E W I T H V E L O C I T Y P U L L - U P H O M O G E N E O U S W I T H O R A P E B A N K E D G E W I T H V E L O C I T Y S A G H O M O G E N E O U S W I T H D I F F R A C T I O N S detectable limits of depositional units. It involves the study of amplitude, frequency, and phase of the wavelet under a given set of geologic conditions, and as such, involves modeling ideal depositional environments and conditions. Within the southern outer borderland, age assignments and areal correlation of key seismic reflectors and sequences were based on ties to microfossil-dated lithologies from a series of exploratory wells along the Santa Rosa-Cortes Ridge, and seafloor dart-cores and dredges. Numerous faults, many with apparent major vertical and/or horizontal displacement, occur within the study area; in such instances, cross-fault correlations were based on geometric and seismic facies similarities within seismic sequences. Two major sequences, or megasequences, can be readily identified on seismic reflection records within the Nicolas Terrane: 1) The Lower Cretaceous through Paleogene megasequence represents forearc fill deposited during the subduction phase, and 2) the Neogene to Recent, namely the lower to middle Miocene, represents subaerial and shallow- water conditions, possibly as a result of uplift due to ridge-trench collision. 109 Secondary sequences were observed within the megasequences, but these cannot be mapped regionally. Late Cretaceous through early Neogene sequences tend to be laterally discontinuous. Since sequences are best recognized on the basis of unconformably-bounding upper and lower surfaces and their lateral, conformable, correlative surfaces (Mitchum and others, 1977), the lateral discontinuities within the Late Cretaceous through early Neogene sequences may represent transitions from shallow- to deep-water environments. Middle Neogene to Recent sequences tend to be laterally discontinuous due to deposition in localized basins that were being formed during this time (Teng, 1985). 5.2 Seismic Stratigraphy of Southern Outer Borderland The southern outer borderland has been divided into two parts, an outer accretionary wedge (Patton Terrane) and an inner forearc basin (Nicolas Terrane), on the basis of apparent differences in basement and Tertiary geology (Crouch, 1981; Howell and Vedder, 1981; Vedder, 1987). These differences are observable in the characteristics of the seismic stratigraphic signatures of the two terranes: 110 5.2.1 Patton Terrane The Patton Terrane can be subdivided into three zones on the basis of a distinctive difference in geomorphology and seismic reflection character (Fig. 5.3). The western zone consists of the Patton Escarpment and Ridge, and is characterized by a lack of coherent internal reflections. The exception is the presence on several seismic reflection records of deep subparallel events representing a possible subducted slab at the base-of-slope. The central zone consists essentially of an elongate, continuous basin (Long Basin) separated midway up its length by Northeast Bank (Pliocene age). The eastern zone is comprised of a series of ridges which may be related to the Ferrelo fault zone. 5.2.1.1 Western Zone Basement rocks within the Patton Terrane do not provide good internal reflections, and tend to be seismically transparent; however, this observation is characteristic of accretionary wedge complexes, because compressional deformation during the subduction process disrupted internal stratification (i.e. von Huene and others, 1986). The features that have been imaged in active subduction margins are the subducted oceanic plate, and thrust wedges or packets of tectonically 111 Fig. 5.3. Geomorphological subdivisions of the Patton Terrane. 112 SO UTHERN C A LIFO R N IA C O N T IN E N T A L B O R D E R L A N D 0 50 100km 1 ________i ________i contours i n meters San Oiego “• Ensenada deformed sediments that are accreted to the landward side of the trench (i.e. Kawamura and Aoki, 1986; Scholl and others, 1986). Evidence for the existence of these features within the study area will be discussed in the following chapter on the structural framework of the southern outer borderland. 5.2.1.2 Central Zone The middle part of the Patton Terrane consists of an elongated, apparently continuous basin separated by Northeast Bank, a late-formed volcanic mound; however, due to the wide seismic line coverage through portions of this area, it should be noted that the middle zone might consist of a series of basins rather than one continuous basin. Based on surface sampling data, the basin fill has been interpreted by previous investigators to be no older than late Oligocene age (Crouch, 1981; Howell and Vedder, 1981; Vedder, 1987). Seismic reflection records indicate that the basin fill within the middle zone tends to be thin, of the order of 700 ms two-way time (TT) or less. Assuming an interval velocity within the late Oligocene- early Miocene of about 2000 m/sec (see chapter 4; also Moore, 1965), that translates to about 700 m or less of fill. 114 At least two, and sometimes three, seismic sequences can be identified within the southern end of the basin. A basal sequence of maximum thickness 250 ms (250 m) is overlain above a toplap truncation surface (unconformity) by a 300 ms (300 m) layer which onlaps the lower sequence in a westward direction (Figs. 5.4A and B). The lower sequence might represent the late Oligocene shelf-slope deposits described by Crouch (1981) and Vedder (1987). Stratigraphically above the middle progradational sequence, the third sequence is characterized on several records by generally weak, flat-lying, discontinuous reflections within an otherwise transparent medium (Fig. 5.4A); in other records (Fig. 5.4B), the third sequence occurs as a mounded feature overlying the low spot in the basin. In seismic sequence analysis, the external geometric character and the internal seismic facies patterns observed in the third sequence would suggest that it is showing a transition from a turbidite-fan deposit to basin fill (refer to Mitchum and others, 1977) . Below the basin fill, there tends to be a distinct difference in the seismic signature of the central versus the western portions of the Patton Terrane; whereas the western zone tends to be seismically transparent, the central zone is generally reflective. Reflection strength 115 Fig. 5.4. Three seismic sequences identified within the basin of the central zone. The uppermost sequence occurs as? A) flat-lying basin fill or B) mound-shaped characteristic of turbidite fans. VXCIemente Is. r tolls S h e p a r d Knoll 000 50 Km Terrane boundary Line segment 1 0 0(1 116 SECONDS LINE 911 mm 3900 4100 Lobated fan deposit 3 km 0 NE LINE 110 SW and continuity vary, from packets of short, discontinuous medium- to high-amplitude events to distinct, continuous high-amplitude events (see Figs. 5.4A and B). Along portions of the central zone, it might be about 440-ms thick, and possibly as thick as 700 ms (Fig. 5.4B). Unfortunately, neither the age of this seismic sequence, nor its relationship to sequences within the Nicolas Terrane, can be determined due to generally poor vertical resolution and lateral continuity. This sequence may be: 1) Late Tertiary shelf and basin fill similar to the region adjacent to Patton Basin (Crouch, 1981; Vedder, 1987)? 2) a less-deformed part of the upthrusted accretionary wedge; or 3) the western continuation of the Upper Cretaceous and Paleogene (UKP) forearc sequence. 5.2.1.3 Eastern Zone The eastern zone consists of a series of northwest- trending en echelon ridges and intervening faults of the Ferrelo fault zone; the zone has been interpreted to be the boundary between the Patton and Nicolas allochthonous terranes on the basis of differences in basement and early Tertiary lithologies (Gibson and others, 1985; Howell and others, 1987). In the northern portion of the study area, in the vicinity of the West Cortes and Tanner Basins, there does appear to be a distinct difference in 119 seismic character of the UKP, from highly continuous, strong-amplitude reflections characteristic of the Nicolas Terrane to poorly defined, weak-amplitude reflections somewhat typical of basement reflections of the Patton Terrane (Fig. 5.5A). However, towards the southern end of the borderland, forearc strata of the Nicolas Terrane can be mapped well into the Patton Terrane (Fig. 5.5B). The crest of the ridge named Outer Ridge, as well as the crests of adjacent ridges such as Southwest Bank, at the southern end of the California Borderland are flat-topped, and seismic facies characteristics of underlying reflections indicate toplap truncation (Figs. 5.6); this suggests that the southern portion of the outer borderland has been subjected to subaerial erosion, or was at least at wave-base. The amount of erosion is uncertain because the steeply dipping, laterally discontinuous reflections are not distinctive enough to identify sequence boundaries. 5.2.2 Nicolas Terrane The Nicolas Terrane is the thick, basin fill of the forearc zone, equivalent to and oftentimes defined as the southward continuation of the Great Valley Sequence of northern and central California (Howell and Vedder, 1981; 120 Fig. 5.5. The Ferrelo fault zone has been identified as a tectonostratigraphic boundary between the Patton and Nicolas Terranes. In the northern end of the study area (A), there appears to be a change in reflectivity across the fault suggesting juxtaposition of differing rock types? however, in the southern end (B), forearc strata appears to continue well into the Patton Terrane. u % Clamanta Is k Knoll 50 Km Tarrant boundary Una sagmant 121 SECONDS 1 6 0 0 1 8 0 0 2 0 0 0 2 2 0 0 Nicolas Terrane Patton Terrane 5 k m g f t - rras;:kr 1 8 &&SsmBmm SECONDS 100 200 300 400 500 6 0 0 i Patton Terrane Nicolas Terrane Ferrelo Fault ^ Zone 5 km -SW LINE 908 NE H to CO Fig. 5.6. Uplift and erosion of the southern end of the borderland resulted in toplap truncation of strata and flat-topped banks such as Outer and Southwest Banks. 124 1300 Southwest Bank Region 1400 Truncated Strata 4- 0 3km LINE 9 0 4 S i * » i BBSM m m 125 Vedder, 1987). South of the California Borderland, the forearc-type strata are believed to extend to Cedros Island and the Vizcaino Peninsula (Minch and others, 1976? Rangin and others, 1983). For descriptive purposes, the Nicolas Terrane can be divided into three zones in order to simplify description of the lithology, stratigraphy, and structures (Fig. 5.7); the division can be justified on the basis of structure and possibly stratigraphy. The western zone is dominantly a basinal zone between the ridges formed by the Ferrelo fault zone and the Santa Rosa-Cortes Anticlinorium, and may be further subdivided on the basis of difference in the seismic character of the Upper Cretaceous and Paleogene (UKP) interval. The middle zone consists of the Santa Rosa-Cortes Anticlinorium itself, and the eastern zone would be the series of basins west of the San Clemente-Santa Cruz high formed by the San Clemente fault zone. Two major seismic megasequences can be identified throughout a large portion of the Nicolas Terrane. The lower megasequence, above the acoustic basement, incorporates strata of Upper Cretaceous through Oligocene age, and is characterized by flat-lying reflections typical of a basin-fill environment (Mitchum and others, 1977). Several distinct sequences can be identified in 126 Fig. 5.7. The Nicolas Terrane can be subdivided into three zones, western, central, and eastern. The western zone may also be subdivided into a northern and a southern portion. 127 1 2 8 34° 3 3 ° Los Angeles SOUTHERN CALIFORNIA CONTINENTAL BORDERLAND O Nicolas Is. Santa Catalina Is 5 0 100km i _____i contours in meters ■tSXCIemente Is. Q San Oiego Central Zone Eastern _ m ° Western Zone Northern Portion ^ w -v Y v 'w e s te rn Zone / -n O O X Southern Portion s ■.Ensenodo ft STUDY AREA 9 3 2 ° - East Cortes Basin which can be tied to exploration wells along the Santa Rosa-Cortes Ridge, but are difficult to correlate throughout the study area due to limited seismic line coverage. The Neogene and Quaternary (NQ) megasequence consists of a lower, north-to-east progradational sequence overlying a regional unconformity at the top of the Paleogene, and an upper basin-fill sequence. 5.2.2.1 Western Zone Within the study area, the western area is defined as the synclinorium made up of a series of basins which include Tanner, Northwest Cortes, West Cortes, and Velero Basins. To the north, the western zone would also include Patton Basin (Crouch, 1981) . On the basis of differences in the reflection patterns within the UKP megasequence, the western zone can be further subdivided into a northern and a southern province. 5.2.2.1.1 Northern Province The UKP of the northern province is typically disrupted in appearance, in which internal reflections, including the base-of-fill reflection, are poorly developed and discontinuous; basins included in this disrupted province are northern West Cortes, Tanner, and 129 Patton. The UKP of the southern province contain the well developed and continuous reflections which are laterally continuous throughout the Nicolas Terrane (Fig. 5.8). The northwest edge of line 21 shows the complexity of the lithology beneath Northwest Cortes Basin (Fig. 5.8; Plate 19). The UKP megasequence is disrupted by a series of reverse faults, and internal reflective character is gradually lost northward. It is uncertain whether the lithology/sequence is equivalent to that in the undisturbed megasequence south of the fault zone or whether there is a change in lithology/stratigraphy due to strike-slip displacement. Along cross-lines 123 and 120, the reflective character of the UKP is obviously different within Northwest Cortes Basin compared to the reflectivity outside of this basin, although weak internal reflections suggest a sedimentary origin (Plates 7 and 8). In map view, the zone of disruption may be associated with; 1) an anticlinal feature which was dragged along northward within the dextral Ferrelo strike-slip fault (Plate 4), or 2) subduction-related backthrusting as the accretionary wedge was pushed into the forearc zone (i.e., Westbrook and others, 1988). Within the NQ megasequence at the north end of West Cortes Basin, at least three sequences can be mapped (Fig. 5.9). The lowermost sequence contains flat-lying 130 Fig. 5.8. Change in internal seismic reflection character, between the reflective UKP megasequence of the southern portion of western Nicolas Terrane and the chaotic reflections of the northern portion of the terrane. Clemente Is. Bleke Knolla Terrene boundary Line segment 131 SECONDS 1 3 0 0 Southern Zone Nothern Zone I ! l < i ) . Fig. 5.9. At least three sequences can be identified within the northern Nicolas Terrane. plemente Is. Blake Knolls Terrane boundary Lins segment 133 SECONDS 100 200 3 0 0 2 - 3 - 4 - 5 - West Cortes Basin 0 3 k m • —■ ■ ■ — ^ * -* NE LINE 9 0 3 SW m t internal reflections characteristic of a basin fill deposit and appears to thicken westward. The top of this sequence appears to be an erosion surface; reflections at the top of the sequence terminate in the form of toplap truncation (Mitchum and others, 1977). The middle sequence in northern West Cortes Basin appears as a progradational sequence which is thickening from east to west over the underlying toplap truncation surface; east-verging reverse faults cutting through the lower sequence suggest that the middle sequence might be due to uplift of a portion of the adjoining Patton Terrane. The topmost sequence lies conformably over a high- amplitude, continuous reflection which might represent a hiatal surface, although underlying strata does not suggest truncation. This upper sequence appears to be a basin-fill, drape-type sequence; internal reflections tend to be flat and relatively continuous, and parallel the underlying hiatal (?) surface. 5.2.2.1.2 Southern Province Unlike the northern province, seismic facies within the UKP megasequence of the southern province are characteristic of the Nicolas Terrane. On seismic line 21, the transition to the Nicolas-type megasequence 135 occurs south of the zone of disruption in the west- central part of the basin (Fig. 5.8). The Nicolas-type UKP megasequence can be mapped to the southern end of the study area (Plate 2); however, sequences within this megasequence cannot be correlated beyond the southern portion of West Cortes Basin due to the lack of seismic line coverage. Three sequences are identified within the UKP megasequence (Fig. 5.10). The lower sequence appears as a prograding (?) wedge which tapers out to the south and east; to the north, this wedge is caught up in the zone of deformation in the west-central part of the basin. The maximum width of this sequence is about 150 ms. Overlying the lower sequence is a thick sequence of about 900 ms, which contains high-amplitude, continuous internal reflections in the central part of the basin adjacent to the disrupted zone, but discontinuous reflections southward. Weak, downlapping reflections within the lower part of this sequence indicate that the lower portion might consist of progradational facies. The top of this sequence is more reflective adjacent to the disturbed zone in the middle of the basin; however, amplitude strength along this reflection decreases southward. The seismic character might indicate a lateral change from an environment of nondeposition to deep- 136 Fig. 5.10. Three seismic sequences can be identified within the UKP megasequence within the southern west Nicolas Terrane. Clemente Is. Terrane boundary Lina segment I I 1 137 SECONDS 700 800 900 1000 West Cortes Basin 5 km w 09 LINE 21 SE — . SliS ' " ' ' « ' r a ww,mw« I"***1 ' r. T v . v ; v.«ww«w,;^ 232353904823482348535353484848535348534848485353534853534823022301230123482348232353232348534848484823232348235348484823235323012301230048232353485323020202485348535353535323482348230153020101534823234823234800530023235353235323532353010202 0201022301534823482348234823484823534801482323002301230123235323905353534848 water, continuous deposition; the geometry and internal reflection character of the sequence above this reflector suggests that it might be a basin deposit. Correlation of this sequence to the exploratory wells, coupled with its progradational nature, suggests that it lies within the Upper Cretaceous to Oligocene interval. If so, interval velocities as determined from well logs would be approximately 3.0 km/sec, which would give a maximum thickness of about 1350 m. Sequence number three is the uppermost sequence identifiable within the UKP megasequence, and appears to be a basin-fill sequence; reflections are discontinuous to continuous, but parallels the basal sequence boundary. This sequence is thinnest over an anticlinal feature in the middle of West Cortes Bank (Fig. 5.10) and thickens to a maximum of about 800 ms in the southcentral portion of West Cortes Basin. From exploratory wells along Santa Rosa-Cortes Ridge, it is known that the upper portion of the UKP megasequence is Oligocene age. Interval velocities for Oligocene-age strata tend to be between 2.0 to 2.5 km/sec; assuming an interval velocity of 2.0 km/sec, thickness of sequence number three would vary from about 600 to 800 m. 139 Paleontologic data suggest that Oligocene strata within the study area progressively changed from shallow water to deep water from the northern end of the ridge to the vicinity of Shepard Knoll (Arnal, 1976). Based on seismic facies character as well as paleodepths determined from exploratory wells (see Fig. 3.5), the portion of the Oligocene sequence in the western zone would appear to be a relatively deep-water facies. The total thickness of the UKP megasequence is relatively constant within the western zone, varying in thickness from about 1400 ms in the area of northern West Cortes Basin to 800 ms in Velero Basin. However, this relationship changes considerably towards the southeastern edge of the southern outer borderland; here the UKP has been uplifted and abruptly truncated over a relatively short distance (Fig. 5.11). Within the southern portion, the NQ megasequence overlies a strong, high-amplitude zone which correlates to an Oligocene or early Miocene volcanic interval present in exploratory wells of Cortes Bank. Up to about 700 m of volcanics (at 4.5 km/sec interval velocity) is interpreted to be present in this interval. Three NQ sequences can be identified within the megasequence. Internal reflections in the basal sequence in the central portion of West Cortes Basin consist of 140 Fig. 5.11. Uplift and truncation of the UKP megasequence along the southern edge of west and central Nicolas Terrane. .Clemente Is. Terrane boundary Line segment 141 SECONDS 400 500 600 700 1 i i i Velero Basin 'Truncated Paleogene Strata flat-lying reflections typical of deep-water, basinal deposits; however, at the southern end of West Cortes Basin, and extending south to include Velero Basin, the reflections appear to prograde from southwest to northeast (Fig. 5.12 and Plate 3). Measured thicknesses of the basal sequence range from about 200 m in West Cortes Basin to greater than 500 m along the northwest side of Velero Basin. The top of the lower sequence (sequence 2) is an unconformity or hiatal surface. Strata of the overlying sequence downlap as well as fill irregularities within this surface. The overlying sequence is characterized by localized, bi-directional progradational facies, from sources off Shepard Knoll and Cortes Bank into southern West Cortes Basin (Fig. 5.12B), and may correspond to sequence 2 in the northern portion of the western zone. In the vicinity of Shepard Knoll, a lateral change in this subsequence from progradational, to transparent, to reflection-poor, yet diffraction-rich, facies is interpreted to represent a possible debris-flow regime (transparent mass) grading laterally to turbidity-flow regime (the progradational facies). The initiation of localized deposition within the southern portion probably corresponds to localized 143 ir Fig. 5.12. Neogene prograding reflections overlying the unconformity at the top of the Paleogene: A) Line 904 is a dip line; B) line 21 is a strike line. k Clemente Is. Terrane boundary Lina sagmant I 144 145 Ferrelo Fault Zone Velero Basin Base of Neogene Prograding Sequence LINE 904 SW j iijiijilii liP 4 f » G S £ 2 E ^ M M i 99999999999999999699999999999999999999999999999999999999999999999999999999999 SECONDS 500 400 300 200 100 Prograding Reflections Chaotic Reflections LINE 21 [ g j ! f f S ! f SggB!S»* / -ws; o i;,~ RM88SSK S ^§^«gi f c S f l S s e g f t S ! deposition within the northern portion of the western zone as well as the eastern part of the Nicolas Terrane such as San Nicolas and East Cortes Basins, and indicates the time of formation of the modern borderland basins. Unfortunately, a direct correlation cannot be made due to the inability to map Miocene events across the Santa Rosa-Cortes Ridge. However, correlation of local deposition in the eastern Nicolas Terrane to exploratory wells suggests a time of late middle Miocene or younger. 5•2•2•2 Central Zone; Santa Rosa-Cortes Anticlinorium The series of ridges and banks of the Santa Rosa-Cortes Anticlinorium make up the central zone. Identification and correlation of seismic sequences along the ridge benefited from lithologic descriptions and sonic velocity logs from exploratory wells; however, only three of the wells (75-70-1, P-286, and P-257) could be correlated to seismic reflection records because most of the records do not cross the well locations. The depth time conversions are provided in appendix 1? correlation of the sonic times to seismic line 115 are provided in Plate 17. The deepest primary reflection that can be identified is a distinct but laterally discontinuous event identified at a seismic time of about 3.0 sec in 147 the vicinity of COST well 75-70-1 (Fig. 5.13)? it cannot be mapped very far northwest of well 75-70-1. This reflection is about 50 to 100 ms (112 to 225 m at 4500 m/sec) beneath total depth (TD)? based on interpretation of magnetometer data in the vicinity, basement was inferred to be at a depth of 330 m (about 150 ms at 4500 m/sec) below TD (Paul and others, 1975). The difference of about 100 m (50 ms) between the reflection on the seismic reflection record and magnetometer data fall within the range of errors expected from navigation, location of the well on geophysical records, depth-time conversion of the sonic log, as well as accuracy of the magnetometer interpretation. Four seismic sequences can be identified within the UKP megasequence? however, with the exception of the uppermost Oligocene sequence, the sequences cannot be mapped throughout the southern outer borderland. The difficulties include: 1) Seismic-line spacing that is too wide to interpolate reflections, 2) the presence of high-amplitude, shallow-water multiples along the Santa Rosa-Cortes Ridge which mask primary reflections, and 3) the presence of intrusive volcanic rocks, which appear to be more prevalent towards the northern end of Cortes and Tanner Banks. 148 Fig. 5.13. Seismic sequences are correlated to subsurface geology by means of sonic logs along the Santa Rosa-Cortes Ridge: Al, lithified Upper Cretaceous; A2, Late Cretaceous through lower middle Eocene; A3; upper middle Eocene through Oligocene; Bl, lower Miocene basalt flow; B2, Miocene to Recent undifferentiated. Santa Catalina Is. San Nicolas Is. S a n - V > Clemente Is. Blake'* Knolls Shepard Knoll Terrane boundary Lin* segment 149 Mid. Miocene OligPor Lower Miocene Volcanics / IOOO 1200 _______ /Lower Miocene 75-70-1 200 400 600 Oligocene- 2 Fronciscon 8osemeQ i 3 4 5 km SE — 1 — 1 — 1 — 1 LINE 115 NW 111 1 For the most part, seismic sequences can be identified by a noticeable change in interval velocities at sequence boundaries. The interval velocities can be correlated to interval velocities derived from the sonic logs (Fig. 5.14). Ages of specific velocity intervals in the wells were determined by microfossil zonations as determined by personnel of the exploration companies who owned the wells. The lower sequence is a Late Cretaceous, slightly metamorphosed, lithified section characterized by high interval velocities in excess of 4500 m/sec. It is separated from the overlying, less lithified Late Cretaceous through lower middle Eocene sandstones and siltstones by an abrupt interval-velocity change; the overlying sequence is characterized on sonic logs by increasing interval velocities with depth, from 3000 to 4000 m/sec (see Fig. 5.14). Velocities within the upper middle Eocene through Oligocene lies within the 2000 m/sec range, with no obvious velocity trends; however, there is a sequence boundary at the base of the Oligocene which can be identified within West and East Cortes Basins. The UKP megasequence is capped by a basalt sequence, mapped as a high-amplitude, continuous reflection separating the UKP megasequence from the NQ megasequence 151 I I Fig. 5.14. Correlation of sonic velocities with age of strata along the Santa Rosa-Cortes Ridge. Santa C a ta lin . San Nicolas Is. ■mente Is. Blake^ Knolls fi. i — I Shepard Knoll Terrane boundary Line segment I 152 SEC O N D S (two-way time) O.O-i 75-70-1 P - 286 P-276 P-262 P-257 H U1 1.0 - 2.0 - MJdiocene E. Miocene .L ,E. Miocene •' O' O ' Volcanics .0' .V □ vo lcan o s .o.-K- - _E u.o— y Qi„ ' 1 Vo Iconics L ? • 1 — Diabases M O m O-’ ’ I (?) ’ *••• E. Miocene Miocene n* / / -o Oligocene or E. Miocene fOhgoeene U. Eocene OHgocene U.Eocene M. Eocene Paleocene U. Eocene ZZlVolcanics p- _ Oligocene / / o / / / |M. Eocene M.Eocene L satb aseE o c Paleocene U.Cretaceous uU. Cretaceous (M Q S ,(Cmp) (Cen) Metamorphic Basement (?) U. Eocene / M.Eocene Eocene Paleocene Paleocene Eocene Paleocene conglomerate U.Cretaceous unconformity Diabase 'vSOm Basement (?) i > i i l I 5 I i i i I j i i I I I I L I— J ------I L VELOCITY (km/sec) throughout the southern outer borderland. This unit measures as thick as 300 m along the Santa Rosa-Cortes Ridge. The age of this layer is uncertain; K/Ar dates indicate an age of about 35 Ma, whereas microfossil dates indicate an age of around 24 Ma (Paul and others, 1976; Vedder; 1987). Weathered basalt fragments from well 75-70-1 recovered at the top of this unit indicate that it is an erosional surface. Furthermore, mapping this surface from seismic reflection records of the southeastern edge of the borderland show channeling features, as well as erosional truncation of underlying strata. Unconformably overlying the basalt unit is the NQ megasequence, which consists of early and middle Miocene, lobated and progradational reflections typical of shelf- slope or fan deposits (Mitchum and others, 1977). Interval velocities within the NQ megasequence are generally in the mid-2000 m/sec range for the lower Miocene, and mid-1500 m/sec range for the middle Miocene. The progradational facies thin northeast into the eastern zone of the Nicolas Terrane; however, the total thickness of the NQ megasequence increases due to the buildup of younger (late Miocene (?) through Quaternary) sediments in the modern borderland basins. Towards the 154 western edge of the Santa Rosa-Cortes Ridge, the megasequence thins due to erosion or non-deposition. 5.2.2.3 Eastern Zones San Nicolas-San Clemente Basins The eastern area is the synclinorium between the Santa Rosa-Cortes Ridge and the Santa Cruz-San Clemente Ridge, and consists of the series of basins separated by the Santo Tomas-Blake Knolls. Within this zone, the UKP megasequence thins north and east, up to a major east- west-trending fault at the northern margin of San Nicolas Basin? north of this fault, the UKP megasequence is considerably thicker (Fig. 5.15). This megasequence appears to lap out to the east, although appearance of toplap truncation at the eastern edge of the megasequence suggests that it has been uplifted and eroded along the Santa Cruz-San Clemente Ridge (Fig. 5.16). Towards the south, the UKP megasequence thins as a result of toplap erosion, and is finally truncated along a north-south erosion edge extending from Blake to Shepard Knolls (Fig. 5.17, Plate 2). The location of truncation lies within Vedder's (1987) Catalina-Nicolas Terrane boundary and follows a zone of approximately north-south extensional faults (Plate 4). Across several east-west-trending faults in the eastern zone, it is noted that thickness is not 155 Fig. 5.15. In the eastern zone of the Nicolas Terrane, the UKP megasequence thins to the north and east up to a major fault at the southern edge of the San Nicolas Ridge, then thickens again north of the fault. Santa Catalina Is. San Nicolas Is. San Clemente Is. Blake- 5 Knolls Shepard Knoll Terrane boundary Line segment 156 I 2 3 4 5 §LINE 109 m m > v ] 4400 4600 4800 5000 5200 5400 Base of Neogene San Nicolas Basin 5 km Base of Neogene MU/ QF SECONDS Fig. 5.16. Although the UKP megasequence appear to lap out towards the eastern edge of the Nicolas Terrane, appearance of toplap truncation indicates that there has been uplift and erosion along the Santa Cruz-San Clemente i Ridge. i i Santa Catalina Is. San Nicolas Is.' San Clemente Is. Blake' Knolls Shepard Knoll Terrane boundary Line segment 1 59 1800 2 0 0 0 2200 2400 2600 San Clemente Ridge San Nicolas Basin NQ Erosion Surface ^ (Middle Miocene?) NQ Erosion Surface 5km Top UKP Truncation Surface SW LINE 116 - y . * * > • » • * . - ■&&&&&!• ^ n r: ; r ‘ agB5j»gS%-<3!^3gg33^ggi'«Si^8g^^ gagggasffggg T S ^ T SS^ssgasssgc^aasgfr^^ - * & S & & ! S S 3 £ S » 4 2 & & i 8 t & & S ^ m f U W m m X ^ ^ «S5S8^e«SB«aKSS£28SK^A^*i®ffi«»^^ —2 — 3 — 4 — 5 SECONDS Fig. 5.17. The UKP megasequence is eroded and abruptly truncated along a north-south trend in the southern edge of the borderland: A) Truncation of UKP megasequence at Blake Knolls? B) truncation of UKP megasequence at Shepard Knoll. r Clemente k B la k e 4 Terrane boundary segment r~ 82 0 0 8 4 0 0 8600 88 0 0 Blake Knoll 9 0 0 0 9 2 0 0 NQ UKP UKP Truncation 5km — 2 - 3 — 4 NW LINE 109 H 0\ H - 7 T 5 W ^ " . f r * WWmryn W g « g g j 8 f aafegssg i c S ^ c : a a a s a yws g b s . ^-- w w ^ W : W & 0 ® x^SSSSS&S . a m i • ^ c , * . * - r ^ YV. . ~>>>?g- SECONDS SECONDS 4 0 0 0 4 2 0 0 4 4 0 0 4 6 0 0 4 8 0 0 5 0 0 0 1 1 i i i i Shepard Knoll NQ UKP Truncation 2 KP 3 - 5km * ■ ■ » - i SW LINE 112 NE (B) ON to ( consistent within the UKP megasequence. One such fault is that at the north end of San Nicolas Basin; another is at the south end of the basin, and thickness change is also noted along a major fault along the northern edge of Shepard Knoll. This suggests that major strike-slip motion has occurred along these faults. Unconformably overlying the UKP megasequence are downlapping reflections of the lower sequence of the NQ megasequence. Up to three subsequences representing regressive-transgressive stages have been noticed within the lower sequence, a major stage in early Miocene time and two progressively smaller stages in about middle Miocene or later time (Fig. 5.18). Above the youngest downlapping subsequence, the initiation of basin filling began for upper East Cortes Basin. This is signified by onlapping of overlying reflections against the downlapping reflections on the west side of the basin, and thickening of the flat-lying basinal-type reflections towards the eastern edge of the basin. Seismic facies within the basin fill suggest at least three changes in source areas or mode of deposition; the basal and top subsequences appear to be drape deposits, whereas the middle section appears to be a lobated, possibly turbidite-fan, deposit. Correlating the stratigraphy to well 75-70-1 on Cortes Bank indicates that the timing of 163 Fig. 5.18. Changeover from progradational to flat-lying, onlapping reflections may indicate timing of local basin formation in East Cortes Basin. Three subsequences can be identified in the progradational (PI to P3) as well as the basin (B1 to B3) sequences. Ciemente^fs. Blake Knoll* Terrane boundary Line segment 164 6000 6200 6400 East Cortes Basin — 2 — 3 5km * ■ ■ ■ i-± i . * NE LINE 120 SW 1 L . . _ . . . ’ , _ L _ ^ " ' f t M i m m u K f f l M'fl'irT/n^r * T i ” 'u rn^rFi iTtTI^Tv”^ 1. "TVi : i'rVi( h“\T': > T,^^.' v^''7 • M r >> m - .... r'r?'-t y.... . ' • ■ -^s I i 165 SECONDS basin formation is later than middle Miocene, perhaps late middle Miocene, perhaps even younger (Teng and Gorsline, 1989). In San Nicolas Basin, downlapping reflections of the lower sequence gradually levels out to the northeast, until they become flat-lying, basin-fill-type reflections. The top of the lower sequence is an unconformity characterized in places by a deeply incised erosion surface (see Fig. 5.15). On top of this surface, downlapping reflections indicate a change in depositional source areas, from areas to the north and east as well as the west, and probably represent the initiation of localized deposition. The data in San Nicolas Basin, like other basins in the southern outer borderland, support Teng's (1985) and Teng and Gorsline's (1989) contention that the formation of the modern borderland basins occurred much later than previously thought. 166 Chapter VI STRUCTURAL FRAMEWORK OF THE SOUTHERN OUTER BORDERLAND The California Continental Borderland can be divided into two major crustal domains (Howell and others, 1987? Teng, 1985). The east-west-trending Transverse Ranges domain constitutes the northern end of the borderland? it is interpreted to include the Malibu and Stanley Mountains allochthonous terranes (Gibson and others, 1985). Each terrane is bounded by faults, and characterized by a specific stratigraphy and a distinct structural history (Coney and others, 1980? Howell and others, 1987). The southern half of the borderland consists of the dominantly northwest-trending structures of the Peninsular Ranges allochthon, although it also includes the east-west-trending Santa Monica Mountains, now considered a part of the Malibu terrane (Gibson and others, 1985? Howell and others, 1987? Lund and Bottjer, 1991). This allochthon has been further subdivided into smaller tectonostratigraphic terranes which amalgamated to the allochthon as far back as the Early Mesozoic 167 (Gibson and others, 1985; Fig. 10-1 in Howell and others, 1987). Prior to late Oligocene ridge-trench collision, the tectonic style within the borderland was subduction. This process resulted in the formation of two distinct lithologic regions, an accretionary wedge of tectonic melange, including greywacke, argillite, blueschist, serpentine, and volcanic rocks, and a forearc zone consisting of layered basin sediments. Such a relationship is observed in the lithologies and inferred from seismic reflection records. Since Early Neogene, collision of the mid-ocean ridge with the Mesozoic-Early Tertiary trench, followed by right-slip translation, rotation, and compression of large portions of the borderland, have disrupted the accretionary wedge-forearc relationship; also, the formation of borderland basins have masked much of the pre-Neogene structural fabric (Atwater, 1970, 1989; Crouch, 1979; Hornafius and others, 1986; Howell, 1976; Howell and Vedder, 1987; Teng, 1985; Vedder, 1987). However, deep-penetration seismic reflection records acquired in 1990 and reprocessed late-1970's seismic reflection records, supplemented with surface and subsurface lithologic descriptions, were able to define several major pre-Neogene structural relationships. 168 The following sections of this chapter will; 1) describe the methods used in identifying structures and defining structural trends, 2) describe pre-Neogene structural features within the southern outer borderland, which encompasses the southern portion of the Patton and Nicolas Terranes, and 3) describe Neogene and Quaternary structural features for this region. The objectives are to determine the magnitude of disruption of the pre- Neogene accretionary wedge-forearc, and the nature of the structural discontinuity (the Ferrelo fault zone) between the Patton and Nicolas Terranes which identifies the terranes as allochthonous. 6.1 Methods The nature and trend of structures within the southern outer Continental Borderland were determined by single- and multifold seismic reflection records supplemented with GLORIA digital side-scan data. Faults are best observed on seismic reflection records where they displace strata? seismic reflection records can resolve the type of vertical faulting, i.e. normal or reverse, but any lateral offsets can only be certain if correlative strata on either side of a fault exhibit mismatches in thickness or if there is abrupt termination of strata juxtaposed against acoustic 169 basement. Otherwise, lateral offsets can only be inferred from identification of geologic structures oftentimes associated with strike-slip faults, namely positive and negative flower structures (Harding et al, 1985). In large portions of the outer borderland, igneous or metamorphic rocks crop out on or very close to the seafloor (Vedder, 1987). In such areas, seismic reflection penetration is limited, and generally faults cannot be detected in the subsurface. Surface evidence of faulting, such as seafloor morphology (i.e. scarps or horst and graben features) and bathymetric trends, must be used to substantiate the interpretation. It should be cautioned that although seismic reflection data coverage is the most extensive for this portion of the borderland (many lines within 10 km apart), line spacing can be as far apart as 20 km; fault segments mapped this far apart may not necessarily connect, nor are the fault orientations necessarily correct. Obviously the highest degrees of confidence are assigned to those faults whose line spacings are within 10 km apart; however, it should be noted that fault segments whose line spacing were greater than 20 km apart were matched by the author on the basis of published GLORIA data (EEZ-SCAN 84 Scientific Staff,1986), bathymetric and structural trends, similar structural 170 and/or deformational style, and previous mappings (i.e. Howell and Vedder, 1981? Vedder, 1987). For descriptive purposes, the Patton Terrane can be divided into three distinct zones; from west to east, these are; 1) the generally high-standing Patton Ridge area, 2) a central zone of basins which includes Long Basin, and 3) a series of ridges along the eastern edge of the Terrane perhaps associated with the Ferrelo fault. Similarly, the Nicolas Terrane can be divided into a western series of basins such as Tanner, West Cortes, and Velero, a central Santa Rosa-Cortes Ridge, and an eastern series of basins such as Santa Cruz, San Nicolas, and San Clemente. 6*2 Pre-Neogene Structures Prior to Neogene time, the California Continental Borderland was a convergent margin, which consisted of an active trench, accretionary wedge, forearc, and arc. Within the study area, an accretionary wedge (Patton Terrane) and forearc (Nicolas Terrane) can be identified. In the Patton Terrane, the major structural feature imaged on seismic reflection records appears to be one or more landward-dipping, high-amplitude, continuous reflection(s) at the base of slope, which is interpreted to be the remnant subducted slab and associated thrust 171 faults. This relationship is better observed within the southern portion of the study area (Figs. 6.1A and B), where the reflection(s) can be followed landward to about the break in slope of the Patton Escarpment. Seismic reflection configuration at the base-of- slope on CALCRUST-reprocessed line 902 provided the first indication to the author that the extinct subduction zone was still present. A strong, relatively continuous, high- amplitude reflection can be observed continuing from the base of the seafloor sedimentary layer to well beneath the reflection-poor accretionary wedge complex (Fig. 6.2). South of line 902, line 124 shows a strong set of reflections at the lower Patton Escarpment which could be followed about 12 km eastwards (to about shot 600) ; east of this point, numerous diffractions may be related to a mounded seafloor expression at a change in relief (Fig. 6.3). Seafloor sampling indicates that this feature is volcanic (Kennedy and others, 1987; Vedder and others, 1979)? other mound-like features can also be observed around Northeast Bank, which is composed of Miocene- (?) and Pliocene-age basalt (Hawkins and others, 1971; Vedder and others, 1981), and cutting through what appears to be small sedimentary basins (Plates 6 and 7). Possibly the 172 Fig. 6.1. Strong reflections at the base of the Patton accretionary wedge suggest the presence of the remnant subducted slab: A) CALCRUST-reprocessed line 904; B) line 110. Clemente Is Blake Knolls 50 Km 1000- ^ Terrane boundary Line segment 173 SECONDS 100 200 i 300 i 400 i 4 Disrupted basin fill 5 Remnant slab Oceanic crust 5km LINE 9 0 4 vj f e l l W ^ M I H i f i a M S f c W f l S M U l . . . . . ... ... .. 5D55-5555555555555555555 4700 4900 Thrust (?) Remnant slab LINE 110 I ' r * : 1 T"; ' f - El Remnant slab SECONDS Fig. 6.2. Reprocessed CALCRUST line showing presence remnant subducted slab beneath the Patton Escarpment Clemente Is. Terrane boundary Line segment L L T SECONDS 100 200 4- Patton Escarpment Thrust faults ? 5 — Accretionary prism, — Remnant slab — disrupted sediments _lt\ E 9 0 2 iSSSSsssaWSWeeks ? ? ? * « * £ I n h MffiSjKKiiiiiPW r l i m p n t iDisrupted sediments “ 1 i ; Figure 6.3. Remnant subducted slab can be followed eastwards on line 124 up to a zone of diffractions which ; may be related to volcanic rocks dredged from the 1 overlying seafloor. .Clemente Is. Blake Knolle Terrane boundary Line segment SECONDS 200 400 2 — 600 800 3 basalt 4 5 Remnant slab 6 5km LINE 124 i . T j ! I nwsiiHteft - < # s w i f e i ^ T O j s a g ^ j g L 3 2 S 2 t 2 £ : * ■ ^^^jtrSPWiaJ ^ a s ^ L ^ w i s ^ . g S H r t i i M f t M g y f c ? ! ? ' S i l t M m & s r f f e W ^ f r tei^ v j j r i g > i . ! ! » f , - ^ K f c r . ; Remnant slab wrmMSjmrlfiWi ; S S ? 2 ” » ^ £ i 179 volcanics are using faults as conduits in some of the fault-bounded basins, in the manner of leaky transforms (Legg, 1991). Internal reflections within the accretionary wedge is indeterminable on line 120 except at the base of the escarpment (Plate 8). Here, at a level of about 400 ms beneath the base of the sedimentary section, there is a bright reflection which appears to rise upwards toward the escarpment; however, it cannot be followed into the escarpment, possibly due to the presence of several major (?) faults at the base of the escarpment which may affect continuity. Reprocessed line 904 distinctly emphasizes the internal nature of the accretionary wedge; its appearance is similar to accretionary wedges of subduction margins along the Pacific rim (Fig. 6.1A, Plate 13). High- amplitude, continuous events of the basal thrust, as well as overlying thrust packets or imbricate thrusts, can be followed partway beneath the escarpment; eventual loss of signal can be attributed to signal attenuation beneath a continuously thicker wedge and/or possible disruption due to faulting and/or magmatic intrusion (i.e. Fig. 6.3). A feature to emphasize on line 904 is the disrupted, slightly folded sedimentary fill in front of the accretionary wedge, and the apparently dipping nature of 180 the oceanic crust directly beneath this disrupted zone. It is possible, under the present transpressive plate- tectonic regime, that along more east-west-facing portions of the continental slope, underthrusting might be occurring. At the base-of-slope on line 110, there is a high- amplitude crustal event at a time of 5700 ms which can be projected along a series of weaker, discontinuous but coherent reflections as far eastwards as the slope break. Above this event, reflections which are interpreted to represent imbricate thrust packets are observed. The apparent upbowing of the crustal reflections beneath the accretionary wedge observed along the seismic lines is not structural, but is due to the fact that seismic reflection data are recorded in time (not depth). The accretionary wedge is a high-velocity layer compared to the water layer, and as the accretionary wedge rapidly thickens laterally, the travel time in the wedge is less than the travel time in an equivalent water layer (Fig. 6.4A). As a result, a velocity pull-up occurs. Conversion of the seismic reflection record to a depth section substantially flattens the crustal reflections (Figs. 6.5A and B). The central zone of the Patton Terrane is composed of fault-bounded basins that are filled by sediments no 181 Fig. 6.4. Upward arch in the downgoing slab at the toe of the accretionary wedge on seismic reflection records is an artifact because the seismic reflection data are recorded in time. 182 SECONDS SECONDS LINE 124 3km LINE 9 0 4 4 5 3 km 6 LINE no 2 3 4 5 3km 6 183 Fig. 6.5. Conversion of seismic sections from time to depth flattens the dip of the remnant downgoing slab: A) Time-depth converted seismic reflection record; B) Manually time-depth converted line drawings of seismic sections 124, 904, and 110. i i i 184 DEPTH (KILOMETERS) sw 5000 i LINE 110 4600 i NE Water (v: 1.47 km/sec) Accretionary wedge (v: Top=3 km/sec) (v: Base=4 km/sec)^ - ---------- V J - / WJi' kA H 00 < J 1 . y ; ' . . , : yv ^ ?!yfry< -.. Vy^ £ rT / r^ H > . ■ • ' , ^ .yyiit-y!^ Sediments (v: 2 km/sec) * * ' • - » ■ * ^ * > * - / - ' I ^ t /.1 * L . '" 1 * 1 > 1 I *7 'T^ • “ i nyi , T ^ i ^ * t A y y ^ r Imbricate thrust (?) rr£ • ^ - ■ : ■ ' y : ^rass ■ - Vv • fPT^ h ^ i ^ . ■ . i Hm j r ^ V y v >XvVvi: V/-.V* v 1 *^/ f V-.VI » 7 - ^ - ' ^’ - • - • y ' ■ vi o uj^ M^V * \ i \ * a V r * ‘ ' Vj^ J j A f r / * ■ > « , ; A* » , j <' : y1 '■ ;‘‘ ^ ’nxV''/1 . ’ ,;V^-V //- ; ^ t - i- V ^- , » y > Vr-TT-V ^ j A<,V. - r /,0 -M.^y. *■ * . * . . » > » > « . . 'v. . . ’ ■:w.l< ( » u. ■ * * . . 1 : . ' . j ? . . - » : l » - . /■ » a vc , i^X« Remnant Slab5= w - * - • - ' ■ * - - » * * - - ' | , A: vv A % y • - * • ' • . ^ . v ^ • » s . / • ^ y « ^ . . _ K i i . ' i N . - . > ■ . . a > ■ . ■ > . . . > v » , . . * - s i ( crust ( v* 5 k m / s e c ) ?•'^ ^ <ftV X^Y./ ■ ' ^ * V ^ *4 ~TX ’> *Y1 > ~V»V t J ^ ^■X-S V^V«, " * ■ * • * 4 4 I 1 T'* I 'V4' ■ ’ ’ ■ *^'1 ' i - ' ' ’ *' > ' - J l * r* ' r ' ^ ’ . j i ' 4 4 *^ V4 i 4 //> » - l i ^r' r• : . ' - ^v, V>y\XVs.^ T -.v^. v * ! "•^ .>^^,-..-f .; > r r \v ^ v ^ v ,,C \7 ^ v ? ^v < c v y .- • ,y, y *\<*r* ^ v < ‘>5,';. ..y^ ^ ■ ;... ■ ■ • • • ■ ■ ■ X ,• • ^. » ;, ■ - yyi SvyAy--»v * T . * ? 5 J ^ ? i - S i r , 8 sw - NE LINE 124 2 - 4- 2 7- LINE 904 u j O _ i x i — C L L U Q LINE 110 4- 5 6 5 km (B) 186 older than Oligocene age (Crouch, 1981; Howell and Vedder, 1981). Beneath the basin fill, seismic reflection records exhibit a change in reflectivity, from chaotic to generally parallel, continuous reflections; the data indicate a difference in lithology, suggesting that the central Patton Terrane might be different from that of Patton Ridge (see Fig. 5.4). The nature and orientation of the contact between these possibly contrasting lithologies cannot be determined with certainty due to the limited seismic coverage, but it appears to be irregular. Fault patterns within the Patton Terrane dominantly trend north-northwest, parallel to post- Neogene fault patterns. Within the region of the eastern Patton Terrane- western Nicolas Terrane, the boundary between accretionary wedge lithologies and forearc fill was thought to be an abrupt contact, on the assumption that the Patton and Nicolas Terranes represent two terranes allochthonous to each other (Gibson and others, 1985; Howell and Vedder, 1981; Sedlock and Hamilton, 1991). Crouch (1981) and Vedder (1987) proposed that the series of ridges at the eastern edge of the Patton Terrane are associated with an en echelon style of strike-slip faulting (Wilcox and others, 1973). The present study suggests that there is no apparent sharp boundary 187 separating the rocks of the Patton and Nicolas Terranes but there is a broad zone in which the two terranes overlap. In the southern portion of the borderland, the total width of the Patton Terrane appears to be very narrow, less than 35 km wide? strata of the Nicolas Terrane extend into the Patton Terrane well west of the ”terrane boundary” (Fig. 6.6). The strata of the Nicolas Terrane appear to be uplifted along a series of high-angle vertical and reverse faults associated with the Ferrelo fault system. Towards the northern end of the Velero Basin, toplap truncation of reflections suggests erosion of strata of probably the UKP megasequence (Fig. 6.7). This interpretation is verified by the presence of Cretaceous and Eocene strata on the slope of the east edge of Tanner and West Cortes Basins respectively. East of the Ferrelo fault zone, discontinuity of internal reflections, especially within the lower portion of the UKP megasequence, suggests disruption of strata (Figs. 6.6A-D). More continuous, higher-amplitude reflections which appear to cut across stratal reflections may be low-angle thrust faults, possibly backthrusts associated with an arcward-migrating accretionary prism. In the northern portion of the study area, in the vicinity of north West Cortes and Tanner Basin, there is 188 Fig. 6.6. The Ferrelo fault zone may not be a major tectonostratigraphic boundary between the Patton and Nicolas Terranes; several seismic lines suggest the strata of the forearc continue west beyond the 'terrane boundary': (A and B) Line 110; (C and D) line 908. .Clemente Is. B l a k e Knoll* Terrane boundary Line segment 189 NE SW 2300 2500 2700 2900 3100 3300 I - K-Ferrelo Fault Zone-H Velero Basin Reversed faults Thrust faults (?) Patton Terrane -► ■Nicolas Terrane 5km (A) _ i____ i--------1 NE LINE IIP_____________________________ sw - * 4 * ' « ' r SECONDS 100 2 00 400 500 300 Nicolas Terrane Patton Terrane Top UKP Ferrelo Fault Zone Velero Basin Reverse fault 5km Top UKP Thrust fault Disrupted strata (C) 193 I I Fig. 6.7. Erosion surface at the base of the Neogene (?), and toplap truncation of probably UKP megasequence, indicate rapid uplift and erosion of the outer borderland at the end of the Paleogene or beginning of the Neogene. To the north, along the slope adjacent to West Cortes Basin and the ridge adjacent to Tanner Basin, strata of Cretaceous and Eocene age have been recovered in dart-cores. Clemente Is. B l a k e K n o R a S O Km 1 90A Terrane boundary Line segment 194 SECONDS 1300 Southwest Bank Region 1400 Truncated Strata UKP(?) Truncated strata 4- 0 3km I.---- LINE 904 a djgsgajU i S I W<W f f i ) W t »f r i g mm s§ssw WM 195 possible thinning of the forearc deposit against the Patton Terrane (Fig. 6.8). However, the quality of the seismic data is not good enough to resolve how much of the thinning might be due to toplap truncation. In the area to the north of West Cortes Basin, adjacent to Cortes Bank, there is a change in the vertical-offset relationship between the Patton and Nicolas Terranes. Whereas in the region south of West Cortes Basin, the forearc fill appears to be uplifted relative to the accretionary wedge, the region north of West Cortes Basin appears to be thrusted over the forearc fill, probably during Neogene formation of the northern West Cortes and Tanner Basins (Compare Figs. 6.6A-D and 6.8) . Compared to the good continuity of reflections in the forearc fill east of Cortes Bank, the reflections west of the Bank tend to be discontinuous (i.e., Plates 6 to 8); this is attributed to: 1) disruption of the forearc fill along a deformation front within the Ferrelo fault zone and 2) the increased presence of intrusive and extrusive volcanics as indicated from exploratory wells along the Santa Rosa-Cortes Ridge. The deformation may be due to Neogene strike-slip faulting (Howell and Vedder, 1981; Vedder, 1987). Alternatively, it may be due to pre-Neogene forearc deformation due to its resistance to 196 Fig. 6.8. Possible thinning of forearc strata against the! Patton Terrane. I Clemente Is. Blake Knoll* Shepard Knoll \\J\ n V/|)^ Terrane boundary Line segment 197 2600 2800 3000 Ferrelo fault West Cortes Basin ■Top UKP Base UKP Nicolas Terrane Patton Terrane 0 5 km 1 SW LINE 122 NE 198 SECONDS the arcward motion of the accretionary prism (Lundberg and Reed, 1991; Silver and Reed, 1988? Westbrook and others, 1988). East of the San Clemente-Santa Cruz fault system, the lithologic relationships of the outer borderland are believed to be repeated in the inner borderland (Crouch, 1976; Howell, 1976, Yeats, 1976). This doubling of the accretionary wedge-forearc pair is attributed to major Neogene strike-slip along the San Clemente-Santa Cruz fault and/or extension within the inner borderland? the timing and mechanisms will be discussed in the following section. 6.3 Neoqene and Quaternary Structures According to Teng (1985), the early structures of the borderland have been largely masked by subsequent structures associated with Neogene tectonism. He classified Neogene structures into first order (features whose dimensions are tens of km) and second order (dimensions are only several km). Patton Ridge and Santa Rosa-Cortes Ridge are interpreted as major anticlinoriums and would be examples of first order structures? individual faults and folds within these anticlinoriums would be examples of second-order structures (Figs. 13 and 14 of Teng, 1985). 199 In the present study, seafloor structural trends were identified with the aid of GLORIA side-scanning sonar data (EEZ-SCAN Scientific Staff,1986) and high- resolution single-channel seismic reflection data. Four major Neogene and Quaternary Structural trends were observed within the study area: 1) a major zone of northwest-trending faults and folds within the Patton Terrane and the western half of the Nicolas Terrane, 2) a major zone of east-west-trending structures within the eastern half of the Nicolas Terrane, 3) the northwest- trending Santa Cruz-San Clemente fault zone separating the Nicolas Terrane forearc from the Catalina Terrane accretionary complex, and 4) a zone of north-south extensional faults at the southeast edge of the study area separating truncated Nicolas forearc fill from uplifted Catalina Terrane (Plate 4). Figure 6.9 shows a set of histograms showing the distribution of fault orientations within each zone. 6.3.1 Patton-West Nicolas Fault Trend In the Patton and west Nicolas Terranes, the structures can be grouped into three zones: an outer zone of faults and folds along the eastern edge of the Patton Ridge, which will be designated the Patton Ridge fault system? the Ferrelo fault system, which was interpreted 200 Fig. 6.9. Histograms of fault orientations within specific regions of the outer borderland. Patton - Ferrelo Faults San N ico las-E ast Cortes Faults co b 3 2 Lt_ o cr LU OD Patton R id g e - Long Basin 4 - 2 0 4 0 6 0 8 0 100 0 Ferello Fault 4 - 0 2 0 4 0 6 0 8 0 100 Santa Rosa-Cortes Ridge 0 2 0 4 0 6 0 8 0 100 San Clem ente Basin 4 - 2 0 4 0 6 0 8 0 100 120 0 8-i 4 - Santa Cruz-San Clemente Fault San Clemente Fault i T 1-1 -1 -1 0 2 0 4 0 6 0 8 0 100 N icolas-C atalina Extension Faults 4 - 1 San Clemente Basin i i i ? — i— i— i— n — -■— i 0 2 0 4 0 6 0 8 0 100 FAULT TREND (degrees W from N) 202 to be a tectonostratigraphic boundary (Howell and Vedder, 1981; Vedder, 1987); and a zone essentially trending along the Santa Rosa-Cortes Ridge, designated the Cortes Bank fault system (Plate 4). Faults within the Patton Ridge fault system appear to be short-length, discontinuous segments dominantly trending between N20°W and N40°W (Fig. 6.9; Plate 4). Although the style of faulting is difficult to determine in most cases due to the lack of seismic penetration in accretionary-wedge lithologies, those faults whose dip- slip component can be determined appear to exhibit normal offset (Fig. 6.10). It is assumed that the Patton Ridge fault is a strike-slip fault system like the Ferrelo fault based on interpretation of flower structures at places along the west edge of the central zone where some of these faults cut basin fill (Fig.6.10). The Ferrelo fault system has been described as an en echelon fault system (Vedder, 1987) and as a distinct tectonostratigraphic boundary separating the Patton allochthon from the Nicolas allochthon (Gibson and others, 1985; Howell and Vedder, 1987). Within the study area, fault segments can be long (up to 50 km) but discontinuous, whose dominant trend ranges between N40°W to N50°W (Fig. 6.9). Many of the faults along this zone show a reverse dip-slip component, especially along the 203 I Fig. 6.10. Apparent offset of faults classified within the Patton Ridge fault zone generally appears to be normal displacement. However, structures similar to the flower structures of Harding and others (1985) suggest that the fault zone contains a strike-slip component, similar to the Ferrelo fault zone. \sSan sf uVCIemente Is. Blake Knolls St y-J— 1 S.V Shepard Knoll 50 Km Terrane boundary Line segment I 204 1000 I 900 800 700 600 UKP(?) Long Basin Strike-slip 0 5k . i ■ LINE 903 east faces of ridges or anticlines within the Patton Terrane (Fig. 6.11). These faults may have formed in response to Neogene strike-slip translation, or alternatively, they may have originated as back-thrusts during pre-Neogene subduction (Silver and Reed, 1988) and were reactivated as strike-slip faults in Neogene time. The broad region of northwest-trending faults does not end at the Patton Terrane boundary, but continues east, half-way into the Nicolas Terrane. The Santa Rosa- Cortes fault consists of a series of long (about 50 km) fault segments trending between N30°W and N60°W. The dip-slip component is generally vertical to normal high- angle (Fig. 6.12). The northwest orientation of faults within the Patton and western Nicolas Terranes may indicate a fundamental structural grain associated with the underlying basement, possibly developed within the accretionary wedge or overriding oceanic plate adjacent to the trench during active subduction. 6.3.2 East Nicolas Fault Trend East of the Santa Rosa-Cortes fault system, in the eastern zone of the Nicolas Terrane, there is an abrupt change in fault orientation towards a strong east-west trend (Plate 4? Fig. 6.9). Without exceptions, these faults are all reverse faults, ranging from high-angle 206 Fig. 6.11. Many faults associated with the Ferrelo fault zone show aparent reverse dip-slip. These faults generally do not appear to offset lower Neogene strata; although they may have formed in response to Neogene strike-slip, it is possible that they formed as back-thrusts prior to cessation of subduction. Clemente Is Blake Knolls Terrane boundary Line segment l 1600 1800 2000 • • i Nicolas Terrane----------------------H Patton Terrane- West Cortes Basin /NQ U K P. 0 5km 1 » NE LINE 123 SW SECONDS Fig. 6.12. Faults classified within the Santa Rosa-Cortes i fault zone show apparent vertical or normal throw. u t San J Clemente Is. Knolls 4 § Shepard Knoll 50 Km Terrane boundary Line segment 1700 1800 I Santa Rosa-Cortes Ridge 1900 Santa Rosa-Cortes Fault Zone----- 5 km 0 — 3 SECONDS 643403867^4246509599^058 types in San Nicolas Basin and Santo Tomas-Blake Knolls to low-angle thrusts along the north side of Shepard Knoll (Figs.6.13A and 6.13B). These faults developed in response to the present-day stress regime, which is compression in roughly a north-south direction due to change in plate motion over the last 3 ma (Engebretson and others, 1985? Harbert, 1991; Harbert and Cox, 1989) and the blockage to northward strike-slip translation due to the development of the east-west-trending Transverse Ranges (Hauksson, 1987). However, these are not young faults, but appear to be reactivated strike-slip faults? strata do not match across many of these faults. For instance, mapping the Upper Cretaceous/Paleogene megasequence shows that this sequence thins towards the northeast? but across the reverse fault at the southern limb of the San Nicolas high, the UKP megasequence is thick again (Fig. 6.13A). It is possible that these faults may have initially formed in response to mid-ocean ridge collision and rifting at the southern end of the borderland. 6.3.3 Santa Cruz-San Clemente Fault Trend The northwest-trending Santa Cruz-San Clemente fault system (SCSCF) forms the eastern edge of the study area and separates the accretionary wedge-forearc pair of the 211 Fig. 6.13. Reverse faults and folds associated with ! east-west-trending structures of the eastern Nicolas | Terrane. However, an early, strike-slip phase is | indicated by thickness changes of strata across the j faults: A) Line 109 is a dip-line to structures; B) line 116 is a strike-line to structures. Santa Catalina Is. San Nicolas I s . ' San - V > Clemente Is. Shepard^Knoll f 50 Km Tarrana boundary Lina sag man t 212 i ________ 213. 4400 4600 4800 5000 5200 5400 fflg w E S S E S (A) SECONDS 214 2400 2000 San Nicolas Basin M B W » tizS t*j * y3 > £ S S ; « V ^ - * A » \ i ^TmVW^ • . v jrSEfv^^Tk SECONDS outer borderland from that of the inner borderland. Unlike the broad zone of discontinuous fault segments of the aforementioned fault systems, the SCSCF is essentially a narrow zone characterized by long fault segments through much of its length, although detailed mapping of this system by Legg (1985) shows branching as well as secondary faults associated with it. The trend of the fault system range between N40°W to N50°W. Along the San Clemente Ridge, the fault appears vertical; where dip is observed, it suggests a normal dip-slip component. East of the ridge, along Fortymile Bank, the dip-slip component appears to be reverse. 6.3.4 Nicolas-Catalina Extensional-Fault Trend Along the southeast corner of the study area southwest of the SCSCF, there is a zone of essentially north-south-trending, short, and discontinuous normal faults which define the boundary between the Upper Cretaceous-Paleogene forearc fill of the Nicolas Terrane from accretionary rocks of the Catalina Terrane? it generally follows the terrane boundary of Howell and Vedder (1981). Most of the faults show a high-angle normal dip-slip component and their orientation, relatively parallel to the trend of the extinct mid-ocean ridge at the base-of-slope in the southern end of the 215 borderland (Plate 4; Fig. 6.14), may be related to the post-collision rifting which initiated strike-slip translation in the borderland (Crouch, 1979; Legg, personal commun., 1991). This does not rule out the possibility that these faults may sole out as detachment faults at depth, as suggested by Bohannan (personal commun., 1991), Howell and others (1987), and Sedlock and Hamilton (1991), or that detachment faults are present. For instance, deep, shallow-dipping reflections beneath Blake Knoll may be detachment faults (Fig. 6.15). Anticlinal and synclinal folds associated with the fault systems for the most part parallel adjacent fault traces. Only along one fault system, the Santa Rosa- Cortes fault, do folds appear to intersect the fault traces at an en-echelon angle (Plate 4). 216 Fig. 6.14. Apparent extensional faulting associated with the Nicolas-Catalina Terrane boundary. Clemente h Blake Knolls Terrane boundary Line segment 217 SECONDS 4500 4700 4900 5100 5300 5500 San Clemente j. Ridge San Clemente Basin NQ 2 3 UKP (?) 4 5km - I L x X X X LINE 120 SW v,•■,■’• > »,,;ii . > • v " -t .l.VtlI1 V II/V.1 »>i V t 1,' ^ 1 Tl'> ’tVTO-lf ! ■ | ' « w s - 1 ’ ••'7T : . 7 t n r . n • • v / ^ ^ r f r « r if t r 'r i Viyr[frrrii T ,',,Y**T= ^ irrirm^ ^ H l^ 7 T l^ t .u k . li : Fig. 6.15. Shallow-dipping reflections beneath the UKP megasequence are shallow-dipping extensional faults. .Clemente Is. Terrane boundary Line segment 219 SECONDS 00 i 200 1300 1 4 0 0 i i Blake Knoll 15 0 0 1 2 3 NO NQ UKP UKP * * ■ ■ 5 km NE LINE 20 SW to to o ^ v ^ » « 3 a a s s ^ : s s ^ . * e * s 5 ^ a a « « « « s e « ? i t t 3 e s ! S ! ^ 3 S ( s s s s ^ e s f M « » » w § fc T In I ,'f*“ i « ; '■ > ~ » WkU *< !» « Iliji'W I ■ * ,* • * , < ,1 miw-<’ '-,%S''i-:■%. • - , “-> - < - •!>*„<*,,»-<•£- - ..,.« ^ Kilfl'JllfaViltf',JW ftt V •"rf'.X, IfiM'WlVi 000100000001000100000200530123012302020000005300000200012301010153020200000002010000002302020202000153010210000202020201000200004801480101004801020102020201010111020248 01010100000001010001000201000101020123022301000100020201000102000202530123000001005302000202000000010202010048024802010101005300482353014853480200010202000000 55555555555555555555555555555555555555T555T5555555555555X5555555555555555 Chapter VII DISCUSSION Prior to Neogene time, western North America was the site of active subduction, probably as far back as the Late Jurassic Period, It was postulated that oblique subduction translated numerous allochthonous terranes northwards along the edge of western North America, especially during the period when the Kula-Farallon Ridge was actively spreading (about 85 to 42 Ma). However, it should be noted that from about 42 to 30 Ma, plate-motion models suggest subduction that was perpendicular to the Farallon North American plate boundary (Atwater, 1989? Carlson, 1982; Engebretson and others, 1985); northward translation should have been limited during this period. The tectonic scenario changed to dextral transform motion in late Oligocene time as the Pacific-Farallon mid-oceanic ridge approached and eventually collided with the Paleogene trench, and changed again in late Pliocene to dextral transpression as a result of changing plate motions. The effects of these processes as interpreted from seismic reflection records and borehole information 221 will be discussed within the context of the borderland in general and the study area in particular. 7.1 Convergent Margin Prior to Oligocene time, the area of the California Continental Borderland was part of the active subduction zone along the western edge of North America (Fig. 7.1). The zone consisted of an accretionary wedge-forearc pair, which is represented within the southern outer borderland by the Patton and Nicolas Terranes; the associated volcanic arc is believed to be the Peninsular Ranges (Howell and others, 1987). From seismic reflection records, coupled with bore hole information, a model of a subduction complex that still appears relatively intact today can be constructed for the outer borderland (Fig. 7.2). At the base of the Patton Escarpment, the remnant downgoing slab can be imaged on many seismic reflection records as far inboard as the upper slope break (Lee and others, 1989). The average slope gradient of about 11°, along the Patton Escarpment, lies within the range of angles found in many wedge tapers of active subduction zones around the Pacific Basin (Crouch, 1981); it is probably not a fault scarp, as originally proposed (i.e. Emery, 1954; Parker, 1971; Shepard and Emery, 1941). 222 1 Fig. 7.1. Diagram of the California Continental Borderland during the period of active subduction LEGEND Accretionary wedge sediments Continental/Oceanic crust Forearc sediments Blueschist 223 94 224 Pre - 3 0 Ma 111 Exposed 2 0 km extension compression compression Fig. 7.2. A composite interpretation of lines 904, 110, and 908 showing possible accretionary wedge-forearc relationship preserved within the southern outer borderland. 225 226 r° I in * 3 SECONDS •g o) cn ^ u ro o I Z m 7.1.1 Accretionary Wedge The Patton Terrane constitutes the accretionary wedge? lithologic descriptions within this terrane indicate similarities to those of the Franciscan Formation of northern and central California, as well as the Eugenia Formation of Baja California (Barnes, 1984; Crouch, 1981? Minch and others, 1976? Vedder, 1987). The similarities include the presence of high-pressure/low- temperature metamorphic rocks (blueschists) found along the eastern margin of the Patton Terrane (Vedder, 1987). The question of how deeply-buried, high-pressure metamorphic rocks were emplaced, uplifted, and exposed within accretionary wedges has been a subject of debate. Gibson and others (1985) interpreted the blueschist outlier within the outer borderland as a separated part of the Catalina Terrane, trapped between the Patton and Nicolas Terranes. Crouch (1981) interpreted the outlier as evidence of an arcward increase in metamorphic grade, similar to that observed in the Franciscan Formation of northern California (Ernst, 1975)? however, simple tectonic overpressure by frontal accretion cannot generate the pressures to form blueschists (Cloos, 1982? Platt, 1985)? burial pressures of at least 6 kb at greater that 20 km depth are required (Cloos, 1982? Hyndman, 1972). 227 Other mechanisms that has been proposed include: 1) Buoyant rise of subducted slabs of crustal material relative to higher-density mantle rocks (Ernst, 1974, 1975, 1984)? 2) return flow of subducted material along the downgoing slab as a result of forced convection (Cloos, 1982); 3) underplating and exhumation by uplift (Hsu, 1991; Platt, 1975); and underplating and exhumation by extension (Platt, 1986). The first two mechanisms has been rejected because; 1) in the first method, it is questionable whether buoyancy will be sufficient to bring subducted material back to the surface unless subduction ceases, and 2) in the second method, while the mechanism may be able to bring up blocks of blueschist, it is doubtful whether this mechanism can bring up regionally coherent terranes of blueschist (Platt, 1986). Hsu (1991) and Platt (1975) envisioned the third mechanism, erosion, as the main method of bringing blueschists up to the surface. But the problem with with this theory as applied to the outer borderland lies in the fact that, although exploratory well information and seismic reflection records indicate that the accretionary wedge-forearc was subjected to intermittent subaerial exposures and shallow-water conditions during Late Cretaceous through Paleocene time, there is no evidence to suggest massive uplift and erosion of the Patton 228 Terrane during this period. It was not until subduction ceased in Neogene time that progradational sequences were identified which originated from sources within the Patton Terrane. Platt (1986) proposed the forth mechanism, that underplating of sediments or crustal slices will thicken an accretionary wedge and raise, or 'jack up', high- pressure/ low- temperature, underplated rocks in the process (Fig. 7.3A). Eventually, a condition will be reached whereby the wedge is overthickened. Deformation of deeply-buried rocks, either by ductile flow or dislocation creep, will result in longitudinal extension in the back half of the wedge (Fig. 7.3B-D). Platt's (1986) model is attractive because; 1) it explains how blueschists can be found along the landward edge of the wedge, and 2) within extensional basins of the Patton Terrane, such as Long Basin, the present study has identified reflective zones beneath the more recent basin fill that may represent older sedimentary strata. It is proposed that the central zone of the Patton Terrane represents the zone of longitudinal extension and basin formation developed during active subduction. 229 Fig. 7.3. High-pressure metamorphic rocks can be exhumed within an accretionary wedge by underplating subducted sediments? the underplated rocks are 'jacked up' by extension within the central wedge region when it becomes overthickened (from Platt, 1986). 230 compression backthrust frontal imbrication underplating maintains stable profile out-of-sequence thrust reactivated forward thrust |g{g forward and backthrusts underthrust sediment fore-arc basin Km buttress underplating by duplex formation, folding, and thrusting compression extension accreted i Stage A 10- low rate of frontal accretion 30- lOkb large-scale underplating « o - s o J extension and thinning accreted in A accreted in B lateral displacement law newly accreted •---5.5kb - uplifted isobars 3q- -- 10kb 40- J late thrusts early syn-metamorphic underplating structures cut by post-metamorphic listric normal faults Continued underplating jacking-up high-P rocks s o J cross-cutting sets of normal faults 5.5 lateral displacement 1 0- 5.5 5.5kb - law 30- 10kb nappe of high-pressure rocks 40- continued underplating Vertical and horizontal scales equal 100km 7.1.2 Forearc Basin The Nicolas Terrane comprises the forearc basin within the outer borderland. Sedimentary strata range in age from Late Cretaceous to Recent, with a major, regional unconformity at the base of the Neogene. Exploratory well data and seismic reflection records indicate that the forearc, as well as the accretionary wedge, was subjected to intermittant, subaerial exposure and shallow-water conditions during Late Cretaceous through Paleocene time, and deep-water conditions in the Eocene. Shallow-water conditions are suggested by unconformities and limestone lenses identified in the wells, and hiatal surfaces on seismic reflection records; deep-water environment is suggested mainly from microfossil and lithologic descriptions from the wells? Deep-water, Eocene strata exposed on San Nicolas Island and dredged from the seafloor indicate that the strata probably represent a deep-sea fan deposit, but seismic reflection records within the study area do not show seismic facies patterns which clearly define a submarine fan system. 7.1.3 Patton-Nicolas Terrane Boundary In previous interpretations of borderland geology, geologists defined the Patton accretionary wedge and the 232 Nicolas forearc basin as two allochthonous terranes (Gibson and others, 1985? Howell and Vedder, 1979; Vedder, 1987); the timing of their juxtaposition was uncertain (Gibson and others, 1985). The terranes were defined on the basis of differences in basement and pre-late Tertiary lithologies (Coney and others, 1980; Vedder, 1987). Geologists favored an ophiolitic basement for the Nicolas Terrane based on comparison with the Great Valley Sequence of northern and central California or from indirect evidence. In central California, the Franciscan assemblage is typically overlain by the Coast Range Ophiolite (Dickinson and Seely, 1979; Howell and Vedder, 1981). On Santa Cruz Island, and along coastal portions of southern California, basal metagabbro clasts in Miocene breccia change composition upwards to blueschist and greenschist clasts, suggesting downward erosion of basement from ophiolitic to Franciscan rocks (Howell and Vedder, 1981). Also, various models of seismic refraction data show that a fit can be made between modeled and actual velocities if an ophiolitic slab is underthrusted by Franciscan rocks (Shor and Raitt, 1956; Howell and others, 1985). More recently, field work and interpretation of seismic reflection data of the western Great Valley 233 Sequence in central California suggest another form of structural relationship between an accretionary wedge and forearc. Eastward-tapering tectonic wedges from the accretionary wedge itself were interpreted to underlie the western margin of the Great Valley Sequence (i.e., Wentworth and others, 1984; Wentworth and Zoback, 1989). Silver and Reed (1988) first proposed the concept of tectonic wedges to explain arc-ward tapering wedges interpreted from seismic reflection records of offshore central America; recent reinterpretation of seismic reflection data suggest that the Barbados subduction complex could be explained in terms of backstopping of the accretionary wedge, deformation of the wedge- bordering forearc, and west-verging thrusts (Westbrook and others, 1988). The results of this study also suggest that such a relationship exists for the outer borderland, which is essentially the southern continuation of the accretionary wedge-forearc of northern and central California. Forearc strata appear deformed within about a 10-km deformation zone east of the Patton Terrane. Towards the arc (eastwards), the accretionary wedge appears to taper again, forming a tectonic wedge that is obducted over ophiolitic (?) basement; ”Franciscan-type1 1 metamorphic basement encountered in two exploratory wells along the 234 Santa Rosa-Cortes Ridge may have drilled into the top of this taper. Many of the high-angle reverse faults generally found on the eastern edge of the Ferrelo fault system may have initially formed as backthrusts. Backthrusts develop in response to arc-ward migration of the accretionary wedge due to the compressive forces of subduction (Langseth and Moore, 1990; Silver and Reed, 1988). The forearc fill adjacent to the accretionary wedge behaves as a "backstop" (Silver and Reed, 1988), overriding the wedge and at the same time resisting the arc-ward migration of the wedge by deforming (Fig. 7.4). 7.1.4 Model of Convergent Margin Formation A conceptual model of convergent margin formation envisions that as oceanic lithosphere moves away from a spreading ridge, it cools, thickens, and becomes more dense as a result of thermal contraction. This will eventually result in downward bending of the thickening lithosphere? near-surface rocks are placed in tension, and block-faulting occurs (Fig. 7.5A). Eventually a stage is reached whereby the lithosphere becomes gravitationally unstable with respect to the underlying mantle rocks. The lithosphere founders and begins to sink into the earth's interior (Turcotte 235 Fig. 7.4. Schematic model of modern subduction complexes, showing the similarities in accretionary wedge-forearc relationships to that of the southern outer borderland (modified from Langseth and Moore, 1990). 236 237 H— Accretionary Prism — H Backstop -----------Forearc H Arc H— Trenchward increase in forearc deformation backthrust underplated sediment Fig. 7.5. Schematic diagrams of the development of the accretionary wedge and forearc in the outer borderland: A) Initial stage of formation involving compression and sagging of older, dense lithosphere? B) initiation of subduction and formation of accretionary wedge and backstop bulge; C) development of backthrusting; D) initiation of transform motion after ridge-trench collision, possibly along faults and fractures created during subduction. 238 DEPTH (KILOMETERS) (A) Sediment Lithosphere Oceanic cruat Fracturea Accretionary wedge Forearc basin 10 - 2 0 High-pressure metamorphism 30 Compression Extension Compression Backthrust 20 30 - 'Jacked up’ metamorphic rocka (D) Ferrelo fault T •v . 17* ^ Newport-lnglewood fault San Clemente fault 1® X X Lower lithosphere 20 - Mantle Spreading Center 50 K m 239 and Schubert, 1982). Some of the seafloor sediments are subducted with the oceanic crust, but accretion and underplating of the remaining sediments formed the accretionary wedge (Fig. 7.5B). Extensional basins may form within the wedge. High-pressure, blueschist rocks underplated to the base of the wedge may overthicken the wedge and cause it to expand laterally (Platt, 1986). Over time, a forearc basin is created behind the developing accretionary wedge, which tends to be dragged arcwards (east) by the underlying, subducting slab (Figs. 7.5C and 7.5D). Disruption of Nicolas forearc strata within an approximately 10-km deformation zone east of the Patton Terrane, and the presence of early-formed reverse faults, many which do not offset strata apparently younger than early Miocene, suggest that backthrusting may have occurred within forearc strata adjoining the accretionary wedge. However, the degree to which oblique subduction, principally prior to 42 Ma, disrupted the accretionary complex, and the timing and mechanism of backthrusting, are difficult to evaluate. Prior to 42 Ma, oblique subduction resulted in the northward migration of allochthonous terranes, especially during the period 85 to 42 Ma, when the northward- spreading Kula-Farallon Ridge was active. However, from 240 42 to 30 Ma, FaralIon-North American relative plate motion was about perpendicular, thus little translation should have occurred. It is possible that forearc strata mapped across the Patton-Nicolas terrane boundary represent strata deposited over the last 10 Ma. The timing and mechanism of backthrusting is uncertain. In general, as the accretionary wedge is built up over time. The accretionary wedge should be dragged arcwards during active subduction, thus forearc deformation should be a continuous process (Figs. 7.5B to 7.5D). However, the Barbados accretionary complex suggests no backthrusting until relatively late in forearc formation. Then rapid forearc deformation occurred adjacent to the accretionary wedge, either as a result of a change in subduction rate, initiation of a new subduction zone, or a change in sediment supply (Torrini and Speed, 1989). 7.2 Transform Margin By the late Oligocene, as the Pacific-Farallon spreading ridge (PFR) approached the trench, the Farallon plate began to fracture into a series of smaller plates (Atwater, 1989). The time of initial ridge-trench collision is uncertain, but appears to range between 26 to 29 Ma? the point of collision is believed to be in the 241 vicinity of southern and Baja California (Atwater, 1970, 1989; Crouch, 1981; Legg, 1991). After collision, subduction would cease, and dextral strike-slip of as much as 1000 km, coupled with melting of the thin, young oceanic slab beneath the accretionary wedge (formation of a slab window) would effectively remove any evidence of the slab (Atwater, 1970; Dickinson and Snyder, 1979). 7.2.1. Remnant-Slab Problem This originally simple scenario changed in 1989, when several CALCRUST-reprocessed U.S. Geological Survey seismic reflection records imaged high-amplitude events beneath the toe of the accretionary wedge which were interpreted to represent the remnant slab (Lee and others, 1989); subsequently, additional seismic reflection records acquired by the U.S. Geological Survey confirmed the existence of the slab. Other investigations, along the continental margin of central California (McCulloch, 1989; McIntosh and others, 1991; Ewing and Talwani, 1991; Meltzer and Levander, 1991) also showed the presence, on seismic reflection records, of a subducted slab beneath the accretionary wedge. The presence of a seemingly intact accretionary wedge at the base of the continental margin severely limits the amount of dextral translation of the Patton 242 Terrane, as envisioned by the original Atwater (1970) model? however, this problem was addressed in later ridge-trench-collision models (i.e., Atwater, 1989; Sedlock and Hamilton, 1991). These later models emphasized the jagged nature of mid-ocean ridges; segments of ridges are separated, oftentimes over long distances, by transform faults. Such ridge segments may be even more prevalent during the initial phase of ridge- trench collision, due to fracturing of the Farallon Plate as the continent encroached on it (Atwater, 1989). The collision process is envisioned to occur in a piecemeal way (Sedlock and Hamilton, 1991). The initial collision at between 26 to 29 Ma occurred in the vicinity of northern Baja California (Fig. 7.6A); by 20 Ma, another ridge segment collided with the trench further south, forming the short-lived Monterey Plate. It was not until about 18 Ma ago, when a third segment collided with the trench at the present location of the southern end of the borderland, that translation and rotation became active within the borderland (Figs. 7.5D and 7.6C). Concomitant with collision was the initiation of widespread volcanic activity, beginning from about 24 to 26 Ma (Howell and Vedder, 1981? Sedlock and Hamilton, 1991? Vedder, 1987). Within the area of Cortes Bank, the Paleogene section along the Santa Rosa-Cortes Ridge was 243 Fig. 7.6. Piecemeal collision of ridge segments, rather than a single collision event, may explain the preservation of the remnant subducted slab beneath the accretionary wedge in the southern outer borderland: A) Initial collision between 26 to 29 Ma; B) A second collision at around 20 Ma resulting in formation of Monterey plate? C) Collisional event at 18 Ma affected processes within the borderland, resulting in major translation and rotation. Map symbols: CP, Colorado Plateau; F, Ferrelo fault? JDF, Juan de Fuca plate; G, Guadalupe plate? M, Monterey plate? NIC, Nicolas Terrane? NICR, Newport-Inglewood-Rose Canyon fault zone; PE, Patton Escarpment? R, Rinconada fault? RS, Russel fault? Mtj, Mendocino triple junction? SA, San Andreas fault; S ANA, Santa Ana terrane; sCR southern Coast Ranges; SLE, Santa Lucia Escarpment; SN, Sierra Nevada batholith? wTR, western Transverse Ranges. 244 245 30 Ma CP marine forearc basin nonmarine strata in old forearc basin bight PACIFIC FARALLON NORTH AM ERICA 20 Ma SN CP JDF \ RS sCR PACIFIC 18 Ma SN CP JDF PACIFIC unnamed fragments of ^ northern Guadalupe plate 0 500 km B capped by a thick layer of submarine basalt flow, dated at late Oligocene-early Miocene (Paul and others, 1976), which was before uplift began in the southern end of the borderland. Younger basalts have been recovered at Northeast Bank, to the west of Cortes Bank; dates on these basalts indicate an age of early Pliocene (Hawkins and others, 1971), although Vedder and others (1981) suggest that Miocene volcanics may underlie the Pliocene volcanics based on their seafloor samples. Other investigators (Doyle and Gorsline, 1977; Krause, 1965) found still younger volcanic rocks of 2 Ma and younger offshore of northernmost Baja California, leading Legg (1991) to propose that a middle Miocene thermal event gradually progressed from west of southern California southeastward across the Baja California borderland. 7.2.2. Subducted Ridge Controversy A major controversy centers around what happens to the spreading center after ridge-trench collision. Atwater (1970, 1989) suggested that the ridge, along with the Farallon Plate and subduction zone, were annihilated; Dickinson and Snyder (1979) showed the development of a slab window, seaward of the triple junction, where asthenospheric mantle would directly underlie the North American continent. However, the present study, as well 246 as studies in central and northern California, demonstrate the presence of a remnant slab beneath the continental margin of North America. In central California, the preservation of the subducted slab is due to the presence of a trapped fragment of the Farallon Plate, beneath and seawards of the accretionary wedge (Ewing and Talwani, 1991; Lonsdale, 1992? Heltzer and Levander, 1991). In the southern California Continental Borderland, magnetic anomaly patterns at the base of the accretionary wedge do not indicate the presence of any trapped Farallon fragments, but suggest that the subducted slab beneath the wedge is part of the Pacific Plate (Atwater, 1970, 1989? Lonsdale, 1992). Assuming that the magnetic anomaly patterns are correct, then either the Pacific-Farallon spreading center and a portion of the Pacific Plate was subducted beneath the accretionary wedge (i.e., Cande and Leslie, 1986? Cande and others, 1987? Tennyson, 1989), or else the wedge expanded over the Pacific Plate as a result of extensional collapse then longitudinal compressive stresses were removed after cessation of subduction (i.e., Platt, 1986? Willett, 1991). Studies of an active ridge-trench collision presently occurring offshore in southern Chile indicate that a ridge is being subducted, and it is tectonically 247 eroding off the overlying accretionary wedge in the process. As a consequence, the wedge narrows and the trench slope steepens at the point of collision (Cande and Leslie, 1986? Cande and others, 1987). Concomitant with ridge subduction were late Neogene and Quaternary uplift, plutonism, and volcanism on the adjacent continental margin (Forsythe and Nelson, 1985? Forsythe and others, 1985). The studies mentioned above, as well as other recent studies (i.e., Collet and Fischer, 1991; Gardner and others, 1992) suggest that the ridge may travel some distance beneath the wedge before it is annihilated or its' influence is no longer felt. Once subducted, elevated temperatures in the mantle would prevent magma from congealing at the spreading center? the ridge erodes and begins to pull apart, forming a slab window (Cande and Leslie, 1986; Dickinson and Snyder, 1979? Forsythe and others, 1985? Thorkelson and Taylor, 1989). 7.2.3 Collision-Related Uplift As the mid-oceanic ridge approached the trench and began to subduct, the region of the accretionary wedge at the point of impingement was uplifted (Fig. 7.7). The uplift may be attributed to three factors: 1) The advent of thin, buoyant, young crust (Legg, 1991)? 2) the 248 Fig. 7.7. Regional uplift at 20 to 24 Ma as a result of ridge-trench collision. Concomitant with uplift was the beginning of volcanic activity in the region; basalt flows at the top of the Paleocene recorded an Oligocene or early Miocene age. Also, the oldest progradational deposits within the region are early Miocene. LEGEND o o a Accretionary wedge sediments Continental/Oceanic crust 'V Forearc sediments Blueschist 249 250 2 4 - 2 0 Ma EH Exposed 0 20 k m 1 _________ i subduction of high-standing ridges and seamounts, some measuring more than 4 km above the seafloor (Fig. 7.8); and 3) flexures within the subducting crust caused by the juxtaposition of older, denser, and thicker crust against the young, buoyant, and thin crust along transform faults where ridge segments are offset. Glazner and Schubert (1985) attributed northwards streamflow in the Mohave region of southern California to flextural uplift due to the passage of the subducted Mendocino Fracture Zone. Within the study area, the result of this doming event is observed in planation and creation of banks along the top of the accretionary wedge (Fig. 7.7). Along the contact between accretionary wedge and forearc, uplift resulted in erosional truncation of forearc strata. Early to Middle Miocene sediments shed from the region of the accretionary wedge prograded northeastwards towards depressions in the vicinity of the present-day San Nicolas and Velero Basins. At about 18 Ma, as the spreading center impinged and was dragged beneath the accretionary wedge, the southeastern end of the southern California borderland was domed upwards along a north-south trend (Fig. 7.9). This major uplift exposed the Upper Cretaceous-Paleogene forearc, which was subsequently eroded off along a zone 251 Fig. 7.8. Cross-sections AA', BB', and CC' across the extinct Guadalupe spreading center. Many ridges within the center reached 1000 m in height; however, Guadalupe Island rises more that 4000 m above the seafloor. Collision of these features with the accretionary wedge can result in uplift and tectonic erosion of the wedge. 252 D E P T H (KILOMETERS) W A Extinct mid-ocean ridge B 0 - Extinct rmid-ocean ridge 4 G uadalupe Is. Extinct mid-ocean ridge 4 - to m W Continental margin Continental margin C' Fig. 7.9. Ridge-Trench collision and subsequent ridge subduction at 18 Ma may be responsible for uplift and truncation of the Upper Cretaceous through Paleogene section in the southern end of the borderland. LEGEND e o ca Accretionary wedge sediments Forearc sediments Continental/Oceanic crust /V -'V Blueschist 254 255 18 -17 Ma H Exposed ! . e ! O Y -O 20km trending from present-day San Clemente Island to south of Shepard Knoll. The exact timing of formation of the present borderland basins is speculative. According to Atwater (1970, 1989), initial strike-slip motion would occur at the continental margin, and migrate inland over time. Thus basin formation would begin furthest offshore at about 23 Ma, whereas onshore basins would not form until later (about 18 to 16 Ma). However, paleomagnetic work by Luyendyk and Hornafius (1987) and Hornafius and others (1986) indicate that rotation of the Transverse Ranges did not begin until middle Miocene time, and Sedlock and Hamilton's (1991) model suggest that strike-slip movement did not begin in the borderland until after collision of a third ridge segment with the trench at around 18 Ma. Regardless of timing, most models of borderland formation involves rotation of the Transverse Ranges as well as northwest-trending strike-slip (i.e. Crouch, 1979; Luyendyk and Hornafius,1987; Hornafius and others, 1986); more recent models also attempt to explain the exhumed blueschists of the Catalina Terrane in terms of major inner-borderland extension and/or detachment faulting as well (Bohannon, personal commun., 1991; Howell and others, 1987; Legg, 1991; Sedlock and Hamilton, 1991). 256 7.2.4 Development of Neoaene Fault Trends The structural map within the southern outer borderland shows four distinct fault trends (Plate 4). The outer margin, which includes the Patton and the western half of the Nicolas Terrane, is characterized by a series of northwest-trending fault zones. The overall trend of the outermost Patton Ridge fault zone appears to be more northerly than that of the Ferrelo fault zone, which in turn trends more northerly than the Santa Rosa- Cortes fault; it suggests the possibility of some counterclockwise rotation of the southern half of the outer borderland over time if Atwater's (1970, 1989) model, showing initial strike-slip beginning at the continental margin after ridge-trench collision, is correct. The eastern half of the Nicolas Terrane is characterized by faults that trend noticeably east-west? the boundary where this change in fault patterns between the Patton and western Nicolas Terranes and the eastern Nicolas Terrane occurred may represent the boundary between accretionary wedge basement to the west and ophiolitic basement to the east. For example, the northwest trend of the Patton and western Nicolas Terranes may represent the original accretionary-wedge fabric of reverse faults and/or oblique faults that were 257 perpendicular to the compressional axis (Legg, personal commun., 1991), whereas the fabric of the eastern Nicolas Terrane may be due to possibly older transform faults on the ophiolitic basement. The results from this study clearly indicate that the Ferrelo fault zone does not constitute a distinct boundary between the Patton and Nicolas Terrane. For the southernmost portion of the study area, it has been shown that strata of the Nicolas Terrane can be mapped well west of the inferred fault zone, and lithologic information from two wells that possibly bottom in Franciscan-type basement may suggest that in portions of the so-called Nicolas Terrane, rocks of the Patton Terrane may also be found. Along the southeastern edge of the outer borderland, a zone of extensional faults which trend nearly north-south lies within the zone of seafloor spreading as envisioned by Crouch (1979), or seafloor rifting as envisioned by Legg (1991) (Fig. 7.10). The nature of these faults and the suggestion of counterclockwise rotation of the outer margin of the outer borderland may favor oblique rifting, or leaky transform (Legg, personal commun., 1991), after ridge-trench collision and doming of the southeastern end of the borderland, as the mechanism for borderland formation. According to Arnal 258 Fig. 7.10. By 16 Ma, volcanic activity has proceeded along the Santa Cruz-San Clemente Ridge, and rifting has opened up the southern end of the borderland. LEGEND e e ca Accretionary wedge sediments Continental/Oceanic crust rv Forearc sediments Blueschist 259 260 16 - 12 Ma Exposed 0 20km 1 __________i and Vedder (1976) and Vedder (1987), water depths began to increase within the southern outer borderland near the end of the Miocene, which may be related to the rifting in the southeastern edge of the borderland. The northwest-trending Santa Cruz-San Clemente fault system forms the eastern boundary of the outer borderland; along this fault zone or its predecessor, it was thought that juxtaposition of the Nicolas Terrane against the Catalina Terrane occurred by: 1) Inner- borderland rifting with the Santa Cruz-San Clemente fault system acting as the principal transform fault (Legg, 1991); a combination of detachment and strike-slip (Sedlock and Hamilton, 1991); or 3) strike-slip (Crouch, 1979; Hornafius and others, 1986; Teng, 1985). The first theory has rapidly fallen into disfavor as lithologic and seismic reflection evidence continue to mount suggesting extensional faulting and exhumation as the probable mechanisms for the presence of the Catalina Terrane. Assuming that interpretations of seafloor magnetic anomaly patterns and bathymetries are correct, the major translation episode affecting the outer borderland would be from the period of 18 to about 12 Ma, when the Guadalupe Ridge was actively rifting, or spreading open, the southern end of the California Continental Borderland. If the subduction complex at the southern end 261 of the California Continental Borderland corresponds to the complex in the Vizcaino Peninsula, the total amount of translation , approximately 500 km, would correspond to the narrow region between these two points of northern Baja California, where seafloor sampling found young volcanic rocks and Late Tertiary to Recent sediments (Plate 1; Doyle and Gorsline, 1977). However, if symmetrical seafloor spreading is assumed, then the amount of translation for the outer California Continental Borderland should only be about 250 Ion. 7.2.5 Development of Modern Borderland Basins The formation of the modern borderland basins within the southern outer borderland can be observed on seismic reflection records as a distinct, vertical change in seismic facies; from generally unidirectional progradational reflections in an unrestricted north and east direction, to multidirectional progradational and onlap reflections, usually above a distinct unconformity, in the modern, restricted basins. Although no age dates are available for this event, correlation of seismic sequences in which age dates were available suggest that the time of basin formation is, in fact, young; possibly late middle Miocene or later (Fig. 7.11). This finding is 262 Fig. 7.11. Formation of the modern borderland basins appear to be young, possibly late middle Miocene or younger. Compressional deformation, in the form of folds and reverse faults along east-west-trending structures, may be related to post-Miocene change in plate motion and obstruction to northwestward translation due to blockage by the Transverse Ranges. Map symbols: C, Cortes Bank; EC, East Cortes Basin; L, Long Basin; SC, Santa Cruz Basin; SCI, San Clemente Island; SNI, San Nicolas Island; T, Tanner Bank; V, Velero Basin; WC, West Cortes Basin. LEGEND e o a Accretionary wedge sediments Forearc sediments Continental/Oceanic crust / V 'V Blueschist 263 34 264 10 - 0 Ma H Exposed SNI SCI 1 / ) /EC / /A 9 y IJ T > fc C sc 2 0 km 0 i____________ i similar to the results of Teng (1985); Teng and Gorsline (1989), and Legg (1991). 7.3 Transpressive Margin A relative change in motion between the North American and Pacific plates from an azimuth of relative motion of from 319°E to 331°E (Harbert, 1991) since about 5 Ma resulted in transpressive strike-slip along the San Andreas fault system and uplift of mountain ranges and folding of strata in central California (Engebretson and others, 1985; Harbert and Cox, 1987; Harbert, 1991). Evidence of compressive structures can also be found throughout southern California. For instance, investigations of faults adjoining the San Andreas fault suggest that portions of the Mohave block may be undergoing compression (Henyey and others, 1989; Li and Henyey, 1989), and recent studies in the Transverse Ranges indicate that young compressional features may be due to the presence of a subduction zone beneath the ranges (Namson and Davis, 1988; Humphreys and others, 1984). Within the southern outer borderland, compressional or transpressional structures tend to be the dominant features which affect the modern seafloor topography (Fig. 7.11). These features tend to be localized in a 265 region between the Ferrelo and the Santa Cruz-San Clemente fault zone; within this region, the fault trends generally range from N30°W to N80°W, placing these structures at an oblique or perpendicular to the direction of plate motion. Evidence from seismic reflection records indicate that faults initially formed as strike-slip faults now appear as reverse faults, and normal faults tend to be bisected by later-formed reverse faults. In two areas, the Patton Ridge fault system and the zone of extension in the southeastern part of the borderland, most of the fault segments tend to show a normal dip-slip component? these faults are mostly oriented in a N20°W to N30°W angle, parallel to the direction of plate motion. 266 Chapter VIII SUMMARY During the last several decades, the plate tectonics history of western North America was interpreted based on magnetic anomaly patterns, onland mapping, and high- resolution seismic reflection data. Very little information was published about the offshore subsurface geology; much of the knowledge was based on sparse surface geologic information extrapolated to depth, and interpretations based on unavailable "proprietary" data. The necessity for oil exploration in the mid-1970s fueled a major research effort within the California Continental Borderland on the part of the U.S. Geological Survey and other academic institutions. New geologic information was collected which suggested that an accretionary wedge-forearc pair existed within the outer borderland, west of San Clemente Island; the accretionary wedge-forearc relationship was repeated within the inner borderland, east of San Clemente Island (Howell, 1976; Howell and others, 1974). This pairing of the accretionary wedge-forearc complex was believed to be the result of strike-slip motion along a major transform 267 fault, the proto-Santa Cruz-San Clemente fault zone, after early Miocene ridge-trench collision (also Crouch, 1979? Hornafius and others, 1986? Teng, 1985). By the early 1980s, the simple relationship between the accretionary wedge and forearc had become suspect, as work by Coney and others (1980), and later Gibson and others (1985), Lund and Bottjer (1992), and Vedder (1987) suggested that large portions of the continental margin of North America were allochthonous, subjected to major lateral translations since Cretaceous time. The idea of a doubled, but genetically-related, accretionary wedge- forearc pair gave way in favor of four distinct, allochthonous terranes. More recent research efforts within the borderland have focused on the relationship between the inner and outer borderland. The inner zone consists of high-grade metamorphic rocks (blueschists), compared to the sedimentary and lower-grade metamorphic rocks (greenschists) of the outer borderland. Many researchers believed that some form of detachment faulting is necessary to uplift and exhume the once-deeply-buried blueschists. The present study used U.S. Geological Survey 24-channel seismic reflection records, correlated to seafloor sediment samples as well as data from recently 268 released exploratory wells, to examine structures and stratigraphies within the outer borderland; included were several U.S. Geological Survey seismic reflection records collected in 1978 and 1979 that were reprocessed at USC to improve data quality. Lithologic examination of the southern outer borderland confirms that this region can be explained in terms of an outer accretionary wedge complex and an inner forearc zone. The outer zone, termed the Patton Terrane, is composed of lithologies of accretionary wedge, such as greywacke, low-grade greenschists, and limited exposures of blueschist. The inner zone, termed the Nicolas Terrane, is dominantly composed of sandstones and shale, with minor occurrences of limestone. In the western edge of the Nicolas Terrane, exploratory wells drilled along the Santa Rosa-Cortes Ridge suggest that part of the basement may be "Franciscan-type" metamorphics, rather than ophiolitic rocks as indicated by rocks exposed on Santa Cruz Island, seismic refraction studies, and comparisons with similar environments in central California and the Vizcaino Peninsula. Within the southern outer borderland, volcanic extrusive rocks are common in the Neogene section, and unconsolidated sediments of Quaternary age are often found as basin fill. 269 Analysis of the 24-channel, seismic reflection records identified two seismic megasequences, an Upper Cretaceous through Paleogene (UKP) section and a shallow Neogene to Quaternary (NQ) section; they are separated by a high-amplitude event which represents a major unconformity at about the end of Oligocene or early Miocene time. Continuity of reflections is poor and wide grid-spacing created problems with identifying events within the UKP section, but several seismic records suggest greater complexity within this section than envisioned by a simple fan. In proximity to the wells, several boundaries, at the Oligocene-Eocene, mid-Eocene, and Upper Cretaceous, can be identified on the basis of distinctive velocity increases on sonic logs. Seismic sequences within the NQ megasequence indicate that a major portion of the lower sequence represents a series of regressive-transgressive events marked by progradational-type seismic facies; the indications are that during late Oligocene-early Miocene time, there was extensive uplift to the west, probably from the vicinity of the Patton Terrane. Only within San Nicolas and Velero Basins are there indications that progradational, early Miocene strata transit into flat-lying basin-fill facies. Within other basins, the flat-lying basin-filled facies tend to occur within 270 successively younger strata. A major unconformity within the late middle Miocene or Late Miocene (?) in San Nicolas Basin might represent the changeover from tectonic uplift to strike-slip; whereas previous strata appear to be derived from the southwest, sedimentation from present-day localized highs seems to dominate above the late middle or late Miocene unconformity. Several seismic reflection records imaged a zone of high-amplitude reflections descending beneath the Patton Escarpment. It is suggested that this zone represents the remnant of the old subducted plate. If this is the case, the southernmost evidence of the subducted plate would represent a piercing point indicating approximately how far the Patton Terrane had traveled, since south of this point, there would be no subduction zone, but new seafloor. The Ferrelo fault zone does not appear to represent a tectonostratigraphic boundary between two allochthons of differing basement and Tertiary geology. At the southern end of the outer borderland, the UKP megasequence can be mapped over into the Patton Terrane? possibly deeper reflections within the central zone of the Patton Terrane may contain UKP as well. The change in fault trend between the northwest- trending faults of the Patton and west Nicolas Terrane 271 versus the east-west-trending faults of east Nicolas Terrane might represent a fundamental change in the underlying basement. Possibly "Franciscan-type” basement underlies the western half of the Nicolas Terrane as well as the Patton Terrane, and the northwest-trending faults represent the structural grain of the subduction margin; the east-west trends within the eastern half of the Nicolas Terrane may represent the structural grain of the ophiolitic basement, perhaps older transforms. Interpretation of lithologic, stratigraphic, and structural data suggests a new geologic model for the southern outor borderland which is similar to recent models proposed for the Coast Ranges-Great Valley region of central California and the Barbados Islands, which show backthrusting, or tectonic wedging, of the arcward edge of the accretionary wedge over ophiolite basement. Along the southeastern end of the borderland, a nearly north-south set of extensional faults within the location of Vedder's (1987) Catalina-Nicolas Terrane boundary may be related to seafloor spreading, rifting, or detachment faulting that exhumed the Catalina Terrane. Previous models of borderland formation advocated strike-slip motion along major terrane boundaries as the method for juxtaposing allochthonous tectonostratigraphic units, which differ in basement and early Tertiary 272 lithologies. However, based on the observations presented in this research, a new tectonic model is needed to explain the formation of the California Continental Borderland. The new model would argue that prior to 30 Ma, the area was undergoing active subduction. With the approach of the spreading ridge about 29 Ma, major uplift occurred, as a result of: 1) the approaching young, hot, buoyant spreading center; 2) subduction of ridges and seamounts associated with the spreading center; and/or 3) possibly a flexure related to a step created by the Mendocino fracture. By about 18 Ma, uplift has spread to the vicinity of the southern borderland, extending east to the Shepard and Blake Knolls areas; concomitant with uplift was the deposition of shallow-water progradational-type facies towards the northeast, into the proto-San Nicolas and proto-Velero Basins. It would appear that strike-slip activity and uplift of the Catalina Terrane did not begin until about late middle- or late Miocene, with the appearance of a major, regional unconformity surface; this was apparently the first appearance of deposition from sources to the north or northeast. Strike-slip motion was thought to be along major, northwest-trending transform faults such as the Ferrelo 273 and the Santa Cruz-San Clemente fault zones (Crouch- , 1981; Howell, 1976? Hornafius and others, 1986? Legg, 1985? Teng, 1985). However, evidence of major strike-slip motion along the more east-west trending faults within the eastern zone of the Nicolas Terrane suggests that translation of segments of the Nicolas Terrane with respect to the Catalina Terrane may have been along the more east-west-trending faults. Coincident with strike-slip was the development of an approximately north-south-trending rift zone or nacent spreading center south of San Clemente Island? possibly some of the east-west-trending faults are transform faults connected to this north-south rift zone. This rift might be related to the Guadalupe spreading center, which lies directly south of the rift. Since the Guadalupe spreading center was supposedly extinct by about 12 Ma, much of the tectonic activity involving strike-slip probably occurred during late middle Miocene. 274 REFERENCES Arnal, R.E., 1976, Miocene paleobathymetric changes of the Santa Rosa-Cortes Ridge Area, California Continental Borderlands in Howell, D.G., Aspects of the Geologic History of the California Borderland? American Association of Petroleum Geologists, Pacific Section Misc. Pub. 24, p. 60-79. Atwater, T.M., 1970, Implications of plate tectonics for the Cenozoic tectonic evolution of western North America: Geological Society of America Bulletin V . 81, p . 3515-3536. , 1989, Plate tectonic history of the northeast Pacific and western North America, in Winterer, E.L., Hussong, D.M., and Decker, R.W., eds., The Eastern Pacific Ocean and Hawaii: Boulder, Colorado, Geological Society of America, The Geology of North America, v. N, p. 21-72. Atwater, T., and Molnar, P., 1973, Relative motion of the Pacific and North American plates deduced from sea-floor spreading in the Atlantic, Indian, and South Pacific Oceans; in Kovach, R.L., and Nur, A., eds., Preceedings of the Conference on Tectonic Problems of the San Andreas Fault System: Stanford University Publications on the Geological Sciences v. 13, p. 136-148. Barnes, D.A., 1984, Volcanic arc derived, Mesozoic sedimentary rocks, Vizcaino Peninsula, Baja California Sur, Mexico, in Frizzell, Jr., ed., Geology of the Baja California Peninsula: Pacific Section, Society of Economic Paleontologists and Mineralogists, p. 119-130. Berkhout, A.J., 1982, Seismic migration: Imaging of acoustic energy by wave field extrapolation: Amsterdam, Elsevier Scientific Publishing Co., v. 1, 351 p. Bottjer, D.J., and Link, M.H., 1984, A synthesis of Late Cretaceous southern California and northern Baja California paleography, in Crouch, J.K., and Bachman, S.B, eds., Tectonics and sedimentation 275 along the California margin: Society of Economic Paleontologists and Mineralogists, Pacific Section, v. 38, p. 171-188. Cande, S.C., and Leslie, R.B., 1986; Late Cenozoic tectonics of the southern Chile Trench: Journal of Geophysical Research, v. 91, p. 471-496. Cande, S.C., Leslie, R.B., Parra, J.C., and Hobart, M., Interaction between the Chile Ridge and Chile Trench: Geophysical and geothermal evidence: Journal of Geophysical Research v. 92, p. 495-520. Carey, S.M., and Colburn, I.P., 1978, Late Cretaceous sedimentation in the Santa Monica Mountains, California, in Howell, D., and McDougall, K., eds., Mesozoic paleogeography of the western United States: Society of Economic Paleontologists and Mineralogists, Pacific Section, p. 547-558. Carlson, R.L, 1982, Cenozoic convergence along the California coast: A qualitative test of the hot-spot approximation: Geology, v. 10, p. 191-196. Claerbout, J.F., 1985, Imaging the earth's interior: Oxford, Blackwell Scientific Publication, 398 p. Cloos, M., 1982, Flow melanges: Numerical modeling and geologic constraints on their origin in the Franciscan subduction complex, California: Geological Society of America Bulletin, v. 93, p. 330-345. Colburn, I.P., 1973, Stratigraphic relations of the southern California Cretaceous strata: Society of Economic Paleontologists and Mineralogists, Pacific Section Guidebook, Fall 1973, p. 45-73. Cole, M.R., 1975, Eocene sedimentation and paleocurrents, San Nicolas Island, California: Guidebook, Geological Society of America Cordilleran Section Meeting, 32 p. Collet, J.Y., and Fisher, M.A., 1989, Formation of forearc basins by collision between seamounts and accretionary wedges: An example from the New Hebrides subduction zone: Geology, v. 17, p. 930-933. 276 Coney, P.J., Jones, D.L., and Monger, 1980, Cordilleran suspect terranes: Nature, v. 288, p. 329-333. Crouch, J.K., 1979, Neogene tectonic evolution of the California Continental Borderland and western Transverse Ranges: Geological Society of America Bulletin, v. 90, p. 338-345. , 1981, Northwest margin of the California continental borderland: Marine geology and tectonic evolution: American Association of Petroleum Geologists Bulletin v. 65, p. 191-218. Crouch, J.K, Bachman, S.B., and Shay, J.T, 1984, Post- Miocene compressional tectonics along the central California margin, in Crouch, J.K., and Bachman, S.B., eds., Tectonics and sedimentation along the California margin: Society of Economic Paleontologists and Mineralogists, Pacific Section, p. 37-54. Crowell, J.C., 1974, Origin of late Cenozoic basins in southern California; in Dickinson, W.R., ed., Tectonics and Sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication 22, p. 190-204. Davis, T.L. and Namson, J., 1989, A cross section of the Los Angeles area: seismically active fold and thrust belt, the 1987 Whittier Narrows earthquake, and earthquake hazard: Journal of Geophysical Research v. 94, p. 9644-9664. Davis, T.L., Namson, J., and Yerkes, R.F., 1989, A cross section of the Los Angeles area: Seismically active fold and thrust belt, the 1987 Whittier Narrows earthquake, and earthquake hazard: Journal of Geophysical Research, v. 94, p. 9644-9664. Dickinson, W.R., and Seely, R.D., 1979, Structure and stratigraphy of fore-arc regions: American Association of Petroleum Geologists Bulletin, v. 63, p. 2-31. Dickinson, W.R., and Snyder, W.S., 1979, Geometry of subducted slabs related to San Andreas transform: Journal of Geology, v. 87, p. 609-627. 277 Doyle, L.J., and Gorsline, D.S., 1977, Marine geology of Baja California, Mexico: American Association of Petroleum Geologists Bulletin, v. 61, p. 903-917.. Emery, K.O., 1954, General geology of the offshore area, southern California, in Jahns, R.J., ed.: Geology of southern California, ch. 2, p. 107-111. Emery, K.O., and Shepard, F.P., 1946, Lithology of the sea floor off southern California: Geological Society of America Bulletin v. 56, p. 431-478. Engebretson, D.C., Cox, A., and Gordon, R.G., 1985, Relative motions between oceanic and continental plates in the Pacific Basin: Geological Society of America Spec. Paper 206, 59 p. Ernst, W.G., 1975, Systematics of large-scale tectonics and age progressions in Alpine and circum-Pacific blueschist belts: Tectonophysics, v. 26, p. 229-246. , 1984, Californian blueschists, subduction, and the significance of tectonostratigraphic terranes: Geology, v. 12, p. 436-441. Espey, H.R., 1982, Seismic signal processing: Houston, Geotran Inc., 338 p. Ewing, J., and Talwani, M., 1991, Marine deep seismic reflection profiles off central California: Journal of Geophysical Research, v. 96, p. 6423-6433. EEZ-SCAN 84 Scientific Staff, 1986, Atlas of the Exclusive Economic Zone, western conterminous United States: United States Geological Survey Miscellaneous Investigation Ser.1-1864, 162 p. Forsythe, R.D., and Nelson, E.P., 1985, Geological manifestations of ridge collision: Evidence from the Golfo de Penas-Tiatao Basin, southern Chile: Tectonics, v. 4, p. 477-495. Forsythe, R.D., Nelson, E.P., Reading, M.E., Hewe, M., Soffia, J.M., Carr, M.J., Harasumour, S., and Mpodozis, C., 1986, Pliocene near-trench magmatism in southern Chile: A possible manifestation of ridge collision: Geology, v. 14, p. 23-27. Gibson, J.D., Howell, D.G., Fuis, G.S., and Vedder, J.G., 1985, North American continent-ocean transect C-3: Pacific Abyssal Plain to the Rio Grande Rift: 278 Geological Society of America Centennial Continent/Ocean Transect 5, 23 p. Gorsline, D.S., 1981, Deposition in active margin basins: With examples from the California Continental Borderland: in Gorsline, D.S., ed., Deposition in active margin basins: Society of Economic Paleontologists and Mineralogists, Pacific Section, v. 54, 51 p. Gorsline, D.S. and Emery, K.O., 1959, Turbidity currents deposits in San Pedro and Santa Monica Basins off southern California: Geological Society of America Bull. v. 70, p. 279-288. Haner, B.E., 1971, Morphology and sediments of Redondo Submarine Fan, southern California: Geological Society of America Bulletin, v. 82, p. 2313-2334. Haq, B.T., Hardenbol, J., and Vail, P.R., 1987, Chronology and fluctuating sea levels since the Triassic: Science, v. 235, p. 1156-1167. Harbert, W., 1991, Late Neogene relative motions of the Pacific and North American Plates: Tectonics, v. 10, p. 1-15. Harbert, W. and Cox, A., 1987, Late Neogene motion of the Pacific plate: Journal of Geophysical Research, v. 94, p. 3052-3064. Harding, T.P., Vierbuchen, R.C., and Christie-Blick, N., 1985, Structural styles, plate-tectonic settings, and hydrocarbon traps of divergent (transtensional) wrench faults, in Biddle, K.T., and Christie-Blick, N., eds., Strike-slip deformation, basin formation, and sedimentation: Society of Economic Paleontologists and Mineralogists Spec. Pub. 37, p. 51-77. Harrison, J.C., von Huene, R.E., and Corbato, C.E., 1966, Bouguer gravity anomalies and magnetic anomalies off the coast of southern California: Journal of Geophysical Research, v. 71, p. 4921-4941. Hauksson, E., 1990, Earthquakes, faulting, and stress in the Los Angeles Basin: Journal of Geophysical Research, v. 95, p. 15365-15394. Hauksson, E., and Jones, L.M., 1989, The 1987 Whittier Narrows earthquake sequence in Los Angeles, southern 279 California: Seismological and tectonic analysis: Journal of Geophysical Research, v. 94, p. 9569-9589. Hauksson, E., and Saldivar, G.V., 1989, Seismicity and active compressional tectonics in Santa Monica Bay, southern California: Journal of Geophysical Research, v. 94, p. 9591-9606. Hawkins, J.W., Allison, E.C., and MacDougall, D, 1971, Volcanic petrology and geologic history of Northeast Bank, southern California borderland: Geological Society of America Bulletin, v. 82, p. 219-228. Hawkins, J.W., and Divis, A.F., 1975, Petrology and geochemistry of mid-Miocene volcanism on the San Clemente and Santa Catalina Islands and adjacent areas of the southern California borderland: Geological Society of America Bulletin, Abstracts with Programs, v. 7, p. 323-324. Henyey, T.L., Li, Y.G., and Leary, P.C., 1989, Crustal seismic imaging of the San Bernardino Uplift/Mojave Desert transition zone: Transactions, American Geophysical Union v. 70, p. 1206. Higgins, R.E., 1976, Major-element chemistry of the Cenozoic volcanic rocks in the Los Angeles Basin and vicinity: in Howell, D.G., ed., Aspects of the Geologic History of the California Continental Borderland; American Association of Petroleum Geologists Pacific Section Misc. Pub. 24, p. 212-227. Hill, D.J., 1976, Geology of the Jurassic basement rocks, Santa Cruz Island, California and correlation with other Mesozoic basement terranes in California, in Howell, D.G., ed., Aspects of the geologic history of the California continental borderland: American Association of Petroleum Geologists Miscellaneous Publication 24, p. 16-46. Hornafius, J.S., Luyendyk, B.P., Terres, R.R., and Kamerling, M.J., 1986, Timing and extent of Neogene tectonic rotation in the wsetern Transverse Ranges, California: Geological Society of America Bulletin v. 97, p. 1476-1487. Howell, D.G., 1976, A model to accomodate 1000 kilometres of right-slip, Neogene displacement in the southern California area, in Howell, D.G., ed., Aspects of 280 the geologic history of the California continental borderland: American Association of Petroleum Geologists Miscellaneous Publication 24, p. 530-540. Howell, D.G., Champion, D.E., and Vedder, J.G., 1987, Terrane accretion, crustal kinematics, and basin evolution, southern California, in Ingersoll, R.V., and Ernst, W.G., eds., Cenozoic basin development of coastal California, Rubey Volume 6: Englewood Cliffs, New Jersey, Prentice-Hall Inc., p. 242-258. Howell, D.G., and Vedder, J.G., 1981, Structural implications of stratigraphic discontinuities across the southern California borderland, in Ernst, W.G., ed., The geotectonic development of California, Rubey Volume 5): Englewood Cliffs, New Jersey, Prentice-Hall Inc., p. 535-558. Hsu, K.J., 1991, Exhumation of high-pressure metamorphic rocks: Geology, v. 19, p. 107-110. Hubbard, R.J., Pape, J., and Roberts, D.G., 1985, Depositional sequence mapping to illustrate the evolution of a passive continental margin, in Berg, O.R., and Woolverton, D.G., eds., Seismic stratigraphy II: American Association of Petroleum Geologists Memoir 39, p. 93-115. Humphreys, E., Clayton, R.W., and Hager, B.H., 1984, A tomographic image of mantle structure beneath southern California Geophysical Reserch Letters, v. 11, p. 625-627. Hyndman, D.W., 1972, Petrology of igneous and metamorphic rocks: New York, McGraw-Hill Book Company, 533 p. Jahns, R.H., 1954, Geology of the Peninsular Range Province, southern California and Baja California, in Jahns, R.H., ed., Geology of southern California: California Division of Mines Bulletin, v. 170, p. 29-52. Junger, A., 1976, Tectonics of the southern California Borderland: in Howell, D.G., ed., Aspects of the Geologic History of the Californi Continental Borderland; American Association of Petroleum Geologists, Pacific Section Misc. Pub. 24, p. 486-498. Kawamura, T., and Aoki, Y., 1986, The Nankai Trough margin, in von Huene, R.E., ed., Seismic images of 281 modern convergent margin tectonic structure: American Association of Petroleum Geologists Studies in Geology, no. 26, p. 54-56. Kennedy, M.P., Greene, H.G., and Clarke, S.H., 1987, Geology of the California Continental Margin: Explanation of the California Continental Margin Geologic Map Series: California Division of Mines and Geology Bulletin 207, 110 p. Lee, C.F., 1977, Reflection profiling and trace metal geochemistry of the Newport Submarine Canyon region, Newport Beach, California [M.S. thesis]: Los Angeles, California State University, 258 p. Lee, C.F., Henyey, T.L., and McRaney, J.K., 1989, Evidence for Late Cenozoic tectonic deformation at the base of slope of the outer Continental Borderland, southern and Baja California: Transactions, American Geophysical Union v. 70, p. 1345. Lee, C.F., and Henyey, T.L., 1990, Compressional deformation in outer Continental Borderland Basins, Southern California: Geological Society of America Abstracts with Papers, p. A327. Lee, C.F., Okaya, D., and Henyey, T.L., 1990, Seismic reflection records of the Cortes Basins region, southern California Borderland; evidence for a late Oligocene-early Miocene sea-level lowstand: American Geophysical Union Transactions, v. 71, p. 1632. Legg, M.R., 1985, Geologic structure and tectonics of the Inner California Continental Borderland, offshore northern Baja California, Mexico [Ph.D. thesis]: Santa Barbara, California, University of California, 410 p. , 1991, Developments in understanding the tectonic evolution of the California continental borderland, in Osborne, R., ed., From shoreline to abyss: Society of Economic Paleontologists and Mineralogists Special Publication 46, p. 291-312 (in press). Legg, M.R., Luyendyk, B.P., Mammerickx, J., de Moustier, C., and Tyce, R.C., 1989, Sea Beam survey of an active strike-slip fault: The San Clemente fault in the California Continental Borderland: Journal of Geophysical Research, v. 94, p. 1727-1744. 282 Li, Y.G., and Henyey, T.L., 1989, Mapping an overthrust near the San Andreas fault using a combination of seismic reflection profiling, ray-tracing and gravity modeling: Transactions, American Geophysical Union v. 70, p. 1214. Lund, S.P., and Bottjer, D.J., 1991, Paleomagnetic evidence for microplate tectonic development of southern and Baja California: American Association of Petroleum Geologists Mem. 47, in press. Lund, S.P., Bottjer, D.J., Whidden, K.J., and Powers, J.E., 1991, Paleomagnetic evidence for the timing of accretion of the Peninsular Ranges Terrane in southern California: Geological Society of America Abstracts with Programs, v. 23, p. 74. Lundberg, N., and Reed, D.L., 1991, Continental margin tectonics: Forearc processes, in Shea, M.A., ed., Reviews of Geophysics Supplement: American Geophysical Union, p. 794-806. Luyendyk, B.P., and Hornafius, J.S., 1987, Neogene crustal rotations, fault slip, and basin development in southern California, in Ingersoll, R.V., and Ernst, W.G., eds., Cenozoic basin development of coastal California: Englewood Cliffs, New Jersey, Prentice-Hall, Inc., p. 259-283. Mann, D.M., McCulloh, T.H., and Crouch, J.K., 1980, Multichannel seismic-reflection profiles collected in 1978 in the eastern Pacific Ocean off the California coast south of Point Conception: United States Geological Survey Open-File Rept. 80-9466, 3 p. Nardin, T.R., 1981, Seismic stratigraphy of Santa Monica and San Pedro basins, California Continental Borderland; Late Neogene history of sedimentation and tectonics [Ph.D. thesis]: Los Angeles, University of Southern California, 295 p. Nardin, T.R., Hein, F.J., Gorsline, D.S., and Edwards, B.D., 1979, A review of mass movement processes, sediment and acoustic characteristics in slope and base-of-slope systems versus canyon-fan-basin floor systems, in Doyle, L., and Pilkey, 0., eds., Geology of the continental slope: Society of Economic Paleontologists and Mineralogists Special Publication 27, p. 61-73. 283 Mattinson, J.M., and Hill, D.J., 1976, Age of plutonic basement rocks, Santa Cruz Island, California, in Howell, D.G., ed., Aspects of the geologic history of the California continental borderland: American Association of Petroleum Geologists Miscellaneous Publication 24, p. 53-58. McCulloch, D.S., 1989, Evolution of the offshore central California margin, in Winterer, E.L., Hussong, D.M., and Decker, R.W., eds., The Eastern Pacific Ocean and Hawaii: Boulder, Colorado, Geological Society of America, v. N, p. 439-470. McIntosh, K.D., Reed, D.L., Silver, E.A., and Meltzer, A.S., 1991, Deep structure and structural inversion along the central California continental margin from EDGE seismic profile RU-3: American Association of Petroleum Geologists Memoir 47 (in press). Meltzer, A.S., and Levander, A.R., 1991, Deep-crustal reflection profiling offshore southern central California: Journal of Geophysical Research, v. 96, p. 6475-6491. Minch, J.C., Gastil, G., Fink, W., Robinson, J., and James, A.H., 1976, Geology of the Vizcaino Peninsula, in Howell, D.G., ed., Aspects of the geologic history of the California continental borderland: American Association of Petroleum Geologists Miscellaneous Publication 24, p. 136-195. Mitchum, R.M., Vail, P.R., and Sangree, J.B., 1977, Seismic stratigraphic interpretation of seismic reflection patterns in depositional sequences, in Payton, C.E., ed., Seismic stratigraphy: American Association of Petroleum Geologists Memoir 26, p. 117-133. Moore, D.G., 1969, Reflection profiling studies of the California Continental Borderland - Structure and Quaternary turbidite basins: Geological Society of America Special Paper 107, 142 p. Mount, V.S., and Suppe, J., 1987, State of stress near the San Andreas fault: Implications for wrench tectonics: Geology, v. 15, p. 1143-1146. Namson, J., and Davis, T., 1988, Structural transect of the western Transverse Ranges, California: Implications for lithospheric kinematics and seismic risk evaluation: Geology, v. 16, p. 675-679. 284 Parker, F.S., 1971, Petroleum potential of southern California offshore, in Cram, I.H., ed., Future petroleum provinces of the United States: American Association of Petroleum Geologists Memoir 15, v. 1 p. 178-191. Paul, R.G., Arnal, R.E., Baysinger, J.P., Claypool, G.E., Holte, J.L., Lubeck, C.M., Patterson, J.M., Poore, R.Z., Slettene, R.L., Sliter, W.V., Taylor, J.C., Tudor, R.B., and Webster, F.L., 1976, Geological and operational summary, southern California deep stratigraphic test OCS-CAL 75-70 No. 1, Cortes Bank area, offshore southern California: United States Geological Survey Open-File Rept. 76-232, 65 p. Platt, J.P., 1975, Metamorphic and deformational processes in the Franciscan Complex, California: Some insights from the Catalina Schist terrane: Geological Society of America Bulletin, v. 86, p. 1337-1347. , 1986, Dynamics of orogenic wedges and the uplift of high-pressure metamorphic rocks: Geological Society of America Bulletin, v. 97, p. 1037-1053. Rangin, C., Girard, D., and Maury, R., 1983, Geodynamic significance of Late Triassic to Early Cretaceous volcanic sequences of Vizcaino Peninsula and Cedros Island, Baja California, Mexico: Geology, v. 11, p. 52-556. Reddihough, R., and Hyndman, R.D., 1989, Queen Charlotte Islands margin, in Winterer, E.L., Hussong, D.M., and Decker, R.W., eds., The Pacific Ocean and Hawaii: Boulder, Colorado, Geological Society of America, The Geology of North America, v. N, p. 403-411. Reynolds, S., and Gorsline, D.S., 1987, Nicolas and Eel submarine fans, California Continental Borderland: American Association of Petroleum Geologists Bulletin v. 71, p. 452-463. Sangree, J.B., and Widmier, J.M., 1977, Seismic interpretation of clastic depositional facies, in Payton, C.E., ed., Seismic stratigraphy: American Association of Petroleum Geologists Memoir 26, p. 165-184. Scholl, D.W., McCarthy, J., and Ryan, H., 1986, Forearc margin, central Aleutian Ridge, in von Huene, R.E., 285 ed., Seismic images of modern convergent margin tectonic structure: American Association of Petroleum Geologists Studies in Geology, no. 26, p. Schwalbach, J.R., 1982, A sediment budget for the northern region of the California Continental Borderland [M.S. thesis]: Los Angeles, University of Southern California, 212 p. Sedlock, R.L., and Hamilton, D.H., 1991, Late Cenozoic tectonic evolution of southwestern California: Journal of Geophysical Research, v. 96, p. 2325-2351. Severinghaus, J., and Atwater, T., 1989, Cenozoic geometry and thermal condition of the subducting slabs beneath western North America, in Wernicke, B., ed., Basin and Range Extension: Geological Society of America Memoir. Shepard, F.P., and Emery, K.O., 1941, Submarine topography off the California coast; canyons and tectonic interpretations: Geological Society of Amerca Special Paper 31, 171 p. Sheriff, R.E., and Geldart, L.P., 1982, Exploration seismology: Data Processing and interpretation: Cambridge, Cambridge University Press, v. 2, 221 p. Shor, G.G., Jr., and Raitt, R.W., 1958, Seismic studies in the southern California borderland: Congreso Geologico Internacional, XXa Seccion, 1956, Mexico, Seccion IV, Segundo Tomo, p. 243-259. Silver, E.A., and Reed, D.L., 1988, Backthrusting in accretionary wedges: Journal of Geophysical Research, v. 93, p. 3116-3126. Sloss, L.L., 1991, The tectonic factor in sea level change: A countervailing view: Journal of Geophysical Research, v. 96, p. 6609-6617. Teng, L.S., 1985, Seismic stratigragraphic study of the California Continental Borderland basins: Structure, stratigraphy, and sedimentation [Ph.D. thesis]: Los Angeles, California, University of Southern California, 197 p. Teng, L.S., and Gorsline, D.S., 1989, Late Cenozoic sedimentation in California Continental Borderland basins as revealed by seismic facies analysis: 286 Geological Society of America Bulletin v.101, p. 27-41. Tennyson, M.E., 1989, Pre-transform early Miocene extension in western California: Geology, v. 17, p. 792-796. Thorkelson, D.J., and Taylor, R.P., 1989, Cordilleran slab windows: Geology, v. 17, p. 833-836. Torrini, R., Jr., and Speed, R.C., 1989, Tectonic wedging in the forearc basin-accretionary prism transition, Lesser Antilles forearc: Journal of Geophysical Research, v. 94, p. 10549-10584. Turcotte, D.L., and Schubert, G., 1982, Geodynamics: Applications of continuum physics to geological problems: New York, John Wiley and Sons, 450 p. Unruh, J.R., Ramirez, V.R., Phipps, S.p., and Moores, E.M., 1991, Tectonic wedging beneath fore-arc basins: Ancient and modern examples from California and the Lesser Antilles: GSA Today, v. 1, p. 185, 188-190. Vail, P.R., Mitchum, R.M., and Thompson, S., 1977a, Global cycles of relative changes of sea level, in Payton, C.E., ed., Seismic stratigraphy: American Association of Petroleum Geologists Memoir 26, p. 83-97. , 1977b, Relative changes of sea level from coastal onlap, in Payton, C.E., ed., Seismic stratigraphy: American Association of Petroleum Geologists Memoir 26, p. 63-81. Vedder, J.G. 1987, Regional geology and petroleum potential of the southern California Borderland: in Scholl, D.W., Grantz, A., and Vedder, J.G., eds., Geology and Resource Potential of the Continental Margin of Western North America and Adjacent Ocean Basins, Beaufort Sea to Baja California: Circum-Pacific Council for Energy and Mineral Resources, Houston, Texas, Earth Science Series v. 6, p. 403-447. Vedder, J.G., Arnal, R.E., Barron, J.A., Bukry, D., Crouch, J.K., and Lee-Wong, F., 1981, Composition and correlation of bedrock and sedoiment cores, R/V Sea Sounder cruise S3-79-SC, May 1979, California 287 Continental Borderland: United States Geological Survey Open-File Rept. 81-744, 74 p. Vedder, J.G., Arnal, R.E., Bukry, D., Barron, J.A., and Lee-Wong, F., 1979, Descriptions of dart core samples, R/V Samuel P. Lee Cruise L2-78-SC, May 1978, California Continental Borderland: United States Geological Survey Open-File Rept. 79-936, 46 p. Vedder, J.G., Beyer, L.A., Junger, A., Moore, G.W., Roberts, A.E., Taylor, J.C., and Wagner, H.C., 1975, Preliminary report on the geology of the continental borderland of southern California: United States Geological Survey Misc. Field Studies Maps MF-624, 34 p., 9 sheets. Vedder, J.G., Crouch, J.K., Arnal, R.E., Bukry, D., Barron, J.A., and Lee-Wong, F., 1977, Descriptions of pre-Quaternary samples, R/V Ellen B. Scripps, September 1976, Patton Ridge to Blake Knolls, California Continental Borderland: United States Geological Survey Open-File Rept. 77-474, 19 p. Vedder, J.G., and Howell, D.G., 1976, Review of the distribution and tectonic implications of Miocene debris from the Catalina Schist, California Continental Borderland and adjacent coastal areas: in Howell, D.G., ed., Aspects of the Geologic History of the California Continental Borderland: American Association of Petroleum Geologists, Pacific Section, Miscellaneous Publication 24, p. 326-342. von Huene, R.E, 1986, Seismic images of modern convergent margin tectonic structure: American Association of Petroleum Geologists Studies in Geology no. 26. von Huene, R.E., Fischer, M., and Miller, J., 1986, The eastern Aleutian continental margin, in von Huene, R.E., ed., Seismic images of modern convergent margin tectonic structure: American Association of Petroleum Geologists Studies in Geology, no. 26, p. 20-23. von Huene, R.E, Suess, E., and Leg 112 Shipboard Scientists, 1988, Ocean Drilling Program Leg 112, Peru continental margin: Part 1, Tectonic history: Geology v. 16, p. 934-938. 288 Wentworth, C.M., 1987, New evidence on the state of stress of the San Andreas fault system: Science, v. 238, p. 1105-1111. Wentworth, C.M., Blake, M.C., Jr., Jones, D.I., Walters, A.W., and Zoback, M.D., 1984, Tectonic wedging associated with emplacement of the Franciscan assemblage, California Coast Ranges, in Blake, M.C., Jr., ed., Franciscan geology of northern California: Society of Economic Paleontologists and Mineralogists, Pacific Section, v. 43, p. 163-173. wentworth, C.M., and Zoback, M.D., 1989, The style of late Cenozoic deformation at the eastern front of the California Coast Ranges: Tectonics, v. 8, p. 237-246. Westbrook, G.K., Ladd, J.W., Buhl, P., and Bangs, N., 1988, Cross section of an accretionary wedge: Barbados Ridge complex: Geology, v. 16, p. 631-635. Wilcox, R.E., Harding, T.P., and Seely, D.R., 1973, Basic wrench tectonics: American Association of Petroleum Geologists Bulletin v. 57, p. 74-96. Yeats, R.S., Haq, B.U., and others, 1981, Site 468: Patton Escarpment, in Yeats, R.S., Haq, B.U., and others, eds., Initial reports of the deep-sea drilling project: Washington D.C., United States Government Printing Office, p. 113-129. Yilmaz, 0, 1987, Seismic Data Processing: Tulsa, Oklahoma, Society of Exploration Geophysicists, 526 p. Zoback, M.D., Zoback, M.L., Mount, V.S., Suppe, J., Eaton, J.P., Healy, J.H., Oppenheimer, D., Reasenberg, P., Jones, L., Raleigh, C.B., Wong, I.G., Scotti, 0., and Wentworth, C., 1987, New evidence on the state of stress of the San Andreas fault system: Science, v. 238, p. 1105-1111. 289 WELL NO. 70-75-1 Depth Interval Total Time Interval (m) Transit (sec) Velocity Time (sec) (m/sec) 115 0.1534 0.1534 1460 226 0.1412 0.2946 1572 232 0.0099 0.3016 1742 241 0.0118 0.3134 1547 259 0.0240 0.3374 1524 341 0.1807 0.5181 1825 372 0.0321 0.5502 1905 384 0.0107 0.5609 2258 405 0.0245 0.5854 1742 418 0.0103 0.5975 2033 436 0.0164 0.6139 2225 445 0.0093 0.6232 1976 476 0.0270 0.6502 2258 488 0.0132 0.6634 1848 497 0.0067 0.6701 2722 503 0.0060 0.6761 2033 515 0.0108 0.6869 2258 546 0.0280 0.7149 2178 564 0.0150 0.7299 2439 585 0.0161 0.7460 2651 597 0.0080 0.7540 3049 652 0.0414 0.7954 2651 674 0.0192 0.8146 2225 695 0.0175 0.8321 2439 793 0.0429 0.8750 4550 851 0.0284 0.9034 4065 869 0.0079 0.9113 4690 905 0.0317 0.9430 2310 927 0.0156 0.9586 2722 951 0.0184 0.9770 2651 972 0.0168 0.9938 2540 1073 0.0825 1.0763 2439 1110 0.0276 1.1039 2651 1140 0.0240 1.1279 2540 1189 0.0343 1.1622 2849 1274 0.0616 1.2238 2772 1305 0.0244 1.2482 2499 1341 0.0395 1.2787 2401 1366 0.0191 1.2978 2540 1390 0.0200 1.3178 2439 1433 0.0309 1.3487 2772 -M Mio/L Mio -volcanics -Oli/U Eo i 290 1451 0.0122 1.3609 2989 1466 0.0112 1.3721 2722 1482 0.0450 1.3836 2651 1506 0.0176 1.4012 2772 1530 0.0152 1.4164 3209 1646 0.0700 1.4864 3314 1683 0.0216 1.5080 3387 1701 0.0104 1.5184 3504 1747 0.0270 1.5454 3387 1783 0.0228 1.5682 3209 1802 0.0108 1.5790 3387 1823 0.0136 1.5926 3143 1847 0.0144 1.6070 3387 1857 0.0057 1.6127 3209 1866 0.0055 1.6182 3314 1905 0.0221 1.6403 3587 1976 0.0400 1.6803 3504 1997 0.0119 1.6922 3587 2049 0.0279 1.7201 3718 2082 0.0191 1.7392 3504 2128 0.0240 1.7632 3811 2183 0.0277 1.7909 3959 2195 0.0069 1.7978 3587 2213 0.0090 1.8070 3959 2241 0.0153 1.8223 3587 2347 0.0539 1.8762 3959 2378 0.0164 1.8926 3718 2418 0.0221 1.9147 3587 2445 0.0135 1.9282 4065 2497 0.0238 1.9520 4355 2515 0.0080 1.9600 4550 2536 0.0091 1.9691 4690 2585 0.0231 1.9922 4234 2591 0.0027 1.9949 4550 2 622 0.0130 2.0079 4690 2713 0.0377 2.0456 4839 2729 0.0067 2.0523 4550 2774 0.0210 2.0733 4355 2796 0.0094 2.0827 4500 2808 0.0052 2.0879 4690 2829 0.0084 2.0963 5081 2854 0.0104 2.1067 4690 2869 0.0067 2.1134 4550 2887 0.0078 2.1212 4690 2921 0.0147 2.1359 4550 2963 0.0169 2.1528 5081 2972 0.0037 2.1565 4917 2982 0.0045 2.1610 4065 2994 0.0052 2.1662 4690 3033 0.0161 2.1823 4917 3061 0.0121 2.1944 4550 -U Eo/M Eo -Pal/UK -UK unc 291 3073 0.0052 2.1996 4690 3131 0.0235 2.2231 4917 3165 0.0143 2.2374 4690 3186 0.0094 2.2468 4550 3201 0.0062 2.2530 4917 3241 0.0169 2.2699 4690 3305 0.0282 2.2981 4550 3314 0.0041 2.3022 4355 3329 0.0068 2.3090 4550 292 WELL NO. P-286 Depth Interval Total Time Interval Description (m) Transit Time (sec) (sec) Velocity (m/sec) 155 0.2120 0.2120 1460 318 0.2100 0.4220 1555 434 0.1196 0.5416 1935 549 0.0904 0.6320 2556 -Volcanics 633 0.0442 0.6762 3793 (565 m) 731 0.0858 0.7620 2281 856 0.1036 0.8656 2390 -Top 01i 978 0.9620 0.9618 2573 (878 m) 1082 0.0834 1.0452 2472 Top Eo 1173 0.0700 1.1152 2613 (969 m) 1356 0.1402 1.2554 2609 -U Eo/M Eo 1508 0.0923 1.3477 3300 (1357 m) 1649 0.0798 1.4275 3515 1737 0.0480 1.4755 3684 -Top Pal 1859 0.0644 1.5399 3778 (1756 m) 1972 0.0604 1.6003 3735 2124 0.0718 1.6721 4238 2276 0.0698 1.7491 4368 -K (?) 2408 0.0678 1.8097 3876 (2314 m) 2465 0.0274 1.8371 4184 2508 0.0250 1.8621 3415 2680 0.0822 1.9443 4206 2833 0.0740 2.0183 4136 -UK unc 2926 0.0390 2.0573 4737 (2847 m) 3026 0.0450 2.1023 4445 3150 0.0542 2.1565 4590 3245 0.0418 2.1983 4551 3296 0.0240 2.2223 4217 3442 0.0624 2.2847 4671 3618 0.0740 2.3196 4754 WELL NO. P-276 Depth Interval Total Time Interval Descriptioi (m) . Travel Time (sec) (sec) Velocity (m/sec) 147 0.2017 0.2017 1460 374 0.2517 0.4534 1800 -volcanics 636 0.1880 0.6415 2793 (399 m) 820 0.1538 0.7953 2394 -U Eo 975 0.1254 0.9207 2470 (960 m) 1125 0.1160 1.0367 2586 1197 0.0528 1.0895 2714 1359 0.1064 1.1959 3054 -U Eo/M Eo 1502 0.0842 1.2801 3382 (1433 m) 1654 0.0856 1.3657 3561 1807 0.0812 1.4469 3754 1959 0.0782 1.5251 3899 2111 0.0780 1.6031 3909 2264 0.0764 1.6795 3990 2416 0.0764 1.7559 3990 -Pal 2569 0.0762 1.8321 4001 (2424 m) 2721 0.0748 1.9069 4076 -UK unc 2874 0.0630 1.9699 4840 (2829 m) 3026 0.0626 2.0325 4870 WELL NO. P-2 62 Depth Interval Total Time Interval Description (m) Transit (sec) Velocity Time (sec) (m/sec) 326 0.4473 0.4473 1460 508 0.2012 0.6485 1800 584 0.0662 0.7147 2303 759 0.1514 0.8661 2316 875 0.0908 0.9569 2552 -Volcanics 916 0.0204 0.9773 4035 (898 m) 1040 0.0960 1.0733 2572 -Top Eo (?) 1194 0.1202 1.1935 2562 (1165 m) 1346 0.1148 1.3083 2656 1498 0.1146 1.4229 2660 -U Eo/M Eo 1636 0.0918 1.5147 2989 (1590 m) 1767 0.0756 1.5903 3468 1922 0.0866 1.6769 3591 2004 0.0464 1.7233 3548 2126 0.0644 1.7877 3787 2291 0.0850 1.8727 3874 2416 0.0652 1.9379 3834 2576 0.0898 2.0277 3565 2596 0.0100 2.03769 3963 2706 0.0560 2.0937 3920 -UK unc 2733 0.0180 2.1055 4651 (2728 m) 2870 0.0594 2.1649 4619 2965 0.0428 2.2080 4416 WELL NO. P-257 Depth Interval Total Time Interval (m) Transit (sec) Velocity Time (sec) (m/sec) 165 0.2259 0.2259 1460 539 0.4160 0.6419 1800 646 0.0790 0.7209 2701 798 0.1070 0.8279 2849 890 0.0598 0.8877 3059 1079 0.1302 1.0179 2904 1199 0.0798 1.0977 3018 1362 0.0938 1.1915 3478 1500 0.0786 1.2701 3491 1591 0.0536 1.3237 3413 1667 0.0520 1.3757 2931 1713 0.0262 1.4019 3491 1850 0.0794 1.4813 3457 1948 0.0532 1.5345 3668 2094 0.0880 1.6225 3326 2170 0.0410 1.6635 3718 2384 0.1102 1.7737 3873 2567 0.0856 1.8593 4274 2634 0.0316 1.8909 4245 2734 0.0404 1.9313 4981 2771 0.0158 1.9471 4631 2872 0.0434 1.9905 4636 2902 0.0120 2.0025 5081 2948 0.0180 2.0205 5081 2957 0.0034 2.0239 5380 Description -M Eo (396 m) -Eo/Pal -UK unc (?) CJ) 0 z 0 (.) w CJ) SW 0 600 2 3 4 5 800 - 1000 - ? Ln.901 1200 1400 San Nicolas Basin -- - ~"""""'"'~-.,,= --- - --- ?-------? r----~~----- ' LINE 116 Ln.109 1600 1800 2000 2200 2400 2600 2800 San Clemente Ridge PLATE A 3000 3200 0 5km NE 0 4 5 (j) [Tl () 0 z 0 (j) .~ ~·,j (/) 0 z 0 (.) w (/) SW ., 2 3 4 5 6 LINE 908 -- ? - ~-- =-- = ------- - --- - Plate 15 A NE -=--=-=- 2 ? -;:..::? / - --- ---- ? ? /~ ;.- U> fT1 () 3 0 z 0 U> 4 5 6 0. 000 0. 200 0. 400 o. 600 0. 800 1. 000 1. 200. 1. 400 1. 600 1. 800 2. ODO 2. 200 2.400 2.600 2. 800 3. ODO 3. 200 3. 400 3.600 3. 800 4. 000 4. 200 4. 400 4. 600 4. 800 s. 000 5. 200 5. 400 5.600 5.800 6.000 6. 200 6. 400 6. 600 6.800 7.000 SW -------------------------------------·------------ ------------------------------------------ KSHIH LINE 908 Plate 158 --'----------~--------------------------------- - -------------------- ------------·--- KSHIH 0.000 0. 200 0. 400 0. 600 0.800 I. 000 1.200 1. 400 I. 600 I. 800 2. 000 2. 200 2. 400 2. 600 2. 800 3. 000 3. 200 3. 400 3. 600 3. 800 4. ODO 4. 200 4. 400 4. 600 4. 800 5. 000 5. 200 5. 400 5. 600 5. 800 6. 000 6. 200 6. 400 6. GOO 6. 800 7. 000 (/) 0 z 0 u w (/) 3400 NW 045:tr. 257 I # It- 3~ I- 4- • 3200 I I 3000 I 2800 I -- - ~ - -- - - ~~---1 LINE 115 286 1400 I - - I - --- - =-.-.--~ 1200 I ---- - --- ~- - 1000 I -- - - .= - - - Ln.120 75-70-1 soo'' I -- -=- --=- - -- - -- ,-- ---==- - - - -- - 600 I 2100z 400 I 0 200 I Plate 17A 2000z 5km SE . 0 0 . - 4 Cfl fTl () 0 z 0 (fl CD I'- QJ +- 0 a.. ~ u cc I- en Lf) .-f .-f w z 1--1 _J CL D u w 1 (/) 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700. 2800 2900 3000 3100 3200 3300 ~ 3400 CL D u ,_-, ~ ISl 0 "' r ~ ISl I 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650 1100 "' ~ ISl 0 • ill "" "' "' . - I ISl "' . .... "' ISl "' "' . ru (T) I I g!j g!j . ru "' ISl "' "' "' "" Lf) I g!j g!j " "' ISl "' . (0 I "' ISl "' ~I ~! " ISll "' . r Cf) 0 z 0 u w Cf) NW 0 2 3 4 5 - _ __.,.. -- -- - -- ____.... --- -- 100 200 T 400 11 300 T 500 -~ -- - ..... ______ ..,--= - =-- - LINE 16 ...__ - -- - - -- 600 Ln.911 700 Velero Basin - ;;:;:: :::- --=- --- -- - - 800 900 1000 0 5km Plate 18A SE 1100 2 3 4 5 Cf) JTl (") 0 z 0 en CDP 52 56 60 64 68 72 76 80 84 88 92 96 100 104 108 112 116 120 124 128 132 136 140 14 4 148 152 156 160 164 168 172 176 180 184 188 192 196 200 204 208 212 216 220 224 228 232 236 240 244 248 252 256 260 264 268 272 276 280 284 288 292 296 300 304 308 312 316 320 324 328 332 336 340 344 348 352 356 360 364 368 372 376 380 384 388 392 396 400 404 408 412 416 420 424 428 432 436 440 444 448 452 456 460 464 468 472 476 480 484 488 492 496 500 504 508 512 516 520 524 528 532 536 540 544 548 552 556 560 564 568 572 576 580 584 588 592 596 600 604 608 612 616 620 624 628 632 636 640 644 648 652 656 660 664 668 672 676 680 684 688 692 696 700 704 708 712 716 720 724 728 732 736 740 744 748 752 756' 760 764 768 772 776 780 784 788 792 796 800 804 808 812 816 820 824 828 832 836 840 844 848 852 856 860 864 868 872 876 880 884 888 892 896 900 904 908 912 916 920 924 928 932 936 940 944 948 952 956 960 964 968 972 976 980 984 988 992 996 1000 1004 1008 1012 1016 1020 1024 1028 1032 1036 1040 1044 1048 1052 1056 1060 1064 1068 1072 1076 1080 1084 1088 1092 1096 1100 1104 COP ~mmmmmmmmmmmmmmmrnrnmmm~~~~~~~A~~wwwwwwwwwwNNNNNNNNNN----------aooaoooooo owm~mm~wN-owm~mm~wN-owoo~mmAWN-o~oo~mm~wN-owoo~mm~wN-o~m~mm~wN-o~oo~mm~wN-o ~mmmmmmrnmmmmmmmmmmmmm~A~~~~~~~~WWWWWWWWWWNNNNNNNNNN----------0000000000 ....................................................................... ornoo~mm~wN-arnoo~mm~wN-a~oo~mm~wN-a~oo~~m~wN-ornoo~mm~wN-arnoo~mm~wN-o~oo~mm~wN-o z ~ r z rrl 0) en ITI "'U 0 - CD CD CD Cf) 0 z 0 u w Cf) Plate 19A NW Ln.120 LINE 21 Ln.906 Ln.911 Ln.905 SE t Ln.114 OrlT4_0_0~~~~~~~~1_30r0"-~~~~~~-'-12,o~o.;;;__~~~~~~~~l~IO~O'--~~~~~~-""'l~Or0~0~~~~~~~~9~0~0.;;;__~~~~~~~-.-~8~0~0'--~~~~~~-'-~~~~~'--''--~~-=r=-~~~~~~~-=T-"-~~~~~~~~---'-r=-~~~~~~~-=r-=-~~~~~~~~=r"'-~~~~~~~-.:.:;=-~~~----. 700 600 500 400 300 200 100 0 =-~-···- West Cortes Basin -- -- --- -- -- - 2 -- -=- = - - --- -- -- = -- -'-==--- - -- - - =- --- -- =- - - --- --- - - -- = -- ---=--~ - - - -- - 4 -- ---- -- - 0 5km 5 5 • • (/) !'Tl (") 0 z 0 (/) CDP 1396 1392 1388 1384 1380 1376 1372 1368 1364 1360 1356 1352 1348 1344 1340 1336 1332 1328 1324 1320 1316 1312 1308 1304 1300 1286 1292 1288 1284 1280 1276 1272 1268 1264 1260 1256 1252 1248 1244 1240 1236 1232 1228 1224 1220 1216 1212 1208 1204 1200 1196 1192 1188 1184 11 []0 11 r/6 1172 1168 1164 1160 1156 1152 11 48 1144 11 40 1136 1132 1128 1124 1120 111 6 1112 1108 11 04 11 00 1096 1092 1088 1084 1080 1076 1072 1068 1064 1060 1056 1052 1048 1044 1040 1036 1032 1028 1024 1020 1016 1012 1008 1004 1000 996 992 988 984 980 976 972 968 964 960 956 952 948 944 940 936 932 928 924 920 916 912 908 904 800 896 892 888 884 880 876 872 868 864 860 856 852 848 844 840 836 832 828 824 820 816 812 808 804 800 796 792 788 784 780 776 772 768 764 760 756 752 748 744 740 736 732 728 724 720 716 712 708 '704 700 696 692 688 684 680 676 672 668 664 660 656 652 648 644 640 636 632 628 624 620 616 612 608 604 600 596 592 588 584 580 576 572 568 564 560 556 552 548 544 540 536 532 528 524 520 516 512 508 504 500 496 492 488 484 480 476 472 468 464 460 456 452 448 444 440 436 432 428 424 420 416 412 408 404 400 396 392 388 384 380 376 3"/2 368 364 360 356 352 348 344 340 336 332 328 324 320 316 312 308 304 300 296 29~ 288 284 280 276 272 268 264 260 256 252 248 244 240 236 232 228 224 220 216 212 208 204 200 196 192 188 184 180 176 172 168 164 160 156 152 148 144 140 136 132 128 124 120 116 112 1 08 1 04 100 96 92 88 84 BU 76 72 68 64 60 56 52 COP ~mmmmmmmmmmmmmmmmmmmm~A~~~~~~~~WWWWWWWWWWNNNNNNNNNN----------0000000000 ....................................................................... owrn~mm~wN-owm~mm~wN-owrn~mm~wN-owrn~mm~wN-owrn~mm~wN-owrn~mm~wN-omrn~mm~wN-o ~mmmmmmmmmmmmmmmmmmmm~~~~~~~~~~wwwwwwwwwwNNNNNNNNNN----------0000000000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . owrn~mm~wN-owrn~mm~wN-omrn~mm~wN-omrn~mm~wN-omrn~mm~wN-omrn~mm~wN-omrn~mm~wN-o z :E r z (TJ I\) en ITI "'U 0 it (.0 (D

Asset Metadata

Creator Lee, Calvin Fong (author)

Core Title Seismic sequence stratigraphy and structural development of the southern outer portion of the California Continental Borderland

Contributor Digitized by ProQuest (provenance)

Degree Doctor of Philosophy

Degree Program Geological Sciences

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

Tag Marine Geology,OAI-PMH Harvest

Language English

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c29-355125

Unique identifier UC11218816

Identifier DP28602.pdf (filename),usctheses-c29-355125 (legacy record id)

Legacy Identifier DP28602.pdf

Dmrecord 355125

Document Type Dissertation

Rights Lee, Calvin Fong

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