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Structural Reactivation and Overprinting of the Eastern Rand

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STRUCTURAL REACTIVATION AND OVERPRINTING OF THE EASTERN RAND THRUST COMPLEX, MOJAVE DESERT, SOUTHERN CALIFORNIA A Thesis Presented to the Faculty of California State Polytechnic University, Pomona In Partial Fulfillment Of the Requirements for the Degree Master of Science In Geological Sciences By Andrew H. McLarty 2014 ACKNOWLEDGMENTS This research represents devotion and perseverance in something I truly love, not only as a student but as a professional geologist. I am extremely pleased to present the following to California State Polytechnic University, Pomona. I want to personally thank my advisor Dr. Nourse for recommending this project to me; words do not do justice for what you have taught me. Over the past 20+ years I have had many mentors, who have made me the geologist I am today. Those of which are; my father, Mark McLarty, Dr. Tarman, Dr. Jessey, Dr. Herber, and last but not least Dr. Nourse, thank you all. This process of being a grad student at this stage of my life has been difficult at times and it would not have been possible without the support, love and guidance from my wife, Sarah, and my two children, Madison and Kylie. Sarah, you have reminded me numerously not to give up, reminded me to be persistent, bear down, and to focus on my goals, thank you Michi and my lil ones, for all your love and support. I would also like to thank, fellow grad student, Logan Wicks for all your help, not only in the field, but with other aspects regarding this project. His abilities as a geologist and field geologists, to observe and recognize geologic aspects in the field is astonishing. Thank you for your help “Buzzzz” it means a lot to me and I will never forget it. I would also like to thank my brother Jeff, for spending multiple weekends with me in the field. I taught him how to read and measure striations and lineations in the field and he was amazed about some of our geologic field findings. You were a huge help to me in the field. Thank you Jeffrey for the numerous weekends spent in the field and time away from your family and other obligations. iii I would also like to thank my thesis advisor Dr. Jonathan Nourse. I am extremely grateful for your help, guidance, excellent mentorship, limitless patience, and continued friendship. Thank you for introducing me to something you have been thinking about for 25+ years and for allowing me to endeavor on an aspect of your doctorate dissertation, it really means a lot me. Last but not least, I would like to thank my father, Mark McLarty, your support, guidance, help in the field, review, suggestions, more review, can never be repaid. Thank you for all your help and everything you have taught me as a geologist and as a father. iv ABSTRACT Presented herein is a 1:3000 scale geologic map of the Rand Thrust Complex (RTC) in the eastern Rand Mountains, California. Geologic study was conducted (Sections 19-20, T30S, R40E of the Johannesburg 7.5 minute quadrangle) to deduce the mechanisms, extent and timing of fault displacements, and to better understand the regional geologic context. The location of previously mapped geologic contacts was refined with GPS technology, new outcrop observations and structural measurements. The RTC consists of four tectonic plates each separated by low angle faults. Each plate is well exposed within the study area along the south-dipping flank of an east-trending antiform. The deepest unit, the ~80 Ma old Rand Schist (Plate I) includes an assemblage of greenschist, metagraywacke, metachert and metabasalt. Plate I is structurally overlain by the Johannesburg Gneiss (Plate II) that includes garnet-amphibolite grade granitic and dioritic orthogneiss, marble and calcsilicate. An 87 Ma old Late Cretaceous hornblende- biotite-sphene granodiorite (Plate III; Atolia Granodiorite) is sandwiched between Plates II and IV (a structurally higher biotite alkali granite). The basal contact of Plate IV is defined by a zone of quartz-rich ultramylonite that locally truncates fault contacts between the lower plates. Structural data collected from mylonitic and brittle fault fabrics document multiple movements that post-date metamorphism of the Rand Schist. The earliest post-metamorphic shearing records the transport of Plate III over Plate I with intervening Plate II retrograded to greenschist facies and mylonitized to produce NW- trending lineations. A later ductile shearing episode is recorded by shallow NE-dipping mylonites in Plate IV that display consistently oriented lineation with associated microstructures and asymmetric folds indicating top-to-the-N NE shear. Several v generations of brittle faults crosscut the low-angle shear zones. These include: (1) a south-dipping brittle-ductile shear zone with chlorite and epidote alteration that transects Plate III and records normal and strike-slip displacement, (2) several E SE-dipping sinistral-oblique normal faults that displace the west end of the RTC and cause significant clockwise rotation of older lineations, and (3) E NE-dipping normal faults associated with quartz breccia and limonitic striations. The observed crosscutting structural pattern is consistent with elements of Mojave Desert tectonic history: Late Cretaceous-early Tertiary crustal shortening, Miocene extension, and Late Cenozoic transrotation and associated transtension that is ongoing today. The Rand Mountains occupy a key location near the intersection of three right-lateral faults of the Eastern California Shear Zone with the left-lateral Garlock Fault. Brittle faults of the range record the evolution from extensional to transrotational-transtensional tectonic regimes. vi TABLE OF CONTENTS Signature Page .................................................................................................................... ii Acknowledgments.............................................................................................................. iii Abstract ............................................................................................................................... v List of Tables ...................................................................................................................... xi List of Figures ................................................................................................................... xii Introduction ......................................................................................................................... 1 Purpose and Objectives ........................................................................................... 1 Location .................................................................................................................. 2 Geologic Context and Unresolved Issues ........................................................................... 5 Chronology of Other Works .............................................................................................. 10 Geologic Field Investigations ........................................................................................... 12 Methods and Procedures ....................................................................................... 12 Rand Schist (Plate I) ............................................................................................. 13 Johannesburg Gneiss (Plate II) ............................................................................. 15 Atolia Granodiorite (Plate III) .............................................................................. 16 Leucocratic Alkali Granite (Plate IV) ................................................................... 18 Intrusive Complexes ............................................................................................. 19 Rand Thrust Faults ................................................................................................ 20 vii Rand Fault I............................................................................................... 20 Rand Fault II ............................................................................................. 21 Rand Fault III ............................................................................................ 22 Cross Faults A through E ...................................................................................... 24 Cross Fault A............................................................................................. 25 Cross Fault B............................................................................................. 27 Cross Fault C............................................................................................. 29 Cross Fault D ............................................................................................ 30 Cross Fault E ............................................................................................. 31 Cross Fault F ............................................................................................. 32 Structural Blocks 1 through 6 ............................................................................... 32 Block 1 ...................................................................................................... 32 Block 2 ...................................................................................................... 35 Block 3 ...................................................................................................... 35 Block 4 ...................................................................................................... 35 Blocks 5 and 6
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