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Structural Evolution of an Extensional Terrane Margin: Case Studies from the Colorado UNIVERSITY OF CALIFORNIA Santa Barbara Structural Evolution of an Extensional Terrane Margin: Case Studies from the Colorado River Extensional Corridor, Southeastern California, USA A Thesis submitted in partial satisfaction of the requirements for the degree Master of Science in Earth Science by Justin Ronald Newmann Committee in charge: Professor Phillip B. Gans, Chair Professor Zachary C. Eilon Professor Kristin D. Morell December 2019 The thesis of Justin Ronald Newmann is approved. ____________________________________________ Zachary C. Eilon ____________________________________________ Kristin D. Morell ____________________________________________ Phillip B. Gans, Committee Chair December 2019 Structural Evolution of an Extensional Terrane Margin: Case Studies from the Colorado River Extensional Corridor, Southeastern California, USA Copyright © 2019 by Justin Ronald Newmann iii ACKNOWLEDGEMENTS Thank you first and foremost to my inimitable advisor Dr. Phillip B. Gans, who has taught me more in the last six years than I ever could’ve imagined. Phil, your guidance, leadership, patience, and care has enabled my growth as a scientist and person, and set me up for a lifetime of curiosity driven experience. Thank you. I would also like to thank Dr. Zachary Eilon and Dr. Kristin Morell for serving on my committee and giving me feedback and advice whenever I sought it. Special thanks to Mom and Dad for their constant support and for exposing me to the wonders of the Earth from the very beginning. Extra special thanks to Dad for making me walk across the Grand Canyon when I was 15, sparking my interest in the natural sciences. Thanks also to Evan Monroe and Alex Wrobel for all the good times across the Basin and Range, the stimulating conversations, and the dusty jam sessions; and to Alaina Rosenthal-Guillot for assisting me in the field. Thanks to the entire Earth Science department for making me into a geologist and to my family at the Cheetah House for their love and support. Finally, special thanks to my guitar, bandmates, and the power of music for keeping me sane in trying times and allowing me to express myself in ways scientific writing never will. iv ABSTRACT Structural Evolution of an Extensional Terrane Margin: Case Studies from the Colorado River Extensional Corridor, Southeastern California, USA by Justin Ronald Newmann The lower Colorado River extensional corridor (CREC) is an area of extreme crustal extension (>100%) which borders areas of moderate to minimal (<15%) extension. Regionally extensive detachment (low-angle normal) fault models commonly applied to the CREC define the margin of the extensional terrane as a discrete “breakaway” where the detachment fault originally intersected the surface. Results of field investigations along the CREC margin reveal locally complex deformation histories inconsistent with simple detachment models. In the Little Piute and northwest Piute Mountain areas, the dominant extension direction (NW-SE) is orthogonal to the commonly observed extension direction in the CREC (SW-NE), multiple generations of strike slip and oblique normal faults are present, and deformation occurred after roughly 17.7 Ma. These observations suggest a complex strain field evolved throughout the Miocene along the western margin of the CREC. Westward tilting of the Piute Mountain block indicates extensional deformation persisted beyond the previously hypothesized v CREC breakaway. In the Piute Range, west-directed normal faults dip steeply (60°) and have cutoff angles near 90° showing no obvious relationship to east-dipping detachment faults documented to the south (Homer Mountain) and east (Newberry Mountains). In all study areas the observed deformation history is more complex than expected in simple detachment fault breakaway models, suggesting current models are inadequate in describing deformation patterns along the margin of highly extended terranes. vi TABLE OF CONTENTS 1. Introduction and Geologic Background ..................................................................... 1 1.1 Introduction ............................................................................................ 1 1.2 Geologic Background ............................................................................ 4 1.2.1 General Geologic History ............................................................. 4 1.2.2 Miocene Evolution of the CREC .................................................. 7 1.2.3 Margins of the CREC ................................................................. 10 1.2.4 Fault Models in the CREC .......................................................... 11 1.3 Objectives of this Study ....................................................................... 14 1.4 Old Woman-Piute Mountains Region ................................................. 15 1.4.1 Area Overview ............................................................................ 15 1.4.2 Major Structures in OWPM Area ............................................... 15 1.4.3 Previous Interpretations of Major Structures .............................. 18 1.4.4 Miocene Stratigraphy of OWPM area ........................................ 20 1.4.5 Timing of Volcanism and Deformation ...................................... 20 1.4.6 Relationship to CREC................................................................. 21 1.4.7 Study Areas ................................................................................. 22 2. Geology of the Southern Little Piute Mountains Study Area .................................. 23 2.1 Area Overview ..................................................................................... 23 2.2 Miocene stratigraphy ........................................................................... 23 2.2.1 Overview ..................................................................................... 23 vii 2.2.2 Lower Sedimentary Section ........................................................ 26 2.2.3 Middle Volcanic Section ............................................................ 30 2.2.4 Upper Sedimentary Section ........................................................ 35 2.2.5 Tectono-stratigraphic Summary ................................................. 38 2.3 Structural geology ................................................................................ 38 2.3.1 Overview ..................................................................................... 38 2.3.2 Description of Fault Slip Data .................................................... 40 2.3.3 Description of Fault Kinematic Data .......................................... 42 2.3.4 Interpretation of Fault Data ........................................................ 44 2.3.5 Regional Tectonic Scenarios for Faulting .................................. 46 2.4 Stratigraphic and structural summary of LPM area in the context of the CREC ......................................................................................................... 47 2.4.1 Overview ..................................................................................... 47 2.4.2 Stratigraphic Summary ............................................................... 47 2.4.3 Structural Summary .................................................................... 48 2.4.4 Relation to CREC Breakaway Models ....................................... 48 3. Geology of the Northwest Piute Mountains Study Area ......................................... 50 3.1 Area Overview ..................................................................................... 50 3.2 Miocene Stratigraphy ........................................................................... 50 3.2.1 Overview ..................................................................................... 50 3.2.2 Description of Map Units ........................................................... 50 3.2.2 Interpreted Stratigraphic History ................................................ 53 viii 3.3 Structural geology ................................................................................ 56 3.3.1 Overview ..................................................................................... 56 3.3.2 Fault Data .................................................................................... 57 3.3.3 Extensional Folding .................................................................... 60 3.3.4 Relationship Between Folding and Faulting ............................... 60 3.3.4 Magnitude of Extension .............................................................. 63 3.4 Summary of NWPM Area ................................................................... 65 3.4.1 Stratigraphic Summary ............................................................... 65 3.4.2 Structural Summary .................................................................... 65 3.4.3 Relation to CREC Breakaway .................................................... 66 4. Geology of the Piute Range Study Area .................................................................. 67 4.1 Area Overview ..................................................................................... 67 4.2 Miocene stratigraphy ........................................................................... 70 4.2.1 Overview ..................................................................................... 70 4.2.2 Stratigraphic Description ............................................................ 71 4.2.3 Interpreted Stratigraphic History ...............................................
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