Examination of Exhumed Faults in the Western San Bernardino Mountains, California: Implications for Fault Growth and Earthquake Rupture
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Utah State University DigitalCommons@USU All Graduate Theses and Dissertations Graduate Studies 5-2005 Examination of Exhumed Faults in the Western San Bernardino Mountains, California: Implications for Fault Growth and Earthquake Rupture Joseph R. Jacobs Follow this and additional works at: https://digitalcommons.usu.edu/etd Part of the Geology Commons Recommended Citation Jacobs, Joseph R., "Examination of Exhumed Faults in the Western San Bernardino Mountains, California: Implications for Fault Growth and Earthquake Rupture" (2005). All Graduate Theses and Dissertations. 5246. https://digitalcommons.usu.edu/etd/5246 This Thesis is brought to you for free and open access by the Graduate Studies at DigitalCommons@USU. It has been accepted for inclusion in All Graduate Theses and Dissertations by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. EXAMINATION OF EXHUMED FAULTS IN THE WESTERN SAN BERNARDINO MOUNTAINS, CALIFORNIA: IMPLICATIONS FOR FAULT GROWTH AND EARTHQUAKE RUPTURE by Joseph R. Jacobs A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in Geology Approved: James P. Evans Susanne U. Janecke Major Professor Committee Member Peter T. Kolesar Laurens H. Smith, Jr. Committee Member Interim Dean of Graduate Studies UTAH STATE UNIVERSITY Logan, Utah 2005 ii ABSTRACT Examination of Exhumed Faults in the Western San Bernardino Mountains, California: Implications for Fault Growth and Earthquake Rupture by Joseph R. Jacobs, Master of Science Utah State University, 2005 Major Professor: Dr. James P. Evans Department: Geology The late Miocene Cedar Springs fault system is a high-angle transpressional system in the Silverwood Lake area, western San Bernardino Mountains, southern California. This thesis presents the study of oblique-slip faults with modest amounts of slip, which represent the early stages of fault development by using slip as a proxy for maturity. A structural and geochemical characterization is provided for six fault zones ranging from 39 m of slip to 3.5 km of offset in order to develop a model of fault zone geometry and composition. Basic geometric and kinematic results are provided for an additional 29 small-displacement (cm- to m-scale) faults. The main faults of this study can be divided into the fault core composed of sheared clay gouge and microbreccia, the primary damage zone made up of chemically altered rock with microstructural damage and grain-size reduction, and the secondary damage zone, which is characterized by an increased fracture density relative to the host rock. Although there appears to be a general increase in fault core thickness with increasing slip, the correlation is iii insignificant when analyzing all faults. Both the primary and secondary damage zones appear to thicken with increased slip on the main fault. Overall, the structure and composition of the faults studied here are similar to those of larger strike-slip and reverse faults. This indicates that the fault core develops early in a fault’s history. Subsequent slip appears to be focused along these narrow zones, with some deformation accumulating in the damage zone. Whole-rock geochemical analyses typically show a reduction in the abundance of Na, Al, K, and Ca in the fault core and primary damage zone relative to the host rock. This indicates enhanced fluid-rock interactions in these zones. Calculations of the energy consumed to produce the chemical alteration in the fault core indicate that a considerable amount of the total earthquake energy may be lost to alteration. This thesis concludes that fault processes are similar throughout the different stages of development, and the study of relatively small-displacement faults can therefore be used to understand fault evolution through time and the processes of larger faults in the brittle crust. (226 pages) iv ACKNOWLEDGMENTS First and foremost, I would like to thank my advisor, Jim Evans, for the valuable support on this project. Jim has been an incredible resource of information and this thesis would not have possible without him. This work was funded through the Southern California Earthquake Center (SCEC) Cooperative Agreement No. 02HQAG0008 issued by the US Geological Survey under CFDA No. 15.807 and I greatly appreciate their financial assistance and interest in the study of exhumed fault zones. I would like to thank Pete Kolesar for assistance with x-ray diffraction methods and interpretations. Pete also helped me immensely with calculating the energy associated with chemical alterations. Susanne Janecke has also been very helpful in improving previous drafts of this thesis and I appreciate her insightful reviews. Discussions with Judy Chester and Ron Biegel helped to gain understanding into fault zone processes. Additional gratitude is given to Tony Williams and Jason Kneedy for a weekend of field assistance. Last, but not least, I would like to acknowledge the many friendships (too many to mention here) that have made my stay here very enjoyable. Stefan Kirby and I started our work here at the same time and it was good to have a friend going through the same experiences as me. Alex Steely, Scott Friedman, and Ben Kessel have also been good friends that all helped to make getting this degree a little easier. Thanks to everyone. Joseph Jacobs v CONTENTS Page ABSTRACT ........................................................................................................................ ii ACKNOWLEDGMENTS ................................................................................................. iv LIST OF TABLES ............................................................................................................ vii LIST OF FIGURES ......................................................................................................... viii CHAPTER 1. INTRODUCTION .......................................................................................1 1-1. Introduction .............................................................................1 1-2. Methodology ............................................................................4 1-3. Seismicity and Los Angeles Basin Analog ..............................7 1-4. Summary of Work ...................................................................8 References ..............................................................................10 2. STRUCTURAL AND GEOCHEMICAL CHARACTERIZATION OF MULTIPLE REVERSE FAULTS IN THE WESTERN SAN BERNARDINO MOUNTAINS, SOUTHERN CALIFORNIA ................16 Abstract ................................................................................16 2-1. Introduction ...........................................................................17 2-2. Mesoscopic Analysis .............................................................27 2-3. Microstructure .......................................................................36 2-4. Mineralogy and Geochemistry ..............................................40 2-5. Earthquake Energy Budget ....................................................47 2-6. Discussion ..............................................................................51 2-7. Conclusions ...........................................................................59 References ..............................................................................62 3. THE CEDAR SPRINGS FAULT SYSTEM: ANALYSIS OF MICRO- SEISMICITY AND COMPARISON TO THE PUENTE HILLS BLIND- THRUST SYSTEM .................................................................................118 Abstract ..............................................................................118 3-1. Introduction .........................................................................119 3-2. Microseismicity ...................................................................120 3-3. Eastwood Fault and Puente Hills Blind-Thrust Analog ......122 vi 3-4. Conclusions .........................................................................126 References ............................................................................128 4. CONCLUSIONS .....................................................................................139 References ............................................................................144 APPENDICES .....................................................................................................147 APPENDIX A: Slip calculations for constrained faults ..........................148 APPENDIX B: Thin section descriptions ................................................167 APPENDIX C: X-ray diffraction patterns ...............................................170 APPENDIX D: Whole-rock geochemistry raw data ...............................202 APPENDIX E: Principal component analysis .........................................204 vii LIST OF TABLES Table Page 2-1 List of all faults where true slip is constrained ......................................................71 2-2 Summary of XRD results for all samples ..............................................................72 2-3 Summary of geochemical trends of all oxides in the Grass Valley fault zone ......74 2-4 Summary of geochemical trends of trace elements and LOI in the Grass Valley fault zone ................................................................................................................74 2-5 Summary of geochemical trends of oxides in the Eastwood fault zone ................75 2-6 Summary of geochemical