ABSTRACT THE STANISLAUS GROUP IN A BEHEADED DRAINAGE: TECTONICS AT THE MARGIN OF THE SIERRA NEVADA MICROPLATE West of Bridgeport Valley near the Sierra Nevada crest, the Little Walker Caldera erupted Late Miocene Stanislaus Group lavas (Table Mountain Formation) and ignimbrites (Eureka Valley Tuff). Remnants of these rocks are now distributed from the western Sierra Nevada foothills across the range and into the Walker Lane. This wide distribution is attributed to the lavas flowing down paleochannels, and provides an excellent marker for post-emplacement deformation in the region. Priest (1978) documented a thick section of these lavas along Flatiron Ridge and other peaks surrounding Buckeye Canyon, including four members in stratigraphic order: Lower, Large Plagioclase, Two-Pyroxene, and Upper Member. 40Ar/39Ar geochronology indicates these Table Mountain Formation Lavas erupted from 10.4-9.5 Ma. Lithologically similar lavas have been identified near Rancheria Mountain, geochemical and paleomagnetic data support this correlation. The lavas flowed down a now-beheaded late Miocene drainage, supporting a westward shift of the Sierra Nevada crest since the Late Miocene. Paleomagnetic data from Priest’s Measured Section reveal that the Upper Member and the Two-Pyroxene Member are reversed polarity. The Large Plagioclase Member and the Lower Member flows are normal polarity. Based on remanence, lithology, and geochemistry, we correlate the Upper Member with the Rancheria Mountain lavas, requiring the presence of a 30 km long paleochannel for the lava to flow down. The Sierra Nevada crest currently divides these ii outcrops of Stanislaus Group lavas. Since lava flows downhill from its source, and Priest’s Measured Section is the most proximal locality, the crest of the Sierra Nevada must have been near to or east of Priest’s Measured Section in the late Miocene. Using the established paleomagnetic reference direction for the Eureka Valley Tuff members, we demonstrate ~15° clockwise vertical-axis rotation at sites stepping from Mono Basin towards the Sierra Crest graben. A cross section that passes from Rancheria Mountain through the Sierra Crest, Priest’s Measured Section and Boone Canyon reveals total down-to-the-northeast offset of about 3500 meters. Numerous north-trending faults in the area cutting Stanislaus Group rocks indicate that deformation through the area was accommodated by a combination of fault offset and vertical-axis rotation. Rosalie Power Schubert May 2017 THE STANISLAUS GROUP IN A BEHEADED DRAINAGE: TECTONICS AT THE MARGIN OF THE SIERRA NEVADA MICROPLATE by Rosalie Power Schubert A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology in the College of Science and Mathematics California State University, Fresno May 2017 APPROVED For the Department of Earth and Environmental Sciences: We, the undersigned, certify that the thesis of the following student meets the required standards of scholarship, format, and style of the university and the student's graduate degree program for the awarding of the master's degree. Rosalie Power Schubert Thesis Author Christopher Pluhar (Chair) Earth and Environmental Sciences John Wakabayashi Earth and Environmental Sciences Keith Putirka Earth and Environmental Sciences For the University Graduate Committee: Dean, Division of Graduate Studies AUTHORIZATION FOR REPRODUCTION OF MASTER’S THESIS x I grant permission for the reproduction of this thesis in part or in its entirety without further authorization from me, on the condition that the person or agency requesting reproduction absorbs the cost and provides proper acknowledgment of authorship. Permission to reproduce this thesis in part or in its entirety must be obtained from me. Signature of thesis author: ACKNOWLEDGMENTS Thank you to my advisor Christopher Pluhar for his insight and guidance throughout this project. Thanks to my committee members John Wakabayashi and Keith Putirka for their thoughtful feedback and advice. Thanks to my intrepid field assistants: Julie Reith, Kou Yang and Trevor Gledhill. Your assistance with sample collecting and packing was in invaluable. And thanks to my fiancé Andrew and my parents for always being there for me. Funding provided by: Fresno State University College of Science and Math Fresno Gem and Mineral Society Fresno State Graduate Net Initiative RGrant TABLE OF CONTENTS Page LIST OF TABLES ................................................................................................. vii LIST OF FIGURES ............................................................................................... viii LIST OF PLATES .................................................................................................... x INTRODUCTION .................................................................................................... 1 LITERATURE REVIEW ......................................................................................... 6 Farallon Subduction .......................................................................................... 6 Present-day Tectonics of North America .......................................................... 7 Uplift of the Sierra Nevada ............................................................................... 8 Sierra Nevada Drainage Divide ...................................................................... 12 Stratigraphy ..................................................................................................... 12 METHODOLOGY ................................................................................................. 16 Stratigraphy, Lithology, and Petrography ....................................................... 16 Geologic Mapping ........................................................................................... 16 Geochemistry .................................................................................................. 18 ARGUS VI Mass Spectrometer – OSU Argon Geochronology Laboratory .. 19 Paleomagnetism .............................................................................................. 21 RESULTS ............................................................................................................... 25 Lithology ......................................................................................................... 25 Geochemistry .................................................................................................. 27 Mapping and Cross-Section ............................................................................ 28 40Ar/39Ar Geochronology ................................................................................ 33 Paleomagnetism .............................................................................................. 36 DISCUSSION ......................................................................................................... 46 vi vi Page Tilt Corrections for Paleomagnetic Data ........................................................ 46 40Ar/39Ar Geochronology ................................................................................ 47 Paleochannel of the Tuolumne River .............................................................. 47 Vertical-Axis Rotation .................................................................................... 55 Kinematics ....................................................................................................... 57 CONCLUSIONS .................................................................................................... 59 REFERENCES ....................................................................................................... 61 APPENDICES ........................................................................................................ 72 APPENDIX A: PLATES ........................................................................................ 73 APPENDIX B: PALEOMAGNETISM STEREONETS ....................................... 78 LIST OF TABLES Page Table 1: Major element composition of Table Mountain Formation Samples. Data from this study (PMS samples) and Jones (2014) including Rancheria Mountain (EE), Boone Canyon (BOC) and Priest's Measured Section (PMS). ........................................................................ 29 Table 2: Trace element composition of Table Mountain Formation samples. Includes data from Jones (2014) and this study (PMS samples). ............ 30 Table 3: Summary of 40Ar/39Ar Results ................................................................. 34 Table 4: Mean Flow ChRM Paleomagnetic Results .............................................. 37 Table 5: Sample site rotation results. A positive rotation indicates clockwise sense. A positive flattening indicates shallower inclination. .................. 38 LIST OF FIGURES Page Figure 1: Fault map of western United States with color blocks representing geologic provinces. Modified from Stockli et al. (2003). Faults from the USGS’s Geologic Map of North America. ................................ 2 Figure 2: Original extent of the Stanislaus Group and current mapped extent. Modified from Pluhar et al. (2009) and King et al. (2007). ...................... 3 Figure 3: Stanislaus Group nomenclature from Priest (1979). ............................... 13 Figure 4: LeBas (1985) diagram showing the relative silica and alkali composition of Table Mountain Formation samples. Typical Table Mountain Formation and typical Eureka Valley Tuff