ABSTRACT DIVERSE METAMORPHIC TRAJECTORIES, IMBRICATED OCEAN PLATE STRATIGRAPHY, AND FAULT ROCKS, YUBA RIVER AREA, FEATHER RIVER ULTRAMAFIC BELT, CALIFORNIA The Feather River Belt (FRB), the most extensive of the ultramafic belts in the Sierra Nevada, is a north–south trending 150-km-long by 1-8 km wide ultramafic belt and includes related rocks that form the basement of the northern Sierra Nevada of California. These rocks have been long interpreted as a Paleozoic to early Mesozoic "suture" zone (position of former subduction zone) and provide a good opportunity to more closely investigate the details of the rock record associated with subduction processes. Conventional tectonic models cannot explain the spatial-temporal distribution of the metamorphic grade of the FRB. Geologic mapping, petrographic, and electron microprobe analyses reveal a complex spatial and relative time relationships between different lithologies and units of different metamorphic grade. In the greater Middle and North Yuba River area, ultramafic rocks structurally overlie amphibolite, composed of primarily metamafic rocks, that structurally overlies the blueschist facies Red Ant schist (RAS). These tectonic contacts have been isoclinally folded at scales of hundreds of meters to a km. In the North Yuba River area, amphibolite records low- pressure, high-temperature metamorphism with redbrown (high Ti, low Al) amphibole and ilmenite. In the Forest City and Alleghany areas on the North Fork-Middle Fork divide, amphibolite grade rocks comprise imbricates of ocean plate stratigraphy, represented by repeated sheets of metabasites, metacherts, and metaclastics. These rocks include zones of cataclasites with pseudotachylites (frictional melts generated by fault movement). The Alleghany amphibolites ii appear to comprise two slabs with contrasting metamorphic history. In one slab, olive green amphibole, apparently representing HP-HT metamorphism (rutile cores in ilmenite) is overgrown by redbrown amphibole formed during LP-HT conditions, whereas the other slab has redbrown amphibole cores mantled by olive green amphibole. One slab may record ridge subduction followed by renewed subduction (LP-HT before HP-HT), whereas the other may record subduction initiation in young oceanic lithosphere (HP-HT) followed by ridge subduction (LP-HT). In addition to the internal complexity in the Yuba River area, the FRB exhibits along-strike variation in the timing and nature of metamorphism. The northernmost FRB in the North Fork Feather River displays HP-HT metamorphism without a second HT event and has yielded Ar-Ar hornblende ages of 240 Ma. In contrast the Yuba River area records multiple HT events with Ar- Ar ages for LP-HT metamorphism of about 340 Ma. This demonstrates that the suture represented by the FRB may record multiple subduction initiation events as well as a ridge subduction event, with significantly different tectonic history along strike. Nobuaki Masutsubo May 2013 DIVERSE METAMORPHIC TRAJECTORIES, IMBRICATED OCEAN PLATE STRATIGRAPHY, AND FAULT ROCKS, YUBA RIVER AREA, FEATHER RIVER ULTRAMAFIC BELT, CALIFORNIA by Nobuaki Masutsubo 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 2013 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. Nobuaki Masutsubo Thesis Author John Wakabayashi (Chair) Earth and Environmental Sciences Stephen Lewis 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 First of all, I would like to thank the Geological Society of America for the GSA research grant , which covered most of my fieldwork costs. I am grateful for the rGrant from Fresno State Associated Student Inc. that covered a large portion of the electron microprobe work. Also, I would like to thank everyone involved with the Faculty Sponsored Student Research award, which made it possible to present my research results at several GSA and AGU conferences. I was very fortunate to have my advisor, John Wakabayashi. He was always positive, encouraging, and inspiring to the students with his passion and enthusiasm for geosciences, fishing, and fermentation sciences. Also, I would like to thank my other thesis committee members: Stephen Lewis and Keith Putirka for their constructive reviews and comments, and all their support. Special thanks to Sarah Roeske and Nick Botto for their guidance with the electron microprobe at UC, Davis. I really appreciated my field partner, Gerardo Torres, who was crazy enough to come with me twice during two week-long field expeditions. Without his company, I could not have made friends at the mining town of Alleghany. To Tobyn and his mother, thank you for your hospitality at Casey’s Place in Alleghany. Finally, I could not make it without the great support from my wife, Annie. I love you so much. My three daughters, Maria (Sakura), RoseAnna (Sayuri), and Alice (Sumire), are always adorable, and their smiles always give me the encouragement I need. TABLE OF CONTENTS Page LIST OF TABLES .................................................................................................. vi LIST OF FIGURES ................................................................................................ vii INTRODUCTION .................................................................................................... 1 GEOLOGICAL SETTING ....................................................................................... 5 Feather River Belt ............................................................................................. 5 Red Ant Schist ................................................................................................... 9 Shoo Fly Complex ........................................................................................... 10 Calaveras Complex ......................................................................................... 11 METHODS ............................................................................................................. 13 RESULTS ............................................................................................................... 15 Structural and Lithologic Relationships .......................................................... 15 Petrography ..................................................................................................... 26 Electron Microprobe Analysis ........................................................................ 39 Thermobarometry ............................................................................................ 47 DISCUSSION ......................................................................................................... 51 Are the Amphibolite Grade Rocks of the Yuba Rivers Area Part of A Metamorphic Sole? .............................................................................. 51 Do the Amphibolite-Grade Rocks Record Subduction Initiation? ................. 51 Origin of LP-HT Metamorphism .................................................................... 52 Two Amphibolite Slabs with Different Relative PT Histories ....................... 52 Along Strike Variation: Multiple Subduction Initiation Events? ................... 53 Complexity in a Suture Zone Recorded in the FRB ....................................... 54 REFERENCES ....................................................................................................... 56 LIST OF TABLES Page Table 1: Electron Microprobe data of Sample A5 and A1-35b, Garnet ................ 40 Table 2: Electron Microprobe data of Sample F2 and A1-7b, CPX ...................... 42 Table 3: Electron Microprobe data of Sample A2-30b and A5, CPX.................... 43 Table 4: Electron Microprobe data of Sample A3-13and A3-14, CPX ................. 44 Table 5: Electron Microprobe data of Sample AMinRA, F2, and A1, Amphibole ................................................................................................ 44 Table 6: Electron Microprobe data of Sample A1-7b and A1-35b, Amphibole .... 45 Table 7: Electron Microprobe data of Sample A2-30b, Amphibole ...................... 46 Table 8: Electron Microprobe data of Sample F2, A1-7b, and A1, Plagioclase .... 47 LIST OF FIGURES Page Figure 1: (a) The field area in the context of ophiolites and general bedrock geology in California. Adapted from Masutsubo and Wakabayashi (2010). (b) Distribution of radioisotopic dates from the Feather River Ultramafic Belt. Adapted from Smart and Wakabayashi (2009). (c) Regional geology of the field study area from Burnett and Jennings (1962). ....................................................................................................... 6 Figure 2: (a) A