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Rock Properties and Structure Within the San Andreas Fault Observatory Utah State University DigitalCommons@USU All Graduate Theses and Dissertations Graduate Studies 5-2012 Rock Properties and Structure Within the San Andreas Fault Observatory at Depth (SAFOD) Borehold, Northwest of Parkfield, California: In Situ Observations of Rock Deformation Processes and Fluid-Rock Interactions of the San Andreas Fault Zone at ~ 3 km Depth Kelly Keighley Bradbury Utah State University Follow this and additional works at: https://digitalcommons.usu.edu/etd Part of the Geology Commons Recommended Citation Keighley Bradbury, Kelly, "Rock Properties and Structure Within the San Andreas Fault Observatory at Depth (SAFOD) Borehold, Northwest of Parkfield, California: In Situ Observations of Rock Deformation Processes and Fluid-Rock Interactions of the San Andreas Fault Zone at ~ 3 km Depth" (2012). All Graduate Theses and Dissertations. 1410. https://digitalcommons.usu.edu/etd/1410 This Dissertation 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]. ROCK PROPERTIES AND STRUCTURE WITHIN THE SAN ANDREAS FAULT OBSERVATORY AT DEPTH (SAFOD) BOREHOLE NORTHWEST OF PARK- FIELD, CALIFORNIA: IN SITU OBSERVATIONS OF ROCK DEFORM- ATION PROCESSES AND FLUID-ROCK INTERACTIONS OF THE SAN ANDREAS FAULT ZONE AT ~ 3 KM DEPTH by Kelly Keighley Bradbury A dissertation submitted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY in Geology Approved: ____________________ ____________________ Dr. James P. Evans Dr. Anthony Lowry Major Professor Committee Member ____________________ _____________________ Dr. John Shervais Dr. Susanne Janecke Committee Member Committee Member ____________________ ____________________ Janis Boettinger Dr. Mark R. McLellan Committee Member Vice President for Research and Dean of Graduate Studies UTAH STATE UNIVERSITY Logan, Utah 2012 ii Copyright © Kelly Keighley Bradbury 2012 All Rights Reserved iii ABSTRACT Rock Properties and Structure Within the San Andreas Fault Observatory at Depth (SAFOD) Borehold, Northwest of Parkfield, California: In Situ Observations of Rock Deformation Processes and Fluid-Rock Interactions of the San Andreas Fault Zone at ~ 3 km Depth by Kelly Keighley Bradbury, Doctor of Philosophy Utah State University, 2012 Major Professor: Dr. James P. Evans Department: Geology This project examines the composition, structure, and geophysical properties of rocks sampled within the San Andreas Fault Observatory at Depth (SAFOD) borehole drilling experiment near Parkfield, California. Cuttings, sidewall cores, spot-core, and whole-rock core are examined from the meso- to micro-scale to characterize the near- fault environment at shallow crustal levels (0-4 km) along the central segment of the San Andreas fault. The central segment deforms by contiuous aseismic creep and microseismicity. An integrated approach utilizing core-logging, detailed structural core mapping, petrology, microstructural analyses, whole-rock geochemistry, borehole geophysics, and analog field studies is followed. iv At SAFOD, fractured granitic rocks and arkosic sediments are identified west of the San Andreas fault zone on the Pacific Plate; whereas sheared fine-grained sediments, ultrafine black fault-related rocks, and serpentinite-bearing fault gouge are present within and northeast of the fault zone on the North American Plate. Here, the fault consists of a broad zone of variably damaged rock containing localized zones of highly concentrated shear that often juxtapose distinct rock-types. Two zones of serpentinite-bearing clay gouge, each meters-thick are found in two locations where active aseismic creep was identified in the borehole. The gouge is composed of Mg-rich clays, serpentinite (lizardite ± chrysotile) with notable increases in magnetite, and Fe-, Ni-, and Cr-oxides/hydroxides and Fe-sulfides relative to the surrounding host rock. Organic carbon is locally high within fractures and bounding slip surfaces. The rocks adjacent to and within the two gouge zones display a range of deformation including intensely fractured regions, block- in-matrix fabrics, and foliated cataclasite structure. The blocks and clasts predominately consist of competent sandstone and siltstone embedded in a clay-rich matrix that displays a penetrative scaly fabric. Mineral alteration, veins, fracture-surface coatings, and slickelined surfaces are present throughout the core, and reflect a long history of syn- deformation and fluid-rock reaction that contributes to the low-strength and creep in the meters-thick gouge zones. Evaluation of borehole geophysical data and elastic modulii for the lithologic and structural units identified in the SAFOD Phase 3 core reveal a correlation between composition and textures and the structural and/or permeability architecture of the SAF at SAFOD. Highly reduced velocity and elastic modulii surround the two serpentinite- v bearing gouge zones, the Buzzard Canyon fault to the southwest, and another bounding fault to the northeast. Velocity and elastic moduli values on the Pacific Plate or southeast of the active fault trace intersected by SAFOD are much higher relative to the values measured on the North American Plate, or northeast of the fault trace. Within and adjacent to the two active gouge zones, the rock properties are highly variable over short distances, however, they are significantly lower relative to material outside of the fault zones. This research contributes critical evidence for rock properties and slip behavior within an active plate boundary fault. Results from this research and the SAFOD experiment help to constrain numerous hypotheses related to fault zone behavior and earthquake generation within central California. (222 pages) vi PUBLIC ABSTRACT Rock Properties and Structure Within the San Andreas Fault Observatory at Depth (SAFOD) Borehold, Northwest of Parkfield, California: In Situ Observations of Rock Deformation Processes and Fluid-Rock Interactions of the San Andreas Fault Zone at ~ 3 km Depth by Kelly Keighley Bradbury, Doctor of Philosophy Utah State University, 2012 Major Professor: Dr. James P. Evans Department: Geology The San Andreas Fault Observatory at Depth (SAFOD) is a scientific drilling experiment situated along the central creeping segment of the San Andreas Fault, near Parkfield, California, and north of a segment of the fault that has experienced large historical earthquakes. Drilling into active fault zones allows scientist’s to examine in situ rock samples and to record real-time data. The main goal of this study is to characterize the geologic setting and rock properties of the San Andreas fault at ~ 3 km depth in the SAFOD borehole. In this region, the fault deforms nearly continuously through aseismic creep and small earthquakes. By sampling and characterizing the rocks from this location of the fault, we can begin to identify the features associated with fault-related deformation processes in vii xthe shallow crust; revealing the nature of the earth’s crust in the near-fault environment and yields insight into the mechanisms associated with earthquake generation along an active strike-slip fault. It is also useful to seismologists for developing well-constrained, predictive earthquake models. Project costs are ~ $175,000 funded primarily by NSF-Earthscope grant EAR- 0454527 to Dr. James P. Evans with additional support provided by the Geology Department and national scholarships to the student. Costs are associated with travel to examine core at the U.S.G.S. Core Lab in Menlo Park, CA and the IODP Gulf Coast Repository in College Station, TX; lab work, and sample processing and analyses at USU and Washington State University; field work travel plus an assistant, and collection and processing of field samples; and expenses associated with Teaching and Research Assistantships appointed to Kelly K. Bradbury during the course of this research. viii ACKNOWLEDGMENTS I am grateful to my advisor, Dr. James P. Evans, for guiding me through the process of completing a dissertation. I would like to thank Dr. Evans for his endless patience, understanding, financial/mental/emotional support, and his unique ability to encourage and mentor his students while encouraging complete autonomy in their work. My Committee Members: Anthony Lowry, John Shervais, Susanne Janecke, and Janis Boettinger, not only provided significant contributions to my education and to the improvement of this dissertation, but have provided excellent examples of scientific integrity and inquiry, the importance of maintaining a sense of humor within the rigors of academia, and are all truly an inspiriation. I appreciate numerous insightful discussions concerning the SAFOD Project with Fred and Judi Chester from Texas A&M. I would also like to thank the rest of the Geology Department Faculty and Staff, especially Marsha Hunt and Jean Daddow for their neverending support and assistance. Thirteen Thank Yous to my family, especially Bill and Ember, for their love, support, and endless patience. Also to my four-legged friends, whose love and joy for life and runs on the trails helped me maintain sanity throughout graduate school. To all of my friends, running partners, and USU Geology graduate students, I appreciate your comradery, flow of knowledge and endless humor. The majority of this project was funded by an NSF-Earthscope grant EAR-
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