Composition and Structure of the San Andreas Fault Observatory at Depth (SAFOD) Phase III Whole-Rock Core: Implications for Fault Zone Deformation and Fluid-Rock Interactions

We examine the composition and texture of whole-rock core from ~ 3 km depth in the San Andreas Fault Observatory at Depth (SAFOD) borehole, which provides a unique opportunity to characterize in situ rock properties of the near-fault environment, and how these properties vary in an area where deformation is accommodated by aseismic creep and high-rates of microseismicity. Detailed petrography and microstructural analyses coupled with X-Ray Diffraction and X-ray Fluorescence techniques are used to describe composition, alteration, and textures. All samples record multiple generations of cataclastic deformation in a complexly deformed and altered sequence of fine-grained sheared rocks. Localized shears bound multi-layered zones of medium to ultra-fine grained cataclasite. Phacoidal clasts or porphyroclasts comprised of serpentinite, quartz, and older cataclasite are embedded within the comminuted phyllosilicate-rich gouge. The intensity of damage-related features and the development of a pervasive anastomosing fabric increases towards and within the two active slip zones near ~ 3192 and 3302 m MD. Foliated fabrics alternating with discrete fractures suggest a mixed-mode style of deformation including both ductile flow and brittle deformation processes during fault zone evolution. Deformation at high-strain rates is suggested by the presence of crack-seal veins in clasts, the presence of porphyroclasts, and the development of S-C fabrics in the phyllosilicate- rich gouge. Evidence for fluid-rock interaction across the fault zone is indicated by depletion of Si and enrichment of MgO, FeO, and CaO; with significant clay alteration and/or growth of neo-mineralized vein fillings and fracture surface coatings. Shear localization may decrease porosity and inhibit fluid flow whereas fracturing may locally facilitate fluid migration and/or chemical alteration within the fault zone. These results constrain hypotheses related to fault zone behavior and broaden our understanding of the processes controlling earthquake nucleation and/or energy adsorption within the SAF. Based on the similarity of our observations to previous results from surface exposures of the SAF, we emphasize the importance of exhumed fault zone studies as proxies for understanding deformation and seismicity in the shallow crust