Rock Properties and Internal Structure of the San Andreas Fault near ~ 3 km Depth in the SAFOD Borehole Based on Meso- to Micro-scale Analyses of Phase III whole rock core

We examine the relationships between rock properties and structure within ~ 41 m of PHASE III whole-rock core collected from ~ 3 km depth along the SAF in the San Andreas Fault Observatory at Depth (SAFOD) borehole, near Parkfield, CA. Direct mesoscale observations of the core are integrated with detailed petrography and microstructural analyses coupled with X-Ray Diffraction and X-Ray Fluorescence techniques to document variations in composition, alteration, and structures that may be related to deformation and/or fluid-rock interactions. Across the low velocity zone (LVZ) defined by borehole geophysical data, lithologies are comprised of a heterogeneous sequence of fine-grained sandstones, siltstones, mudstones, and shales with block-in-matrix textures and pervasively foliated fabrics. More competent clasts within the block-in-matrix materials exhibit pinch-and-swell shaped structures with crosscutting veins that do not extend into the surrounding phyllosilicate-rich matrix. Narrow fault strands at 3192 and 3302 m bound the LVZ and correspond to sites of active casing deformation (aseismic creep). Here, the rock consists of ~ 2 m thick serpentinite-bearing phyllosilicate gouge with a pervasive penetrative scaly clay fabric and phacoidal-shaped clasts. Bounding these two active slip surfaces are highly sheared and comminuted ultrafine-grained black fault rocks with abundant calcite veins parallel and oblique to the foliation trend. Localized shear surfaces bound multi-layered zones of medium to ultra-fine grained cataclasite in the near-fault environment and record multiple generations of brittle deformation processes. Deformation at high-strain rates is suggested by the presence of crack-seal veins in clasts within the block-in-matrix materials, the presence of porphyroclasts, and the development of S-C fabrics in the phyllosilicate-rich gouge. Across the fault(s) and related damage zones, foliated fabrics alternating with discrete fractures suggest a mixed-mode style of deformation including both ductile and brittle deformation processes during the temporal and spatial evolution of the fault zone. 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. Results reveal the complex internal structure and fluid-rock interactions within the San Andreas Fault at shallow crustal levels and provide a geologic context, which can be used for further core-based studies and experimental analyses.