Lithology and internal structure of the San Andreas fault at depth. based on characterization of Phase 3 whole-rock core in the San Andreas Fault Observatory at Depth (SAFOD) borehole
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We characterize the lithology and structure of the spot core obtained in 2007 during Phase 3 drilling of the San Andreas Fault Observatory at Depth (SAFOD) in order to determine the composition, structure, and deformation processes of the fault zone at 3. km depth where creep and microseismicity occur. A total of approximately 41. m of spot core was taken from three separate sections of the borehole; the core samples consist of fractured arkosic sandstones and shale west of the SAF zone (Pacific Plate) and sheared fine-grained sedimentary rocks, ultrafine black fault-related rocks, and phyllosilicate-rich fault gouge within the fault zone (North American Plate). The fault zone at SAFOD consists of a broad zone of variably damaged rock containing localized zones of highly concentrated shear that often juxtapose distinct protoliths. Two zones of serpentinite-bearing clay gouge, each meters-thick, occur at the two locations of aseismic creep identified in the borehole on the basis of casing deformation. The gouge primarily is comprised of Mg-rich clays, serpentinite (lizardite chrysotile) with notable increases in magnetite, and Ni-Cr-oxides/hydroxides relative to the surrounding host rock. The rocks surrounding the two creeping gouge zones display a range of deformation including fractured protolith, block-in-matrix, and foliated cataclasite structure. The blocks and clasts predominately consist of sandstone and siltstone embedded in a clay-rich matrix that displays a penetrative scaly fabric. Mineral alteration, veins and fracture-surface coatings are present throughout the core, and reflect a long history of syn-deformation, fluid-rock reaction that contributes to the low-strength and creep in the meters-thick gouge zones. 2011 Elsevier B.V.