MICRO-SCALE COMPOSITION AND TEXTURE OF FAULT ROCKS FROM SAFOD CORE: EVIDENCE FOR DEFORMATION PROCESSES AND FLUID-ROCK INTERACTIONS

Drilling of the San Andreas Fault Observatory at Depth (SAFOD) borehole near Parkfield, California confirmed the presence of serpentinite at ~ 3 km depth within the San Andreas Fault [SAF]. We examine the fine-scale composition and texture of fault rocks from the Phase 3 whole-rock core sampled between 3187.4 to 3301.4 m measured depths in the borehole to provide insight into the physical and chemical properties, structural architecture, and fluid-rock interactions of the fault. Petrography and microstructure analyses, electron microbeam studies (SEM), mineralogical (XRD), and geochemical (ICPMS-XRF) data are used to characterize SAF fault rocks and two analog sites within serpentinite-bearing rocks at San Simeon and Goat Rock State Park, California. In the SAFOD samples, the fault-related rocks consist of juxtaposed lenses of foliated siltstone and shale with block-in-matrix fabric, black cataclasite to ultracataclasite, and sheared serpentinite-bearing, fine foliated fault gouge. Meter-thick zones of sheared rock and fault gouge correlate to the sites of active borehole casing deformation and are characterized by multiple discrete slip surfaces or anastomosing shear zones that surround conglobulated- to lens-shaped compacted clay and serpentinite clasts. The gouge matrix is composed of Mg-rich clays saponite ± palygorskite, and serpentinite minerals lizardite ± chrysotile. Whole-rock chemistry data show increases in Fe-, Mg-, Ni-, and Cr-oxides and hydroxides, Fe-sulfides, and C-rich material. The faults sampled in the field consist of m-thick zones of cohesive to non-cohesive, serpentinite-bearing foliated clay gouge and black fine-grained fault rock and represents sheared Franciscan Formation. The foliated clay gouge is composed primarily of saponite and serpentinite and shows increases in Ni- and Cr-oxides and C-rich material. Micro- to meso-scale fabrics observed in the SAFOD core exhibit unique textures that are common in deformed serpentinites and attributed to distributed deformation and/or aseismic deformation with episodic seismic slip. These observations indicate that the fault zone experienced a complex history of transient fluid-rock interactions, including reducing fluids, that may contribute to the low-strength and aseismic behavior in the active SAF at this location.