MINERALOGIC AND TEXTURAL ANALYSIS OF FAULT-RELATED ROCKS AT DEPTH: ANALYSIS OF SAFOD ROCKS AND THE TOP OF THE SEISMOGENIC ZONE

We examine drill cuttings, electric image logs, borehole geophysical data, and X-ray diffraction data from the San Andreas Fault Observatory at Depth (SAFOD) boreholes to determine the lithology and deformational textures in the fault zones and host rocks associated with the San Andreas Fault at depth. Cutting samples represent the lithologies from 1.7 km map distance and 3.2 km vertical depth adjacent to the San Andreas Fault. We analyzed 266 grain mount thin-sections at an average of 30 m cuttings sample and identify Quaternary and Tertiary (?) sedimentary rocks in the upper 700 m of the holes; granitic rocks from 760 – 1920 m measured depth; arkosic and lithic arenites, interbedded with siltstone sequences, from 1920 to ~3150 m measured depth; and interbedded siltstones, mudstones, and shales at 3150 - 3987 m measured depth. We infer the presence of at least five fault zones composed of damage zone and fault core on the basis of percent of cataclasite abundances, presence of grains with deformation, and presence of alteration phases. The largest faults to occur at: 1920 meters measured depth (mmd) where Salinian granodiorite on the southwest is juxtaposed against an arkosic section, 2530 mmd where a clay-rich unit lies between the upper and lower arkoses and is a diffuse >65 mmd wide low-velocity, high gamma, clay-rich fault zone with numerous sheared clay-filled veinlets; at 3067 mmd, where arkosic rocks are faulted against and fine-grained sedimentary rocks, and where cored rock exhibit brittle damage, and the 3150-3350 mmd zone, which exhibits increased amounts of alteration, cataclasis, low seismic velocities, and mineralogic anomalies. If the deeper zone of cataclasite and alteration intensity connect to the surface trace of the San Andreas fault, then this fault zone dips 80–85° southwest, and consists of multiple slip surfaces in a damage zone up to 250–300 m thick. This is supported by borehole geophysical studies, which show this area is a region of low seismic velocities, reduced resistivity, and variable porosity. The microstructures and alteration textures observed in the borehole are clearly associated with slip at the top of the seismic region of the SAF, and are similar to textures observed in exhumed faults, lending credence to using exhumed faults as proxies for faults at depth.