[1] Mineralogical and geochemical changes in mudrock cuttings from two segments of the San Andreas Fault Observatory at Depth (SAFOD) drill hole (3066-3169 and 3292-3368 m measured depth) are analyzed in this study. Bulk rock samples and hand-picked fault-related grains characterized by polished surfaces and slickensides were investigated by X-ray diffraction, electron microscopy, and geochemical analysis. The elemental changes in fault-related grains along the sampled San Andreas Fault are attributed to dissolution of detrital grains (particularly feldspar and quartz) and local precipitation of illite-smectite and/or chlorite-smectite mixed layers in fractures and veins. Assuming ZrO 2 and TiO 2 to be immobile elements, systematic differences in element concentrations show that most of the elements are depleted in the fault-related grains compared to the wall rock lithology. Calculated mass loss between the bulk rock and picked fault rock ranges from 17 to 58% with a greater mass transport in the shallow trace of the sampled fault that marks the upper limit the fault core. The relatively large amount of element transport at temperatures of $110-114°C recorded throughout the core requires extensive fluid circulation during faulting. Whereas dissolution/precipitation may be partly induced by the disequilibrium between fluids and rocks during diagenetic processes, stress-induced dissolution at grain contacts is proposed as the main mechanism for extensive mineral transformation in the fault rocks and localization of neomineralization along grain interface slip surfaces.

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