• At elevated temperatures, many carbon-rich fault zones are subjected to thermal maturation, fluid-rock interactions, and/or shear-induced phase transformations, often yielding various fault weakening agents. • Black carbonaceous material documented by Bradbury et al. (2011; 2015) in SAFOD Phase III core exhibits intense comminution, shear-induced slip localization, fragmented shear zones, stylolites, calcite-cemented breccia, calcite veins of varying trace element chemistry, and calcite vein fragments in the wall rock. • Presence of calcite-cemented carbonaceous ultracataclasites in creeping segments of the Central Deforming Zone (CDZ) and Southwest Deforming Zone (SDZ) of the San Andreas Fault raises questions regarding the source of the carbonaceous material, and the nature of thermochemical reactions and fluid-rock interactions that promote dynamic weakening and strength recovery in carbon-bearing fault gouges during the seismic cycle. Overview Fault microstructures and geochemistry Micro X-ray fluorescence (μXRF) map of carbonaceous SAFOD Phase III gouge using synchrotron radiation. (a) Deformation-related microtextures illuminated by variations in Fe concentration in the gouge. (b) Close-up of boxed area in (a). Yellow zones represent calcite veins (Fe-depleted relative to the adjacent matrix). Note the undulatory form (precipitation during shear?) and incremental growth of calcite veins (different growth stages from a single fluid phase?). Scale bar is in µm. Composition, alteration, and texture of fault-related rocks from SAFOD core and surface outcrop analogs. Pure and Applied Geophysics 172, 1053-1078. [1] Wiersberg, T., Erzinger, J., 2005. Real-time fluid and gas monitoring during drilling of the SAFOD Main Hole in Park-field, CA. Eos Trans. AGU, 86(52), Fall Meet. Suppl., Abstract T23E-02. [2] Schieber, J., 2011. Iron sulfide formation. Encyclopedia of Geobiology, Springer Verlag. J. Reitner & V. Thiel (Eds.) 486-502. 500 µm Microstructures in carbonaceous material (CM) of SAFOD Phase III core (cross-polarized light) consist of a network of stylolites at (primarily) calcite interfaces. Note the fine-grained, fibrous nature of (re-precipitated?) calcite-diffusive material transport. SAMPLE (m MD) Weight [%] C δ 13 C (‰) SAMPLE (m MD) Weight [%] C δ 13 C (‰)
