ANALYSIS OF SAN ANDREAS FAULT ZONE COMPOSITION AND STRUCTURE; SYSTEMATIC MINERALOGIC AND STRUCTURAL STUDIES

We examine the fault structure, composition, and kinematics of the San Andreas Fault from sites that range from surface exposures to exhumed sites from 5+ km depth, and samples from cores at depth in order to determine how this major fault evolved, and how slip localization occurred. Study sites include surface exposures in the Coast Ranges, the Transverse Ranges, and the Mecca Hills; subsurface samples come from the SAFOD and Cajon Pass drillholes. The protolith lithologies includes Franciscan serpentinite, granitoid gneiss, granodiorite, clastic sedimentary rocks, and the Pelona-Orocopia Schist. Common elements for the fault includes the presence of clay-rich foliated fault-related rocks, within which mm- to cm-thick slip surfaces that are characterized by intense comminution, and shear within clay-rich zones. Alteration minerals include saponite, palygorskite, sepiolite, and zeolites, all of which exhibit low coefficients of friction. Thin and reworked veins and zones of neomineralization is evidence for fluid-associated deformation, and we propose that seismically generated heat produces the activation energies of reaction. We observe nm- to mm-thick syn-kinematic clay coatings on slip surfaces and on individual clay grains of the Schleicher-van der Pluijm-type slip surfaces that we infer to record instantaneous slip. At upper levels of the faults, clay-calcite coatings are common within faults that juxtapose crystalline rock on Tertiary to early Quaternary sediments. Thin clay-calcite surfaces exhibit plastically deformed sheared calcite. At the mesoscopic scale some of the shallow level faults exhibit curved slip vectors, or multiple slip vector orientations that reflect highly heterogeneous stresses on the fault surfaces. These complex alteration and geochemical signatures are observed at depth in the SAFOD and Cajon Pass cores, and indicate that the mechanical structure of the fault zone is highly altered in the upper 5-7 km. This may give rise to fault zones with zones of low values of elastic moduli 10’s m thick, distinct mechanical boundaries, extremely narrow principal slip surfaces, and high attenuation coefficients that might greatly affect predicted damage and ground shaking along the surface traces of faults.