Microstructural and Geochemical Indications of Potential for Seismic Events in the Creeping Segment of the San Andreas Fault in Central California

The presence of fluid overpressure microstructures including open-cavity blocky calcite veins, gouge fluidization microstructures, and implosion microbreccias have been noted in previous studies of core samples from the San Andreas Fault Observatory at Depth (SAFOD) but, are often considered to be exhumed relicts of past transient events in a currently inactive SAF damage zone. This study is aimed at questioning the inactivity of the SAF damage zone and its long-term decouplement from the actively creeping core of the fault. We used Energy Dispersive Spectroscopy (EDS), Cathodoluminescence (CL), Electron Backscattered Diffraction (EBSD) and Field Emission SEM to acquire microstructural, geochemical, and luminescence data from a suite of 20 SAFOD core samples spanning 3187m to 3311m Measured Depth including samples from the actively creeping Southwestern and Central Deforming Zones respectively known as the SDZ and CDZ. The results indicate that 1) multiple generations of gouge fluidization injections and implosion microbreccias are present in the SDZ and CDZ, 2) the trace element signatures of vein-calcite related to pressure solution cleavage and those in the fluid overpressure microstructures differ and that only the analysis of the latter vein calcites indicates a core-damage zone fluid exchange, and 3) the Mg/Ca ratio in the fluid overpressure microstructures increases with decreasing distance to the SDZ and CDZ, and proximity to clusters of minor fault zones in the damage zone. These findings are further supported by evidence of intermittent deformation by pressure solution and cataclasis and estimates of P-T conditions of deformation. These results are consistent with intermittent seismic events in an otherwise aseismically creeping fault zone as well as potential for the future seismicity in the currently creeping segment of the SAF.