FLUID-ROCK INTERACTIONS AND DISTRIBUTED DEFORMATION ALONG SHALLOW HEMATITE-CLAY FAULTS IN THE SOUTHERNMOST SAN ANDREAS FAULT SYSTEM

Along the southernmost San Andreas fault (SSAF) system through Mecca Hills, slip is concentrated in shallowly exhumed Fe-oxide-rich clay faults, highlighting the role these structures play in accommodating shallow deformation. The main San Andreas fault is delineated by a thick zone of red clay gouge with sandstone phacoids. The Painted Canyon fault, an exhumed voluminous fault in a flower structure with the SSAF, comprises basement damage zones with 10 cm-wide clay gouge with internal layers of euhedral hematite crystals and spatially-associated, <1 cm-thick, delicate, mixed hematite-clay slip surfaces. Prior textural and (U-Th)/He thermochronometry investigation of pure hematite fault surfaces in the Painted Canyon fault, suggests nm-scale hematite plates form within the upper <2 km of the crust. Here, we integrate prior work with new field observations, scanning electron microscopy, and energy dispersive x-ray spectroscopy to document the composition and textures of these mixed hematite-clay faults.

Clay gouge zones in the Painted Canyon fault are Fe, Al, Mg, K rich, and heterogeneous surfaces reflect a decrease in Si, Al, and Mg content with increasing Fe-oxide phases (i.e., hematite). Prior deformation experiments indicate clay displays velocity strengthening behavior and a low coefficient of friction, properties that are thought to promote deformation at subseismic rates. Hematite exhibits a low coefficient of friction and velocity strengthening to velocity neutral behavior at slow slip rates despite the lack of crystal bound water. Some hematite slip surfaces have injection veins into host rock or calcite, suggesting initial precipitation via fluid overpressure. Mixed hematite-clay fault surfaces reflect fluid-host rock interaction and evolving rheology with changing composition. For example, the low permeability and Fe-content of clay minerals or nearby host rock may have promoted fluid pressurization and fluids enriched in Fe precipitated hematite when conditions were favorable. We suggest neoformed hematite localizes deformation and promotes slow slip. This has implications for the evolution of the shallow SSAF, where recent shallow slow slip events are observed to propagate and where earthquake energy may be dampened by distributed slip along minor hematite-rich fault surfaces.