How Fault Rocks Form and Evolve in the Shallow San Andreas Fault

Randolph Williams, Christie D. Rowe, Kristina K. Okamoto, Heather M. Savage, & Erin Eves

Published November 17, 2021, SCEC Contribution #11744

We document the mechanical and geochemical processes of fault rock development in the shallow San Andreas fault (Mojave segment), and quantify their importance in shaping the mineralogy, grain size, fabric, and frictional characteristics of gouge. Through a combination of field and laboratory analysis of an extensive suite of shallow (<150 m) drill cores, we show that fault rocks evolved from a granodiorite protolith via three main processes: distributed microfracturing/pulverization; cataclastic flow and incipient fabric development; and production of authigenic illite/smectite during fluid-rock interaction. The interdependence of these mechanical and geochemical processes results in a diverse suite of fault rocks, and causes significant changes in frictional strength. Spatial variations in the effects of these mechanisms, as manifested in fault-rock mineralogy and geochemistry, indicate marked variations in their relative contribution to fault-rock evolution. These data reveal a complex San Andreas fault with multiple principal slip zones and damaged and altered rock hosting numerous interconnected secondary slip surfaces. The resulting picture of the San Andreas Fault zone suggests a substantial departure from the simple structures envisioned for near-surface seismogenenic faults in numerical models is required, and may inform future efforts to forecast peak ground accelerations during southern California earthquakes.

Williams, R., Rowe, C. D., Okamoto, K. K., Savage, H. M., & Eves, E. (2021). How Fault Rocks Form and Evolve in the Shallow San Andreas Fault. Geochemistry, Geophysics, Geosystems, 22(11). doi: 10.1029/2021GC010092.