HOW FAULT ROCKS FORM AND MATURE IN THE SHALLOW SAN ANDREAS FAULT

We document the mechanical and geochemical pathways of fault-rock development in the shallow San Andreas fault, and quantify their importance in shaping the mineralogy, grain size, fabric, and frictional characteristics of gouge. Using a combination of field and analytical techniques, we show that fault rocks evolved from granodiorite protolith in three main stages: initial distributed microfracturing/pulverization; subsequent cataclastic flow and incipient fabric development; and finally, production of authigenic illite/smectite during fluid-rock interaction. The interdependence of mechanical and geochemical processes operating during these phases results in a remarkably diverse suite of fault rocks (in terms of grain size and fabric, texture, color, mineralogy and bulk composition; hereafter summarized as "character"). Laboratory friction analyses indicate that the frictional strength of these fault-related rocks varies dramatically from relatively strong pulverized rocks (coefficient of friction ~0.75) to profoundly weak clay-rich gouges (coefficient of friction ~0.09). Using these observations, we propose a new definition of fault-rock "maturity" as the establishment of "steady state" mineralogical, geochemical, and microstructural characteristics during deformation. Fundamentally, this steady state reflects changes in the magnitude and/or efficacy of mechanical deformation and geochemical reaction, beyond which additional changes are unlikely if ambient conditions remain unchanged. We find that gouge samples from the primary strands of the San Andreas in core samples evolved along an identical path, and are of comparable maturity, to gouge samples from a nearby low-displacement (<5 m) fault developed in identical protolith. Thus, it appears that fault rock maturation occurs remarkably early in fault development. These observations suggest that changes in frictional strength, which are profound in our fault rocks and the result of phyllosilicate production during maturation/development, also occurred relatively early during faulting.