Poster #029, San Andreas Fault System (SAFS)
HOW FAULT ROCKS FORM AND MATURE IN THE SHALLOW SAN ANDREAS FAULT
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Poster Presentation
2020 SCEC Annual Meeting, Poster #029, SCEC Contribution #10666 VIEW PDF
ult 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.
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