Gouge Development in the San Andreas Fault from Lake Elizabeth core samples

Heather M. Savage, Randolph Williams, & Christie D. Rowe

Published August 15, 2018, SCEC Contribution #8780, 2018 SCEC Annual Meeting Poster #171

Fault gouge is produced through a variety of processes, including pulverization, fracturing, grinding, weathering reactions, hydrothermal alteration, precipitation of cements, compaction, dilation, and possibly flash heating. We seek to quantify the meaning of fault rock ‘maturity’ by relating mineralogical and textural characteristics of gouge at different stages of evolution to frictional properties through triaxial shearing experiments. Using core samples from the shallow San Andreas Fault near the Elizabeth Tunnel in the Sierra Pelona Mountains, we track the evolution of fault gouge using grain size, bulk chemistry, microstructure and mineralogy to document the evolution from wall rock to fault core. We have selected samples adjacent to, and genetically related to, the most abundant North American wall rock to the SAF, which is a Mesozoic tonalite-granodiorite, a relatively simple rock characteristic of continental crust. Laser diffraction grain size analyses (n=60) reveal bimodal grain size distribution in most samples, tracking cataclasis of framework silicates (quartz and feldspar) in the coarser population (>5-10 µm) and authigenic clay appearance in the finer fraction (1-2 µm). Bulk chemistry of fault gouges (n=90) roughly co-varies with changes in grain size distribution, indicating that fluid-rock interaction and precipitation of authigenic clays and zeolites (principally smectite and laumontite) is accompanied by hydration of the rock and preferential enrichment in Ca and Mg. X-ray diffraction analysis (n=100) of bulk gouge samples and clay separates demonstrate that phyllosilicate enrichment in damaged wall rock predates significant cataclasis, as chlorite and illite / illite-smectite are present in mechanically ‘intact’ samples, and abundant components added to fault gouge through cataclasis. Authigenic Ca-rich smectites increase to 10-20% or more of clay gouge samples. Our preliminary results show that grain size evolution depends on both cataclastic grain size reduction of detrital grains and also on the rate and style of authigenic gouge mineral development. Cataclasis and mineral transformations are inter-dependent and drive incredible diversity in the style of fabric/foliation in the gouge, which we predict will have significant effects on strength and frictional behavior.

Citation
Savage, H. M., Williams, R., & Rowe, C. D. (2018, 08). Gouge Development in the San Andreas Fault from Lake Elizabeth core samples. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)