HOW DOES LITHOLOGY INFLUENCE SOUTHERN SAN ANDREAS FAULT SLIP BEHAVIOR?

What lithologies and deformation processes promote the propagation of earthquake ruptures and/or nucleate transient creep events through shallow fault rocks? The southern San Andreas fault (SSAF) through Coachella Valley comprises a zone of red clay gouge that narrows and widens up to a hundred meters along its trace. This segment has not experienced a major (M_w ~7) earthquake in ~300 years, but it accommodates some plate motion as transient shallow creep events. Previously published geodetic data indicate creep in the Mecca Hills occurs off the main fault trace. We hypothesize that the exhumed clay gouge is an analog for material present at shallow depths along the SSAF and subsidiary faults, and that this gouge influences the slip mode and structural evolution along the fault. We test this hypothesis by pairing observations from natural fault rocks with friction and healing experiments on natural gouge.

In the Mecca Hills, the phacoid-bearing, heterogeneous red clay gouge with networks of discrete mm- to cm-scale slip surfaces is present along the moderate to steeply-dipping main SSAF and transpressional faults that project into it. X-ray diffraction analyses (n = 25 samples) show the gouge comprises ~20-50% clay (~50% illite, ~20% kaolinite, ~20% smectite), ~30% quartz, and ~10-30% feldspar. A gouge sample (including phacoids) from the mouth of Painted Canyon was deformed in a suite of experiments using a direct shear apparatus at Utah State University over a series of slide-hold-slide and velocity steps. The gouge has a coefficient of friction of ~0.45-0.50 at room humidity and temperature, and ~0.25-0.30 when saturated. This weakness relative to the surrounding sandstone indicates that slip should localize within the clay gouge during past and future earthquakes. Saturated gouge exhibits slow and stick slip behavior at plate tectonic driving rates and a transition to stable sliding at faster rates. Slide-hold-slide experiments show that the frictional healing rate of wet gouge is ~zero. Together with our observations of transitional frictional stability, the healing behavior implies saturated gouge can promote shallow creep transients, consistent with geodetic data.