Hydrothermal frictional strengths of rock and mineral samples relevant to the creeping section of the San Andreas Fault

We compare frictional strengths in the temperature range 25-250 degrees C of fault gouge from SAFOD (CDZ and SDZ) with quartzofeldspathic wall rocks typical of the central creeping section of the San Andreas Fault (Great Valley sequence and Franciscan Complex). The Great Valley and Franciscan samples have coefficients of friction, mu > 035 at all experimental conditions. Strength is unchanged between 25 and 150 degrees C, but mu increases at higher temperatures, exceeding 0.50 at 250 degrees C. Both samples are velocity strengthening at room temperature but show velocity-weakening behavior beginning at 150 degrees C and stick-slip motion at 250 degrees C. These rocks, therefore, have the potential for unstable seismic slip at depth. The CDZ gouge, with a high saponite content, is weak (mu = 0.09-0.17) and velocity strengthening in all experiments, and mu decreases at temperatures above 150 degrees C. Behavior of the SDZ is intermediate between the CDZ and wall rocks mu < 0.2 and does not vary with temperature. Although saponite is probably not stable at depths greater than similar to 3 km, substitution of the frictionally similar minerals talc and Mg-rich chlorite for saponite at higher temperatures could potentially extend the range of low strength and stable slip down to the base of the seismogenic zone. Published by Elsevier Ltd.