Olivine friction at the base of oceanic seismogenic zones

[ 1] We investigate the strength and frictional behavior of olivine aggregates at temperatures and effective confining pressures similar to those at the base of the seismogenic zone on a typical ridge transform fault. Triaxial compression tests were conducted on dry olivine powder ( grain size <= 60 mu m) at effective confining pressures between 50 and 300 MPa ( using Argon as a pore fluid), temperatures between 600 degrees C and 1000 degrees C, and axial displacement rates from 0.06 to 60 mu m/s ( axial strain rates from 3 x 10(-6) to 3 x 10(-3) s(-1)). Yielding shows a negative pressure dependence, consistent with predictions for shear enhanced compaction and with the observation that samples exhibit compaction during the initial stages of the experiments. A combination of mechanical data and microstructural observations demonstrate that deformation was accommodated by frictional processes. Sample strengths were pressure-dependent and nearly independent of temperature. Localized shear zones formed in initially homogeneous aggregates early in the experiments. The frictional response to changes in loading rate is well described by rate and state constitutive laws, with a transition from velocity-weakening to velocity-strengthening at 1000 degrees C. Microstructural observations and physical models indicate that plastic yielding of asperities at high temperatures and low axial strain rates stabilizes frictional sliding. Extrapolation of our experimental data to geologic strain rates indicates that a transition from velocity weakening to velocity strengthening occurs at approximately 600 degrees C, consistent with the focal depths of earthquakes in the oceanic lithosphere.