Dynamic weakening of serpentinite gouges and bare surfaces at seismic slip rates

To investigate differences in the frictional behavior between initially bare rock surfaces of serpentinite and powdered serpentinite (gouge) at subseismic to seismic slip rates, we conducted single-velocity step and multiple-velocity step friction experiments on an antigorite-rich and lizardite-rich serpentinite at slip rates (V) from 0.003m/s to 6.5m/s, sliding displacements up to 1.6m, and normal stresses (sigma(n)) up to 22MPa for gouge and 97MPa for bare surfaces. Nominal steady state friction values ((nss)) in gouge at V=1m/s are larger than in bare surfaces for all sigma(n) tested and demonstrate a strong sigma(n) dependence; (nss) decreased from 0.51 at 4.0MPa to 0.39 at 22.4MPa. Conversely, (nss) values for bare surfaces remained similar to 0.1 with increasing sigma(n) and V. Additionally, the velocity at the onset of frictional weakening and the amount of slip prior to weakening were orders of magnitude larger in gouge than in bare surfaces. Extrapolation of the normal stress dependence for (nss) suggests that the behavior of antigorite gouge approaches that of bare surfaces at sigma(n)60MPa. X-ray diffraction revealed dehydration reaction products in samples that frictionally weakened. Microstructural analysis revealed highly localized slip zones with melt-like textures in some cases gouge experiments and in all bare surfaces experiments for V1m/s. One-dimensional thermal modeling indicates that flash heating causes frictional weakening in both bare surfaces and gouge. Friction values for gouge decrease at higher velocities and after longer displacements than bare surfaces because strain is more distributed.