Constitutive relations for antigorite-rich fault gouge under conditions of high pore fluid pressure

Geophysical observations indicate that pore fluid pressure is near-lithostatic along many plate boundary faults, and in some cases these faults exhibit slow slip behavior. Experimental studies show the frictional constitutive behavior of fault rocks to be a function of slip rate, temperature, effective normal stress, lithology, and pore fluid chemistry. However, less is known about the role of pore fluid pressure, independent of effective normal stress, on frictional constitutive relations. We present an experimental study of frictional sliding in antigorite fault gouge under conditions of high pore fluid pressure. We conducted rate-stepping tests on a 1 mm thick layer of antigorite-rich gouge sandwiched between porous sandstone or steel driving blocks in the triaxial configuration. Experiments were conducted at room temperature and 100 °C, effective normal stresses of 10 MPa and 70 MPa, pore pressures from 5 MPa to 125 MPa, slip rates from 0.12 to 4.94 µm/s, and displacements to 6 mm. We observe that antigorite-rich fault gouge is steady-state rate-strengthening when sheared at 10MPa effective normal stress and rate-weakening to marginally rate-strengthening at 70MPa effective normal stress. At constant effective normal stress, the magnitude of rate-strengthening or rate-weakening increases and decreases, respectively, with increasing pore fluid pressure. These results indicate pore fluid pressure should be explicitly incorporated into frictional relations used to constrain fault slip behavior in regions where pore fluid pressure is high.