Frictional properties of actinolite-chlorite gouge at hydrothermal conditions

Exhumed subduction zones frequently show widespread actinolite-chlorite (Act-Chl) dominated compositions, suggesting that this may play an important role in controlling megathrust fault slip. We investigate the frictional behavior of simulated Act-Chl (85:15) gouge mixtures derived from natural mafic metamorphic rocks, under hydrothermal conditions using a ring shear deformation apparatus. Experiments were performed at effective normal stresses (sigma(eff)(n)) of 50-200 MPa, pore fluid pressures (P-f) of 50-200 MPa, at temperatures (T) of 23-600 degrees C. In each experiment we applied a shear displacement (x) of similar to 10 mm at a constant sliding velocity (v) of 10 mu m/s, followed by v-stepping in the range 0.3-100 mu m/s, and slide-hold-slide (SHS) tests with hold times (t) ranging from 3 s to 3000 s. We quantified the rate- and state-dependent friction parameter (a-b), and investigated the effect of t on fault healing (Delta mu pk). The results showed no effects of temperature on the coefficient of friction (mu approximate to 0.6-0.7), or on (a-b), with some experiments showing persistent, displacement-hardening or -weakening trends. Nonetheless, effects of v and of normal stress (sigma(n) = sigma(eff)(n) + P-f) on (a-b) fall into three temperature regimes: (1) T = 23-100 degrees C, (2) T = 200-400 degrees C, and (3) T = 500-600 degrees C. In Regimes (1) and (3), (a-b) > 0 for all conditions tested, whereas in Regime (2), (a-b) <= 0, at sigma(eff)(n) = P-f = 50 MPa and v = 0.3-3 mu m/s. We discuss on the origin of persistent displacement-hardening or -weakening trends observed, and assess the implications of our data for subduction zone seismogenesis. Extrapolation of (a-b)-data using multiple linear regression suggests that high pore pressure ratios (>0.9) are needed to promote seismogenesis in faults cutting actinolite-chlorite compositions.