Velocity Weakening of Simulated Augite Gouge at Hydrothermal Conditions: Implications for Frictional Slip of Pyroxene‐Bearing Mafic Lower Crust

To understand the velocity‐weakening behavior of pyroxene as a major constituent in mafic rocks under hydrothermal conditions, we employed augite (clinopyroxene, (Ca 0.577 Na 0.110 Mn 0.004 Mg 0.886 Fe 2+ 0.193 Fe 3+ 0.037 Al VI 0.173 Ti 0.019 )[Al IV 0.137 Si 1.863 O 6 ]) as simulated gouge sample material to run velocity stepping sliding tests at temperatures of 101–607 °C, confining pressure of 127 MPa with 30 MPa pore pressure and axial loading rates of 0.04–1.0 μm/s. The friction coefficient was found to range from 0.71 to 0.73 with no systematic temperature dependence. Velocity‐weakening behavior occurred at temperatures above 203 °C, with low velocity dependence or ( b‐a ) value around 0.0012, corresponding to b/a values of 1.15–1.18. Inferred b values in the rate and state friction laws exhibited an increasing trend with temperature increase from ~0.0056 to ~0.0098 up to 403 °C, indicating activation of Arrhenius‐type creep at the frictional contacts. Microstructural observations on deformed samples show ubiquitous precipitated particles suggesting intergranular pressure solution, and no evidence for deformation by crystalline plasticity was observed. This fact, together with the comparison between experimental data and model prediction, suggests that the process of intergranular pressure solution at frictional contacts may have governed the evolution effect for augite. The minor velocity weakening of augite combined with previous data on plagioclase is consistent with the modest velocity weakening seen in mafic rocks, such as quartz‐free oceanic basalt. Such a minor velocity‐weakening property tends to favor slow slip events on the downdip portion of the subduction interface below currently locked seismogenic zones, such as in southwestern Japan, northern Cascadia, and Mexico.