Frictional strength and strain weakening in simulated fault gouge: Competition between geometrical weakening and chemical strengthening

Despite the importance of hydromechanical effects in fault processes, not much is known about the interplay of chemical and mechanical processes, in part because the conditions are difficult to simulate in the laboratory. We report results from an experimental study of simulated fault gouge composed of rock salt sheared under conditions where pressure solution is known to operate. At sliding velocities above 10 m/s and high shear strains (>5), friction measurements show that layers of rock salt weaken significantly and ultimately slide unstably (i.e., stick-slip). Microstructural observations show the presence of a zone of comminuted grains along shear zone boundaries, forming boundary-parallel Y shears at high sliding velocities. Samples deformed at low sliding velocities do not show boundary-parallel shear but rather exhibit low porosity passive regions isolated by dilational zones in the Riedel shear orientation. We posit that the significant strain weakening observed at high sliding velocities is caused by severe grain size reduction as shear localization develops, i.e., by frictional wear, ultimately leading to the development of a throughgoing boundary parallel Y shear. Unstable slip is probably related to rupture on this Y shear surface with intermittent healing of the asperities by pressure solution. Furthermore, the data show that the weakening and subsequent unstable slip can be delayed (i.e., occur at higher strains) by lower sliding velocities, larger initial grain sizes, lower normal stresses, and the presence of fluids. This suggests a competition between mechanical wear and chemical processes. Our data highlight the importance of hydrothermal processes in tectonic faulting.