Healing of simulated fault gouges aided by pressure solution: Results from rock analogue experiments

Slide-hold-slide friction experiments are reported on fault gouges of salt and salt-muscovite mixtures to investigate the effects of fluids and phyllosilicates on strength gain. Healing rates of salt gouges in the presence of saturated brine are an order of magnitude higher than dry salt and water-saturated quartz at 65 degrees C. Fault gouges consisting of salt-muscovite (80:20) mixtures show healing rates half that of 100 wt % salt; this is consistent with the effects of lower porosity and reduced dilation resulting from lower friction associated with muscovite. Half of the strength gain can be attributed to dilational work. The remainder of the strength gain can be explained by a microphysical model of compaction via pressure solution. Our model predicts the rate of contact area growth and of frictional restrengthening. The model predicts the observed rate of restrengthening for long hold periods for wet salt but underestimates the values for shorter hold periods. The short time response is attributed to strengthening of the grain boundary, elevating the resistance to frictional sliding on its interface, which is likely to be operative at longer hold periods as well but is masked by the strength gain owing to the increase in contact area. Our observations are consistent with an increased resistance to sliding of the contact at short term, the growth of the contact beyond this, and dilational hardening at all hold durations. To predict the magnitude and rates of healing in natural fault gouges under hydrothermal conditions, knowledge of the "state'' of the fault gouge is required.