While there is significant evidence for healing in natural faults, geothermal reservoirs, and lab experiments, the thermal, hydraulic, mechanical, and chemical interactions that influence healing are poorly understood. We present preliminary results of triaxial slide-hold-slide experiments to constrain rates and mechanisms of healing. Experiments were conducted on gouge composed of Westerly granite and on bare surfaces of Westerly granite and Eureka quartzite. Tests were run at 22, 100, and 200 °C. In some experiments, we also determined the in-plane fluid transmissivity. In bare surface experiments we observe that restrengthening depends on both time and temperature. At 200 °C the simulated fractures restrengthen at a rate of Δμ/Δlog(tholdhold) = 0.009/decade while at 22 °C the healing rate is ~ 0.002/decade. In the gouge experiments restrengthening appears to be independent of temperature. This may be related to the heterogenous mineral composition and thickness of the gouge layer which could allow shearing to be accommodated in unhealed zones. In the experiments, an overall reduction in fluid transmissivity is observed but sliding periods are often associated with increases in the fluid transmissivity. The transmissivity reduction tends to be greater at 200 °C relative to room temperature. Our preliminary results suggest that multiple healing mechanisms are operating under hydrothermal conditions.
Healing in rock refers to the loss of memory of the pre-healed, or damaged, state due to a variety of thermal, hydraulic, mechanical, and chemical (THMC) processes such as mineral transformations, pressure solution, dissolution and precipitation, gouge densification and cementation, and sealing. Evidence of healing occurring in the upper crust includes increases in seismic velocity during the interseismic period following an earthquake (Li et al., 2003, 2006; Vidale & Li, 2003), decreases in permeability around faults and in geothermal reservoirs (Kitagawa et al., 2007; Minissale et al., 2008; Xue et al., 2013), and the relations between stress drop and earthquake recurrence interval (Marone et al., 1995; Beeler et al., 2001; Chaves et al., 2020). There is considerable interest in the time- and temperature-dependent healing due to its implications for fault stability as well as permeability and heat transfer in enhanced geothermal systems. Time-dependent restrengthening of fault friction is an integral component of rate- and state-dependent friction constitutive relationships (see Marone, 1998a for a review) and has received significant attention in room temperature laboratory experiments (Dieterich, 1972; Dieterich & Kilgore, 1994; Karner & Marone, 2001; Ryan et al., 2018). These experiments are often conducted at nominally dry conditions and short timescales-conditions that are not particularly relevant to THMC processes that are prevalent at earthquake nucleation depths and in geothermal reservoirs (Sibson, 1983, Tsang, 1999).