We present results from an experimental program designed to simulate interseismic healing of fault gouge at midcrustal depths. Simulated fault gouge specimens consisting of a quartzo-feldspathic sand were deformed in triaxial shear at temperatures up to 250 degrees C. Specimens were loaded to steady state sliding and then subjected to a hold period of up to 2 days at a reduced level of axial load During the hold periods, the aqueous pore fluid was cycled through the gouge layer, allowing for continuous permeability measurements. It was found that the specimens would progressively "seal" dining hold periods. Sealing rates were found to be faster at higher temperature and also at earlier stages of the experiments. Subsequent reloading showed that strengthening, or "healing," had occurred and that the amount of this healing was closely related to the net amount of sealing, rather than to holding time. During reloading, some recovery of permeability occurred, although every hold period resulted in a net loss of permeability. Based on postexperimental pore fluid analysis, observation of the indurated and deformed gouge, and associated work from a concurrent study, we conclude that the sealing was a result of secondary mineral precipitation and that the healing resulted from the cementation of grains by the precipitating minerals. This "precipitation sealing" mechanism is distinct from mechanisms that have been observed to cause healing or "aging" in dry friction experiments. Thus, healing from precipitation sealing is a likely mechanism for explaining the underprediction of laboratory estimates of healing based on dry friction laws relative to healing observed in natural faults. Precipitation sealing also may contribute to maintenance of abnormally high pore fluid pressures within fault gouge zones.
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