HEALING AND SEALING OF A SIMULATED FAULT GOUGE UNDER HYDROTHERMAL CONDITIONS - IMPLICATIONS FOR FAULT HEALING

We present results from an experimental program designed to simulate i nterseismic healing of fault gouge at midcrustal depths. Simulated fau lt gouge specimens consisting of a quartzo-feldspathic sand were defor med in triaxial shear at temperatures up to 250 degrees C. Specimens w ere 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 peri ods, the aqueous pore fluid was cycled through the gouge layer, allowi ng for continuous permeability measurements. It was found that the spe cimens would progressively ''seal'' dining hold periods. Sealing rates were found to be faster at higher temperature and also at earlier sta ges 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, althou gh 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 concl ude that the sealing was a result of secondary mineral precipitation a nd that the healing resulted from the cementation of grains by the pre cipitating minerals. This ''precipitation sealing'' mechanism is disti nct from mechanisms that have been observed to cause healing or ''agin g'' in dry friction experiments. Thus, healing from precipitation seal ing is a likely mechanism for explaining the underprediction of labora tory estimates of healing based on dry friction laws relative to heali ng observed in natural faults. Precipitation sealing also may contribu te to maintenance of abnormally high pore fluid pressures within fault gouge zones.