The strength and stability of crustal faults are not adequately addressed by the widely used two-mechanism rheologic models of the crust based on Byerlee's law and power law creep. The models neglect the fluid-assisted mechanisms of deformation that are important to strength of the crust and do not describe the variation in rate dependence of friction that governs the stability of fault slip. Several distinct mechanisms of frictional slip are defined on the basis of variations in frictional behavior and microfabric of simulated fault gouge in laboratory experiments. State variable constitutive relations are used in a multiple-mechanism formulation to describe the rate and temperature dependence of three friction mechanisms in wet quartz gouge at elevated temperatures. This multimechanism description of friction is substituted for Byerlee's law in the two-mechanism models to generate a multimechanism rheologic model for the crust. The multimechanism model issued to predict the rate and temperature dependence of crustal strength and the slip rate and depth (temperature) conditions over which each mechanism dominates. Application of the model to strike-slip faults in the crustal illustrates that the thickness of the actively shearing zone within faults is a critical parameter governing fault strength and stability. The model predicts that frictional strength at midcrustal depths is significantly reduced relative to Byerlee's law only for thick fault zones. The reduction in strength is due to the operation of a strain rate sensitive friction mechanism involving combined cataclasis and solution transfer. The rheologic model predicts that only very thin faults display the rate dependent characteristics necessary for initiation of seismic slip to any significant depth in the crust. ¿ American Geophysical Union 1995