An analysis of physicochemical processes in saturated silt-clay gouge indicates that this type of fault zone material can account for for the following phenomena: (1) the nonlinear mechanical behavior indicated by certain geophysical measurements along the San Andreas fault zone, (2) the low stress drops associated with earhtquakes to several kilometers' depth, and (3) the recurrence of creep-induced instabilities at shallow depths along fault zones. A rheological model is described for a gouge consisting of colloidal size clay platelets with absorbed water, brittle silt size particles, and 'free' pore water. Recurrence of shallow earthquakes or accelerated creep is explained in the model by thixotropic hardening of the colloidal phase following shear deformation, i.e., by electrochemical reorientation of clay platelets from a dispersed structure to a face-to-edge type of structure during a quiescent period. The silt phase must support part of the effective mean stress for thixotropic hardening to occur at several kilometers' depth. The peak shear strength of the gouge in this case is expressed in functional form by Sp=f<&kgr;Pe+?c tan &psgr;e; ?s tan &psgr;s>, where &kgr;, Pe, and &psgr;e are Hvorslev parameters; ?c is the effective stress in the colloidal phase, acting normal to the shear zone; ?s is the effective stress in the silt phase, acting normal to the shear zone; and &psgr;s is the 'friction angle' of the silt phase. The peak shear strength is time dependent owing to viscous type contacts between absorbed water layers surrounding the colloidal platelets. The time-dependent nature of Sp may be resonsible for certain nonlinear behavior noted in fault zones and for the small stress drops associated with earthquakes occurring at several kilometers' depth.