Experimental Relationships of Clay Gouge to Faulted Rock Properties

Abstract

Weak clay minerals have been suggested as important members of fault gauge to explain aseismic creep. The Bombolakis silt-clay model seeks to account for aseismic creep and recurrent stress drops along natural faults by the electrical orientation of clay sheets and thixotropic hardening. Experimental evidence here using bentonite clay supports the Bombolakis model, and further, the relationships of water content, clay content, and the amount of previous shear are indicated as the primary factors influencing the stable sliding behavior of silt-clay and pure clay gouges. The silt fraction dominates the pre-sliding frictional characteristics while the clay reduces the peak shear stress and dominates the stability of the sliding mode. Clay content appears to enhance the homogeneity of deformation within the gouge whether dry or saturated. Water content in the silt-clay system imposes a weakened, initial, non-linear yield on the saturated specimen but decreases in effect with further shear displacement. A significant work-hardening characterizes the sliding behavior of silt-clay gouges containing at least 50% clay. The presence of this behavior depends upon the content of 50% or more clay by volume to insure that deformation takes place within the gouge and not just at the gouge-rock interface.