Examination of exposed sections of active fault zones often reveals the presence of abundant clayey materials. Observations in deep tunnels, in deep mines, and deep boreholes have also revealed abundant clays on faults at depths to 1.24 mi (2 km). Experimental clay mineralogists have shown that the stability fields of montmorillonite, kaolinite, and chlorite may extend to conditions existing at mid-crustal depth. The question whether clays can exist down to appreciable depths has been addressed by several authors with convincing arguments in support of the hypothesis that clays can exist on faults down to depths of 6 mi (10 km) or more. Interpretation of the seismic velocities in the San Andreas fault zone, considering laboratory data, is also consistent with the hypothes s that large quantities of clayey gouge may be present along the fault zone to depths of 6 mi (10 km) or more.

If clayey gouges are indeed present in significant quantity along faults at mid-crustal depth, their mechanical properties would influence the behavior of faults. When the fault surfaces are separated by a layer of montmorillonite or vermiculite, the frictional resistance to sliding becomes very low. However, the presence of dry illite, chlorite, and kaolinite does not significantly lower the frictional resistance, and violent releases of energy may occur at high pressures. At high pressures dry clays are capable of sustaining considerable deviatoric stresses; the strength is characterized by having a peak value and a gradual decline to a residual value as strain is increased at a constant rate. Saturation of the clayey gouge, however, significantly lowered the shear resistance and st bilized sliding.

Laboratory studies of soil at low pressure have shown that the deformation and failure of soils involve time-dependent rearrangement of matter and the theories developed for rate-dependent processes are applicable to the study of soil deformation. Work on the time-dependent deformation of clayey gouge at high pressures, however, has just begun. New experimental results have shown that the bulk compressibility of clays at high pressure exhibits pronounced time-dependent characteristics.

End_of_Article - Last_Page 1708------------