CLAY FABRIC INTENSITY IN NATURAL AND EXPERIMENTAL CLAY-RICH FAULT ROCKS – IMPLICATIONS FOR CONDITIONS AND PROCESSES DURING FAULT SLIP

Clay-rich gouges in fault zones have long been recognized as significant in understanding the processes of fault slip and fault creep. It is typically assumed that clay-rich fault rocks form in fault zones dominated by brittle processes and that the crystallographic preferred orientations of clay-rich fault rocks should therefore reflect their origins in zones of shear and presumably high differential stress. X-ray Texture Goniometry (XTG), which directly measures the crystallographic preferred orientation, or fabric intensity of clay gouge using a modified X-ray diffraction unit, allows for the quantification of the intensity of the mineral fabric within gouge zones. New XTG work on clay gouges from a variety of tectonic and mineralogical environments, along with earlier data sets shows the surprising result that fabrics developed in natural fault gouge clays are uniformly weak relative to the host rock and similar to those found in weakly consolidated mudstones and poorly-lithified shales. Neither the mineralogy of the dominant clay phase nor the tectonic environment of the fault in question significantly affects fabric intensity. Both clay-rich gouges dominated by detrital minerals, such as chlorite, and those dominated by authigenic clays, such as smectite and the 1Md polytype of illite, have similarly weak fabrics. In contrast, experimental work on artificially-deformed smectitic gouges indicates an increase in fabric intensity with increasing normal stress, indicating that these clay-rich rocks are sensitive to stresses as predicted by passive reorientation processes. These fabric studies on natural and laboratory samples indicate the respective roles of neocrystallization and reorientation in fault rocks as a function of dominant deformation mechanisms.