FAULT GOUGE MINERALOGY AS A DRIVER FOR VARIABLE FRICTIONAL RESPONSE IN A LOW-ANGLE NORMAL FAULT SETTING

The physics of low angle detachments continue to intrigue geoscientists. Classical Andersonian fault mechanics precludes active extension on planes dipping < 40 degrees, yet numerous examples of such structures have been found in orogens around the world. Workers have suggested that abnormally weak fault zones may influence active slip on low-angle detachments. Recognizing the complications in explaining low angle normal faulting using classical fault mechanics, we investigate the frictional properties and clay mineralogy of natural fault gouge from selected locations along low-angle normal faults in the southern portion of Panamint Valley. Gouge samples were collected along an ~8.5 kilometer North-South transect stretching from Jail Canyon to a canyon approximately 1 kilometer North of Big Horn Canyon. Gouge samples were recovered by removing weathered surface debris and carving out 8¨x 8¨ blocks of fault gouge. All samples were crushed, milled and sieved to produce a uniform particle size distribution ranging from 30 to 350 microns. Quantitative X-ray diffraction analyses were achieved for all samples by employing the TOPAS diffraction interpretation tools. Total clay fractions vary from 15% to 62%. In the experiments we sheared 7mm thick layers of fault gouge in a servo-controlled biaxial deformation rig, using a double-direct-shear configuration at room temperature and humidity. The experiments consisted of an identical series of velocity steps and slide-hold-slide load cycles over a range of normal stresses to define the Coulomb-Mohr failure criteria and the friction constitutive properties as a function of slip velocity and state. Friction values range from ~0.2 to 0.6. These friction coefficients correlate with the clay content of the gouge and suggest that an abundance of clay in the fault rock may significantly weaken the fault zone. These results are evaluated in the context of classical fault mechanics and competing models for active slip on low angle normal faults.