Variations in Frictional Behavior of Fault Gouge Along a low Angle Normal Fault System.

The Panamint Valley fault system contributes up to ~2.0 mm/yr of slip to the Eastern California Shear Zone, and has been interpreted to have developed as a low-angle detachment system. More recent studies of young scarps suggest that the fault currently accommodates oblique normal slip. Recognizing the complications in explaining low angle normal faulting using classic fault mechanics we investigate frictional properties 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 small canyon approximately 1 kilometer North of Big Horn Canyon. We investigate potential variations in the frictional behavior of gouges from these fault zones. Gouge samples were recovered by removing weathered surface debris and carving out roughly 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. In the experiments we sheared 7mm thick layers of fault gouge in a servo-controlled biaxial deformation machine, 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. Stress-strain curves show that shear strength exhibits a peak followed by shear at a steady-state friction level or slight strain weakening during load cycles run at normal stresses <20 MPa. At normal stresses from 30-50 MPa, the fault gouges exhibit a transition to strain hardening. For normal stresses <50 MPa, shear strength increases predictably in a stepwise manner as normal stress is increased. For higher normal stress (> 100 MPa in most samples, depending on clay content), this stepwise increase decays and shear strength becomes nearly independent of normal stress. The steady-state coefficient of friction ranged from ~0.2 to 0.6. Initial results indicate velocity strengthening frictional behavior, steady-state frictional strength increases with increasing slip velocity, for normal stresses from 5-150 MPa. A few of the gouges exhibit velocity strengthening at 5 MPa and a transition to velocity weakening at normal stresses from 10-20 MPa, with a transition back to velocity strengthening above 30 MPa. We studied the magnitude of friction velocity dependence as a function of normal stress. We produced thin sections of the sheared layers for detailed microstructural analysis. The Coulomb failure parameters and the friction behavior are related to microstructures preserved in the deformed gouge layers. We also relate deformation behavior to mineralogical differences between samples, which were determined using X-ray diffraction (XRD).