Influence of Protolith Composition and Sliding Velocity on the Microfabric of Fault Gouge: Experimental Results

The relation between fault gouge fabric and fault-slip behavior remains a central unknown in our knowledge of fault and earthquake mechanics. The linkage of cause to effect remains cryptic in part because natural gouges are highly variable (i.e., particle size distribution, mineralogy, microfabric), due to the heterogeneity of protoliths and differences in cumulative slip. To help isolate key variables, we conducted a series of direct shearing tests in a double direct shear configuration, on specimens of artificial powdered gouge. The experiments were conducted under room temperature and humidity conditions (RH = 14.4-32.8%), using ~3 mm-thick (prior to shearing) layers with nominal contact areas of 5x5 cm. Layers were sheared between grooved steel forcing blocks designed to minimize slip at the layer boundary. We tested four gouge compositions: illitic shale, chlorite schist, a 50:50 mixture of smectite and quartz, and Westerly granite. Three load point sliding velocities were applied (1.15, 11.5, and 115 microns/sec), and all experiments were conducted under a normal stress of ~50 MPa. As expected, the coefficients of friction vary as a function of both composition and sliding velocity. The smectite:quartz mixture is consistently weakest (0.38-0.39) and granite gouge is consistently strongest (0.60-0.61). Gouges of illitic shale and chlorite schist yielded larger differences as a function of sliding velocity, 0.40-0.53 and 0.48-0.58, respectively. To characterize the microfabric that developed within each sample during shearing, the twelve experimental wafers were analyzed by X-ray texture goniometry (XTG) and imaged (uncoated) using an FEI Quanta 600 scanning electron microscopy in low vacuum mode (80 Pa). SEM images were shot parallel and perpendicular to the shear plane at high voltage (30 kV) with spot size of 3.0 and working distance of 10 mm. After processing the digital images (1000X magnification), we ran statistical analyses of the apparent long-axis orientations, with an average of 660 grains per view (i.e., graphical standard deviation of the azimuths). Preferred orientations are much stronger on views perpendicular to the shear plane, whereas grains on shear-parallel surfaces are randomly oriented. Within each gouge composition, samples subjected to faster sliding velocity developed more highly preferred grain orientations. Among the four compositions, both SEM and XTG show that chlorite schist gouge developed the most highly preferred orientation, granite and smectite:quartz the least. Faster sliding velocity weakens the fabric of chlorite-rich gouge but strengthens the fabric of illite-rich gouge. Although the alignment of microfabric in natural fault gouge is undoubtedly enhanced by repeated slip events and growth of diagenetic phases, our experiments show that protolith mineralogy, especially the abundance and types of phyllosilicates, is a key prerequisite to preferred grain orientation and fabric development.