The role of fault zone fabric and lithification state on frictional strength, constitutive behavior, and deformation microstructure

We examine the frictional behavior of a range of lithified rocks used as analogs for fault rocks, cataclasites and ultracataclasites at seismogenic depths and compare them with gouge powders commonly used in experimental studies of faults. At normal stresses of similar to 50 MPa, the frictional strength of lithified, isotropic hard rocks is generally higher than their powdered equivalents, whereas foliated phyllosilicate-rich fault rocks are generally weaker than powdered fault gouge, depending on foliation intensity. Most samples exhibit velocity-strengthening frictional behavior, in which sliding friction increases with slip velocity, with velocity weakening limited to phyllosilicate-poor samples. This suggests that lithification of phyllosilicate-rich fault gouge alone is insufficient to allow earthquake nucleation. Microstructural observations show prominent, throughgoing shear planes and grain comminution in the R1 Riedel orientation and some evidence of boundary shear in phyllosilicate-poor samples, while more complicated, anastomosing features at lower angles are common for phyllosilicate-rich samples. Comparison between powdered gouges of differing thicknesses shows that higher Riedel shear angles correlate with lower apparent coefficients of friction in thick fault zones. This suggests that the difference between the measured apparent friction and the true internal friction depends on the orientation of internal deformation structures, consistent with theoretical considerations of stress rotation.