Friction of foliated fault gouge with a biotite interlayer at hydrothermal conditions

To investigate the influence of foliation and structure within fault zones, synthetic fault gouges of quartz containing various thin layers of biotite were sheared in a triaxial testing system at hydrothermal conditions to acquire the faulting behaviour, along with quartz-biotite mixtures tested for comparison. First, in Series-1 experiments, gouges were sheared at a temperature of 100 °C, with effective confining pressure of 200 MPa, pore pressure of 30 MPa and loading rate of 1.22 μm/s. The results revealed that strength of quartz-biotite mixture weakened linearly with increasing biotite proportion, whereas the gouges with a biotite interlayer weakened much more significantly even for biotite content as little as ~5 wt%. Series-2 experiments were performed on simulated gouge with 15 wt% biotite interlayer at temperatures of 25-600 °C, effective normal stress of 200-230 MPa and pore pressure of 30 MPa, with stepped shear rates of 0.0488 to 1.22 μm/s. At effective normal stress of 200 MPa, the simulated foliate gouge showed a transition from velocity strengthening to velocity weakening around 200 °C, which persisted up to 600 °C. Stick-slips above 400 °C were associated with minor a-b averagely of ~−0.0006 and submicron dc values. At effective normal stress of 230 MPa and temperature of 600 °C, stick-slip motions evolved with slip distance to viscous-like behaviour at a displacement of ~3 mm. Microstructural observation on deformed samples revealed that shear deformation mainly occurred within the biotite interlayer in most cases, with surrounding quartz gouge only slightly deformed but increasingly compacted as temperature increased. Viscous-like behaviour corresponded to migration of dominant shear deformation to the quartz gouge, as indicated by the development of Riedel shears therein. Our results may help constrain fault strength and depth range of seismogenesis within fault zones associated with biotite in form of foliations and/or interconnected networks.