On the role of phyllosilicates on fault lubrication: Insight from micro-and nanostructural investigations on talc friction experiments

Exposures of mature faults at the Earth's surface show that in the shallow crust some of co-seismic slip occurs within a narrow phyllosilicate-rich gouge material. Mechanical behavior of phyllosilicates sheared at seismic slip-velocities remains a complex issue as suggested by the experimental and theoretical studies reported so far. Talc represents a simple case for common phyllosilicates in natural faults and therefore we chose talc as an analogue to better understand the mechanical behavior of clay-rich mature crustal faults. We present a series of high-velocity friction experiments conducted on talc at 1.31 m s^-1 and normal stresses of 0.3-1.8 MPa for wet and dry conditions. At 1 MPa normal stress, both wet and dry experiments show a slip-weakening behavior, however, with a higher decreasing rate in wet conditions. Dynamic shear stress evolves from a peak value of 0.24-0.52 MPa down to a residual state value of 0.08-0.18 MPa in wet conditions, and from a peak value of 0.46-1.17 MPa down to a residual state value of 0.12-0.62 MPa in dry conditions. Based on a detailed microstructural analysis down to the nanoscale, we propose thermal-pressurization as a possible slip-weakening mechanism for wet conditions. On the contrary, in dry conditions the long-lasting weakening is interpreted to be due to a combination of i) progressive disappearance of geometrical incompatibilities, ii) solid lubrication of talc lamellae, and iii) powder lubrication by nanometric aggregates. We conclude that initial gouge humid conditions and inherited fabric from past sliding may have large influence on the slip-weakening for subsequent slip.