Velocity Weakening to Velocity Strengthening Transition Controlled by Temperature, Effective Normal Stress and Sliding Rate in Frictional Sliding of a Phyllosilicate-Rich Mylonite from Longmenshan Fault Zone

Under hydrothermal conditions, we performed experiments of frictional sliding of a phyllosilicate-rich mylonite from Longmenshan fault zone. In addition to quartz and plagioclase, the mylonite includes 60wt. % phyllosilicate which are mainly chlorite(~36%) and muscovite(~24%), thus it is of interest to see how these phyllosilicates affect the properties of frictional sliding of the fault rock, especially the steady-state rate dependence that is important in determining sliding stability. With effective normal stresses from 100 MPa to 300MPa and temperatures from 100 to 600 ^oC, we found transition from velocity weakening(VW) to velocity strengthening(VS) as temperature increases at effective normal stresses of 200MPa and 300MPa, and higher effective normal stress was found to promote the VW-VS transition, manifested by reducing the transition temperature from 600 ^oC to 400 ^oC when effective normal stress is raised from 200MPa to 300MPa. The VW-VS transition was also found to depend on the applied loading rate in our experiments where response to rate steps between 0.2 μm/s and 1 μm/s exhibits VW behavior whereas rate steps between 0.2 μm/s and 0.04 μm/s corresponds to VS behavior.

In the VW cases at 200MPa effective normal stress, the fabrics of deformed gouges observed with SEM show both small faults (Riedel and P shears) and localized shear zones(LSZ). The LSZs are strips of gouge where the grain size of minerals are significantly reduced, which appear parallel or sub-parallel to the main fault boundary, with a prominent boundary to the gouge matrix and a width of 20-25 μm. The fabrics below 200 ^oC are dominated by through-going R1 shears and some P shears, whereas the fabrics at 300 and 400 ^oC are fault-free and dominated instead by LSZs. At 600 ^oC, R1 shears appeared again along with LSZs. The LSZs are characterized with nano mineral particles of 20-30nm, which are considered here to be the result of a triggered stick-slip motion. In the VS regime at normal stress of 300MPa, the fabric at 600 ^oC are totally different to the 500 ^oC case. At 500 ^oC, the deformed gouge is characterized with curved R1 shears, along with a single LSZ. The fabric at 600 ^oC is dominated by a fabric similar to S-C fabric, along with a few R1 shears. The apparent S-C fabric indicates enhancement of ductile deformation by elevated temperature.