T11B-4559:
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
Abstract:
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.