Frictional behavior of simulated biotite fault gouge under hydrothermal conditions

We investigated frictional properties of biotite under hydrothermal conditions by shearing simulated biotite gouge sandwiched between saw-cut driving blocks, using a testing system with argon as confining medium. Experiments were conducted under an effective normal stress of 200 MPa, with pore pressure of 30 MPa. Temperatures were varied from room temperature to 600 degrees C, with shear displacement rate stepped at standard velocities (1.22-0.122 mu m/s) and slow velocities (0.244-0.0488 mu m/s) to acquire rate dependence of friction. Our results show that the friction coefficient of biotite ranges from 0.25 to 0.44, significantly lower than that of muscovite documented in previous studies. The steady-state rate dependence shows velocity strengthening at temperatures of 25 degrees C to 200 degrees C, and transitions to velocity weakening at temperatures above 300 degrees C. The velocity weakening is quite minor with (a - b) ranging from 4 x 10(-4) to 9 x 10-4, and stick-slip motions at temperatures from 400 degrees C to 600 degrees C are found to be related to submicron de values corresponding to critical stiffness higher than the actual stiffness surrounding the fault, thus leading to unstable slips. Microstructures of deformed samples show three types corresponding respectively to three different temperature ranges. While brittle structures are dominantly associated with deformation at 100 degrees C, significant plastic deformation occurred at higher temperatures. Deformation at temperatures of 200 degrees C to 400 degrees C are characterized with shear fracture zones accompanied with plastic deformation mostly associated with basal glide. Intensively deformed zones are the characteristic structures for samples deformed at 500 degrees C to 600 degrees C, accompanied with shear fracture zones and moderately deformed zones. It is evident that biotite alone is not weak enough to explain a weak fault like the San Andreas Fault, thus overpressured pore fluid or another weaker mineral is needed to produce a weak fault with apparent friction coefficient of 0.1-0.2. The around-neutral velocity weakening found for biotite may be appropriate in modeling fault creep transients in the mid crust. (C) 2014 Elsevier B.V. All rights reserved.