Dynamic Weakening of Serpentinite Gouges and Bare-Surfaces at Seismic Slip Rates

Thursday, 18 December 2014
Brooks Proctor1, Thomas M Mitchell2, Greg Hirth1, David L Goldsby3, Federico Zorzi4, John D Platt5 and Giulio Di Toro4, (1)Brown Univeristy, Providence, RI, United States, (2)University College London, London, United Kingdom, (3)University of Pennsylvania, Geology, Philadelphia, PA, United States, (4)University of Padua, Padua, Italy, (5)Harvard University-SEAS, Cambridge, MA, United States
Serpentinite is a common rock type in oceanic transform faults. Our ability to understand and predict seismicity generated along these faults depends on models for the frictional behavior of serpentinite at conditions spanning the seismogenic zone. To investigate differences in the frictional behavior between serpentinite bare-rock surfaces and powdered serpentinite (‘gouge’) at sub-seismic to seismic-slip rates, we conducted rotary shear experiments on an antigorite-rich and lizardite-rich serpentinite at slip rates (V) from 0.003 m/s to 6.5 m/s, sliding displacements up to 1.6 m, and normal stresses (σn) up to 22 MPa for gouge and 97 MPa for bare surfaces. Nominal steady-state friction values (μnss) in gouge at V= 1 m/s are larger than in bare surfaces for all σn tested and demonstrate a strong σn dependence; μnss decreased from 0.51 at 4.0 MPa to 0.39 at 22.4 MPa. Conversely, μnss values for bare surfaces remained ~0.1 with increasing σn and V. Additionally, the velocity at the onset of frictional weakening and the amount of slip prior to weakening were orders of magnitude larger in gouge than in bare surfaces. Extrapolation of the normal stress dependence for μnss suggests that the behavior of antigorite gouge approaches that of bare surfaces at σn ≥ 60 MPa. X-ray diffraction revealed dehydration reaction products in samples that frictionally weakened. Microstructural analysis revealed highly localized slip zones with melt-like textures in some cases gouge experiments and in all bare-surfaces experiments for V ≥ 1 m/s. 1-D thermal modeling indicates that flash heating is the primary process causing initial weakening in bare surfaces. Flash heating also occurs in gouge, however because strain is more distributed, dynamic weakening occurs at higher velocities and after larger displacements. Our findings suggest that at shallower depths (<5 km) the frictional behavior of faults will be strongly affected by the presence of unconsolidated gouge.