title: Dynamic weakening of serpentinite gouges and bare surfaces at seismic slip rates.
abstract: To investigate differences in the frictional behavior between initially bare rock surfaces of serpentinite and powdered serpentinite ("gouge") at subseismic to seismic slip rates, we conducted single-velocity step and multiple-velocity step friction 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 atV= 1 m/s are larger than in bare surfaces for allσntested and demonstrate a strongσndependence;μnssdecreased from 0.51 at 4.0 MPa to 0.39 at 22.4 MPa. Conversely,μnssvalues for bare surfaces remained ∼0.1 with increasingσnandV. 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 forμnsssuggests that the behavior of antigorite gouge approaches that of bare surfaces atσn≥ 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 forV≥ 1 m/s. One-dimensional thermal modeling indicates that flash heating causes frictional weakening in both bare surfaces and gouge. Friction values for gouge decrease at higher velocities and after longer displacements than bare surfaces because strain is more distributed.
  
  
  CONCLUSIONS
  Gouge friction approaches that of bare surfaces at high normal stressDehydration reactions and bulk melting in serpentinite in < 1 m of slipFlash heating causes dynamic frictional weakening in gouge and bare surfaces.
authors.affiliation: Department of Geological Sciences, Brown University Providence, Rhode Island, USA.
authors.forename: B P
authors.initials: BP
authors.lastname: Proctor
authors.affiliation: Istituto Nazionale di Geofisica e Vulcanologia Roma, Italy ; Now at Department of Earth Sciences, University College London London, UK.
authors.forename: T M
authors.initials: TM
authors.lastname: Mitchell
authors.affiliation: Department of Geological Sciences, Brown University Providence, Rhode Island, USA.
authors.forename: G
authors.initials: G
authors.lastname: Hirth
authors.affiliation: Department of Geological Sciences, Brown University Providence, Rhode Island, USA ; Now at Department of Earth and Environmental Sciences, University of Pennsylvania Philadelphia, Pennsylvania, USA.
authors.forename: D
authors.initials: D
authors.lastname: Goldsby
authors.affiliation: Department of Geological Sciences, Padova University Padova, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Zorzi
authors.affiliation: School of Engineering and Applied Sciences, Harvard University Cambridge, Massachusetts, USA.
authors.forename: J D
authors.initials: JD
authors.lastname: Platt
authors.affiliation: Istituto Nazionale di Geofisica e Vulcanologia Roma, Italy ; Department of Geological Sciences, Padova University Padova, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Di Toro
keywords: dynamic weakening; flash heating; high-velocity friction; pseudotachylyte; rapid metamorphism; serpentinite