title: 'Melt welt' mechanism of extreme weakening of gabbro at seismic             slip rates.
abstract: Laboratory studies of frictional properties of rocks at slip velocities approaching                 the seismic range                     (∼0.1-1 m s(-1)),                 and at moderate normal stresses (1-10 MPa), have revealed a                 complex evolution of the dynamic shear strength, with at least two phases of                 weakening separated by strengthening at the onset of wholesale melting. The second post-melting weakening phase is governed by                 viscous properties of the melt layer and is reasonably well understood. The initial phase of extreme weakening, however, remains a                 subject of much debate. Here we show that the initial weakening of gabbro is                 associated with the formation of hotspots and macroscopic streaks of melt                 ('melt welts'), which partially unload the rest of the slip                 interface. Melt welts begin to form when the average rate of frictional heating                 exceeds 0.1-0.4 MW m(-2),                 while the average temperature of the shear zone is well below the solidus                 (250-450 °C). Similar heterogeneities in stress and                 temperature are likely to occur on natural fault surfaces during rapid slip, and to                 be important for earthquake rupture dynamics.
authors.affiliation: Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093, USA. kmbrown@ucsd.edu
authors.forename: Kevin M
authors.initials: KM
authors.lastname: Brown
authors.forename: Yuri
authors.initials: Y
authors.lastname: Fialko