During seismic slip, the elastic strain energy released by the wall rocks drives grain fragmentation and flash heating in the slipping zone, resulting in formation of (nano)powders and melt droplets, which lower the fault resistance. With progressive seismic slip, the frictional melt covers the slip surface and behaves as a lubricant reducing the coseismic fault strength. However, the processes associated to the transition from grain fragmentation to bulk frictional melting remain poorly understood. Here we discuss in situ microanalytical investigations performed on experimentally produced solidified frictional melts from the transition regime between grain fragmentation and frictional melting. The experiments were performed on granitic gneiss at seismic slip rates (1.3 and 5 m/s), normal stresses ranging from 3 to 30 MPa. At normal stresses <12 MPa, the apparent friction coefficient μapp (shear stress versus normal stress) evolves in a complex manner with slip: μapp decreases because of flash weakening, increases up to a peak value μp1 ~0.6–1.0, slightly decreases and increases again to a second peak value μp2 ~0.44–0.83, and eventually decreases with displacement to a steady-state value μss ~0.3–0.45. In situ synchrotron observations of the solidified frictional melt show abundance of ultrafine quartz grains before μp2 and enrichment in SiO2 at μp2. Because partial melting occurs on the ultrafine quartz grains and, as a consequence, it suggested that the second re-strengthening (μp2) is induced by the higher viscosity of the melt due to its enrichment in Si from melting of the ultrafine quartz grains derived from grain fragmentation.
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