The coexistence of pseudotachylyte, a brittle fault rock representing seismic slip, and mylonite, a plastic or semiplastic fault rock associated with aseismic slip, has been debated for its origin in earthquake-generating faults. Deformation experiments on halite gouge at seismic slip rates show that mylonite forms simultaneously with frictional melt (pseudotachylyte) in the brittle regime, equivalent to upper crustal conditions. The plastic deformation of halite, resulting in mylonite, is facilitated by the conduction of heat from a slip-concentrated molten layer formed by frictional heating at seismic slip rates. Thus mylonites, which are (1) kinematically consistent and (2) just at the contact with pseudotachylyte, might have formed together with the pseudotachylyte in the brittle regime during seismic slip.

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