High-speed slip experiments performed on Westerly granite using friction welding apparatus reveal that comminution is an essential precursor to melting by friction. Observations of slip surfaces via analytical scanning electron microscopy (SEM) document the following sequence of events occurring in 2 s with increasing velocity (up to 2 m/s): fracture; progressive comminution; surface melting of mineral fragments; fragment-to-fragment adhesion; and, finally, production of a fragment-laden, melt-supported suspension. Explosive dehydration and melting of the epidote-group mineral allanite indicates that temperatures of at least 1000 °C were realized at the interface. This is corroborated by calculation of the temperature rise for the known operating conditions. Contrary to earlier proposals, these results show that comminution and frictional melting are complementary and not mutually exclusive processes. Depending on the velocity–shear stress–displacement relations prevailing during frictional slip, rocks produced in seismogenic zones can be predominantly comminuted wall rock or fragment-melt mixes (pseudotachylytes).

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