PSEUDOTACHYLYTE CONTROVERSY - FACT OR FRICTION

High-speed slip experiments performed on Westerly granite using fricti on 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): fractu re; progressive comminution; surface melting of mineral fragments; fra gment-to-fragment adhesion; and, finally, production of a fragment-lad en, melt supported suspension. Explosive dehydration and melting of th e epidote-group mineral allanite indicates that temperatures of at lea st 1000 degrees C were realized at the interface. This is corroborated by calculation of the temperature rise for the known operating condit ions. Contrary to earlier proposals, these results show that comminuti on and frictional melting are complementary and not mutually exclusive processes. Depending on the velocity-shear stress-displacement relati ons prevailing during frictional slip, rocks produced in seismogenic z ones can be predominantly comminuted wall rock or fragment-melt mixes (pseudotachylytes).