Quasi-equilibrium melting of quartzite upon extreme friction

Abstract

The friction on fault planes that controls how rocks slide during earthquakes decreases significantly as a result of complex fault-lubrication processes involving frictional melting. Fault friction has been characterized in terms of the preferential melting of minerals with low melting points-so-called disequilibrium melting. Quartz, which has a high melting temperature of about 1,726 degrees C and is a major component of crustal rocks, is not expected to melt often during seismic slip. Here we use highvelocity friction experiments on quartzite to show that quartz can melt at temperatures of 1,350 to 1,500 degrees C. This implies that quartz within a fault plane undergoing rapid friction sliding could melt at substantially lower temperatures than expected. We suggest that depression of the melting temperature is caused by the preferential melting of ultra-fine particles and metastable melting of beta-quartz at about 1,400 degrees C during extreme frictional slip. The results for quartzite are applicable to complex rocks because of the observed prevalence of dynamic grain fragmentation, the preferential melting of smaller grains and the kinetic preference of beta-quartz formation during frictional sliding. We postulate that frictional melting of quartz on a fault plane at temperatures substantially below the melting temperature could facilitate slip-weakening and lead to large earthquakes.