A PHYSICAL BASIS FOR THE FRICTIONAL MELTING OF SOME ROCK-FORMING MINERALS

Under conditions of coseismic slip (0.1-2.0 m s-1) within the upper crust, differences in the shear yield strengths, fracture toughnesses, melting points and, to a lesser extent, thermal conductivities of rock-forming minerals result in a hierarchy of friction melting susceptibilities. In many so-called pseudotachylytes it is observed that phyllosilicates and amphiboles are preferentially consumed to form the melt phase, whilst quartz and feldspar tend to survive as clasts. This can be explained in terms of the disequilibrium "flash" melting of those mineral phases possessing the lower shear yield strengths, fracture toughnesses and thermal conductivities rather than by eutectic melting that would otherwise generate equilibrium minimum melts indicative of igneous processes. At high strain rates typical of coseismic faulting, the mechanical behaviour of minerals is primarily controlled by their fracture toughnesses, with progressive communication during slip leading to fusion. The friction melting dependence on the mechanical properties of minerals indicates that there may be an important relationship between wall-rock lithology and earthquake magnitude if frictional melting results in fault lubrication.