VISCOSITY DETERMINATIONS OF SOME FRICTIONALLY GENERATED SILICATE MELTS - IMPLICATIONS FOR FAULT ZONE RHEOLOGY AT HIGH-STRAIN RATES

Analytical scanning electron microscopy has been used to determine the major element compositions of some natural and artificial silicate gl asses and their microcrystalline equivalents derived by the frictional melting of intermediate to acid protoliths. The data show that the ma trices of the friction melts (which cool to form pseudotachylytes) are relatively basic and hydrous, even when their protoliths are intermed iate to acid. This is because frictional fusion involves the selective comminution and nonequilibrium melting of minerals based on their ind ividual mechanical properties and melting points, not the formation of minimum melts through equilibrium mineral interaction. This means tha t hydrous ferromagnesian minerals (e.g., micas and amphiboles) melt pr eferentially to form the liquid matrix, while feldspars and especially quartz more readily survive as clasts. Pseudotachylytes generated by frictional melting are therefore not bulk melts, and as clast-melt sus pensions, they cannot be considered as simple Newtonian fluids. The ca lculated viscosities of the friction melts are low. For example, at 12 00-degrees-C, most friction melts possess zero-shear suspension viscos ities of 10(2) - 10(4) dPa s (1 dPa s = 1 P). This is equivalent to th e viscosities of tholeiitic and alkaline basaltic magmas at the same t emperature. These viscosities are maximum determinations because, as c last-melt suspensions, friction melts may undergo shear thinning and e xhibit pseudoplasticity at high shear rates (i.e., during slip on a fa ult surface). Contrary to earlier suggestions, where the bulk melting of intermediate to acid protoliths was believed to result in the gener ation of viscous friction melts that could act to inhibit continued sl iding, this work shows that most pseudotachylytes are partial melts po ssessing low viscosities. The formation of highly fluid suspensions du ring slip may have profound effects on the dissipation of stored strai n energy in the rocks surrounding a fault. Interface lubrication could facilitate an increase in the slip rate and the rate of energy dissip ation. This would be manifest as an increase in high-frequency seismic wave radiation and vibrational.