BRANCHING FRACTURES IN HIGH SPEED LAB EXPERIMENTS AND FIELD EXPOSURES: AN INDICATION OF HIGH CO-SEISMIC SLIP RATES?

It has recently been proposed by Sagy et al. (2001, Journal of Structural Geology 23, 1223-1239) that fractures with a tree-like, branching morphology may be characteristic of dynamic fractures that propagate at velocities approaching the Rayleigh wave speed. Here we present results of microstructural analyses of larvakite granite samples subjected to high velocity friction tests. Samples were subjected to varying normal stress and slip rates for up to 28 seconds, and all samples produced pseudotachylyte, except one, which was stopped after 2 seconds. When examined under thin section, all samples contain fractures oblique to the slipping surfaces that have a branching morphology. The fractures branch away from the slipping surface, consistent with the experimental results in epoxy (op. cit.). We compare these experimental microstructures to microfractures around pseudotachylyte-bearing faults that cut granite in the Sierra Nevada, California. Larger-scale branching fractures can also be observed in field exposures of the same faults. These preliminary results suggest that the presence of branching fractures at microscopic and field scales may be a useful indicator of high slip rates during a seismic event.