DYNAMIC DEFORMATION OF QUARTZ IN THE LANDSLIDE OF KOFELS, AUSTRIA

The Kofels landslide is a rare example of a ductile deformation proces s at very high strain rate as compared to usual tectonic deformation i n the crust. The landslide-induced deformation occurred in a narrow sh ear zone and the resulting friction raised temperature up to melting, leading to the formation of a thin layer of pumice which lubricated th e slip of the large mountain piece. We investigated by transmission el ectron microscopy (TEM) the defect microstructure (dislocations, twins , subgrain boundaries ...) in quartz grains at various distances from this friction surface. At a few centimetres away from the pumice, quar tz grains in the wall gneiss show a high density of dislocations, most often in glide configuration. In quartz grains closer to the pumice ( one cm or less) the dislocation density decreases while well organized subgrain boundaries begin to build a microstructure typical of high t emperature recovery. The crystalline quartz fragments in the pumice ar e severely annealed and numerous tiny water bubbles are precipitated o n the dislocations. Pure silica glass is also detected between the qua rtz grains. No defects characteristic of shock metamorphism are detect ed although the landslide process must have released an energy compara ble with the impact of a large meteorite (almost-equal-to 30 m wide) r eaching the earth at almost-equal-to 20 km/s. Our observations thus co nfirm that landslide processes release their energy much more slowly t han impacts in such a way that this energy is dissipated in the form o f heat without radiation or shock wave effects. The pumiceous glass of Kofels formed by usual high-temperature melting and not by shock meta morphism as previously suggested by some authors.