Dynamic deformation of quartz in the landslide of Kofels, Austria

Abstract

The Kofels landslide is a rare example of a ductile deformation process at very high strain rate as compared to usual tectonic deformation in the crust. The landslide-induced deformation occurred in a narrow shear zone and the resulting friction raised temperature up to melting, leading to the formation of a thin layer of pumice which lubricated the slip of the large mountain piece. We investigated by transmission electron 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, quartz 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 temperature recovery. The crystalline quartz fragments in the pumice are severely annealed and numerous tiny water bubbles are precipitated on the dislocations. Pure silica glass is also detected between the quartz grains. No defects characteristic of shock metamorphism are detected although the landslide process must have released an energy comparable with the impact of a large meteorite (≃ 30 m wide) reaching the earth at ≃ 20 km/s. Our observations thus confirm that landslide processes release their energy much more slowly than impacts in such a way that this energy is dissipated in the form of heat without radiation or shock wave effects. The pumiceous glass of Kofels formed by usual high-temperature melting and not by shock metamorphism as previously suggested by some authors