Microscopic model of rock melting beneath landslides calibrated on the mineralogical analysis of the Köfels frictionite

Friction rocks are produced by concentrated shear and partial melting at the Earth surface. Although the commonest examples are known from earthquake-generated layers (pseudotachylytes), such rocks may also derive from the slippage of rock avalanches. While numerous studies have been dedicated to earthquake-generated pseudotachylytes from a petrological, geochemical and physical viewpoint, fewer investigations have focused on the corresponding rock avalanche rocks (also termed frictionites), especially concerning the mechanics of formation and its relationship to the mineralogical composition of the original rock. In this work, we introduce a numerical model for the melting of a crystalline micro-breccia (gouge) in the shear layer of a rock avalanche due to heat generated at the sliding surface. The motion of the landslide is calculated and the resulting frictional heat is used to compute the melting of the crystalline gouge. We constrain the model based on calibration of the Köfels frictionite in Austria, the best-known example of landslide-frictionite association. We have collected samples of frictionite and of the original rock consisting of gneiss rich in alkaline feldspar and have analysed it chemically and mineralogically. Not only should the model calculate the temperature increase to fuse the average gneissic rock, we also require that the simulated percentages of mineral species should reasonably reproduce the data. The compositional data of both the original rock and the frictionite, whose mineralogical species have different melting temperatures, latent heats and thermal conductivities, allow us to constrain the numerical simulations. A second aspect considered in this work is the particular pumiceous texture derived from growth and coalescence of bubbles as a consequence of decreasing ambient pressure. The simulations show a satisfactory agreement with data but also discrepancies that are probably due to the limitations of the numerical model and to the irregular grain shape of the rock gouge in the real data. Simulations indicate that although temperatures were higher than the melting temperature of all the species, complete melting of the original crystals was not reached because of the limited duration of the landslide flow. The model could be of interest for future quantitative investigations of landslide frictionites.