LAUSR

Abstract The Arequipa volcanic landslide deposit to the east of Arequipa (Peru) originated from the Pichu Pichu volcanic complex, covering an area >100 km2. The debris avalanche deposit exhibits internal flow structures and basal pseudotachylytes. We present field, microstructural and chemical observations from slip surfaces below and within the deposit which show varying degrees of strain localisation. At one locality the basal shear zone is localised to a 1–2 cm thick, extremely sheared layer of mixed ultracataclasite and pseudotachylyte containing fragments of earlier frictional melts. Rheological modelling indicates brittle fragmentation of the melt may have occurred due to high strain rates, at velocities of >31 m s−1 and that frictional melting is unlikely to provide a mechanism for basal lubrication. Elsewhere, we observe a