Pseudotachylyte (frictionite) at the base of the Arequipa volcanic landslide deposit (Peru): Implications for emplacement mechanisms

The Arequipa volcanic landslide deposit (Peru) displays a pseudotachylyte (or frictionite) at its basal contact with an older ignimbrite. The pseudotachylyte consists of a dark 1-cm-thick vitreous layer that shows striations on its upper surface. It contains small mineral clasts from both the landslide rocks and the underlying ignimbrite embedded in an optically isotropic matrix. TEM observations revealed a clast-melt suspension with a lower black obsidian-like vein made of chemically heterogeneous glass. The pseudotachylyte is interpreted as the result of partial melting due to frictional heating during landslide emplacement. This is the third pseudotachylyte reported at the base of a landslide deposit and the first in a volcanic environment. Calculations show that basal melting only accounts for a small proportion of the total energy released during landslide emplacement, indicating that landslide transport was dominated by dissipative stresses affecting the whole landslide body and not concentrated only in a narrow basal layer. Melting most likely occurred during a short frictional episode that marked the transition from nondepositional to depositional conditions.