Experimental generation of volcanic pseudotachylytes: Constraining rheology

Abstract

Volcanic systems are highly dynamic environments that comprise rocks and magmas, which, in the process of strain localisation (e.g., in catastrophic flank collapse or dome-building events), are candidate materials for the occurrence of frictional melting and the formation of pseudotachylytes. We evaluate the frictional behaviour of a plagioclase-, two-pyroxene- and glass-bearing andesite and introduce an approach to constrain the rheology of frictional melts. Frictional slip at a rate of 1.3m/s under an axial stress of 1.5MPa induces heating at a rate approximating 130°C per metre of slip and frictional melting of the andesite occurs at >1000°C, which corresponds to a peak in shear stress. With continuing displacement, the shear stress decreases by ∼25% from the peak strength and stabilises while the temperature equilibrates to 1230–1290°C. The shear stress for the system is congruent with a non-Arrhenian temperature-dependent frictional melt rheology exhibiting a non-Newtonian viscosity evolving from approximately 104.2 to 103.4Pas. Post-experiment analysis shows a gradation from 1) the host rock, to 2) an extraordinary zone of unmelted, yet viscously deformed material, to 3) a thin, outer region of chemically heterogeneous protomelts, and to 4) an inner region of chemically homogeneous frictionally generated/modified melt in the core of the slip zone. We discuss the role and identification of frictional melting in volcanic systems and the implication of viscous remobilisation of magmatic plugs during frictional slip of glass-bearing volcanic materials.