Constitutive Laws for Etnean Basement and Edifice Lithologies

Abstract

The mechanical dynamics of volcanic systems can be better understood with detailed knowledge on strength of a volcanic edifice and subsurface. Previous work highlighting this on Mt. Etna has suggested that its carbonate basement could be a significant zone of widespread planar weakness. Here, we report new deformation experiments to better quantify such effects. We measure and compare key deformation parameters using Etna basalt, which is representative of upper edifice lava flows, and Comiso limestone, which is representative of the carbonate basement, under upper crustal conditions. These data are then used to derive empirical constitutive equations describing changes in rocks strength with pressure, temperature, and strain rate. At a constant strain rate of 10^-5 s^-1 and an applied confining pressure of 50 MPa, the brittle-to-ductile transitions were observed at 975 °C (Etna basalt) and 350 °C (Comiso limestone). For the basaltic edifice of Mt. Etna, the strength is described with a Mohr-Coulomb failure criterion with μ ~ 0.704, C = 20 MPa. For the carbonate basement, strength is best described by a power law-type flow in two regimes: a low-T regime with stress exponent n ~ 5.4 and an activation energy Q ~ 170.6 kJ/mol and a high-T regime with n ~ 2.4 and Q ~ 293.4 kJ/mol. We show that extrapolation of these data to Etna's basement predicts a brittle-to-ductile transition that corresponds well with the generally observed trends of the seismogenic zone underneath Mt. Etna. This in turn may be useful for future numerical simulations of volcano-tectonic deformation of Mt. Etna, and other volcanoes with limestone basements.