3-D FEM-based thermal modeling of frictional melt cooling and constraints on microstructures in pseudotachylytes vein (original) (raw)

Abstract

ABSTRACT Pseudotachylytes are solidified friction-induced melts recording a seismic rupture and made of survivor clasts suspended in a glassy-like matrix . The pseudotachylyte microstructure enables to estimate several earthquake source parameters (e.g. seismic energy budget at a point of a fault). However, these estimates require the discrimination among pre-seismic, co-seismic and post-seismic microstructures. Modeling the thermal evolution of the frictional melt might help in understanding the formation and preservation of pseudotachylytes microstructures. A 3-D model for the thermal evolution of friction melt layers and of survivor clasts within the melt is proposed here. The model is based on Finite Element commercial code COMSOL Multiphysics™. The model boundary conditions are based on a pseudotachylyte belonging to the Gole Larghe Fault Zone in the granitic Adamello batholith, Italy, and which has been previously used to estimate the earthquake surface energy from the internal fragmentation observed in plagioclase survivor clasts . In our model, we have considered clasts with different composition (quartz and plagioclase), size (from 0.1 to 2.0 mm), position within the melt layer and initial temperature (250, 850 and 1000°C). The pseudotachylytes vein is 6 mm thick, and the initial melt temperature is in the range of 1450 and 1750°C. Results show that the cooling rate of clasts suspended in the frictional melt strongly depends on the position of the clast within the vein. For example, the core of a plagioclase clast with initial temperature of 850°C, immersed in a 1750°C melt, never reaches complete melting (i) at the vein center, in the case of a radius larger than 1.5 mm, and (ii) at the vein margin, whatever its size. In particular, clast portions closer to the contact with the wall rock never undergo melting. As a consequence, the plagioclase clast internal fragmentation, referable to the initial stages of coseismic slip, can be potentially preserved from melting, allowing the estimate of the surface energy and thus the estimate of the local energy budget of an earthquake.

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