Viscous Shear Heating Instability in Fine-Grained Shear Zones

Ethan Coon, Columbia University

Localized deformation in shear zones is an ubiquitous observation in both mantle ophiolites and crustal outcrops. Characteristic observations of these zones include shear localized to very narrow regions and a decreased grain size in the region. While much work has been undergone analyzing and modeling the runaway instability associated with shear heating, these works depend upon a region of finite temperature perturbation that must be imposed by outside factors. In this work, we consider shear zones deforming under grain-size sensitive creep mechanisms seeded from pre-existing variations in grain size. These zones stay localized for long periods of time and allow multiple quasi-periodic instabilities. These instabilities, while occurring in viscous materials, reach speeds and stress drops of seismic magnitudes. This mechanism provides an attractive alternative to dehydration embrittlement for explaining intermediate depth earthquakes, especially those occurring in a narrow thermal window within the mantle section of subducting oceanic plates.

Abstract Author(s): Ethan T. Coon, Peter B. Kelemen, Greg Hirth, and Marc Spiegelman