Spontaneous strain localisation in a viscoelastic material owing to thermal softening

Numerous studies emphasize the significance of thermal softening induced by shear heating and the concurrent generation of ductile shear zones in geological processes spanning various spatial and temporal scales. Thermal softening is proposed as a primary mechanism in the formation of tectonic plate boundaries and as a potential mechanism of intermediate-depth and deep-focused earthquakes, where large confining pressure inhibits brittle fracture. In the latter scenario, seismic velocities during shearing are achieved through the spontaneous viscous dissipation of stored elastic energy, a phenomenon known as self-localizing thermal runaway (SLTR) 1. Resolving SLTR poses a formidable numerical challenge due to the multiscale nature of the process. The slow initial stage of differential stress buildup and strain localization is succeeded by the rapid development of a thin shear zone, necessitating high spatial and temporal resolution. Prior numerical studies of self-localizing thermal runaway were confined to 1D, restricting applications to less realistic geometries and simpler model setups. In this study, we explore the viscoelastic effects of spontaneous flow localization through numerical modeling in 2D. We develop a fully coupled thermo-mechanical solver utilizing a novel conservative energy formulation. Our work demonstrates the potential for SLTR instability in various 2D model setups. Through systematic numerical experiments, we compare the onset of localization with 1D predictions. Additionally, we investigate the role of heterogeneities in the distribution of material properties on the development of a ductile shear zone. 1 Braeck, S., Podladchikov, Y. Y. (2007). Spontaneous thermal runaway as an ultimate failure mechanism of materials.Physical Review Letters,98(9), 095504.