Rapid Ductile Strain Localization Due To Thermal Runaway.

Thermal runaway is a ductile localization mechanism that has been linked to deep‐focus earthquakes and pseudotachylyte formation. In this study, we investigate the dynamics of this process using one‐dimensional, numerical models of simple shear deformation. The models employ a visco‐elastic rheology where viscous creep is accommodated with a composite rheology encompassing diffusion and dislocation creep as well as low‐temperature plasticity. To solve the nonlinear system of differential equations governing this rheology, we utilize the pseudo‐transient iterative method in combination with a viscosity regularization to avoid resolution dependencies. To determine the impact of different model parameters on the occurrence of thermal runaway, we perform a parameter sensitivity study consisting of 6,000 numerical experiments. We observe two distinct behaviors, namely a stable regime, characterized by transient shear zone formation accompanied by a moderate (100–300 K) temperature increase, and a thermal runaway regime, characterized by strong localization, rapid slip and a temperature surge of thousands of Kelvin. Nondimensional scaling analysis allows us to determine two dimensionless groups that predict the model behavior. The ratio tr/td ${t}_{\mathrm{r}}/{t}_{\mathrm{d}}$ represents the competition between heat generation from stress relaxation and heat loss due to thermal diffusion while the ratio Uel/Uth ${U}_{\text{el}}/{U}_{\text{th}}$ compares the stored elastic energy to thermal energy in the system. Thermal runaway occurs if tr/td ${t}_{\mathrm{r}}/{t}_{\mathrm{d}}$ is small and Uel/Uth ${U}_{\text{el}}/{U}_{\text{th}}$ is large. Our results demonstrate that thermal runaway is a viable mechanism driving fast slip events that are in line with deep‐focus earthquakes and pseudotachylyte formation at conditions resembling cores of subducting slabs. Plain Language Summary: Thermal runaway is a mechanism that concentrates material deformation into thin layers without breaking the material and has been linked to earthquakes more than 70 km below the surface. This study uses one‐dimensional computer models with a complex material behavior and conducts 6,000 numerical experiments to investigate the influence of different parameters like temperature, deformation rate and material properties. Results show two distinct behaviors, namely a stable regime with slow sliding and a temperature rise of 100–300 K or thermal runaway with fast movement and a temperature increase of a few thousand Kelvin. We find two dimensionless ratios that are combinations of the input parameters and can predict the behavior. The ratio tr/td ${t}_{\mathrm{r}}/{t}_{\mathrm{d}}$ compares heat production to heat loss, and the ratio Uel/Uth ${U}_{\text{el}}/{U}_{\text{th}}$ compares stored elastic energy to thermal energy. If the first ratio is small and the second ratio is large, thermal runaway occurs. Our results show that thermal runaway could cause fast deformation events like earthquakes and produce thin layers of molten rock in conditions that are typical for subducting plates. Key Points: Numerical investigation of thermal runaway in visco‐elastic material with diffusion creep, dislocation creep and low‐temperature plasticityNondimensional scaling analysis reveals two dimensionless groups governing the occurrence of thermal runawayDuctile localization can cause slip events in line with deep earthquakes and pseudotachylyte formation at subducting slab core conditions [ABSTRACT FROM AUTHOR]