From strong plates to weak boundaries: strain localization in the lithospheric mantle with low- to high-temperature dislocation creep

Abstract

Plate-like behavior in mantle convection models is commonly obtained using a strongly temperature-dependent viscosity together with a low yield stress that caps lithospheric strength. Yet, alternative mechanisms for limiting strength, including low-temperature plasticity, have been proposed. Here, we investigate how different rheological formulations promote lithospheric-scale strain localization, using 2-D thermo-mechanical simulations of upper mantle extension. We test a suite of olivine flow laws (diffusion creep and dislocation creep at low- and/or high-temperature), with and without a yield-stress cap, and introduce diagnostics that attribute plate weakening to either increasing strain rate (mechanical weakening) or increasing temperature (thermal weakening). Without a yield-stress cap, deformation remains distributed in all cases, indicating that weakening of the whole lithosphere is required for strain localization. In such simulations, a new extensional plate boundary develops in two stages: progressive narrowing of the deforming zone followed by rapid thinning of the weakened lithosphere. Across plate ages of 10–100 Myr, localization depends weakly on age but strongly on divergence rate, primarily through Stage-1 narrowing, and is controlled more by the stiffest lithospheric region than by the presence of a shallow weak layer. Including dislocation creep allows both mechanical and thermal weakening to operate within the deforming plate, enhancing feedbacks and accelerating localization relative to yield-stress plus diffusion creep. This implies that yield-stress/diffusion-only rheologies may overestimate break-up timescales in whole-mantle convection models. Low-temperature dislocation creep weakens the plate at 800–1000 K, providing a physically grounded mechanism for limiting strength in this temperature range. Finally, we propose that thermal weakening may in some cases promote rapid strength loss and accelerated extension during the late stages of natural rift evolution.