Deformation memory in the lithosphere: A comparison of damage-dependent weakening and grain-size sensitive rheologies

Strain localization in the lithosphere and the formation, evolution, and maintenance of resulting plate boundaries play a crucial role in plate tectonics and thermo-chemical mantle convection. Previously activated lithospheric deformation zones often appear to maintain a “memory” of weakening, leading to tectonic inheritance within plate reorganizations including the Wilson cycle. Different mechanisms have been proposed to explain such strain localization, but it remains unclear which operate on what spatio-temporal scales, and how to best incorporate them in large-scale mantle convection models. Here, we analyze two candidates, 1), grain-siz...  more

Strain localization in the lithosphere and the formation, evolution, and maintenance of resulting plate boundaries play a crucial role in plate tectonics and thermo-chemical mantle convection. Previously activated lithospheric deformation zones often appear to maintain a “memory” of weakening, leading to tectonic inheritance within plate reorganizations including the Wilson cycle. Different mechanisms have been proposed to explain such strain localization, but it remains unclear which operate on what spatio-temporal scales, and how to best incorporate them in large-scale mantle convection models. Here, we analyze two candidates, 1), grain-size sensitive rheology and, 2), damage-style parameterizations of yield stress which are sometimes used to approximate the former. Grain-size reduction due to dynamic recrystallization can drive localization in the ductile domain, and grain growth provides a time-dependent rheological hardening component potentially enabling the preservation of rheological heterogeneities. We compare the dynamic weakening and hardening effects as well as the timescales of strength evolution for a composite rheology including grain-size dynamics with a pseudo-plastic rheology including damage- (or “strain”-) dependent weakening. We explore the implications of different proposed grain-size evolution laws, and test to which extent strain-dependent rheologies can mimic the weakening and hardening effects of the more complex micro-physical behavior. Such an analysis helps to better understand the parallels and differences between various strain-localization modeling approaches used in different tectonics and geodynamics communities. More importantly, our results contribute to efforts to identify the key ingredients of strain-localization and damage hysteresis within plate tectonics and how to represent those in planetary-scale modeling.