Grain size evolution in mantle convection models promotes continuous rather than episodic tectonics

Abstract

A long-persistent caveat of geodynamic models with Earth-like tectonic behavior is the need of an ad hoc yield stress lower than any laboratory-inferred rock strength. The actual physical processes that govern the strength of the lithosphere are still debated. Grain size reduction due to dynamic recrystallization promotes local weak zones in the lithosphere thereby facilitating lithospheric breakdown and continuous mobile-lid tectonics. Grain growth should instead (re-)strengthen the lithosphere and inhibit this regime. By modeling mantle convection in a spherical annulus, we analyze the impact of grain size evolution (GSE) on the global tectonic style. GSE, together with composite rheology, modifies the diagnostics within one tectonic regime. Indeed, considering grain growth enables high viscosity in the lower mantle, with broad stable plumes. Grain reduction, on the other hand, increases the number of subduction zones, also making them shorter-lived. We find that grain size reduction suppresses episodic behavior and facilitates surface mobility over a range of lithospheric yield stresses, but GSE has no discernible effect on the transition to stagnant-lid tectonics. Moreover, increased grain growth does not result in higher episodicity either. These findings support the relevance of grain size evolution for stabilizing mobile-lid tectonics vs episodic tectonics, and may explain lower mantle features and reconcile geophysical observations; but they also cast doubt on the potential of damage to explain mobile-lid tectonics up to laboratory-inferred strengths.