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

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. Grain size reduction due to dynamic recrystallization provides local weak zones in the lithosphere thereby promoting 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. We find that grain size reduction suppresses episodic behavior and facilitates surface mobility over a range of lithospheric yield stresses, but GSE has no discernable effect on the transition to stagnant-lid tectonics. Moreover, increased grain growth does not result in higher episodicity either. GSE, together with composite rheology, modify the diagnostics within one tectonic regime. These findings support the importance of grain size evolution for stabilizing mobile-lid tectonics, but also cast doubt on the potential of damage to explain mobile-lid tectonics up to laboratory-inferred strengths.