Effects of composite rheology on surface tectonic behavior in numerical models of whole-mantle convection

Abstract:

Earth’s lithosphere behavior is tied to the properties and dynamics of mantle flow. In particular, upper mantle rheology controls the coupling between the lithosphere and the asthenosphere, and therefore partly dictates Earth’s tectonic behavior. It is therefore important to understand how Earth’s upper mantle deforms to study the evolution of plate tectonics. The presence of seismic anisotropy in the uppermost mantle suggests the existence of mineral lattice-preferred orientation (LPO) caused by the asthenospheric flow. Together with laboratory experiments of mantle rock deformation, this indicates that Earth’s uppermost mantle can deform in a non-Newtonian way, through dislocation creep. Although such a deformation mechanism can significantly impact both mantle flow and the surface tectonic behavior, most numerical studies of whole-mantle convection use a viscoplastic rheology involving diffusion creep as the only deformation mechanism in the mantle.

Here, we investigate the effects of using a composite rheology (with both diffusion and dislocation creep) on the surface tectonic behavior in 2D- and 3D-cartesian whole-mantle convection models self-generating plate tectonics. We vary the proportion of dislocation creep in the mantle by imposing different temperature- and depth-dependent transitional stresses between diffusion and dislocation creep. We investigate how the amount of dislocation creep in the upper-mantle impacts the planform of convection and favors a plate-like or a stagnant-lid behavior in models with different surface yield stresses. In particular, we show that for a given surface yield-stress promoting plate-like behavior in diffusion-creep-only models, increasing amounts of dislocation creep in the uppermost mantle lead to stagnant-lid convection. We also discuss the spatio-temporal distribution of dislocation creep in the uppermost mantle of our models in light of the spatial distribution of seismic anisotropy in Earth’s upper-mantle.