Strain-weakening rheology in Earth’s lower mantle and its control on mantle convection and tectonics

Abstract:

Rocks in the Earth’s interior consist of different mineralogical phases with different rheological properties. In Earth’s lower mantle, the main rock constituents are bridgmanite (Br) and smaller amounts of ferropericlase (Fp). Bridgmanite is substantially stronger than ferropericlase, and lower mantle rheology may be highly dependent on the relative mineral abundances and spatial distribution of these two phases. It has been suggested that for bridgmanite-depleted compositions, the viscosity decreases with accumulating strain due to the interconnection of the weaker ferropericlase minerals. This implies that deformation may localize in the lower mantle, potentially aiding the formation and preservation of compositionally distinct and “hidden” reservoirs away from these regions of localized deformation. Moreover, such lower-mantle dynamics may affect surface dynamics over long timescales.

To better understand the feedback between the viscosity structure obtained by deforming Br-Fp aggregates and the dynamics of mantle convection, we present 2D numerical models of thermomechanical convection in a spherical annulus geometry that includes a new implementation of strain-weakening (SW) rheology. This macro-scale SW rheology is based on micro-scale solutions found in prior studies. We test the first-order effect of SW rheology on global-scale patterns of mantle convection, and the dynamics of plumes and slabs. Preliminary results show that SW rheology increases convective vigour in the lower mantle and to a lesser extend in the upper mantle. Moreover, it affects the dynamic coupling of the lower- and upper mantle, and to some extend even surface dynamics. This implies that lower mantle rheology is not only important for convection in the mantle, but also for tectonic behavior over time.