Influence of convection regimes of two-layer thermal convection with large viscosity contrast on the thermal and mechanical states at the interface of the two layers: Implications for dynamics in the present-day and past Earth

Abstract

This paper reports on results for the thermal and mechanical states at the interface of two-layer thermal convection in two-dimensional (2-D) spherical geometry solved by numerical calculations. The two-layer system was composed of a highly viscous layer (HVL) and a low-viscosity layer (LVL) underneath. The two end-member convection regimes were studied by varying two free model parameters, which control the degree of layering in HVL convection and separate the HVL into the upper and lower parts. One of the regimes was a nearly whole-layer convection regime in which the upwelling and downwelling plumes easily penetrated into another layer in the HVL, while the other was a so-called hybrid convection regime, which represented a transitional regime between the whole-layer convection and the double-layer convection. The spatiotemporal analyses of convection behavior showed that the lateral scale of HVL convection and the resultant lateral scale of thermal heterogeneity beneath the HVL–LVL interface tended to be larger in the hybrid convection regime than those in the whole-layer convection regime. On the other hand, the fluctuation of shear-stress at the HVL–LVL interface was more time-dependent in the hybrid convection regime, whereas the mechanical heterogeneity near the HVL–LVL interface was larger in the whole-layer convection regime. The present results on the differences in the scale of dynamically determined thermal and mechanical states beneath the HVL–LVL interface between the two end-member convection regimes may apply to issues on the relationship between thermal and mechanical conditions at the Earth’s core–mantle boundary and the strength of the geomagnetic field.