THE EFFECTS OF INSULATION ON MANTLE DYNAMICS AND EARTH’S THERMAL EVOLUTION

The Earth’s mantle is bracketed by continental lithosphere above and Large Low Velocity Provinces (LLVPs) below. Both continental lithosphere and LLVPs impact the efficiency of convective heat transfer within the mantle. For example, prior studies find continents to behave like mantle insulators by buffering the amount of heat leaving the system, increasing mantle temperatures over time. Other studies suggest that LLVPs behave as core insulators, reducing heat transport from the core to the mantle and, consequently, decreasing mantle temperatures over time. Past studies focused mainly on the investigating mantle convection with one insulator (either top or bottom), however, a gap remains that considers two insulators operating in tandem. Do these two insulating surfaces cancel each other out? Or does one outcompete the other? To answer and better understand these questions, we first model the mantle with a single insulator. Here we present results from a suite of mantle convection simulations using the finite element code ASPECT. These simulations are isoviscous and use a two-dimensional spherical shell geometry with seven levels of mesh refinement. The model continents and LLVPs were imposed as two regions of constant viscosity, density, and thickness, one at the top (continent) and one at the bottom (LLVP) of the model mantle. To quantify the impact of insulation we examined two different conditions: (1) a system with only a top insulator, (2) a system with only bottom insulator. Within each of these conditions, we examined a range of surface average coverages of the two regions, from full surface area coverage to no surface area coverage. We found that continents reduce surface heat flux and heat the mantle with increasing surface area coverage. On the other hand, LLVPs inhibit basal heat flux and decreasing internal mantle temperatures with increasing surface area coverage. We then discuss the impacts of changes to internal temperature on mantle dynamics and the thermal evolution of the Earth, Finally, we discuss implications on the origin and morphology of LLVPs as well as the impacts of core-mantle boundary heterogeneity on the geodynamo and geomagnetic reversals.