The effect of convection planform on the toroidal–poloidal energy ratio

This study investigates the influence of thermal convection planform geometry on the pattern of surface deformation in a dynamic model of plate–mantle coupling. The lithosphere is viewed as a distinct rheological layer and is modeled as a thin, non-Newtonian sheet that is dynamically coupled to a three-dimensional, convecting Newtonian layer. The results of numerical experiments show that planforms which possess a high degree of symmetry excite only small amounts of toroidal energy in the thin non-Newtonian sheet and thus have low toroidal–poloidal energy ratios. Asymmetric planforms tend to possess toroidal–poloidal energy ratios that are one to two orders of magnitude larger than symmetric cases. The results of this study suggest the precipitous drop in the toroidal–poloidal energy ratio for the Earthʹs surface velocity field that occurred in the late Cretaceous–early Cenozoic resulted from the late Cretaceous superplume which organized mantle flow into a higher degree of symmetry.