THE EFFECT OF CONVECTION PLANFORM ON THE TOROIDAL-POLOIDAL ENERGY RATIO

Abstract

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 Earth's 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.