Generation of plate-like behavior and mantle heterogeneity from a spherical, viscoplastic convection model

How plate tectonics arises from mantle convection is a question that has only very recently become feasible to address with spherical, viscoplastic computations. We present mainly internally heated convection results with temperature-dependent viscosity and explore parts of the Rayleigh number (Ra) yield pstress (sigma(y)) phase space, as well as the effects of depth-dependent sigma(y), bottom heating, and a low-viscosity asthenosphere. Convective planform and toroidal-poloidal velocity field ratio (TPR) are affected by near-surface viscosity variations, and TPR values are close to observed values for our most plate-like models. At the relatively low convective vigor that is accessible at present, most models favor spherical harmonic degree one convection, though models with a weaker surface viscosity form degree two patterns and reproduce tomographically observed power spectra. An asthenospheric viscosity reduction improves plate-like nature, as expected. For our incompressible computations, pure bottom heating produces strong plumes that tend to destroy plates at the surface. This implies that significant internal heating may be required, both to reduce the role of active upwellings and to form a low-viscosity zone beneath the upper boundary layer.