Numerical models of convection in the upper mantle

Abstract

Two-dimensional numerical models of steady state convection show that convection cells of aspect ratio as large as 8.6 are possible for variable viscosity convection in the upper mantle. Our models include the effects of variable viscosity, viscous dissipation, internal heating, heat flow through the bottom, and the adiabatic gradient. The large aspect ratio of the convection cells is primarily due to the large viscosity contrast between the lithosphere and the asthenosphere. It appears possible for multiple convection cells to occur in a low-viscosity zone while the surface velocities give the appearance of a single cell. The details of the viscosity law relevant to mantle materials and conditions are presently uncertain but are of crucial importance; temperature, viscosity, and flow patterns are inextricably entwined. Convection decreases the overall temperature gradient; consequently, generally accepted temperatures for most of the mantle are too high. The controversies over plate-mantle decoupling and passive versus active plates are probably due to oversimplifications that disregard hydrodynamic concepts.