Three-Dimensional Mantle Convection With Continental Crust: First-Generation Numerical Simulations

Abstract

The potential effects of continental crust on mantle convection are explored using three-dimensional numerical simulations. The simulations model the coupling between a thin, deformable layer of chemically buoyant crust and a deep layer of thermally convecting and chemically dense mantle. Simulations begin with a crustal layer embedded within the upper thermal boundary layer of a mantle convection roll in a 1 × 1 × 1 domain. Convective stresses cause crust to thicken above a sheetlike mantle downwelling. For mild convective vigor an initial crustal thickness variation is required to induce three-dimensional mantle instability below the zone of thickening. Instability is characterized by the formation of local driplike thermals that exist within the large-scale thermal roll associated with crustal thickening. In terms of surface effects the drips cause significant deviations from local Airy compensation. After initial thickening the crustal accumulation that forms serves as an analog to a continent. Its presence leads to mantle flow patterns different from the thermal roll that occurs in its absence. Large lateral thermal gradients are generated at the edge of a crustal accumulation, which, as a result, becomes the initiation site for small-scale thermals that circulate below a model continent. Eventually, these instabilities induce a restructuring of large-scale mantle flow morphology from a roll to a square cell. Although preliminary, the simulations support the idea that crustal thickening of the type associated with mountain belt formation can alter local mantle flow patterns that can affect subsequent crustal deformation. Thus problems of continental dynamics and local mantle dynamics may be more strongly coupled than is generally presumed. The ability of crustal accumulations to cause a change in large-scale convective planform also suggests that continental crust may have low-order effects on global mantle convection patterns despite its seemingly trivial relative volume.