3-DIMENSIONAL NUMERICAL SIMULATIONS OF CRUSTAL DEFORMATION AND SUBCONTINENTAL MANTLE CONVECTION

3-D simulations of mantle convection allowing for continental crust ar e explored to study the effects of crustal thickening on lithosphere s tability and of continents on large-scale mantle flow. Simulations beg in with a crustal layer within the upper thermal boundary layer of a m antle convection roll in a 1 x 1 x 1 Cartesian domain. Convective stre sses cause crust to thicken above a sheet-like mantle downwelling. For mild convective Vigor an initial crustal thickness Variation is requi red to induce 3-D lithospheric instability below the zone of crustal c onvergence. The amplitude of the required Variation decreases with inc reasing convective vigor. Morphologically, instability is manifest in the formation of drip-like thermals that exist within the large-scale roll associated with initial crustal thickening. A strong surface sign ature of the drips is their ability to cause deviations from local Air y compensation of topography. After the initial thickening phase, the crustal accumulation that forms serves as a model analog to a continen t. Its presence leads to mantle flow patterns distinctly different fro m the steady-state roll that results in its absence. Large lateral the rmal gradients are generated at its edge allowing this region to be th e initiation site for continued small-scare thermal instabilities. Eve ntually these instabilities induce a restructuring of large-scale mant le flow, with the roll pattern being replaced by a square cell. Althou gh preliminary and idealized, the simulations do show the fluid dynami cal plausibility behind the idea that significant mantle variations ca n be generated along the strike of a largely 2-D mountain chain by the formation of the chain itself. The ability of a model continent to ca use a change in fundamental convective planform also suggests that the effects of continental crust on mantle convection may be low-order de spite the seemingly trivial volume of crust relative to mantle. (C) 19 97 Elsevier Science B.V.