A self-consistent 2-D model for the dip angle of mantle downflow beneath an overriding continent

We demonstrate how the inclination of descending mantle flow is affected by the presence of a continent overriding the downwelling zone. The mantle is modelled by a fluid having a temperature- and pressure-dependent viscosity which describes a low viscosity asthenosphere and a higher viscosity lower mantle. The fluid occupies a square x:y=10:1 box heated from below. In the absence of overlying continents thermal convection sets in with cold downw ellings being nearly vertical. A continent is placed on the convecting mant le and starts to drift. The continent is assumed to be a thick rigid plate. We consider models of two types, Model 1 involves a free-floating continen t. The continent is pulled by viscous forces and approaches a cold downwell ing nearby. Interactions between the convective mantle and the drifting con tinent lead to an inclined downwelling zone. Model 2 involves a continent m oving horizontally with a given velocity. When the continent approaches a d ownwelling, it alters the flow and causes the downwelling to deviate from t he vertical. The dip angle of the downflow depends on the velocity of the o verriding continent and can take values ranging from 0 to 90 degrees. (C) 1 999 Elsevier Science Ltd. All rights reserved.