A SELF-CONSISTENT 2-D MODEL FOR THE DIP ANGLE OF MANTLE DOWNFLOW BENEATH AN OVERRIDING CONTINENT

Abstract

We demonstrate how the inclination of descending mantle flow is affected by thepresence of a continent overriding the downwelling zone. The mantle is modelled by a fluidhaving a temperature- and pressure-dependent viscosity which describes a low viscosityasthenosphere and a higher viscosity lower mantle. The fluid occupies a square x:y=10:1 boxheated from below. In the absence of overlying continents thermal convection sets in with colddownwellings being nearly vertical. A continent is placed on the convecting mantle and starts todrift. The continent is assumed to be a thick rigid plate. We consider models of two types. Model1 involves a free-floating continent. The continent is pulled by viscous forces and approaches acold downwelling nearby. Interactions between the convective mantle and the drifting continentlead to an inclined downwelling zone. Model 2 involves a continent moving horizontally with agiven velocity. When the continent approaches a downwelling, it alters the flow and causes thedownwelling to deviate from the vertical. The dip angle of the downflow depends on the velocityof the overriding continent and can take values ranging from 0 to 90 degrees.