MOVING LITHOSPHERIC PLATES AND MANTLE CONVECTION

The coupling between a rigidly moving lithospheric plate and a convecting mantle is investigated using a simple two‐dimensional numerical model that incorporates a horizontally moving upper boundary, simulating the effect of a moving plate, over a fluid layer heated from below. The moving boundary strongly controls the horizontal length scale of convection cells when its velocity is greater than the free convective velocity (i.e. the velocity with which the fluid would convect under a stationary boundary). In a box of aspect ratio 4 (width/depth), a transition in flow structure occurs from several equidimensional convection cells under a slowly moving boundary to a single long convection cell under a rapidly moving boundary. The flow structure transition occurs approximately when Pe/Ra2/3= 0.04, where the Peclet number, Pe, measures the (prescribed) velocity of the upper boundary, and the Rayleigh number, Ra, measures the heating of the fluid layer. Near the transition, the flow tends to be unsteady; this behaviour can be well understood in terms of the instability of the thermal boundary layers, which can be characterized by a local Rayleigh number. Using conventional estimates of mantle parameters, the mantle is either near or above the transition to single‐cell convection, whether upper‐mantle or whole‐mantle convection is assumed. The net tangential force exerted by the fluid on the upper boundary varies approximately linearly with the boundary velocity above the transition, and it is positive (driving) for boundary velocities ranging from the value at the transition to about three times the transition value. Boundary velocities corresponding to zero net force are close to those expected for convection under a free‐slip boundary. When scaled to whole‐mantle convection, the shear stress on the upper boundary is of the order of 10 bar, and above the transition to a single cell the net force on a large plate is sufficient to overcome even kilobars of frictional resistance at plate boundaries. These results indicate that mantle convection and lithospheric plate motions are likely to be strongly coupled, with the rigid moving plates strongly controlling the mantle flow structure, with single cells under the faster plates, but with the mantle convection cells possibly exerting large driving forces on the plates. Copyright © 1979, Wiley Blackwell. All rights reserved