Three-dimensional models of mantle flow across a low-viscosity zone: implications for hotspot dynamics

Abstract

The influence of a shallow low-viscosity zone on the planform of infinite Prandtl number, Rayleigh-Bénard convection has been investigated at Rayleigh numbers ( Ra) ranging from 1.5 × 10 3 to 3 × 10 5. A viscosity contrast of130 has been set between the low- and high-viscosity layers. For Ra exceeding about 3 × 10 4 and no-slip conditions at the top and bottom boundaries, a hexagonal convective planform is systematically observed; free-slip boundary conditions lead to the development of a square convective planform. The flow is ascending along the center of the cells and is strongly narrowed in the low-viscosity layer. A moving top boundary does not destabilize significantly these plumes as long as the drift velocity is lower than the upwelling velocity in the low-viscosity layer. A higher drift velocity of the upper boundary rapidly induces the formation of convective rolls. At Ra of 3 × 10 5, unsteady flow occurs along the descending currents when the convective cells have the horizontal size predicted by the linear theory. This time-dependence is prevented when the cells are smaller, suggesting that there is a competition between the aspect ratio and the steadiness of the cells. Such a reduction of the horizontal dimensions of the cells with increasing Ra has been observed in large ☐ experiments. These results show that a moderate viscosity drop at shallow depth in the mantle, compatible with the geoid and bathymetric data at hotspot swells, induces a marked asymmetry between the upper and lower convective boundary layers. This promotes the development of ascending plumes, large at depth and very narrow—a few tens of kilometers in section—below the lithosphere. These plumes are very stable in time and space. It is inferred that the channelling of the ascending plumes induced by a viscosity drop precludes the development of secondary convection in a top low-viscosity layer.