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

The influence of a shallow low-viscosity zone on the planform of infinite Prandtl number, Rayleigh-Brnard convection has been investigated at Rayleigh numbers (Ra) ranging from 1.5 × 103 to 3 × 105. A viscosity contrast of 1/30 has been set between the low-and high-viscosity layers. For Ra exceeding about 3 x 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 × 105, 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 ceils 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 box 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.