The Formation of Steep-Sided Islands at the Base of the Mantle

Seismic studies have detected the presence of sharp-edged, flat-topped structures at the base of the mantle in areas believed to be upwelling such as beneath southern Africa and the south Atlantic. Numerical models of thermochemical convection have shown that it is possible for dense material at the base of the mantle to pile up and form such structures. Using a simplified mode of D'', based on the assumption that the layer is intrinsically dense, we illustrate the circumstances in which discrete islands of dense material will form. The D'' layer is represented by a thin dense layer which is compositionally distinct from but mechanically coupled to the rest of the lower mantle. The dominant external mechanism governing the flow within the layer and the displacement of its upper boundary is assumed to be tractions on the upper surface resulting from the convecting mantle above. Horizontal density gradients within the layer due to variations in temperature are also taken into account. In the case where the dense layer couples viscously with the driving convection above we observe solutions where the layer is completely swept away beneath areas of mantle downwelling leaving steep-sided `islands' of dense material beneath upwelling regions. Alternatively, in the case where there is thermal coupling between the layers, the dense layer remains ubiquitous and a cusp-like peak forms beneath upwelling regions. We also use the finite element method to obtain 2-D solutions for large boundary deflection where analytic solutions are not possible. We describe how the size and shape of these steep-sided islands varies with the density of the layer and the buoyancy and viscous forces driving the flow.