GRAVITY AND CONVECTION IN A TWO-LAYER MANTLE

Abstract

One of the current models of mantle convection assumes that the circulation takes place in two superimposed layers separated by the 700-km discontinuity. The present study reports two-dimensional numerical experiments carried out to investigate the structural features and gravitational consequences of layered convection. The models have two constant viscosity layers heated from below. The main variable is the viscosity ratio, other free parameters are the layer depths and the over-all Rayleigh number. As the viscosity of the lower layer is increased with respect to that of the upper layer, the flow structure shifts gradually from the dominance of viscous coupling of the layers towards thermal coupling. Viscous coupling prevails when the viscosity ratio is near to one, and produces pairs of cells which turn oppositely in the two layers. Thermal coupling becomes predominant if the viscosity ratio is several hundred or more. In a series of models with a depth ratio 1 : 2, thermal coupling leads to pairs of cells which turn in the same sense in the two layers. During the transition between the extremes, two or more small rolls appear in the thinner upper layer over one cell of the lower fluid.Gravity profiles and boundary deformations have been calculated for each model. The transfer function (admittance) of gravity over top surface topography shows a characteristic variation with the viscosity ratio. In a series with the depth ratio 1 : 4, negative values of the admittance are obtained when the lower viscosity is 10–50 times higher than the upper one. Negative admittance is a typical feature of the subcontinental convection operating behind subduction zones. Two-layer models of this type of convection are able to explain not only the sign of admittance but also the magnitude of observed gravity anomalies.