The structure of 2D mantle convection and stress fields: Effects of viscosity distribution

Abstract

Spatial fields of temperature, velocity, overlithostatic pressure, and horizontal stresses in the Earth's mantle are studied in twodimensional (2D) numerical Cartesian models of mantle convection with variable viscosity. The calculations are carried out for three different patterns of the viscosity distribution in the mantle: (a) an isoviscous model, (b) a fourlayer viscosity model, and (c) a temperatureand pressure� dependent viscosity model. The pattern of flows, the stresses, and the surface heat flow are strongly controlled by the viscosity distribution. This is connected with the formation of a cold highly viscous layer on the surface, which is analogous to the oceanic lithosphere and impedes the heat transfer. For the Rayleigh number Ra = 10 7 , the Nusselt number, which characterizes the heat transfer, is Nu = 34, 28, and 15 in models with constant, fourlayered, and p, Tdependent viscosity, respectively. In all three models, the values of overlithostatic pres� sure and horizontal stresses σxx in a vast central region of the mantle, which occupies the bulk of the entire volume of the computation domain, are approximately similar, varying within ±5 MPa (±50 bar). This follows from the fact that the dimensionless mantle viscosity averaged over volume is almost similar in all these mod� els. In the case of temperatureand pressuredependen t viscosity, the overlithostatic pressure and stress σxx fields exhibit much stronger concentration towards the horizontal boundaries of the computation domain compared to the isoviscous model. This effect occurs because the upwellings and downwellings in a highly viscous region experience strong variations in both amplitude and direction of flow velocity near the horizon� tal boundaries. In the models considered with the parameters used, the stresses in the upper and lower mantle are approximately identical, that is, there is no denser concentration of stresses in the upper or lower mantle. In contrast to the overlithostatic pressure field, the fields of horizontal stresses σxx in all models do not exhibit deep roots of highly viscous downwelling flows.