On the convective instability of real models of the upper mantle and shear flows beneath the continental lithosphere

Abstract

The convective instability of realistic upper mantle models is investigated by a new method, taking into account the dependence of viscosity, thermal diffusivity and the coefficient of the thermal expansion on depth. Two main phase changes in the upper mantle, at depths of about 420 and 630 km, are also considered. It is found that the dependence of viscosity on depth and the presence of phase changes strongly affect the stability of the models considered. However, the dependence of the physical parameters on depth does not explain the considerable increase in the horizontal scale of convective cells. A thermomechanical model of the continental upper mantle beneath the crust, moving with a constant velocity, is also constructed, in which a complex profile of the effective viscosity is presented. The stability of the thermomechanical model is investigated. The picture of instability is very complicated because the mechanisms of thermal and convective instabilities are combined. The regions of stability and instability in the model are given in the plane ( Ra c, q) of the critical Rayleigh number Ra c against the wavenumber q. The increments of the convective instabilities have been calculated, and it is shown that the horizontal scale of convection is governed by the thickness of the asthenosphere.