POSTGLACIAL SEALEVELS ON A SPHERICAL, SELF-GRAVITATING VISCOELASTIC EARTH: EFFECTS OF LATERAL VISCOSITY VARIATIONS IN THE UPPER MANTLE ON THE INFERENCE OF VISCOSITY CONTRASTS IN THE LOWER MANTLE

Abstract

Previous studies of lateral viscosity variations on postglacial rebound often neglect the spherical shape of the Earth and the self-gravity of the solid earth and oceans. In this paper, the consistent sealevel equation is solved by the coupled Laplace-finite element method for a spherical, self-gravitating incompressible Maxwell earth. It confirms the importance of self-gravity in the oceans on sealevel computation near the ice margin where the data are most sensitive to the existence of lateral variations in lithospheric thickness and asthenospheric viscosity. The effects of lateral variations in lower mantle viscosity, asthenospheric viscosity and lithospheric thickness are investigated. This paper confirms the finding of earlier investigations. The combined effect of lateral viscosity variations in the lower mantle with reverse viscosity contrasts in the upper mantle is also investigated. For sealevel data near the center of rebound, the effect of lateral viscosity contrast in the lower mantle can be masked by the existence of a reverse lateral viscosity contrasts in the upper mantle. However, this is not the case for sealevel data just outside the ice margin.