Mantle dynamics, postglacial rebound and the radial viscosity profile

We infer the radial viscosity structure of the Earth's mantle from observat ions of long-wavelength geoid, glacially-induced sea-level changes, and cha nges in the Earth's rotation and gravitational field. We employ a combinati on of forward and formal inverse modeling of long-term mantle circulation d riven by large-scale density differences deduced from seismic tomography. B ased on the resulting unsealed mantle viscosity profiles, we model the lime -dependent glacial isostatic adjustment of the Earth related to past and pr esent changes in the ice-ocean mass imbalance and we deduce scaled mantle v iscosity profiles, which simultaneously fit the long-wavelength geoid const raint and glacially-induced changes of the Earth's shape. Three mantle viscosity profiles are fitting the observational data equally well. All profiles are characterized by a two order of magnitude variation of viscosity within the Earth's mantle. Variations of viscosity in the uppe r mantle are less than one order of magnitude. In the lower mantle, the vis cosity differs significantly with depth for all models. Average viscosities in the upper and lower mantle are around (2 - 5) x 10(20) and (1 - 3) x 10 (22) Pas, respectively. (C) 2000 Elsevier Science B.V. All rights reserved.