A global geoid model with imposed plate velocities and partial layering

Most inversions of the long-wavelength geoid in conjunction with the seismi c tomographic information have so far been carried out under the assumption of either purely whole mantle or perfectly layered circulation. Moreover, modeling the lithosphere as a spherical shell with a uniform low viscosity was found to yield the best fit to the observed geoid. We have tested wheth er a good prediction of the geoid can also be achieved by including two con straints: a semipermeable behavior of the 660-km discontinuity and surface plate velocities equal to the observed ones. The mass transfer between uppe r and lower mantle has been changed by imposing a surface density anomaly a t a depth of 660 km, which is proportional to the mass anomaly needed to ac hieve perfectly layered circulation. On the top of the mantle we assume a s tiff lithosphere that moves with a velocity corresponding to the observed p late motion. The viscosity only varies with depth. Considering a simple thr ee-layer viscosity structure and changing the permeability of the 660-km in terface, we have obtained a satisfactory variance reduction of the geoid da ta (similar to 75% for degrees 2-12). The best fit to the geoid is obtained if the mass transfer across the 660-km boundary is reduced to one third in comparison with the purely whole mantle model. The best fitting viscosity profile is characterized by a clearly defined asthenosphere and a viscosity increase by at least 2 orders of magnitude in the lower mantle. The amplit udes of dynamic topography predicted by our model are remarkably small (sim ilar to 100 m), thus fully compatible with the observation.