Global plate motion and the geoid: a physical model

SUMMARY The effect of lateral variations in the sub‐lithospheric viscosity, i.e. in the lithosphere‐asthenosphere coupling, is incorporated in dynamic earth models driven by plate velocities. If the coupling is stronger under fast‐moving continents, it is shown that a geoid low will develop in their wake and a high in front of them. Thus the well‐known low in the Indian ocean could be explained, at least in part, in terms of induced upper‐mantle dynamics. Similarly, if the thickness of oceanic plates increases with age the models show that trenches should be associated with marked geoid highs, whereas ridges could correspond to much weaker geoid lows. This also seems to agree with some features of the observed geoid at very long wavelengths. The mathematical framework of such dynamic earth models is developed extensively here. These models are characterized by lateral viscosity variations inside an outer shell and a purely radial viscosity structure at greater depth. Their internal flow patterns are only driven by imposed surface velocities, not by internal loads.