Plate tectonics and aspherical Earth structure: the importance of poloidal- toroidal coupling.

We derive poloidal flow internal loading Green functions for imcompressible fluid models of the mantle consisting either of a single constant viscosity spherical shell or of 2 adjacent spherical shells having different viscosities. From the Green function we obtain kernels connecting the surface divergence, the geoid, and the surface topography fields to the lateral density heterogeneity inferred on the basis of the application of seismic tomographic imaging techniques, and with these kernels we argue that both the surface divergence and geoid fields, consisting of harmonic degrees 2-5, may be reasonably fit with only a factor of 8 viscosity increase at a depth of 1200 km.-from Authors