GEOID EFFECTS OF LATERAL VISCOSITY VARIATION NEAR THE TOP OF THE MANTLE - A 2D MODEL

Whereas the oceanic lithosphere is underlain by a low-viscosity layer, the asthenosphere, it appears unlikely that this layer is well develo ped beneath continental cratons. This difference could represent a lat eral viscosity variation of two to three orders of magnitude near the top of the mantle. Previous mantle flow/geoid models have typically ne glected this variation. To investigate the impact of such a lateral st rength variation on mantle flow and the resulting geoid, we solve for density-driven flow in a 2D box of uniform-viscosity fluid. A dichotom ous lateral viscosity variation near the top of the mantle is modeled by having a no-slip boundary condition over a section of the top of th e box, while allowing the remainder of the top to be free-slip. For va rying fractions of no-slip on the upper boundary, and various values o f the wavenumber, depth and phase of the driving density anomaly, mode l geoids are found. In contrast to viscous flow with only a depth-depe ndent viscosity, mode coupling in the geoid is observed. This occurs p rincipally at a wavelength equal to the lengthscale of the lateral vis cosity structure, with amplitudes as large as 20-40% in some cases. Th us, coupling can produce a significant signal in the geoid at a wavele ngth longer than that of the driving load. This result suggests that t he accuracy of models of the geoid that assume a solely radial viscosi ty structure may be limited to the 60-80% level found in recent studie s [1,2]. We speculate that some of the Earth's low-order geoid is due to flow coupling resulting from the degree two to five components of l ateral ocean-continent structure.