Long-wavelength geoid: the effect of continental roots and lithosphere thickness variations

The existence of continental roots and the gradual thickening of the cooling oceanic lithosphere give rise to large-scale rheological heterogeneities in the uppermost mantle. The effect of these heterogeneities on the long-wavelength geoid is investigated using a 3-D mantle flow model involving a low-viscosity asthenosphere beneath the oceanic lithosphere and the tectonically active continental regions and a thick highly viscous lithosphere beneath ancient continents. Below 400 km the mantle viscosity is laterally homogeneous with a lower mantle more viscous than the overlying layer. The mantle circulation is driven by imposed surface velocities NUVEL-1 HS2 (Gripp & Cordon 1990) and by the density anomalies inferred from the tomographic models P16B30 and S16B30 (Masters et nl. 1996). The geoid heights both due to plate motion and due to internal loading differ by as much as 30 per cent between the models with and without lateral viscosity variations. In contrast to what was suggested in previous studies, spherical harmonics 2 and 3 are strongly affected by the lateral viscosity variations. These differences in the forward problem suggest that the response to the inverse problem that consists of finding the profile of viscosity as a function of depth providing the best fit to the geoid should be considerably affected by the lateral viscosity variations. The shear stresses at the base of the plates induced by the imposed surface velocities and those induced by the internal loading are sensitive to the lateral viscosity variations. This suggests that the lateral viscosity variations are very important for understanding the stress field or the forces acting on the plates. The direction of these shear stresses, which should be linked to the anisotropy direction, is very different from the plate motion direction.