Finite element modeling of lateral viscosity heterogeneities and post-glacial rebound

The existence of lateral viscosity heterogeneities of a few orders of magnitude in the Earth mantle has recently been proposed by several authors, although classical post-glacial rebound models are consistent with a rather uniform structure. Tomographic results reveal the presence of lateral heterogeneities in seismic velocities within the mantle which can be correlated with temperature and viscosity variations. Our aim is to analyze the uplift in deglaciated areas, using a realistic horizontal viscosity profile of the mantle, in order to determine the influence of such models on global mantle viscosity evaluation. Since analytical models cannot handle lateral heterogeneities, we used a finite-element numerical approach. The code is a version of TECTON program (Melosh and Raefsky, 1983), suitable for treating the axisymmetric case and modified to take isostatic compensation of the surface load into account. We found that the average mantle viscosity values deviate from the "canonical" value of 1021 Pa · s when lateral variations are taken into consideration. The displacement in the center is controlled by the average viscosity underlying the load. For long wavelengths (heterogeneities of two orders of magnitude), the estimated average viscosity may differ by a factor 7 from the results obtained using radial models. For stronger variation (of four orders of magnitude) the deviation is a factor of 30, although such strong viscosity seems to be inconsistent with the data. Difficulties in data fitting are also encountered for smaller variations when short wavelengths are considered. When the interplay of vertical and lateral stratification is considered, we find that average viscosities in the upper and lower mantle depend on the magnitude and pattern of the heterogeneities in each layer. In particular, strong lateral viscosity contrasts in the upper mantle can overlie a uniform lower mantle. © 1990.