Effects of lateral variations in lithospheric thickness and mantle viscosity on glacially induced surface motion in Laurentia

The effects of lateral variations in lithospheric thickness and mantle viscosity on surface motions in Laurentia are investigated using the Finite Element method. Both a simple axisymmetric ice model and a more realistic ice deglaciation model (ICE4G) are used to investigate the effects of lateral heterogeneity. Beside the laterally homogeneous reference model, six laterally heterogeneous models are used. The lateral heterogeneity in the lithosphere, upper mantle, transition zone and lower mantle are inferred from seismic tomographic model S20A. In general, the magnitude of the effect of lateral heterogeneity is found to depend on the deglaciation history and the relative position between the ice center and the lateral heterogeneity. It is shown that lateral heterogeneity does affect horizontal motion in Laurentia, however because the effects of lateral heterogeneity from different depths give different orientations to the incremental velocity vector, the total effect is not large enough to overwhelm the divergent motion - but just weaken it in southern Laurentia. For land uplift rate, the effect of lateral viscosity variation in the transition zone between 425 and 700 km depth is opposite to that of the upper or lower mantle - thus the net effect of lateral heterogeneity on uplift rate is not very large - but enough to change the shape of the uplift pattern.