Glacial Isostatic Adjustment and Relative Sea Level Changes: the Role of Lithospheric and Upper Mantle Lateral Viscosity Variations

The Earth response to the melting of the late--Pleistocene ice--sheets has been mainly studied by spherically layered models, based on well established analytical methods. In the last few years new approaches have flourished, taking advantage of numerical techniques and massive computer resources. These methods allow to evaluate the effects of non-Newtonian rheologies and lateral viscosity variations on glacial isostatic adjustment (GIA) and relative sea level (RSL) changes. In this framework, we used a 3D FE code to include laterally varying structures both in the elastic lithosphere and in the Newtonian viscoelastic upper mantle. Our spherical models reproduce the global structure of the cratons and account for a low-viscosity layer beneath the oceanic lithosphere. We compare our model results to the RSL trends in the last 6000 years showing that, when all of the data available are jointly considered, homogeneous and laterally heterogeneous models perform in a similar way. This suggests that the effect of lateral viscosity variations cancel out globally, so that the common radially stratified models are a useful tool for GIA predictions on a large spatial scale. When particular subsets of the global RSL data set are considered, belonging to regions which have experienced directly the glacial melting (such as North America and Northern Europe), we find that the RSL can be better, albeit marginally, reproduced by models which include heterogeneous structures. This suggests that a further refinement of the laterally varying features on a regional scale may lead to a better agreement between RSL observations and model predictions.