The impact of a 3-D Earth structure on glacial isostatic adjustment following the Little Ice Age in Southeast Alaska

In Southeast Alaska, extreme uplift rates are primarily caused by glacial isostatic adjustment (GIA), as a result of ice load changes from the Little Ice Age to the present combined with a low viscosity asthenosphere. Current GIA models adopt a one-dimensional (1-D) stratified Earth structure. However, the actual (3-D) structure is more complex due to the presence of a subduction zone and the transition from a continental to an oceanic plate. A simplified 1-D Earth structure may not be an accurate representation in this region and therefore affect the GIA predictions. In this study we will investigate the effect of 3-D variations in the shallow upper mantle viscosity on GIA in Southeast Alaska. In addition, investigation of 3-D variations also gives new insight into the most suitable 1-D viscosity profile.

We test a number of models using the finite element software ABAQUS. We use shear wave tomography and mineral physics to constrain the shallow upper mantle viscosity structure. We investigate the contribution of thermal effects on seismic velocity anomalies in the upper mantle using an adjustable scaling factor, which determines what fraction of the seismic velocity variations are due to temperature changes, as opposed to non-thermal causes. We search for the combination of the scaling factor and background viscosity that best fits the GPS data. Results show that relatively small lateral variations improve the fit with a best fit background viscosity of 5.0×10¹⁹ Pa s, resulting in viscosities at ~80 km depth that range from 1.8×10¹⁹ to 4.5×10¹⁹ Pa s.