The Impact of a 3-D Earth Structure on Glacial Isostatic Adjustment in Southeast Alaska Following the Little Ice Age

In Southeast Alaska, extreme uplift rates are primarily caused by glacial isostatic adjustment (GIA), as a result of ice thickness changes from the Little Ice Age to the present combined with a low-viscosity asthenosphere. Previous GIA models adopted a 1-D Earth structure. However, the actual Earth structure is likely more complex due to the long history of subduction and tectonism and the transition from a continental to an oceanic plate. Seismic evidence indeed shows a laterally heterogenous Earth structure. In this study, a numeral model is constructed for Southeast Alaska, which allows for the inclusion of lateral viscosity variations. The viscosity follows from scaling relationships between seismic velocity anomalies and viscosity variations. We use this scaling relationship to constrain the thermal effect on seismic variations and investigate the importance of lateral viscosity variations. We find that a thermal contribution to seismic anomalies of 10% is required to explain the GIA observations. This implies that non-thermal effects control seismic anomaly variations in the shallow upper mantle. Due to the regional geologic history, it is likely that hydration of the mantle impact both viscosity and seismic velocity. The best-fit model has a background viscosity of 5.0 x 10(19) Pa-s, and viscosities at similar to 80 km depth range from 1.8 x 10(19) to 4.5 x 10(19) Pa-s. A 1-D averaged version of the 3-D model performed slightly better, however, the two models were statistically equivalent within a 2 sigma measurement uncertainty. Thus, lateral viscosity variations do not contribute significantly to the uplift rates measured with the current accuracy and distribution of sites. Plain Language Summary Rapid uplift in Southeast Alaska is caused by past and current melting of glaciers-which is termed "Glacial Isostatic Adjustment" (GIA). Traditionally, GIA models have considered a vertically stratified Earth model. However, the Earth's structure below Southeast Alaska is more complex and laterally variable due to a long history of subduction and tectonism. We aim to reconstruct the mantle viscosity below Southeast Alaska to best match the present-day GPS observations. Viscosity describes how readily the mantle flows when subjected to forces. We prescribe our models with an ice load history derived from ice-mass changes during and after the Little Ice Age. We test several models that predict the 3D viscosity using seismic velocity models and search for a parameter that determines how much of those variations are caused by temperature as opposed to other effects. We show that, the lateral variations in the Earth do not significantly impact the modeled uplift predictions in comparison to a laterally averaged uniform structure. This may be due to limitations of the seismic model. We find that, the viscosity variations are slightly influenced by thermal effects and are more likely due to hydration of the mantle, which is consistent with the tectonic history of this region.