Effect of lithospheric thickness and radial mantle viscosity profile on glacial isostatic adjustment crustal motions in North America

North America is experiencing vertical and horizontal crustal motion due to glacial isostatic adjustment (GIA). To explore these motions across central and eastern North America, GIA modelling was carried out employing the ICE-6 G_C surface loading model. The Earth model response was determined for 500 three-layered mantle viscosity profiles at nine different lithospheric thicknesses, assuming a constrained density and elastic structure. The predictions were compared to observed velocities downloaded from the Nevada Geodetic Laboratory for selected global navigation satellite system sites and were corrected for hydrological loading and current global ice change. The fit is assessed through a root mean square (RMS) calculation of the residual velocities. Scanning across lithospheric thicknesses and viscosity profiles, the preferred models were compiled to assess the overall best fit for vertical, horizontal, and combined crustal motions. The horizontal and combined responses exhibit two optimal viscosity profile ranges dependent on the lithospheric thickness. The viscosity profile for thinner lithospheres (<120 km) is akin to other published profiles but inferred mantle viscosities shift by an order of magnitude at thicker lithospheres (>= 120 km). The optimal viscosity for the vertical velocities is similar to published profiles with a preferred lithosphere thickness of 100 km. Tests with a different loading model (ICE-7G) and without hydrological corrections give similar results. Despite the exhaustive exploration of a constrained parameter space, the significant remaining horizontal residuals (RMS of the preferred model is 0.56 mm/year, RMS of horizontal observations is 1.24 mm/year) suggests the need for more complex Earth models.