Constraining dynamic models in North America using the static gravity field

The negative anomaly present in the static gravity field near Hudson Bay bears striking resemblance to the area depressed by the Laurentide ice sheet during the Last Glacial Maximum, suggesting that it is at least partly due to Glacial Isostatic Adjustment (GIA), but mantle convection and density anomalies in the crust and the upper mantle are also expected to contribute. At the moment, the contribution of GIA to this anomaly is still disputed. Estimates, which strongly depend on the viscosity of the mantle, range from 25 percent to more than 80 percent. Our objective is to find the contributions from GIA and mantle convection, after correcting for density anomalies in the topography, crust and upper mantle. The static gravity field has the potential to constrain the viscosity profile which is the most uncertain parameter in GIA and mantle convection models. A spectral method is used to transform 3D spherical density models of the crust into gravity anomalies. Density anomalies in the lithosphere are estimated so that isostatic compensation is reached at a depth of 300 km. The dynamic processes of mantle flow are corrected for before isostasy is assumed. Upper and lower mantle viscosities are varied so that the gravity anomaly predicted from the dynamic models matches the residual gravity anomaly. We consider uncertainties due to the crustal model, the lithosphere-asthenosphere boundary (LAB), the conversion from seismic velocities to density and the ice history used in the GIA model. The best fit is found for lower mantle viscosities >10²² Pa s.