LOCALIZATION OF THE GRAVITY-FIELD AND THE SIGNATURE OF GLACIAL REBOUND

The negative free-air gravity anomaly centred on Hudson Bay, Canada, s hows a remarkable correlation with the location of the Laurentide ice sheet, suggesting that this gravity anomaly is the result of incomplet e post-glacial rebound(1-3). This region, however, is also underlain b y higher-than-average mantle seismic velocities, suggesting that the g ravity low might result instead from dynamic topography associated wit h convective downwellings(4-7). Here we analyse the global gravity fie ld as a simultaneous function of geographic location and spectral cont ent, We find that the Hudson Bay gravity low is unique, with anomalous ly high amplitude in the spectral band where the power from the Lauren tide ice load is greatest(2) and the relaxation times predicted for vi able models of viscous relaxation are longest(8). We estimate that abo ut half of the Hudson Bay gravity anomaly is the result of incomplete post-glacial rebound, and derive a mantle viscosity model that explain s both this gravity signature and the characteristic uplift rates for the central Laurentide and Fennoscandian regions(6). This model has a jump in viscosity at 670 km depth, comparable to that in dynamic model s of the geoid highs over subducted slabs(4,9), but lacks a low-viscos ity asthenosphere, consistent with a higher viscosity in the upper man tle beneath shields than in oceanic regions.