Subcontinental mantle dynamics: A further analysis based on the joint constraints of dynamic surface topography and free-air graviy

The depth extent, density, and dynamical role of apparent subcontinental ke els are investigated by using constraints provided by the very long-wavelen gth representations of several geophysical fields. We first consider local cross correlations between the nonhydrostatic free-air gravity field, the m ap of the ice sheets at Last Glacial Maximum, and the surface expression of continental cratons. We initially confine our analyses to North America an d observe that equally good local correlations exist between the gravity he ld and either of the others. The case of Eurasia is also considered. We obs erve that correlation analyses of this type cannot be employed to unambiguo usly infer the cause of long-wavelength continental gravity anomalies; ther efore we revert to explicit postglacial rebound and tomography-based viscou s flow modeling of the gravity field. For viscosity profiles that optimally reconcile relative sea level constraints from the Laurentide platform, we find that the rebound process accounts for only 10% of the observed free-ai r gravity low over Hudson Bay. We consider mantle convection as the more li kely source of this gravity anomaly and alternatively investigate the impli cations of assuming that seismically fast, deep structure imaged tomographi cally beneath the continent represents either negatively, neutrally, or pos itively buoyant material. In addition to the gravity constraint we introduc e the independent constraint of continental dynamic surface topography. We infer this new datum by using the Crust 5.1 global model of crustal structu re. Remarkably, continents are found to systematically reside in topographi c depressions of the order of 1-2 km. Within the context of our modeling as sumptions we find that optimal model descriptions of the joint gravity and dynamic surface topography constraints over the continents require deep and dense subcontinental undercurrents.