Upper-mantle thermochemical structure below North America from seismic-geodynamic flow models

High-resolution seismic models of 3-D mantle heterogeneity are interpreted in terms of thermal and compositional anomalies residing in Earth's upper mantle. These anomalies produce density perturbations that drive mantle flow and thus provide the source for convection-related geophysical observables, such as the non-hydrostatic geoid, free-air gravity anomalies and dynamic surface topography. These observables, in turn, may be used to provide constraints on mantle density structure. To resolve as accurately as possible the density structure of the subcontinental mantle below North America, we correct the convection data for the isostatically compensated crustal heterogeneity using a newly compiled crustal data set in Canada, which provides stricter control of crustal structure beneath Canada than previously available global crustal models. The crust-corrected surface topography and free-air gravity fields are compared with those predicted by tomography-based mantle flow models. Through an Occam inversion of the surface topography and gravity data, we obtain inferences of the velocity-density scaling coefficients that characterize the mantle density anomalies below North America. The inferred density anomalies are inconsistent with the presence of thermal anomalies alone, and require simultaneous anomalies in temperature and composition. The anomalies in density and seismic shear velocity are employed to place constraints on the thermochemical structure of the mantle beneath the North American craton. We employ perturbations in the molar ratio of iron, R =X (Fe) /(X (Fe) +X (Mg) ), to quantify the compositional anomalies in terms of iron depletion in the subcontinental mantle. We thereby obtain the first detailed estimates of the extent of basalt depletion in the tectosphere beneath North America. This depletion produces a local balance between positive chemical buoyancy and the negative buoyancy which would otherwise be produced by the colder temperatures of the subcratonic mantle relative to the suboceanic mantle.