High-resolution seismic models of three-dimensional mantle heterogeneity are interpreted in terms of upper mantle thermal and compositional anomalies. These anomalies produce density perturbations that drive mantle flow and corresponding convection-related geophysical observables, such as the nonhydrostatic geoid, free-air gravity anomalies, and dynamic surface topography, and provide constraints on internal mantle density structure. The convection related observables are corrected for the isostatically compensated crustal heterogeneity and compared with those predicted by tomography-based mantle flow models. Occam inversions of the surface topography and gravity data provide inferences of the velocity–density scaling coefficients, which characterize mantle density anomalies below North America. The inferred density anomalies require simultaneous contributions from temperature and composition. The density and seismic shear velocity anomalies place constraints on the thermochemical structure of the mantle beneath the North American craton. Perturbations in the molar ratio of iron, R = XFe/(XFe + XMg), are used to quantify the compositional anomalies in terms of iron depletion in the sub-continental mantle. Estimates of the extent of basalt depletion in the tectosphere beneath North America are obtained. This depletion is interpreted to produce a local balance between positive chemical buoyancy and the negative thermal buoyancy that would otherwise be produced by the colder temperatures of the sub-cratonic mantle relative to its sub-oceanic counterpart.
Upper mantle shear structure of North America