DENSITY STRUCTURE OF THE UPPER MANTLE UNDER NORTH AMERICA

We present a new high-resolution density model for the lithosphere and upper mantle of North America and analyse its geodynamic and tectonic implications. This model is based on an integrated analysis of gravity, seismic refraction, seismic tomography, drill-hole, and geological data. The thickness of sedimentary cover was determined from geological maps, and the average density-depth relationship was estimated for each specific basin from published data. The density model of the consolidated crust (including Moho variations) is derived from seismic determinations. By removing the effect of the crust we calculate the residual ‘mantle’ gravity anomalies and the residual topography which are due to density inhomogeneities in the upper mantle. A joint analysis of these results with seismic tomography data (Van der Lee et al., 2000) leads to a construction of a 3D density model of the upper mantle under North America that is consistent with the residual gravity and produces the dynamic topography which is close to the residual one. The obtained density variations in the upper mantle under North America change significantly with depth. We conclude that they can not be explained solely by variations in temperature (Goes and Van der Lee, 2001) but a substantial part of them should be due to compositional differences. Under the Canadian Shield this difference is negative and is equal on the average to -40±5 kg/m³ which corresponds to 1.2% depletion. The opposite ‘compositional’ anomaly is found in the southern part of North America adjoining to Gulf of Mexico, it exceeds 30 kg/m³. The origin of this anomaly is disputable.