Deep-mantle contributions to the surface dynamics of the North American continent

Abstract

The regional and continental scale dynamics of North America and vicinity are explored using a high-resolution model of mantle flow. The model is constrained by simultaneously inverting global seismic and mantle convection data sets and it includes an explicit treatment of the positive chemical buoyancy of the continental tectosphere. Moreover, it adopts a depth-dependent mantle viscosity structure which reconciles both glacial isostatic adjustment (GIA) and convection data. The flow model successfully reproduces plate velocities and observations of surface gravity and topography, including the continent-scale quasi-linear depression (after corrections for GIA and crustal heterogeneity) extending from northern Alaska to Venezuela. The predictions also match lithospheric flow and stress fields inferred from local and regional measurements of seismic anisotropy and surface deformation. We demonstrate that these signals are largely driven by viscous flow coupled to density anomalies in the deep portions of the upper mantle and within the lower mantle, where the latter may be associated with the descent of the ancient Kula–Farallon plate system and an active mantle upwelling below the Pacific margin of the North American plate. More importantly, the flow calculations elucidate how these large-scale heterogeneities give rise to regional-scale flow and stress patterns below the southwestern U.S. and below the central U.S.