Mantle Thermochemical Variations beneath the Continental United States Through Petrologic Interpretation of Seismic Tomography

Abstract

The continental lithospheric mantle plays an essential role in stabilizing continents over long geological time scales. Quantifying spatial variations in compositional and thermochemical properties of the mantle lithosphere is crucial to understanding its formation and its impact on continental stability; however, our understanding of these variations remains limited. Here we apply the Whole-rock Interpretive Seismic Toolbox For Ultramafic Lithologies (WISTFUL) to estimate thermal, compositional, and density variations in the continental mantle beneath the contiguous United States from MITPS 20, a joint body and surface wave tomographic inversion for Vp and Vs with high resolution in the shallow mantle (60-100 km). Our analysis shows lateral variations in temperature beneath the continental United States of up to 800-900°C at 60, 80, and 100 km depth. East of the Rocky Mountains, the mantle lithosphere is generally cold (350-850°C at 60 km), with higher temperatures (up to 1000°C at 60 km) along the Atlantic coastal margin. By contrast, the mantle lithosphere west of the Rocky Mountains is hot (typically >1000°C at 60 km, >1200°C at 80-100 km), with the highest temperatures beneath Holocene volcanoes. In agreement with previous work, we find that the predicted chemical depletion does not fully offset the density difference due to temperature. Extending our results using Rayleigh-Taylor instability analysis, implies the lithosphere below the United States could be undergoing oscillatory convection, in which cooling, densification, and sinking of a chemically buoyant layer alternates with reheating and rising of that layer.