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.