SES-03. Mantle Thermochemical Variations beneath the Continental United States Through Petrologic Interpretation of Seismic Tomography
Constraining the thermomechanical state of the mantle beneath the continental United States is vital to understand the stability of the lithosphere. Recent studies have shown the potential of combining high-resolution seismic inversions with petrologically informed forward-calculations of seismic wave speed to improve these constraints. Here we apply the Whole-rock Interpretive Seismic Toolbox For Ultramafic Lithologies (WISTFUL) to interpret thermal, compositional, and density variations according to MITPS_20, a joint body and surface wave tomographic inversion for Vp and Vs with high resolution in the shallow mantle. We infer temperature variations beneath the continental United States up to 800–900°C at 60 km and 80 km, with reduced variations at 100 km. East of the Rocky Mountains, the mantle lithosphere is generally cold (350–850°C, 60 km), with higher temperatures (up to 1000°C, 60 km) on the Atlantic coastal margin. Long-wavelength thermal variations are present in these regions, some correlated with surface expressions of rifting events. By contrast, the mantle lithosphere west of the Rocky Mountains is hot (often >1000°C at 60 km and >1200°C at 80 and 100 km), with highest temperatures found under Holocene volcanoes. Our results agree well with temperature and compositional constraints from recently (<10 Ma) erupted mantle xenoliths in the southwestern US. Our results predict colder mantle temperatures at 60 km depth than does primary magma thermobarometry. This discrepancy is likely due to the differences in sampling resolution between seismic and geochemical methodologies, as well as uncertainties in the anelastic formulations. In agreement with previous work, we find that the chemical depletion predicted by our model does not fully compensate for the density difference due to temperature between the eastern and western United States. According to Rayleigh-Taylor instability analysis, the cratonic lithosphere in the eastern United States could be undergoing oscillatory convection at wavelengths of ~75 km and periods of ~700 Myr, which could explain intractratonic basin and rifts.