Mapping the Thermal Lithosphere and Melting Across the Continental US

The thermal regime of continental lithosphere plays a fundamental role in controlling the behavior of tectonic plates. In this work, we assess the thermal state of the North American upper mantle by combining shear-wave velocity models, calculated using data from the EarthScope facility, with empirically derived anelasticity models and basalt thermobarometry. We estimate the depth of the thermal lithosphere-asthenosphere boundary (LAB), defined as the intersection of a geotherm with the 1300 degrees C adiabat. Results show lithospheric thicknesses across the contiguous US vary between similar to 40 km and >200 km. The thinnest thermal lithosphere is observed in the tectonically active western US and the thickest lithosphere in the midcontinent. By combining geotherm estimates with solidus curves for peridotite, we show that a pervasive partial melt zone is common within the western US upper mantle and that partial melt is absent in the eastern and central US without significant metasomatism. Plain Language Summary The lithosphere, which is the upper most mechanical layer of the earth, makes up tectonic plates and can vary in thickness laterally. Thickness variations influence the location of volcanoes and where deformation occurs when forces act on the lithosphere. Lithospheric thicknesses can be estimated by examining the thermal properties of the uppermost mantle. We integrate seismic observations and lab results to calculate temperatures for the upper mantle in the continental US and compare these to geochemical proxies for temperature. We then use these temperature results to estimate lithospheric thickness and map, where melt is predicted within the upper mantle. Results show thin lithosphere and the presence of melt in the western US. Thicker lithosphere and an absence of melt are observed in the central and eastern US.