The Changing Face of the Lithosphere‐Asthenosphere Boundary: Imaging Continental Scale Patterns in Upper Mantle Structure Across the Contiguous U.S. With Sp Converted Waves

Juxtaposed terranes of highly varied tectonic history make up the contiguous U.S.: the tectonically active western U.S., the largely quiescent Archean and Proterozoic cratons of the central U.S., and the Phanerozoic orogen and rifted margin of the eastern U.S. The transitions between these regions are clearly observed with Sp converted wave images of the uppermost mantle. We use common conversion point stacked Sp waves recorded by EarthScope's Transportable Array and other permanent and temporary broadband stations to image the transition from a strong velocity decrease at the lithosphere‐asthenosphere boundary (or LAB) beneath the western U.S. to deeper, less continuous features moving east that largely lie within the lithosphere. Only sparse, localized, weak phases are seen at LAB depths beneath the cratonic interior. Instead, we observe structures within the cratonic lithosphere that are most prominent within the Archean lithosphere of the Superior Craton. The transition from west to east is clearly revealed by cluster analysis, which also shows eastern U.S. mantle velocity gradients as more similar to the western U.S. than the ancient interior, particularly beneath New England and Virginia. In the western U.S., the observed strong LAB indicates a large enough velocity gradient (an average velocity drop of 10 ± 4.5% distributed over 30 ± 15 km) to imply that melt has ponded beneath the lithosphere. Plain Language Summary The contiguous U.S. can be roughly divided into three geological regions. The active western U.S. has many volcanoes and earthquakes. The middle aged eastern U.S. last had such activity during the opening of the Atlantic, hundreds of millions of years ago. The ancient central U.S. has rocks that are up to 3.5 billion years old, and much of it has remained relatively unaltered for at least 1 billion years. We investigate how the nature of the tectonic plate underlying these regions changes, in particular the boundary at the base of that plate (∼50–250 km below Earth's surface). Using earthquake waves that interact with this boundary, we can also clearly distinguish these three regions first defined by surface features. In the western U.S., the earthquake waves see this boundary so strongly that it implies there is a small but significant volume of molten rock at the base of the tectonic plate. Key Points S‐to‐P converted waves image contiguous U.S. lithosphere, including the negative velocity gradient at its base and its internal structu