SES-03. Periodic Lithosphere-Asthenosphere Boundary variations in the southwestern United States as evidence for oscillatory convection

Abstract
The southwestern United States, including the Basin and Range and Colorado Plateau, is enigmatic for high elevations and widespread deformation and volcanism at large distances from modern plate boundaries. Recent petrological interpretation of the seismic tomography beneath the continental United States (Shinevar et al., 2023, EPSL) found that the chemical buoyancy of the mantle lithosphere only compensates ~40% of the negative thermal buoyancy. Rayleigh-Taylor instability analysis predicts that the lithosphere under these conditions undergoes oscillatory convection, a form of small-scale convection in which cooling, densification, and sinking of a chemically buoyant layer alternates with reheating and rising of that layer. This process could have a significant role in the geodynamics and topographic evolution of the southwestern United States as well as major consequences on the thermal and chemical evolution of the continental lithosphere. Oscillatory convection predicts, for the buoyancies estimated beneath the continental United States, variations in the observed Lithosphere-Asthenosphere Boundary (LAB) of wavelengths equal to â…” the unstable layer thickness. To test whether oscillatory convection is occurring under the southwestern United States, we use multitaper methods to estimate spectral energy in the seismically-derived LAB (Golos & Fischer, 2022, G^3). Along transects with the densest station spacing (~20 km), we find significant spectral energy at 40–80 km, in line with wavelengths expected from oscillatory convection. Our results are robust to choices in gridding, number of tapers, and transect orientation.