SS Precursor Imaging Reveals a Global Oceanic Asthenosphere Modulated by Sea-Floor Spreading

The asthenosphere is a weak layer in the upper mantle where geotherm may exceed mantle solidus and partial melt occurs. Although it has been suggested that an increase in seismic wavespeed at about 220 km depth represents the base of the asthenosphere, seismic studies to-date have not been able to provide evidence for the existence of such a global interface in the oceanic regions. In this study, we report observations of SS precursors reflected at this boundary throughout the global oceans. The average depth of the discontinuity is approximately 250 km, with a velocity jump of about 7% across the interface. Finite-frequency tomography of SS precursor traveltimes reveals large depth variations of the discontinuity over short spatial distances, which explains the absence of this discontinuity in previous global stacks. The depth perturbations are characterized by alternating linear bands of shallow and deep anomalies that roughly follow seafloor age contours, indicating a fundamental connection between seafloor spreading and asthenosphere convection. The base of the asthenosphere is smoother under seafloors formed at slow-spreading centers and becomes much rougher under seafloors formed at fast-spreading centers with a spreading rate greater than $\sim 50$ mm/yr. This observation suggests that different geophysical processes at slow and fast spreading centers generate lithospheric plates with different chemical compositions and physical properties, which in turn influences the convection in the oceanic asthenosphere.