Reports

Plate tectonic theory describes a strong and rigid lithospheric lid that translates coherently atop a weaker and more deformable convecting asthenosphere. Determining the depth and pervasiveness of the interface between these two layers, known as the lithosphere-asthenosphere boun-dary (LAB), is key for understanding the formation, evolution, and thermochemical properties of plates and associated tectonics. The exact compositional and thermal mechanisms that control this rheological division remain enigmatic, but seismological imaging of anisotropy — the directional dependence of seismic wave velocity — across the upper mantle provides an essential tool for interrogating the transition in ma-terial properties across the LAB. Seismological interrogations of the oceanic upper mantle beneath the Pacific ocean find evidence for a sharp drop in seismic velocity, known as the Gutenberg (G) discontinuity (1), at 40–100 km depth. The depth of the G roughly coincides with the top of a low velocity zone (LVZ