Origin and geologic significance of Dual Seismic Discontinuities in the upper mantle

Two negative seismic discontinuities are commonly imaged in the continental upper mantle. Beneath thick cratons they are generally interpreted as a mid-lithospheric discontinuity (MLD) and a deeper lithosphere–asthenosphere boundary (LAB). However, similar paired signals beneath tectonically active regions with thinner lithosphere have remained difficult to explain, leading to conflicting interpretations of lithospheric structure. Here we show that integrating thermal lithosphere thickness from multi-observable probabilistic inversions, S-wave receiver functions and melt extraction depths from young basalts resolves these ambiguities and identifies a previously unrecognized origin for paired seismic discontinuities in tectonically active regions. Using eastern China as a data-rich natural laboratory, and drawing on additional tests worldwide, we show that in stable cratons the base of the thermal lithosphere matches the deeper seismic discontinuity, consistent with the conventional interpretation. In thinned, intraplate volcanic regions, however, the shallow discontinuity aligns with both the thermal and geochemical LAB, whereas the deeper discontinuity lies within the asthenosphere. We infer that paired seismic discontinuities in these settings record distinct melt accumulation zones, with melt stored both near the LAB and deeper in the asthenosphere. This framework reconciles long-standing conflicts over lithosphere thickness and shows that integrated thermal, seismic and geochemical constraints can map how melt is stored and transferred through continents, linking upper-mantle structure to volcanism, lithospheric weakening and the long-term evolution of Earth’s interior.