Backfolded slab in the mantle transition zone as a melt source for Mariana petit-spot volcanism

Abstract Petit-spot volcanoes are a distinct class of intraplate volcanism whose melt source remains poorly constrained. Near the Mariana Trench, young edifices atop the incoming Pacific plate are interpreted as petit-spot features. Here we image the upper mantle beneath the central Mariana subduction zone using newly acquired and existing seafloor seismic data. Our model reveals pronounced backward folding of the subducting slab within the mantle transition zone (MTZ)—produced as ongoing subduction and the globally significant IBM trench advance feed slab material into the MTZ faster than its slowly migrating deep limb escapes, so the accumulating slab volume—accommodated by flattening and stacking—mechanically extrudes ambient MTZ material upward. We image this upwelling as a low-velocity channel extending from the MTZ to the incoming lithosphere behind the slab. Upon crossing the 410-km discontinuity, the upwelled hydrous and carbonated material releases a dense supercritical melt that ascends to form an interconnected, volatile-charged carbonate melt network at shallower depths. This deep-sourced, CO₂–H₂O-driven melting framework explains both the imaged low-velocity anomalies and the distinctive carbon-rich, alkaline geochemistry of petit-spot lavas. These findings demonstrate that trench-advance-driven slab folding can generate deep-sourced melts, offering a new perspective on intraplate volcanism and deep volatile cycling.