Segments of flat subduction are recognized in subduction zones, but little is known about the behavior of slabs along the edges of flat slab segments. Well-constrained earthquake data from the northern edge of the Peru flat slab segment reveal that the subducting Nazca slab is more contorted than previously assumed, with a prominent tear developed along a lateral ramp that bounds the Peruvian flat slab beneath southern Ecuador. The slab geometry and associated tear are spatially linked to Sumaco volcano ∼100 km east of the arc front. Sumaco mafic lavas are ultrapotassic, enriched in incompatible elements, and show little evidence for crustal assimilation. These geochemical signatures are consistent with low-degree (≤3%) melting of an upper mantle source comprising depleted mid-oceanic ridge basalt (MORB) mantle within the spinel stability field, and metasomatism of the source by melts derived from the basaltic portion of the slab within the garnet stability field (>80 km depth). We propose that focused melting at the slab edge was enabled by the combination of tearing and the generation of poloidal mantle flow in the proximity of the tear. A possible explanation for this process is the opening of a trapdoor-style tear, which induced a poloidal mantle flow by the gravitational foundering of the relatively old and dense edge of the flat slab segment.

You do not have access to this content, please speak to your institutional administrator if you feel you should have access.