A trapdoor mechanism for slab tearing and melt generation in the northern Andes

Segments of flat subduction are commonly recognised in subduction zones, but little is known about the behaviour 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 developing 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 in the Amazonian Basin, 100 km east of the Andean Belt. 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 MORB mantle (DMM) 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). This focused melting is explained by heating and decompression due to a trapdoor-style sinking of the torn slab edge, generating a poloidal flow that necessarily advects mantle from beneath.