Flat‐Slab Subduction and Overriding Plate Architecture Beneath the Northwestern Andes Revealed by Receiver Functions

The existence of flat‐slab subduction beneath the northern Andes of Colombia, north of ∼5.5°N, has been inferred from the absence of active volcanism and from a Wadati–Benioff zone located >400 km from the trench. Here we use new broadband seismic data from northwestern Colombia to constrain slab geometry, the structure of the overriding South American plate, and their interaction. Teleseismic radial receiver functions acquired along a WSW–ENE profile across the Western and Central Cordilleras image the main lithospheric interfaces. We identify two closely spaced crust–mantle boundaries beneath both cordilleras, vertically separated by <∼20 km. The deeper interface is interpreted as the Moho of the subhorizontal slab, whereas the shallower interface corresponds to the Moho of the overriding plate, here associated with the accreted and underthrust Caribbean Oceanic Plateau (COP). Toward the Magdalena Valley, we image the preserved continental Moho of South America, indicating that the overriding plate Moho is laterally discontinuous and reflects tectonic juxtaposition of distinct crustal domains assembled during long‐lived convergence. Within the overriding plate, an east‐dipping low‐velocity zone is interpreted as underplated, hydrated sediments related to progressive COP underthrusting beneath the Central Cordillera. These observations support prolonged Late Cretaceous–Paleogene collision and underthrusting of the COP, followed by westward migration of subduction and establishment of the present‐day flat slab in the Late Miocene–early Pliocene.