We used seismic tomography to investigate the complex structure of the upper mantle below northwestern South America. Images of slab structure not delineated by previous seismicity studies help us to refine existing tectonic models of subducted Caribbean-Pacific litho sphere beneath the study area. Beneath western Venezuela and Colombia we distinguish two slabs: a Maracaibo and a (redefined) Bucaramanga slab. The Maracaibo slab, coinciding with most of the Bucaramanga slab previously defined by W. D. Pennington, dips in a direction of 150° at an angle of 17° to a depth of 275 km and correlates to the subducted Late Cretaceous oceanic plateau of the Caribbean plate. Farther south, a second slab dips at an angle of 50° in a direction of 125° to a depth of at least 500 km and correlates to the subducted oceanic crust of the Nazca plate and the downdip extension of the Panama island arc. We refer to this slab as the redefined Bucaramanga slab, because it is different from the Bucaramanga slab segment defined by Pennington. The area of the South American plate overriding both slabs is characterized by the absence of an active volcanic are, an anomalouslywide topographically uplifted and tectonically active area, and the northward escape of the Maracaibo block along active strike-slip faults. In support of earlier studies, we attribute this to the underthrusting of the Caribbean oceanic plateau (our shallowly dipping Maracaibo slab) along the base of the South American lithosphere and to the recent collision of the Panama island arc rafted in on more steeply dipping crust of the Nazca plate (our redefined Bucaramanga slab).