We present results of travel time inversions of teleseismic P and S waves recorded at the SECaSA92 (Southeast Caribbean South America 1992) temporary broadband array in northeastern Venezuela and Trinidad. The inversions reveal the unusual structure of the southern termination of the Lesser Antilles subduction zone: A minimum 2% relatively high-velocity anomaly trends WSW from the seismically defined Lesser Antilles slab beneath and NW of the Paria Peninsula to a point below the Venezuelan Serran¿a del Interior, well south of the Caribbean coast. Resolution tests utilizing actual ray geometries and densities of the source data indicate that the regional-scale structure beneath the study area is reasonably well resolved. Thus a detached and detaching subducted South American slab appears to lie beneath continental South America. We infer that oceanic South American lithosphere has been overridden to a significant degree by continental South America. The detached slab now lying beneath continental South America was driven into its current position after detaching from the former eastward striking Mesozoic ocean-continent passive margin as this margin entered the subduction zone. Because oceanic and continental South America are still attached without apparent relative motion between them along the Atlantic passive margin southeast of our study region, the slab must be the actively moving element during continental overriding. Thus the slab and its surrounding mantle (both Caribbean and South America) beneath northeastern South America are mobile and have moved ESE relative to the stable Guyana Shield craton.