Abstract A multichannel seismic survey of the Tagus abyssal plain defines the extent of oceanic crust and the continent-ocean boundary. This boundary is marked by landward-dipping reflectors. The oceanic domain is divided into two parts: a deep eastern basin, where an abandoned earliest Cretaceous spreading center is inferred, and a shallower western basin formed after a ridge jump. This latter oceanic domain may have a western complement in the Newfoundland basin.