The Banda, Celebes and Sulu basins are poorly understood marginal seas located at the junction of the Eurasian, Indian–Australian and Pacific–Philippine Sea plates. The incompleteness of the data sets, the complex geology of the surrounding islands and the late Cenozoic evolution involving subduction, rifting, transform faulting and island arc collision have complicated tectonic interpretation of the region. The Banda basin is underlain by oceanic crust1–3. Bowin et al.2 and Lapouille et al.3 have suggested that it is a trapped oceanic basin which was once continuous with the late Jurassic Argo Abyssal Plain off north-west Australia. Similarly, the Celebes and Sulu basins are also underlain by oceanic crust4–6. On the basis of newly identified magnetic reversal ages (Fig. 1), heat-flow data and on-land geological studies, we suggest that the Celebes and Sulu seas may have been continuous with the Banda basin. This Cretaceous–Eocene basin has been dissected into its present configuration by Tertiary tectonic processes.