The Papuan Ultramafic Belt of eastern New Guinea is one of the largest known examples of the mafic-ultramafic complexes commonly termed ophiolitic. These complexes, widespread in orogenic areas, are characterized by a succession upward from a peridotite-pyroxenite basal layer through gabbro to dolerite, basalt (often pillowed), and deep-sea sediments. Large gravity anomalies associated with the Papuan ophiolites emphasize the similarities between eastern Papua and areas such as Cyprus, New Caledonia, and the Alpine Ivrea zone; but the anomalies, considered together with the mapped geological relations, are difficult to reconcile with any of the commonly proposed modes of ophiolite emplacement. Although some problems remain unresolved, the most likely of the processes considered involves large-scale splitting of oceanic lithosphere as it approaches a subduction zone. This process may be characteristic of the destruction, not of normal oceanic crust, but of young basins formed at the rear of active volcanic arcs.