Until about 1950, the southwest Pacific region was thought by many to be the foundered eastern half of Greater Australia, Tasmantis. The Outer Melanesian arcs defined the northern part of the Tasmantis boundary to the Pacific Ocean basin. Others considered the Inner Melanesian arcs to be the expanded rim of the Australian craton, the outwardly propagated expression of the Tasman Geosyncline. With the surge of exploration after World War II, new data showed that the outer islands could not be matched with island-arc models such as the Sunda Arc, and that New Caledonia, of the inner arcs, seemed increasingly to have Tasman Geosyncline affinities. By 1968, the land and sea data had produced a cluttered and confused picture, ripe for the application of new global tectonic models. After 1968, the Tasman Sea was shown to be a product of Late Cretaceous seafloor spreading and New Caledonia was accepted as part of the expanded rim of eastern Gondwana. The sea floor between the inner and outer chains was relatively young and the product of complex seafloor spreading. Over recent years, the southwest Pacific has in many respects been a showcase of plate tectonic theory. The region is intensely mobile. The obliquely convergent northern Indo-AustralianlPacific plate boundary carries great crustal blocks as allochthons to join others in eastern Indonesia. These crustal blocks also undergo vertical and rotational movement. The outer island chains do not fit the conventional models of an island arc - they are hybrid entities made up of the byproducts of subduction and of exotic terranes; as arcs, they continually change composition, form and configuration, even as they are being built. These outer chains are components of what is here called an accommodation boundary .
