Bathymetric and magnetic data from the Lord Howe Rise — New Caledonia Basin — Norfolk Ridge area form the basis of a remterpretation of the structure and geological history of the Melanesian Complex in relation to the South-West Pacific Ocean. Lord Howe Rise and Norfolk Ridge and their extensions east of New Zealand belong to a system of orogenic belts which developed in succession starting in the Upper Paleozoic marginal to Australia and Antarctica, the source of sediment. Crustal spreading is superimposed on the evolution of the Lord Howe Rise — Campbell and Melanesian geosynclines. This spreading had already begun in the Triassic with the moving apart of Australia and Antarctica and resulted in a foundering and fracturing of the Melanesian Rise and Ridge system and the creation of new ocean basins mainly during the Mesozoic. The movement of individual crustal blocks was directed away from the Indian-Antarctic and Pacific-Antarctic Rises and also from a presumably incipient mid-Tasman Rise. Tension in the marginal zone resulted in the formation of the outer Melanesian Belt, an island arc, volcanic are, and trench complex. Two major fracture systems can be recognised in the South-West Pacific; one with dominant north-east-south-west strikes, reflecting the process of crustal spreading, the other, a major shear zone with dominant north-west-south-east strikes, possibly the effect of the movement of the earth's axis of rotation with respect to the crust.