The New Hebrides back-arc troughs are restricted to the southern and northern rear of the arc. Lack of trough in the central part is related to the compression induced by the collision of the d'Entrecasteaux ridge and the New Hebrides arc. The southern troughs are 50-60 km wide and are composed of one or two grabens striking N130-140 degree , while the northern troughs extend over 70 km wide and are composed of a succession of N-S trending grabens, horsts and half-grabens. Although the location and the main trend (N150 degree ) of the troughs indicate that they are largely guided by the subduction, the direction of extension (N40-45 degree ) is clearly oblique to the volcanic line and thus does not seem related to the subduction process. Formation of the New Hebrides back-arc results from extensional tectonics induced by a major reorganization at 3 Ma of the seafloor spreading of the North Fiji basin, which appears since this time to be developed independently of the subduction process.