Campagne Seapso 2: Origine et nature des fosses arriere-arc des Nouvelles Hebrides

The data base of the cruise consists of 3900 milles of bathymetry, gravimetry and magnetism profiles and 2000 milles of single channel seismic reflection profiles; 31 dredgings were also successfully performed. New Hebrides back-arc troughs are located in the northern and southern part of the rear of the New Hebrides arc. There is no trough in front of the collision/subduction of the d'Entrecasteaux ridge. The objectives of the cruise were to collect more data concerning the structure and the volcanism in this area in order to solve the problem of age and origin of the structures observed at the back of the arc

Structure, age et origine des fosses arriere-arc des Nouvelles-Hebrides (Sud-Ouest Pacifique)

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