Shallow Structure of the New Hebrides Island Arc

Abstract

Marine geological and geophysical studies of the New Hebrides island arc have been made to study (a) the present development of lithospheric plate boundaries, (b) evidence for creation of oceanic crust behind the frontal arc in interarc basins, and (c) evidence for reversal of the arc from east-facing to the present-day west-facing orientation. The arc system is bisected between 13° and 15° S. by the east-west Hazel Holme Fracture Zone which connects the trench and a north-south—trending spreading center (Nova Rise) on the Fiji Plateau near 174° E. The crust on the plateau south of the fracture zone is very young. Narrow interarc basins are present but youthful, south of about 18° S. North of the Hazel Holme Fracture Zone, interarc basins are less well developed and apparently even younger. Most of the Fiji Plateau has apparently been formed by spreading from the Nova Rise rather than within interarc basins associated with the New Hebrides. The tectonics of the central region of the arc system, immediately south of 15° S., appears to be governed by the transform section of the Hazel Holme Fracture Zone and by subduction of the D9Entrecasteaux Fracture Zone into the trench rather than by interarc spreading. In this region, the western and eastern chains of the New Hebrides group have been recently uplifted and tilted toward one another, creating a sedimentary basin. Most data do not support the idea that the eastern island belt in this region, including Maewo and Pentecost islands, is an ancient remnant of an east-facing arc system. These islands have been uplifted only very recently and later than the western islands. Therefore, any east-facing subduction phase must have ceased recently and occurred after subduction beneath the western islands. We suggest instead that the eastern island belt represents an interarc basin floor or a frontal arc uplifted behind the volcanic line.