|Vanuatu Geodynamical Situation|
The Vanuatu archipelago forms the letter Y, which each of the branches has its own complex tectonic history (Macfarlane et al., 1988; Greene et al., 1994).
The Vitiaz Trench marks the locus of the fossil trench that was once associated with subduction of the Pacifique Plate beneath the Australia-India Plate to form the Vitiaz arc (Gill & Gordon, 1973).
Vanuatu was part of the Vitiaz arc earlier than 12 Ma, and the Western Belt magmatism (25-14 Ma) was associated with this phase of westward subduction (Mitchell & Warden, 1971), as the North Fiji back-arc basin began to open (Auzende et al., 1995). This resulted in the formation of the islands of Santo and Malekula.
Eastward subduction of the Australia-India Plate beneath the Pacific Plate then produced the Eastern Belt magmatism (7-4 Ma) represented by the islands of Pentecost and Maewo: Magmatism in the central chain began at 6 Ma and was initially focused on Erromango, Tanna and Anatom in the south (Mitchell & Warden, 1971).
Volcanism subsequently developed along the entire length of the arc and shifted closer to the trench because of a steepening of the subduction zone to its present inclination of 70°. Globally, the depth of the top of the subduction slab beneath most arc is relatively constant at 110 km (Tatsumi & Eggin, 1995) but, in Vanuatu, most of the volcanic front is ~ 150 km above the slab and relatively close (~ 125 km) to the trench axis.
The recent tectonic evolution has been dominated by the collision with the central region of the arc of the D’Entrecasteaux Zone, an Eocene-Oligocene island-arc complex on the subduction plate. The collision began between 1.5 and 3 Ma near Epi and has been migrating northwards as a result of the obliquity of the D’Entrecasteaux Zone relative to the trench (Green et al., 1994).
This has produced a system of transverse wrench faulting along the arc and exposed the older parts of the arc complex. Rear-arc rifts that extend nearly continuously along the arc disappear abruptly opposite the collision zone (Maillet et al, 1995). The GPS data collected over a decade in Vanuatu and New Caledonia give precise crustal motions all along the Vanuatu arc and sheds new light on the tectonics of this area (Calmant et al, 2003).
The islands of Vanuatu consist mainly of volcanic rocks. Most of the Pliocene and Quaternary islands have been formed by volcanic growth, with uplift in a few cases, those containing older Tertiary rocks have resulted entirely from differential elevation of fault bounded blocks (Warden , 1971).