LEG 134

Vanuatu, New Hebrides

The New Hebrides Island Arc is centered in a complex system of active volcanic arcs extending from Papua New Guinea through the Solomon Islands to Vanuatu, where it is displaced eastward along the Hunter Fracture Zone to Tonga, and then continues through the Kermadec Islands to New Zealand. The system is the product of intra-oceanic subduction, possible reversal of subduction polarity, and ridge-arc collision. Although along most of the system, the Pacific Plate is being subducted under the Australia-India Plate, from Papua New Guinea to Vanuatu, the Australia-India Plate is being subducted beneath the Pacific Plate and the North Fiji Basin. During Leg 134, seven sites (Site 827 to Site 833) were drilled along a transect from the Australia-India Plate to the forarc and intra-arc North Aoba Basin.

Each ridge of the d'Entrecasteaux Zone (DEZ), the North d'Entrecasteaux Ridge (NDR) and the South d'Entrecasteaux Seamount Chain (SDC), causes different forearc deformation. The basement rocks of the NDR have a MORB signature and are denser than those of the Bougainville Guyot, the easternmost of the SDC. The sediments and surficial basement rocks of the NDR were apparently scraped off and accreted to the forearc during subduction, forming the Wousi Bank. The SDC impacts the forearc in a different manner; little deformation has occurred in comparison to the NDR collision zone, although the SDC is converging at the same rate and at the same angle. In the collision zone, diagenetic alteration of volcanic sediment has produced major compositional changes in the pore fluids with chloride and calcium concentrations greater than, and sodium, potassium, and magnesium concentrations lower than, seawater values. These variations correspond to structural features, particularly thrust faults and fracture zones. More fluid is present below thrust faults penetrated in the forearc slope of the New Hebrides Island Arc where the NDR impinges upon the arc and faults may have channelled the flow. Low chloride and high methane concentrations characterize deep levels in the forearc slope. Thus, the fluid near the deep thrust fault at the forearc slope may have been derived from the decollement and subduction process.

More than 2 km of middle Miocene to Pliocene sediments and volcaniclastics were recovered from the intra-arc North Aoba Basin. An upper Pliocene or lower Pleistocene unconformity suggests a change in the base of the basin, possibly indicating uplift of Espiritu Santo Island in response to the collision of the DEZ at that time. The upper 400 m of sediment in the center of the basin resulted from active Central Chain volcanism and the occurrence of a volcanic sill on the lower east-central flank of the basin suggests active volcanism along the Eastern Belt from the early Pliocene to the Pleistocene. Diagenetic alteration of these volcanic sediments has produced some of the most altered and concentrated fluids yet recovered on an ODP/DSDP leg.