Neogene tectonic evolution of the Hebrides Island arc: a review incorporating ODP drilling results

Evolution of the New Hebrides Island Arc occurred primarily during the Neogene, when, as part of the Vitiaz Arc it was ripped away from Fiji and Tonga and rotated clockwise (westward) to its present position. A combination of subduction polarity reversal, backarc spreading, and ridge-arc collision is responsible for the present-day configuration of the arc. During Ocean Drilling Program (ODP) Leg 134 drilling in the central New Hebrides Island Arc, where the collision of the d'Entrecasteaux Zone occurs, new information was revealed that resolves questions of timing for d'Entrecasteaux Zone collision, the style of arc deformation associated with colliding ridges, and rates of North Fiji Basin opening. These results constrain late Cenozoic relative plate motion along this section of the Australian (Australia-India) and Pacific Plate boundary. The opening of the North Fiji Basin, the separation of the New Hebrides Island Arc from the Vitiaz Trench, and the counter-clockwise rotation of the arc probably started sometime between 10 and 12 Ma. A mean relative convergence rate of 13.8 to 14.8 cm/yr between the Australian (Australia-India) and Pacific plates, was determined based on the chronostratigraphy of Bougainville Guyot (Site 831). Initial impact of the d'Entrecasteaux Zone may have occurred at -1.5-3.0 Ma in the vicinity of Epi Island and has propagated northward along the forearc at a rate of 2 to 4 cm/yr. Offscraping of material from the North d'Entrecasteaux Ridge is documented by nine major and four minor thrust sheets, located in the d'Entrecasteaux Zone-arc collision area (Sites 827 and 829) on the forearc slope of the central New Hebrides Island Arc, which lithologically and chronologically correlate with sedimentary and volcanic rocks drilled on the North d'Entrecasteaux Ridge (Site 828). The accretion of the ridge material is actively building Wousi Bank, a shallow, rising ridge located offshore of west-central Espiritu Santo Island. Holes drilled in the North Aoba Basin (Sites 831 and 832) indicate that basin depths have varied between abyssal and shallow bathyal depths. One, or possibly two, unconformities drilled along the flanks of the North Aoba Basin suggests two phases of flank inclination along the basin resulting in mass wasting along the flanks. One unconformity, penetrated at a depth of -700 mbsf in Hole 832B, has a 0.13-0.44 m.y. hiatus. This unconformity appears to represent the time that the d'Entrecasteaux Zone collided with the arc from central to southern Espiritu Santo Island, causing major uplift of the island during early Pleistocene time. The d'Entrecasteaux Zone collision has had a major impact on arc magmatism. Regular volcano spacing (80-90 km) occurs for the southern New Hebrides Island Arc volcanoes as far north as Epi. From Epi northwards to Santa Maria this spacing is severely disrupted in the collision zone. Several huge low-profile, Hawaiian-type basaltic shield volcanoes (Ambrym, Aoba) occur along major cross-arc fractures induced by the collision. These volcanoes contain abnormal amounts of primitive picritic and ankaramitic lavas, suggesting relative ease of access for these magmas from their mantle sources to eruption. Furthermore, post-d'Entrecasteaux Zone collision basaltic and andesitic magmas have a similar range of major and trace element compositions as pre-collision magmas, but profoundly different isotopic compositions than the pre-collision lavas. For example, the sill complex from the base of Hole 833B and dated at 3.3 to 3.6 Ma has Pb/Pb and Sr/Sr values of 18.7 to 18.8 and 0.7025-7030, respectively, similar to values for volcanoes in the New Hebrides Island Arc remote from the collision zone. In contrast, ρost-d'Entrecasteaux Zone collision lavas from the Central Belt islands of Santa Maria, Aoba, and Ambrym, and lavas drilled in Hole 832B lithostratigraphic Units I, II, III, and IV have 2 0 Pb/ 0 Pb values of 18.1 to 18.3 and Sr/Sr values around 0.7040. The origin of the "enriched" component that isotopically distinguishes the post-collisional magmas has not been identified, but it is unlikely to be directly derived from the d'Entrecasteaux Zone, as drilled d'Entrecasteaux Zone lavas are isotopically matched to the pre-collision and southern arc lavas. Collision and subduction of the d'Entrecasteaux Zone is currently indenting the forearc (Site 830) and causing uplift of the islands of the Western and Eastern belts of islands. Offscraping of the North d'Entrecasteaux Ridge sedimentary and volcanic rocks is active and northward migration of the d'Entrecasteaux Zone continues to deform new parts of the central New Hebrides Island Arc forearc slope. Erosion of these islands and Pleistocene effusive volcanic products of the Central Chain volcanoes contribute sedimentary fill to the North Aoba Basin at a rate of 286 m/m.y. Soon (-0.5 m.y.) the West Torres Massif and the Loyalty islands will collide with the north-central and southern part of the arc, and new episodes of island elevation, basin formation, and arc fragmentation will occur.