North Fiji Basin

Vanuatu The New Hebrides central chain stretches from Ureparapara island, Banks Islands, in the north to Hunter island in the south. The New Hebrides Trench retreats progressively which causes the southern end the subduction zone to bend eastward. The Australian plate subducts under Vanuatu at the New Hebrides Trench which results in a complex of rifts and transforms in the NFB. The New Hebrides island chain itself is being deformed as buoyant features such as d'Entrecasteaux Ridge and West Torres Plateau are being subducted in this process. NFB is the product of the asymmetric back-arc opening about a hinge point at 11°S, 165°E around which the Vanuatu chain has rotated 28° clockwise during the last 6 , or 6–7.5°/Ma. This rotation has also caused rifting in the northern part of the NFB. Vanuatu can be divided into a southern and a northern tectonic blocks separate from the western NFB block. These blocks are separated by an extensional zone east of the islands chain. Lau In the Lau Basin east of the NFB the Pacific plate is subducting westward under Tonga Trench in the highest rate of back-arc rifting known — where the Louisville seamount chain subducts under the Tonga Trench rifting propagates at . This seamount chain–trench intersection propagates southward at a rate of and, as a consequence, Tonga Islands rotate clockwise at a rate of 9.3°/Ma. Hunter fracture zone The southern margin of the NFB is formed by the Hunter fracture zone and the Hunter Ridge (including Matthew and Hunter Islands, two active volcanoes). The central spreading ridge of the NFB transects Hunter Ridge and a small spreading centre is developing south of it. The Hunter Ridge formed 3 Ma and fossil transform faults in NFB north of the ridge are remains of a spreading ridge that was active before the Vanuatu Trench propagated south of the southern end of Vanuatu, Anatom Island. Vitiaz Trench The northern Melanesian arc collided with the subducted south-eastern segment of the Ontong Java Plateau at 10–8 Ma. This collision reversed the direction of subduction in the Vitiaz Trench and thus initiated the clockwise rotation of the Vanuatu arc and the opening of the NFB at 8–3 Ma. An isolated zone of deep-focus earthquakes towards the middle of the basin would be explained by the continuing slab subduction of the Pacific plate remnant from before 110–8 Ma that cut off when the collision with the Ontong Java Plateau occurred stalling further subduction and reorientating the direction of subduction in the area. ==Intra-basin morphology==

There are two main spreading systems in the central and southern part of the NFB: the Central Spreading Ridge and the West Fiji Rift, both with a variable spreading rate of . In the northern NFB a series of spreading centres stretches along an east–west-trending belt (with spreading rates): the Futuna ( and North Cikobia ( spreading centres, and the Tripartite (, South Pandora, and Hazel-Holmes ridges. Basalts in the Central Spreading ridge are of N--type, indicative of a mature accretionary system, whereas basalts in the northern NFB have an ocean island basalt (OIB) mantle source. It can be divided into four -long segments: • The southernmost segment (21°40'S–20°30'S) stretches trending north–south. It has a complicated morphology with -deep ridges separated by -deep depressions. This makes the exact location of the spreading ridge difficult to identify, but magnetic lineations reveal its presence. The morphology is intermediate between fast and slow spreading ridge. • The north–south segment (21°S–18°10'S) is -long and the least complicated with the axial ridge below wide. A flat-topped dome reaching is cut in half by a graben a few hundred metres wide and tens of metres deep. The dome is flanked by symmetrical grabens. V-shaped pseudofaults at the northern and southern ends are traces of the propagating ridge segment. Northern North Fiji Basin The South Pandora and Tripartite Ridges in the northern NFB are active spreading ridges with -long segments, a -wide volcanic axis, and ordered magnetic lineations running parallel to the ridge. The ridge segments are separated by complex relay zones rather than transform faults. The South Pandora Ridge is divided into five segments averaging in width. The axial valley is partly obscured by faulted and rifted volcanic structures; elongated grabens are typical of slow spreading ridges with steep walls flanking a deep valley. On either side of the ridge there are numerous, large volcanoes; a very thin or absent sedimentary cover over a distance of ; and continuous magnetic lineations indicating a very low half spreading rate (/Ma) during the past 7 Ma. ==Tectonic evolution==

100–45 Ma, after the break-up of Gondwana, a single, almost continuous arc-subduction system existed in the south-west Pacific, from Solomon Islands to New Zealand's North Island. Today only two actively spreading back-arc basins remain in the region: Taupō–Kermadec–Tonga and Hunter–Vanuatu. Other geological structures are remnants of island arcs and back-arc basins mostly from the Eocene and Miocene, including the Vitiaz Trench and the Lau–Colville, Three Kings, and Loyalty ridges. Accordingly, the Loyalty-Three Kings Ridge once formed a single, continuous arc with the Lau-Colville Ridge which is called the Vitiaz arc. The Fiji–New Hebrides region is made of volcanic rock but where volcanism began is uncertain. The region probably formed far south-west of its present location where it was subsequently rifted apart when the South Fiji Basin opened in the Early Oligocene. From the Early Oligocene to Miocene the region was part of an arc that formed the northern margin of the Australian plate. The NFB back-arc basin broke through this margin 12  and has since the Late Miocene rotated the New Hebrides Arc 30° clockwise and Fiji at least 100° counter-clockwise. Slab avalanche Large magnitude earthquakes beneath the NFB have been attributed to a detached slab segment of the subducted Australian plate which collided with the subducting Pacific plate at a depth of 5 Ma. The earthquakes are the result of these colliding slabs settling on the 660 km discontinuity. Beneath Tonga at a depth of the number of earthquakes increases dramatically while the shape of the Pacific becomes complex. Hundreds of these earthquakes occur outside the Wadati–Benioff zone (top of slab) along a horizontal plane. The eastward subduction of the Australian plate (together with the now-fused South Fiji plate) under NFB created the New Hebrides and south Solomon Islands. The slab produced from this subduction stretches steeply down to except at its southern end where it only reaches . The north end of the slab, at the southern Rennell Trough, corresponds to the sharp bend in the andesite line. A detached slab from the east-dipping Australian plate beneath the NFB has slid eastward and collided with the west-dipping Pacific slab. A series of unusual earthquakes below the NFB occur within several such detached slab segments. If these segments are combined and reconstructed back to their original location at the surface, they equal both the NFB and the subducted part of the Australian plate since 12 Ma in area. The Tonga slab is avalanching through the 660 km layer at the southern end of the New Hebrides Aarc and Trench. The Pacific plate has been subducting at the Tonga Trench for a long time which led to an accumulation of slab material at the 660 km layer south of the Vitiaz Trench while the New Hebrides island arc has been pushed southward and clockwise. It also reversed the direction of subduction and opened the NFB back-arc and pushed the Vitiaz slab into the mantle and initiated the subduction at New Hebrides Trench. The slab avalanche was initiated at 8 Ma and most of the material is now located below the 660 km layer. The slab beneath Tonga and Kermadec penetrates into the lower mantle. It is dipping down from Tonga Trench but deflects horizontally at the 660 km discontinuity. There is a detached remnant slab beneath the Vanuatu Trench. At the Kermadec Trench the Pacific plate has been subducting since 40 Ma ==See also==