Margins
Vanuatu
The New Hebrides central chain stretches from Ureparapara island,Lau
In theHunter Fracture Zone
The southern margin of the NFB is formed by the Hunter Fracture Zone and the Hunter Ridge (includingVitiaz Trench
The northern Melanesian arc collided with the subducted south-eastern segment of the Ontong Java Plateau 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 8–3 Ma.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 /yr. In the northern NFB a series of spreading centres stretches along an east–west-trending belt (with spreading rates): the Futuna (/yr) and North Cikobia (/yr) spreading centres, and the Tripartite (/yr), South Pandora, and Hazel-Holmes ridges. Basalts in the Central Spreading ridge are of N-MORB-type, indicative of a mature accretionary system, whereas basalts in the northern NFB have anCentral spreading ridge
The central spreading centre of the NFB is the largest and probably the oldest back-arc basin on Earth. 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. The N15° segment is -long and corresponds to a change in spreading direction from north–south to N15°. The southern part is poorly-defined with an accretion distributed over numerous small volcanoes scattered over a wide area. A double ridge north of 17°55'S flanks a graben -wide, -deep. A shallow massif at the northern tip is cut by a graben -wide, -deep. This part harbours hydrothermal vents. The N15° segment is flanked by curved grabens interpreted as fossil overlapping spreading centres. There is a triple junction at 16°50'N. The northern N160° segment is -long, and has three parts: In the southern part (16°50'S–15°30'S) the spreading axis is a -deep, -wide graben flanked by near-vertical walls. It has the morphology of slow-spreading ridges with an axis cut by a -wide, -high ridge. A bend at 16°10'S offsets the ridge . This part is flanked by a volcanic massif which reaches a depth of less than and is -wide at its southern end but disappears north of 15°30'S. Volcanism and uplift of older oceanic crust was initiated at 1 Ma. The central part (15°30'S–15°00'S) has two en echelon grabens that offset the axis . They form a domain -long, -deep and each graben is made of -long en echelon segments. Magmatic supply here limited to a narrow ridge separating the grabens and accretion has been mostly amagmatic for the past 1 Ma. North of 15°N the central spreading ridge is complex with two branches forming a possible triple junction. The western branch trending N120° is a -wide, -deep graben which cuts through older oceanic crust. The northern branch trending N140° is a -deep ridge and forms a continuation of the N160° segment.West Fiji Rift
The West Fiji area is dominated by a western and an eastern graben separated by a central plateau. The western graben, -wide and -deep, is flanked by a steep western wall but a series of steps on its eastern side and is a propagating rift. A ridge on its western side, reaching less than bsl, is flanked by another graben, -wide and -deep. This system of grabens and ridges, probably the southern extremity of the North Fiji fracture zone, converges in a flat area at the southern end of the western graben, -deep, that is flanked by two pseudofaults -high. The eastern graben, -wide and -deep, is flanked by parallel ridges and depressions over a -wide area. In the central plateau there is a fan-shaped system of ridges and depressions, the centre of which is occupied by a -deep and -wide graben. The sedimentary cover is thin or absent over the entire area. Pillow basalts in both the western and eastern grabens have a composition close to the mid-ocean ridge basalt (MORB) of the central spreading ridge.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. The Tripartite Ridge is divided into three segments oriented in different directions. It is a very young ridge that is propagating into an older domain covered by sediments. The inactive volcanic islandsTectonic 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: Taupo–Kermadec–Tonga and Hunter–Vanuatu. Other geological structures are remnants of island arcs and back-arc basins mostly from the Eocene and Miocene, including Vitiaz Trench and the Lau–Colville, Three Kings, and Loyalty ridges. 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. Today the Pacific Plate is subducting westward along the eastern margin of the NFB, the Tonga-Kermadec Trench. The Australian Plate is subducting eastward along the western margin of the NFB, the New Hebrides Trench. The transition between these opposed subduction systems is the Fiji Fracture Zone, a complex left-lateral succession of ridges and faults north of Fiji that extends into the North Fiji and Lau basins respectively.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 arc 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 MaSee also
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* * * * * * * * * * * * * * * * * * * * {{Coord, -17, 173, display=title Back-arc basins Oceanic basins of the Pacific Ocean