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The North Fiji Basin (NFB) is an
oceanic basin In hydrology, an oceanic basin (or ocean basin) is anywhere on Earth that is covered by seawater. Geologically, ocean basins are large  geologic basins that are below sea level. Most commonly the ocean is divided into basins fol ...
west of Fiji in the south-west Pacific Ocean. It is an actively spreading
back-arc basin A back-arc basin is a type of geologic basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most of ...
delimited by the Fiji islands to the east, the inactive
Vitiaz Trench Vityaz may refer to: * Bogatyr, a folk or epic hero in Slavic culture Aircraft and spacecraft *Vityaz, the call sign of spacecraft Soyuz TM-14 *Russky Vityaz, an aircraft built by Igor Sikorsky in 1913 Military * , a ''Bogatyr'' class screw cor ...
to the north, the Vanuatu/ New Hebrides island arc to the west, and the Hunter fracture zone to the south. Roughly triangular in shape with its apex located at the northern end of the New Hebrides Arc, the basin is actively spreading southward and is characterised by three spreading centres and an oceanic crust younger than 12 . The opening of the NFB began when a slab roll-back was initiated beneath the New Hebrides and the island arc started its clockwise rotation. The opening of the basin was the result of the collision between the
Ontong Java Plateau The Ontong Java Plateau (OJP) is a massive oceanic plateau located in the southwestern Pacific Ocean, north of the Solomon Islands. The OJP was formed around (Ma) with a much smaller volcanic event around 90 Ma. Two other southwestern Pacifi ...
and the Australian Plate along the now inactive Solomon–Vitiaz subduction system north of the NFB. The NFB is the largest and most developed back-arc basin of the south-west Pacific. It is opening in a complex geological setting between two oppositely verging subduction systems, the New Hebrides/Vanuatu and Tonga trenches and hence its ocean floor has the World's largest amount of spreading centres per area. Two opposite-facing systems of deformation partly overlap where the
Australian Australian(s) may refer to: Australia * Australia, a country * Australians, citizens of the Commonwealth of Australia ** European Australians ** Anglo-Celtic Australians, Australians descended principally from British colonists ** Aboriginal Au ...
and Pacific plates meet along a section of the andesite line in the south-west Pacific: east of the NFB the Kermadec-Tonga Arc stretches some north from New Zealand, and west of the NFB the
New Hebrides subduction zone The New Hebrides Trench is an oceanic trench which extends to over deep in the Southern Pacific Ocean. It lies to the northeast of New Caledonia and the Loyalty Islands, to the southwest of Vanuatu, east of Australia, and south of Papua New Gu ...
formed during the opening of the NFB back-arc basin. There are three microplates in the NFB: New Hebrides, Balmoral Reef, and
Conway Reef Conway Reef, known since 1976 by its Fijian name Ceva-I-Ra Reef (pronounced ), is a coral reef of the atoll type. It is at , and it is 2.5 km long in an east–west direction. In the middle of the reef there is a small sand cay, 1.8 meters ...
. Little was known about the NFB before 1985 and in the 1970s the central part of the basin, the only mapped area, was called the North Fiji Plateau.


Margins


Vanuatu

The New Hebrides central chain stretches from Ureparapara island,
Banks Islands The Banks Islands (in Bislama ''Bankis'') are a group of islands in northern Vanuatu. Together with the Torres Islands to their northwest, they make up the northernmost province of Torba. The island group lies about north of Maewo, and include ...
, 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 The d'Entrecasteaux () Ridge (DER) is a double oceanic ridge in the south-west Pacific Ocean, north of New Caledonia and west of Vanuatu Islands. It forms the northern extension of the New Caledonia–Loyalty Islands arc, and is now actively s ...
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 Ma, 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 The Lau Basin is a back-arc basin (also addressed as "interarc basin") at the Australian-Pacific plate boundary. It is formed by the Pacific plate subducting under the Australian plate. The Tonga-Kermadec Ridge, a frontal arc, and the Lau-Colville ...
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 The Louisville Ridge, also known as the Louisville Seamount Chain, is an underwater chain of over 70 seamounts located in the Southwest portion of the Pacific Ocean. As one of the longest seamount chains on Earth it stretches some Vanderkluysen, ...
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 Hunter Island and Matthew Island are two small and uninhabited high islands in the South Pacific, located east of New Caledonia and south-east of Vanuatu archipelago. Hunter Island and Matthew Island, apart, are claimed by Vanuatu as part of ...
, 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 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 an
ocean island basalt Ocean island basalt (OIB) is a volcanic rock, usually basaltic in composition, erupted in oceans away from tectonic plate boundaries. Although ocean island basaltic magma is mainly erupted as basalt lava, the basaltic magma is sometimes modified by ...
(OIB) mantle source.


Central 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 islands
Mitre The mitre (Commonwealth English) (; Greek: μίτρα, "headband" or "turban") or miter (American English; see spelling differences), is a type of headgear now known as the traditional, ceremonial headdress of bishops and certain abbots in ...
and
Anuta Anuta is a small high island in the southeastern part of the Solomon Islands province of Temotu, one of the smallest permanently inhabited Polynesian islands. It is one of the Polynesian Outlier communities in Melanesia. Geography The island ...
are rejuvenated Vitiaz arc volcanoes that formed 2.2 Ma, probably as a consequence of a change in the motion of the Pacific Plate.


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: 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 Ma


See also

*
Geology of the Pacific Ocean The Pacific Ocean evolved in the Mesozoic from the Panthalassic Ocean, which had formed when Rodinia rifted apart around 750  Ma. The first ocean floor which is part of the current Pacific Plate began 160 Ma to the west of the central ...
*
Samoa hotspot The Samoa hotspot is a volcanic hotspot located in the south Pacific Ocean. The hotspot model describes a hot upwelling plume of magma through the Earth's crust as an explanation of how volcanic islands are formed. The hotspot idea came from J. ...
* South Fiji Basin


References


Notes


Sources

* * * * * * * * * * * * * * * * * * * * {{Coord, -17, 173, display=title Back-arc basins Oceanic basins of the Pacific Ocean