North Fiji Basin | |
---|---|
Stratigraphic range: | |
Lithology | |
Primary | mafic basalts |
Location | |
Coordinates | 17°S173°E / 17°S 173°E |
Region | South Pacific |
Country | Fiji |
Type section | |
Named for | Fiji |
The North Fiji Basin (NFB) is an oceanic basin west of Fiji in the south-west Pacific Ocean. It is an actively spreading back-arc basin delimited by the Fiji islands to the east, the inactive Vitiaz Trench to the north, the Vanuatu/New Hebrides island arc to the west, and the Hunter fracture zone to the south. [2] 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 Ma. 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. [1] The opening of the basin was the result of the collision between the Ontong Java Plateau and the Australian plate along the now inactive Solomon–Vitiaz subduction system north of the NFB. [3] 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. [3]
Two opposite-facing systems of deformation partly overlap where the Australian 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 3,000 km (1,900 mi) north from New Zealand, and west of the NFB the New Hebrides subduction zone formed during the opening of the NFB back-arc basin. [4]
There are three small tectonic plates in the NFB: New Hebrides, Balmoral Reef, and Conway Reef. [5]
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. [6]
The New Hebrides central chain stretches 1,200 km (750 mi) from Ureparapara island, Banks Islands, in the north to Hunter island in the south. [7] The New Hebrides trench retreats progressively which causes the southern end the subduction zone to bend eastward. [8] 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 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. [9]
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 10 cm/year (3.9 in/year). This seamount chain–trench intersection propagates southward at a rate of 12.8 cm/year (5.0 in/year) and, as a consequence, Tonga Islands rotate clockwise at a rate of 9.3°/Ma. [10]
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 c. 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. [11]
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. [12] 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. [13]
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 5–8 cm/year (2.0–3.1 in/year). In the northern NFB a series of spreading centres stretches 1,500 km (930 mi) along an east–west-trending belt (with spreading rates): the Futuna (1–4 cm/year (0.39–1.57 in/year) and North Cikobia (2 cm/year (0.79 in/year) spreading centres, and the Tripartite (2–5 cm/year (0.79–1.97 in/year), 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 (OIB) mantle source. [3]
The central spreading centre of the NFB is the largest and probably the oldest back-arc basin on Earth. [14] It can be divided into four 120–200 km (75–124 mi)-long segments:
The West Fiji area is dominated by a western and an eastern graben separated by a central plateau. The western graben, 10 km (6.2 mi)-wide and 4,000 km (2,500 mi)-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 2,000 m (6,600 ft) bsl, is flanked by another graben, 4 km (2.5 mi)-wide and 3,000 m (9,800 ft)-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, 3,500 m (11,500 ft)-deep, that is flanked by two pseudofaults c.500 m (1,600 ft)-high. The eastern graben, 10–12 km (6.2–7.5 mi)-wide and 3,200 m (10,500 ft)-deep, is flanked by parallel ridges and depressions over a 25 km (16 mi)-wide area. In the central plateau there is a fan-shaped system of ridges and depressions, the centre of which is occupied by a 3,000 m (9,800 ft)-deep and 10 km (6.2 mi)-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. [16]
The South Pandora and Tripartite Ridges in the northern NFB are active spreading ridges with 50–100 km (31–62 mi)-long segments, a 10–20 km (6.2–12.4 mi)-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. [17]
The South Pandora Ridge is divided into five segments averaging 20 km (12 mi) 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 c.100 km (62 mi); and continuous magnetic lineations indicating a very low half spreading rate (8 km (5.0 mi)/Ma) during the past 7 Ma. [17]
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. [17]
The inactive volcanic islands Mitre and Anuta are rejuvenated Vitiaz arc volcanoes that formed 2.2 Ma, probably as a consequence of a change in the motion of the Pacific plate. [18]
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 the Vitiaz Trench and the Lau–Colville, Three Kings, and Loyalty ridges. [19] Accordingly, the Loyalty-Three Kings Ridge once formed a single, continuous arc with the Lau-Colville Ridge which is called the Vitiaz arc. [20]
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 c. 12 Ma and has since the Late Miocene rotated the New Hebrides Arc 30° clockwise and Fiji at least 100° counter-clockwise. [4]
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. [21]
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 500 km (310 mi)c. 5 Ma. The earthquakes are the result of these colliding slabs settling on the 660 km discontinuity. [22]
Beneath Tonga at a depth of 350–500 km (220–310 mi) 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. [23] 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 300–350 km (190–220 mi) except at its southern end where it only reaches 150 km (93 mi). The north end of the slab, at the southern Rennell Trough, corresponds to the sharp bend in the andesite line. [24] 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. [25]
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 c. 8 Ma and most of the material is now located 450 km (280 mi) below the 660 km layer. [26]
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 [27]
Oceanic trenches are prominent, long, narrow topographic depressions of the ocean floor. They are typically 50 to 100 kilometers wide and 3 to 4 km below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about 50,000 km (31,000 mi) of oceanic trenches worldwide, mostly around the Pacific Ocean, but also in the eastern Indian Ocean and a few other locations. The greatest ocean depth measured is in the Challenger Deep of the Mariana Trench, at a depth of 10,994 m (36,070 ft) below sea level.
The Tonga Trench is an oceanic trench located in the southwestern Pacific Ocean. It is the deepest trench in the Southern hemisphere and the second deepest on Earth after the Mariana Trench. The fastest plate-tectonic velocity on Earth is occurring at this location, as the Pacific plate is being subducted westward in the trench.
Hunter Island and Matthew Island are two small and uninhabited volcanic islands in the South Pacific, located 300 kilometres (190 mi) east of New Caledonia and south-east of Vanuatu archipelago. Hunter Island and Matthew Island, 70 km (43 mi) apart, are claimed by Vanuatu as part of Tafea Province, and considered by the people of Aneityum part of their custom ownership, and as of 2007 were claimed by France as part of New Caledonia.
The Kermadec Trench is a linear ocean trench in the south Pacific Ocean. It stretches about 1,000 km (620 mi) from the Louisville Seamount Chain in the north (26°S) to the Hikurangi Plateau in the south (37°S), north-east of New Zealand's North Island. Together with the Tonga Trench to the north, it forms the 2,000 km (1,200 mi)-long, near-linear Kermadec-Tonga subduction system, which began to evolve in the Eocene when the Pacific Plate started to subduct beneath the Australian Plate. Convergence rates along this subduction system are among the fastest on Earth, 80 mm (3.1 in)/yr in the north and 45 mm (1.8 in)/yr in the south.
The Izu–Bonin–Mariana (IBM) arc system is a tectonic plate convergent boundary in Micronesia. The IBM arc system extends over 2800 km south from Tokyo, Japan, to beyond Guam, and includes the Izu Islands, the Bonin Islands, and the Mariana Islands; much more of the IBM arc system is submerged below sealevel. The IBM arc system lies along the eastern margin of the Philippine Sea Plate in the Western Pacific Ocean. It is the site of the deepest gash in Earth's solid surface, the Challenger Deep in the Mariana Trench.
The Kermadec–Tonga subduction zone is a convergent plate boundary that stretches from the North Island of New Zealand northward. The formation of the Kermadec and Tonga plates started about 4–5 million years ago. Today, the eastern boundary of the Tonga plate is one of the fastest subduction zones, with a rate up to 24 cm/year (9.4 in/year). The trench formed between the Kermadec–Tonga and Pacific plates is also home to the second deepest trench in the world, at about 10,800 m, as well as the longest chain of submerged volcanoes.
The New Hebrides plate, sometimes called the Neo-Hebridean plate, is a minor tectonic plate located in the Pacific Ocean. While most of it is submerged as the sea bottom of the North Fiji Basin, the island country of Vanuatu, with multiple arc volcanoes, is on the western edge of the plate. It is bounded on the south-west by the Australian plate, which is subducting below it at the New Hebrides Trench. The Vanuatu subduction zone is seismically active, producing many earthquakes of magnitude 7 or higher. To its north is the Pacific plate, north-east the Balmoral Reef plate and to its east the Conway Reef plate.
The Lau Basin is a back-arc 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 Ridge, a remnant arc, sit to the eastern and western sides of the basin, respectively. The basin has a raised transition area to the south where it joins the Havre Trough.
The 1999 Ambrym earthquake occurred on November 27 at 00:21:17 local time with a moment magnitude of 7.4 and a maximum Mercalli intensity of VII. The back arc thrust event occurred within the Vanuatu archipelago, just to the south of the volcanic island of Ambrym. Vanuatu, which was previously known as New Hebrides, is subject to volcanic and earthquake activity because it lies on an active and destructive plate boundary called the New Hebrides Subduction Zone. While the National Geophysical Data Center classified the total damage as moderate, a destructive local tsunami did result in some deaths, with at least five killed and up to 100 injured.
The d'EntrecasteauxRidge (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 subducting in the Vanuatu subduction zone under the Vanuatu/New Hebrides arc. The subduction of the DER is responsible for the anomalous morphology of the central part of New Hebrides arc whose movement more closely matches the north-east direction of the subducting Australian Plate.
The Tonga–Kermadec Ridge is an oceanic ridge in the south-west Pacific Ocean underlying the Tonga–Kermadec island arc. It is a result of the most linear, fastest converging, and seismically active subduction boundary on Earth, the Kermadec–Tonga subduction zone, and consequently has the highest density of submarine volcanoes.
The New Hebrides Trench is an oceanic trench which is over 7.1 km (4.4 mi) 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 Guinea and the Solomon Islands. The trench was formed as a result of a subduction zone. The Australian plate is being subducted under the New Hebrides plate causing volcanism which produced the Vanuatu archipelago.
The Hunter fracture zone is a sinistral (left-lateral) transform faulting fracture zone, that to its south is part of a triple junction with the New Hebrides Trench, and the North Fiji Basin Central Spreading Ridge. The Hunter fracture zone, with the Hunter Ridge, an area with recent volcanic activity to its north, is the southern boundary of the North Fiji Basin. This boundary area in the south-western part of the Hunter fracture zone is associated with hot subduction, and a unique range of volcanic geochemistry.
The Lau–Colville Ridge is an extinct oceanic ridge located on the oceanic Australian Plate in the south-west Pacific Ocean extending about 2,700 km (1,700 mi) from the south east of Fiji to the continental shelf margin of the North Island of New Zealand. It was an historic subduction boundary between the Australian Plate and the Pacific Plate and has important tectonic relationships to its east where very active spreading and subduction processes exist today. It is now the inactive part of an eastward-migrating, 100 million year old Lau-Tonga-Havre-Kermadec arc/back-arc system or complex and is important in understanding submarine arc volcanism because of these relationships. To its west is the South Fiji Basin whose northern bedrock is Oligocene in origin.
The Havre Trough is a currently actively rifting back-arc basin about 100 km (62 mi) to 120 km (75 mi) wide, between the Australian Plate and Kermadec microplate. The trough extends northward from New Zealand's offshore Taupō Volcanic Zone commencing at Zealandia's continental shelf margin and continuing as a tectonic feature, as the Lau Basin which currently contains active seafloor spreading centers. Its eastern margin is defined by the Kermadec Ridge created by Pacific Plate subduction under the Kermadec microplate, while the western margin is the remnant Lau-Colville Ridge.
The Vityaz Trench is an oceanic trench tectonic feature of the South West Pacific Ocean floor.
The South Fiji Basin is a large 4 to 4.7 km deep oceanic basin in the south-west Pacific Ocean, south of Fiji. It was formed from the then Indo-Australian plate and is delimited to the north west by the New Hebrides Trench, and the Hunter fracture zone, to the west by the Three Kings Ridge, to the east by the Lau-Colville Ridge, and to the south by the continental shelf of Zealandia.
The Vanuatu subduction zone is currently one of the most active subduction zones on Earth, producing great earthquakes, with potential for tsunami hazard to all coastlines of the Pacific Ocean. There are active volcanoes associated with arc volcanism.
The Hunter Ridge, is an active volcanic arc oceanic ridge located on the oceanic New Hebrides plate in the south-west Pacific Ocean extending at least 550 km (340 mi). It defines the south-western limit of the North Fiji Basin (NFB) and is an area of unique range in volcanic geochemistry, which transpires to have been due partially to a new, previously unrecognised, subduction zone.
The Norfolk Basin, which has been subdivided into the North Norfolk Basin and South Norfolk Basin, is an ocean floor sedimentary basin between the Norfolk Ridge to the east and the Three Kings Ridge to the west, on the edge of the submerged continent of Zealandia. The northern boundary is the Cook fracture zone and the southern is the Regina ridge projecting from Northland Peninsula, New Zealand. While it has back-arc basin characteristics its formation and structure are not able to be explained by historic back-arc basin theory.