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North Fiji Basin

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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. 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. 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. 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.

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81-670: 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. There are three small tectonic plates in

162-692: A heading of 80° (slightly north of due east, at the Amsterdam transform fault to the south western side of Australian plate), 7 cm (2.8 in) per year with heading 120° (southeast) and 6.6 cm (2.6 in) per year near the Macquarie triple junction which is the south eastern side of the Australian plate. The Capricorn plate at the western side of the Australian plate is moving at 1.9 mm (0.075 in) per year ± 0.5 mm (0.020 in) per year with heading 45° (northwest) relative to

243-726: A bend up towards the north-east via the transform faults of the Hunter Fracture Zone to Fiji . The Australian plate interacts at the southern and south-eastern border of the North Fiji Basin with the microplates of the New Hebrides already mentioned, as well as with the Conway Reef plate and the Balmoral Reef plates . To the west of Fiji the Australian plate interacts in the spreading centre of

324-605: A high U/ Pb ratio. Over time, as U decays to Pb, HIMU Earth materials develop particularly radiogenic (high) Pb/ Pb. If an Earth material has elevated U/ Pb (HIMU), then it will also have elevated U/ Pb, and therefore will produce radiogenic Pb compositions for both the Pb/ Pb and Pb/ Pb isotopic systems ( U decays Pb, U decays to Pb). Similarly, Earth materials with high U/Pb also tend to have high Th/Pb, and thus evolve to have high Pb/ Pb ( Th decays to Pb). Ocean island basalts with highly radiogenic Pb/ Pb, Pb/ Pb, Pb/ Pb are

405-524: 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. In the Lau Basin east of

486-602: A long half-life (i.e., longer than 704 million years) decays to a “radiogenic” daughter isotope. Changes in the parent/daughter ratio by, for example, mantle melting, result in changes in the radiogenic isotopic ratios. Thus, these radiogenic isotopic systems are sensitive to the timing, and degree, of parent/daughter the changed (or fractionated) parent daughter ratio, which then informs the process(es) responsible for generating observed radiogenic isotopic heterogeneity in ocean island basalts. In mantle geochemistry, any composition with relatively low Sr/ Sr, and high Nd/ Nd and Hf/ Hf,

567-504: A mid-ocean ridge, and Samoa , which is located near a subduction zone. In the ocean basins, ocean island basalts form seamounts , and in some cases, enough material is erupted that the rock protrudes from the ocean and forms an island, like at Hawaii , Samoa, and Iceland. Over time, however, thermal subsidence and mass loss via subaerial erosion causes islands to become completely submarine seamounts or guyots . Many ocean island basalts erupt at volcanic hotspots , which are thought to be

648-511: A part of the ancient continent of Gondwana , Australia remained connected to India and Antarctica until approximately 100  million years ago when India broke away and began moving north. Australia and Antarctica had begun rifting by 96  million years ago and completely separated a while after this, some believing as recently as 45  million years ago , but most accepting presently that this had occurred by 60  million years ago . The Australian plate later fused with

729-468: A rate of 9.3°/ Ma . 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

810-600: A spreading ridge that was active before the Vanuatu Trench propagated south of the southern end of Vanuatu, Anatom Island . 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

891-434: Is a referred to as “geochemically depleted”. High Sr/ Sr, and low Nd/ Nd and Hf/ Hf, is referred to as “geochemically enriched”. Relatively low isotopic ratios of Pb in mantle-derived rocks are described as unradiogenic ; relatively high ratios are described as radiogenic . These isotopic systems have provided evidence for a heterogenous lower mantle. There are several distinct “mantle domains” or endmembers that appear in

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972-635: Is about half that of the collision with the Sunda plate, but this would not explain some of the largest and most destructive recent earthquakes and eruptions on the face of the planet. There is oblique convergence of what are now the Pacific and Australian plates at about 11 cm/year (4.3 in/year) near eastern Papua New Guinea. This has resulted in shear complexities, resolved by the formation of multiple microplates and convergence velocity that varies between 2–48 cm/year (0.79–18.90 in/year) where

1053-493: Is being constantly produced within the Earth via alpha decay (of U, Th, and Sm), but He is not being generated in appreciable quantities in the deep Earth, the ratio of He to He is decreasing in the interior of the Earth over time. The early Solar System began with high He/ He and therefore the Earth first accreted with high He/ He. Thus, in plume-derived lavas, high He/ He is an “ancient” geochemical signature that indicates

1134-499: 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

1215-761: Is not parallel to the biogeographical Wallace line that separates the indigenous fauna of Asia from that of Australasia . The eastern islands of Indonesia lie mainly on the Eurasian plate , but have Australasian-related fauna and flora. Southeasterly lies the Sunda Shelf . To the east of Indonesia there appears to be under the Indian Ocean a deformation zone between the Indian and Australian plates with both earthquake and global satellite navigation system data indicating that India and Australia are not moving on

1296-494: Is now possible to classify usefully and more conveniently on high field strength trace elements alone such as barium (Ba), caesium (Ce), rubidium (Rb), niobium (Nb) and terbium (Tb is chosen as proportion about constant in all IOB).  : The geochemistry of ocean island basalts is useful for studying the chemical and physical structure of Earth's mantle. Some mantle plumes that feed hotspot volcanism lavas are thought to originate as deep as

1377-499: Is older than 2.3 Ga, formed prior to the Great Oxidation Event and has resurfaced via mantle plume volcanism. Noble gas isotopic systems, such as He/ He, Ne/ Ne, and Xe/ Xe, have been used to demonstrate that parts of the lower mantle are relatively less degassed and have not been homogenized despite billions of years of mantle convective mixing. Some large, hot mantle plumes have anomalously high He/ He. Since He

1458-428: 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

1539-853: The Alpine Fault . South of New Zealand the boundary becomes a transitional transform-convergent boundary, the Macquarie Fault Zone , where the Australian plate is beginning to subduct under the Pacific plate along the Puysegur Trench . Extending southwest of this trench is the Macquarie Ridge . The southerly side is a divergent boundary with the Antarctic plate called the Southeast Indian Ridge (SEIR). The subducting boundary through Indonesia

1620-592: The Antarctic plate , the African plate and the Indian plate . It is however known from movement studies that this definition of the Australian plate is 20% less accurate than one that assumes independently moving Capricorn , and Macquarie microplates. The northeasterly side is a complex but generally convergent boundary with the Pacific plate . The Pacific plate is subducting under the Australian plate, which forms

1701-900: The Ediacaran (South African Kuunga Orogeny ). As a separate plate, the Australian plate came into being on the breakup of Gondwana with final separation from what is now the Antarctic plate and Zealandia starting in the Early Cretaceous between about 132  million years ago and finishing in the Cenomanian at about 96  million years ago . The separation continued with various authors modelling full separation time based on sea levels and/or biological separation. A currently widely used reference model for plate movement has total separation of Tasmania by 60 million years ago although some had argued historically that this

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1782-642: The Lau Basin with the Niuafo'ou plate and the clockwise rotating Tonga plate under which the Pacific plate is subducting in the Kermadec-Tonga subduction zone . The back arc spreading in the Lau Basin continues almost due south in the line of interaction between the Australian and Tonga plates to the Kermadec plate and on to New Zealand where direct interaction resumes with the Pacific plate south of

1863-532: The New Hebrides plate . As we go south the convergence rate falls from 17 cm/year (6.7 in/year) north of the Torres Islands to 4 cm/year (1.6 in/year) in the central section of the trench, to rise again to 12 cm/year (4.7 in/year) in the south. Very active spreading then resumes in the North Fiji Basin where the edge of the Australian plate makes a transition in

1944-832: The Solomon Sea plate subducts under the South Bismarck plate and Pacific plate at the New Britain subduction zone . To the south of this there is sea floor spreading between the Australian plate and the Woodlark plate in the Woodlark Basin while the subduction of the oceanic crust of the Australian plate occurs to the south east in the New Hebrides Trench of the Vanuatu subduction zone under

2025-532: The Sunda plate (Sundaland plate, previously classified as part of Eurasian plate ) has a maximum convergence velocity of 7.3 cm (2.9 in) per year ± 0.8 cm (0.31 in) per year at the Java Trench decreasing to 6.0 cm (2.4 in) ± 0.04 cm (0.016 in) per year at the southern Sumatra Trench . The eastern collision with the Pacific plate has increasing displacement rates towards

2106-529: The Taupō Volcanic Zone and such direct interaction continues into the Macquarie Fault Zone to the south of New Zealand. There is up to 9.6 cm (3.8 in) per year motion accommodated with complex rotational components in the collision dynamics between the north eastern Australian plate and the rotating Tonga plate , the long thin Kermadec plate and the south western aspects of the Pacific plate. The Pacific plate east to west convergence rates along

2187-476: The Tonga and Kermadec Trenches , and the parallel Tonga and Kermadec island arcs . It has also uplifted the eastern parts of New Zealand's North Island . The continent of Zealandia , which separated from Australia 85  million years ago and stretches from New Caledonia in the north to New Zealand's subantarctic islands in the south, is now being torn apart along the transform boundary marked by

2268-423: The core–mantle boundary (~2900 km deep). The composition of the ocean island basalts at hotspots provides a window into the composition of mantle domains in the plume conduit that melted to yield the basalts, thus providing clues as to how and when different reservoirs in the mantle formed. Early conceptual models for the geochemical structure of the mantle argued that the mantle was split into two reservoirs:

2349-571: The Australia plate. 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 igneous differentiation to produce a range of other volcanic rock types, for example, rhyolite in Iceland , and phonolite and trachyte at

2430-468: The Australian plate that the latest tectonic models suggest is still independent from when it first achieved independent rotation to the then Indo-Australian plate several million years ago, the Macquarie microplate. Data from the 11,800 km (7,300 mi) long Southeast Indian Ridge only became available after about 1985 and this gives a fairly consistent spreading rate between the Antarctic and Australian plates of 6 cm (2.4 in) per year at

2511-399: The Earth's building blocks) compositions. There are two geochemically enriched domains, named enriched mantle 1 (EM1), and enriched mantle 2 (EM2). Though broadly similar, there are some important distinctions between EM1 and EM2. EM1 has unradiogenic Pb/ Pb, moderately high Sr/ Sr, and extends to lower Nd/ Nd and Hf/ Hf than EM2. Pitcairn , Kerguelen - Heard , and Tristan - Gough are

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2592-620: The Mount Barren Group on the southern margin of the Yilgarn Craton and zircon provenance analysis support the hypothesis that collisions between the Pilbara – Yilgarn and Yilgarn – Gawler Cratons assembled a proto-Australian continent approximately 1,696  million years ago (Dawson et al. 2002). Australia and East Antarctica were merged with Gondwana between 570 and 530  million years ago starting in

2673-468: 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. There are two main spreading systems in

2754-416: 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 . Beneath Tonga at a depth of 350–500 km (220–310 mi) the number of earthquakes increases dramatically while

2835-592: 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

2916-568: The NFB: New Hebrides , Balmoral Reef , and Conway Reef . 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. 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. The New Hebrides trench retreats progressively which causes

2997-493: 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. The slab beneath Tonga and Kermadec penetrates into

3078-743: 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. Today the Pacific Plate is subducting westward along

3159-653: The Southeast Indian Ridge between the Australian plate and the proposed Capricorn plate . It is known that the Eastern Pilbara Craton within present day Western Australia , contains some of the oldest surface rocks on earth being pristine crust up to 3.8 billion years ago. Accordingly, the Pilbara Craton continues to be studied for clues as to the commencement and subsequent course of plate tectonics . Depositional age of

3240-622: 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. 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 120–200 km (75–124 mi)-long segments: The West Fiji area

3321-624: 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

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3402-463: The adjacent Indian plate beneath the Indian Ocean to form a single Indo-Australian plate . However, recent studies suggest that the two plates have once again split apart and have been separate plates for at least 3 million years and likely longer. The Australian plate includes the continent of Australia , including Tasmania , as well as portions of New Guinea , New Zealand and the Indian Ocean basin. The continental crust of this plate covers

3483-455: 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

3564-691: 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

3645-448: 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. The South Pandora and Tripartite Ridges in

3726-687: 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. Large magnitude earthquakes beneath

3807-428: The existence of a well-preserved helium reservoir in the deep mantle. The timing of the formation of this reservoir is constrained by observed anomalies of Xe/ Xe in ocean islands basalts, because Xe was only produced by decay of I during the first ~100 My of Earth's history. Together, high He/ He and Xe/ Xe indicate a relatively less degassed, primitive noble gas domain that has been relatively well preserved since

3888-410: The heavy radiogenic isotopic ratio of a melt, which upwells and becomes a volcanic rock on the surface of the Earth, reflects the isotopic ratio of the mantle source at the time of melting. The best studied heavy radiogenic isotope systems in ocean island basalts are Sr/ Sr, Nd/ Nd, Pb/ Pb, Pb/ Pb, Pb/ Pb, Hf/ Hf and, more recently, Os/ Os. In each of these systems, a radioactive parent isotope with

3969-419: The intraplate volcano Fernando de Noronha . Unlike mid-ocean ridge basalts (MORBs), which erupt at spreading centers ( divergent plate boundaries ), and volcanic arc lavas, which erupt at subduction zones ( convergent plate boundaries ), ocean island basalts are the result of intraplate volcanism . However, some ocean island basalt locations coincide with plate boundaries like Iceland, which sits on top of

4050-455: 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 Australian Plate The Australian plate is a major tectonic plate in the eastern and, largely, southern hemispheres. Originally

4131-464: The main component is ancient recycled basaltic oceanic crust which has inherited the trace element and isotopic signatures of a subduction zone dehydration process, with enrichment in high field strength elements. These mantle sources are inferred from differences in radiogenic isotope ratios that magmas inherit from their source rock. Sources have been defined from a combined analysis of strontium (Sr), neodymium (Nd) and lead (Pb) isotopes but it

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4212-402: The middle of India and Australia landmasses, with Australia as the point of reference, presently Australia is moving northward at 3 cm (1.2 in) per year with respect to India consistent with a zone of deformation between the two plates as commented upon earlier. This zone of deformation may actually presently involve some of India. The northwards collision of the Australian plate with

4293-405: The north from a low of less than 0.2 cm (0.079 in) per year at the southern end of the Macquarie Fault Zone , where there is the major plate triple junction with the Pacific and Antarctic plates . Due to vector complexities at the north eastern end of this collision, which includes several spreading centres, it is perhaps simplest to state that the average displacement rate to the north

4374-416: 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. The South Pandora Ridge is divided into five segments averaging 20 km (12 mi) in width. The axial valley

4455-435: The ocean island basalt record. When plotted in multi-isotope space, ocean island basalts tend to form arrays trending from a central composition out to an endmember with an extreme composition. The depleted mantle, or DM, is one endmember, and is defined by low Sr/ Sr, Pb/ Pb, Pb/ Pb, Pb/ Pb, and high Nd/ Nd and Hf/ Hf. The DM is therefore geochemically depleted, and relatively unradiogenic. Mid-ocean ridges passively sample

4536-407: 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 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. 100–45 Ma, after

4617-464: The product of mantle plumes. There are thousands of seamounts that are not clearly associated with upwelling mantle plumes, and there are chains of seamounts that are not age progressive. Seamounts that are not clearly linked to a mantle plume indicate that regional mantle composition and tectonic activity may also play important roles in producing intraplate volcanism. There are various sources identified for ocean island basalt magma in Earth's mantle but

4698-698: The products of HIMU mantle domains. St. Helena , and several islands in the Cook - Austral volcanic lineament (e.g., Mangaia ) are the type localities for HIMU ocean island basalts. The final mantle domain discussed here is the common composition that ocean island basalts trend toward in radiogenic isotopic multi-space. This is also most prevalent mantle source in ocean island basalts, and has intermediate to geochemically depleted Sr/ Sr, Nd/ Nd, and Hf/ Hf, as well as intermediate Pb/ Pb, Pb/ Pb, Pb/ Pb.  This central mantle domain has several names, each with slightly different implications. PREMA, or “Prevalent Mantle”

4779-470: The same physical mantle source, as evidenced by their slightly distinct arrays in radiogenic isotopic multi-space. Thus, hotspots that are categorized as “EM1”, “EM2”, “HIMU”, or “FOZO”, may each sample physically distinct, but compositionally similar, portions of the mantle. Furthermore, some hotspot chains host lavas with wide range of isotopic compositions so that the plume source seems to either sample multiple domains which can be sampled at different times in

4860-546: The same physical reservoir in the deep mantle. Instead, mantle domains with similar radiogenic isotopic compositions sampled at different hotspot localities are thought to share similar geologic histories. For example, the EM2 hotspots of Samoa and Society are both thought to have a mantle source that contains recycled upper continental crust, an idea that is supported by stable isotope observations, including δ O and δ Li. The isotopic similarities do not imply that Samoa and Society have

4941-407: The same vectors northward and have started a process of again separating. This zone is along the northern Ninety East Ridge which implies this area presently is weaker tectonically than the area where originally the Indian and Australian plates merged which is believed to have been further to the north west. There is also deformation in an approximately 1,200 km (750 mi) zone north of

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5022-564: 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 300–350 km (190–220 mi) except at its southern end where it only reaches 150 km (93 mi). The north end of

5103-411: 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

5184-638: The south starts subducting under the Pacific plate at a rate of 3.6 cm/year (1.4 in/year) at the Puysegur Trench , which ends in the south as a long series of transform faults between the two plates called the Macquarie Ridge Complex, commencing with the McDougall Fault Zone and ending with the Macquarie Fault Zone. The south western portion of the zone has the Pacific plate interacting with an area of

5265-419: 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

5346-424: 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

5427-466: The subduction systems with the Kermadec plate, which are perhaps simpler to state, are among the fastest on Earth, being 8 cm (3.1 in) per year in the north and 4.5 cm (1.8 in) per year in the south. At the central Alpine Fault in New Zealand the subduction component of the Pacific plate moving westward is about 3.9 cm (1.5 in) per year. The Australian plate then to

5508-561: The surface expressions of melting of thermally buoyant, rising conduits of hot rock in the Earth's mantle , called mantle plumes . Some such hotspot volcanic chains are believed to have started with the formation of large igneous provinces . Mantle plume conduits may drift slowly, but Earth's tectonic plates drift more rapidly relative to mantle plumes. As a result, the relative motion of Earth's tectonic plates over mantle plumes produces age-progressive chains of volcanic islands and seamounts with

5589-405: The surface in buoyantly rising mantle plumes. Sulfur isotopic analyses have shown mass-independent-fractionation (MIF) in the sulfur isotopes in some plume-derived lavas. MIF of sulfur isotopes is a phenomenon that occurred in Earth's atmosphere only before the Great Oxidation Event ~2.3 Ga. The presence of recycled material with MIF signatures indicates that some of the recycled material brought

5670-405: 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

5751-402: The type localities of EM1.  EM2 is defined by higher Sr/ Sr than EM1, and higher Nd/ Nd and Hf/ Hf at a given Sr/ Sr value, and intermediate Pb/ Pb. Samoa and Society are the archetypal EM2 localities. Another distinct mantle domain is the HIMU mantle. In isotope geochemistry, the Greek letter μ (or mu) is used to describe the U/ Pb, such that ‘high μ’ (abbreviated HIMU) describes

5832-532: The upper mantle and MORBs are typically geochemically depleted, and therefore it is widely accepted that the upper mantle is composed mostly of depleted mantle. Thus, the term depleted MORB mantle (DMM) is often used to describe the upper mantle that sources mid-ocean ridge volcanism. Ocean island basalts also sample geochemically depleted mantle domains. In fact, most ocean island basalts are geochemically depleted, and <10% of ocean island basalts have lavas that extend to geochemically enriched (i.e., Nd/ Nd lower than

5913-497: The upper mantle and entering the lower mantle, which indicates that the lower mantle cannot be isolated. Additionally, the isotopic heterogeneity observed in plume-derived ocean island basalts argues against a homogenous lower mantle. Heavy, radiogenic isotopes are a particularly useful tool for studying the composition of mantle sources because isotopic ratios are not sensitive to mantle melting. According tradition subclassification used Sr-Nd-Pb-Hf-He isotopic ratios. This means that

5994-508: The upper mantle and the lower mantle. The upper mantle was thought to be geochemically depleted due to melt extraction which formed Earth's continents. The lower mantle was thought to be homogenous and “ primitive ”. (Primitive, in this case, refers to silicate material that represents the building blocks of the planet that has not been modified by melt extraction, or mixed with subducted materials, since Earth's accretion and core formation.) Seismic tomography showed subducted slabs passing through

6075-405: The volcanic evolution of a hotspot. Isotopic systems help to deconvolve the geologic processes that contributed to, and in some cases the timing of, the formation of these mantle domains. Some important examples include the presence of crustal fingerprints in enriched mantle sources that indicate that material from Earth's continents and oceans can be subducted into the mantle and brought back up to

6156-749: The whole of Australia, the Gulf of Carpentaria , southern New Guinea , the Arafura Sea , the Coral Sea . The continental crust also includes northwestern New Zealand , New Caledonia and Fiji . The oceanic crust includes the southeast Indian Ocean , the Tasman Sea , and the Timor Sea . The Australian plate is bordered (clockwise) by the Eurasian plate , the Philippine plate , the Pacific plate ,

6237-451: The work involved in determining these plate vectors involves assurance that the points of reference are representative of the plates they are on, as distortion will be likely in areas of tectonic activity. Further assumptions such as there are only 8 plates were made in earlier modelling when as many as 52 may exist, with independent movement, although fair accuracy for larger plate movement can be obtained if only 25 are modelled. In terms of

6318-469: The youngest, active volcanoes located above the axis of the mantle plume while older, inactive volcanoes are located progressively farther away from the plume conduit ( see Figure 1 ). Hotspot chains can record tens of millions of years of continuous volcanic history; for example, the oldest seamounts in the Hawaiian–Emperor seamount chain are over 80 million years old. Not all ocean island basalts are

6399-425: The “Primitive Helium Mantle”, or PHEM. Finally, Hanan and Graham (1996) used the term “C” (for common component) to describe a common mixing component in mantle derived rocks. The presence of a particular mantle domain in ocean island basalts from two hotspots, signaled by a particular radiogenic isotopic composition, does not necessarily indicate that mantle plumes with similar isotopic compositions are sourced from

6480-440: Was as recent as 45 million years ago. The Australian plate, which Australia is on, is moving faster than other plates. The Australian plate is moving about 6.9 cm (2.7 inches) a year in a northward direction and with a small clockwise rotation. The Global Positioning System must be updated due to the movement, as some locations move faster. Technically movement is a vector and requires to be related to something. Much of

6561-414: Was the first term coined by Zindler and Hart (1986) to describe the most common composition sampled by ocean island basalts. Hart et al. (1992) later named the location of the intersection of ocean island basalt compositions in radiogenic isotopic multi-space as the “Focus Zone”, or FOZO. Farley et al. (1992) in the same year described a high He/ He (a primitive geochemical signature) component in plumes as

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