The South Fiji Basin ( also Fiji Basin ) is a large deep oceanic basin in the south-west Pacific Ocean, south of

Fiji Fiji, officially the Republic of Fiji, is an island country in Melanesia, part of Oceania in the South Pacific Ocean. It lies about north-northeast of New Zealand. Fiji consists of an archipelago of more than 330 islands—of which about ...

. It was formed from the then Indo-Australian plate and is delimited to the north west by the

New Hebrides Trench The New Hebrides Trench (perhaps better termed the South New Hebrides Trench) is an oceanic trench which is over deep in the Southern Pacific Ocean. It lies to the northeast of New Caledonia and the Loyalty Islands, to the southwest of Vanuatu ...

, and the

Hunter fracture zone 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, wit ...

, to the west by the

Three Kings Ridge The Three Kings Ridge, sometimes known as Three Kings Rise and more rarely at its southern end as the Three Kings Bank is a deep sea westward facing volcanic arc of Zealandia continental crust extending from northwest of Cape Reinga / Te Rerenga ...

, to the east by the Lau-Colville Ridge, and to the south by the continental shelf of

Zealandia Zealandia (pronounced ), also known as (Māori language, Māori) or Tasmantis (from Tasman Sea), is an almost entirely submerged continent, submerged mass of continental crust in Oceania that subsided after breaking away from Gondwana 83� ...

Geology

Over the basement volcanics is deposited up to of sediment. The thicker sediments of the Kupe Abyssal Plain nearer New Zealand and land commenced to accumulate from the end of the

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

onwards and terminated in the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58 The oldest basalt dredged from the Minerva Abyssal Plain is 26.0 ± 1.0 but it is unknown if this is of basement or later volcanic origin. However drill site 285 in the Minerva Abyssal Plain also has basement volcanics that are dated to 26 Ma. The oldest other drill basement may be 31 Ma, but this has been questioned without the sample being re-examined. Basalts from the South Fiji Basin are little influenced by subduction, although they can have slight enrichment from a slab component, but come from variably enriched mantle, resulting from mixing between enriched mantle, like and depleted mantle, like depleted

MORB A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a div ...

mantle (DMM). To the north is the Minerva Abyssal Plain which has north west limits of the New Hebrides Trench and the Hunter fracture zone. Within this abyssal plain are several seamounts including to the north of the Cook fracture zone, Alison Seamount and Coquille Seamount beyond the end of the Three Kings Ridge. Beyond the Fantail Terrace to the north east of the Three Kings Ridge are a number of seamounts that separate the Minerva Abyssal Plain from the southern Kupe Abyssal Plain, in a Central Ridge structure with little overlying sediment. These include Julia, Marion, Matahourua and Mascarin seamounts. To the north east the Minerva Abyssal Plain is bounded by the Lau Terrace rather than the Lau Ridge to the east of the terrace. The Kupe Abyssal Plain is delimited to its west by the Three Kings Ridge and this western portion contains multiple seamounts including the Sarah group, Margot and Devonport group of seamounts. These tend to give younger dates and there is increasing evidence that the Kupe Abyssal Plain is

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

in age. The seafloor magnetic anomalies of the southern Kupe Abyssal Plain, unlike those further north, have been resolved and indicate NW–SE spreading.

Seamount Geology

Basalt from Julia Seamount was dated at 22.1±1.8 and Alison at 19.3±1.5 Ma. Matahourua, Mascarin, Marion, Coquille and one Margot seamount dedged sample are

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 ...

s. Marion

trachybasalt Trachybasalt is a volcanic rock with a composition between trachyte and basalt. It resembles basalt but has a high content of alkali metal oxides. Minerals in trachybasalt include alkali feldspar, calcic plagioclase, olivine, clinopyroxene and l ...

was dated at 16.2 ± 0.2 Ma and Matahourua basalt at 15 ± 2 Ma. These younger ages must be after spreading had ceased. The Devonport East and Central, Sarah North, Central and West, and one sample from Margot, are trachybasalts or trachyandesite consistent with back arc volcanism and are aged from 19.9 to 21 million years ago.

Seismology

The basin itself is inactive. However its eastern portions in the present

Australian plate The Australian plate is or was a major tectonic plate in the eastern and, largely, southern hemispheres. Originally a part of the ancient continent of Gondwana, Australia remained connected to India and Antarctica until approximately when Indi ...

are over the deeply subducted Pacific plate slab, so many deep ( more than ) and large (more than ) earthquakes have occurred in historic times. Other earthquake activity is confined to the northwestern margins of the basin.

Tectonics

The South Fiji Basin started forming in the

. The historic interpretation based on magnetic anomaly patterns was that spreading involved a triple junction in the Minerva Abyssal Plain. There has been some disagreement if the Kupe Plain Abyssal spreading was in a westward or eastward trend. This means that a full tectonic model consensus can not be reached. However the current best fit model is one of west-dipping Tonga–Kermadec–Hikurangi subduction zone rolling back clockwise, and an approximately east-dipping New Caledonia–Northland subduction zone that rolls back anticlockwise until about 21 million years ago. A maximum half spreading rate of about has been calculated. The Minerva spreading centres persisted into the Miocene and with the 23 Ma age of a basalt from the Cook fracture zone and other samples, have been interpreted to indicate the spreading centres have been active to this date. The northern Norfolk Basin has also been adjacent to this activity in the Cook fracture zone. The New Hebrides Trench has consumed a large area of crust of the South Fiji Basin since the Late Miocene. It is possible that Pacific plate subduction was followed by Early to Middle Miocene back-arc spreading in the southern South Fiji Basin with a pre-Miocene tectonic configuration that would have resembled the present

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-Colvill ...

and

Havre Trough The Havre Trough (Havre Basin) is a currently actively rifting back-arc basin about to wide, between the Australian Plate and Kermadec microplate. The trough extends northward from New Zealand's offshore Taupō Volcanic Zone commencing at Zea ...

. The former Kupe microplate is now fixed to the Australian plate. Presently the north western South Fiji Basin is probably not being subducted under the

New Hebrides plate The New Hebrides plate, sometimes called the Neo-Hebridean plate, is a minor tectonic plate (just larger than a microplate) located in the Pacific Ocean. While most of it is submerged as the sea bottom of the North Fiji Basin, the island count ...

at the New Hebrides Trench but rather the margin is a zone of transformation.

Ecology

The basin seawater has a temperature of about at depth. Fish species known include those from the genus

Pachycara ''Pachycara'' is a genus of marine ray-finned fishes belonging to the Family (biology), family Zoarcidae, the eelpouts. The fishes in this genus are found in the Atlantic Ocean, Atlantic, Indian Ocean, Indian, Southern Ocean, Southern and Pacific ...

(e.g. '' Pachycara moelleri''), large Synaphobranchid,

Halosaur Halosaurs are eel-shaped fishes found only at great ocean depths. As the family Halosauridae, halosaurs are one of two families within the order Notacanthiformes; the other being the deep-sea spiny eels, Notacanthidae. Halosaurs are thought to h ...

s ('' Aldrovandia affinis''),

cusk-eel The cusk-eel family, Ophidiidae, is a group of marine bony fishes in the Ophidiiformes order. The scientific name is from the Greek ''ophis'' meaning "snake", and refers to their eel-like appearance. True eels diverged from other ray-finned fi ...

s ('' Bassogigas'' spp.. '' Bassozetus'' spp., ''

Barathrites iris ''Barathrites iris'' is a species of fish in the family Ophidiidae The cusk-eel family, Ophidiidae, is a group of marine bony fishes in the Ophidiiformes order. The scientific name is from the Greek ''ophis'' meaning "snake", and refers t ...

''), and members of the

Zoarcidae The eelpouts are the ray-finned fish family (biology), family Zoarcidae. As the common name suggests, they are somewhat eel-like in appearance. All of the 300 species are ocean, marine and mostly bottom-dwelling, some at great depths. Eelpouts a ...

family. High-energy macrourids are absent at the limited sites sampled. Amongst crustaceans,

amphipods Amphipoda () is an order (biology), order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods () range in size from and are mostly detritivores or scavengers. There are more than 10,700 amphip ...

including members of the family

Lysianassidae Lysianassidae is a family (biology), family of marine Amphipoda, amphipods, containing the following genera: *''Acontiostoma'' Stebbing, 1888 *''Alibrotus'' Milne-Edwards, 1840 *''Allogaussia'' Schellenberg, 1926 *''Ambasia'' Boeck, 1871 *''Amba ...

, prawns of the genus Benthesicymus ('' Benthesicymus crenatus'', '' Benthesicymus howensis'') and

Aristeidae Aristeidae is a family of Dendrobranchiata decapod crustaceans known as deep-sea shrimps, gamba prawns or gamba shrimps. Some species are subject to commercial fisheries. Genera The following genera are classified under the Aristeidae: *'' Aris ...

('' Cerataspis monstrosus'') are found.

References