The Hikurangi Trough (previously known as the Hikurangi Trench) is a sea floor feature of the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...

off the north-east

South Island The South Island ( , 'the waters of Pounamu, Greenstone') is the largest of the three major islands of New Zealand by surface area, the others being the smaller but more populous North Island and Stewart Island. It is bordered to the north by ...

and the east coast of the

North Island The North Island ( , 'the fish of Māui', historically New Ulster) is one of the two main islands of New Zealand, islands of New Zealand, separated from the larger but less populous South Island by Cook Strait. With an area of , it is the List ...

of New Zealand. It has been forming for about 25 million years and is

turbidite A turbidite is the geologic Deposition (geology), deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing ...

-filled, particularly in its south. This characteristic can be used to distinguish it from the sediment-poor and deeper Kermadec Trench, which is its continuation to the north. Sediment currently passing through the trough represents about 0.5% of the total sediment input to the world oceans. The trough has deep-sea chemosynthetic ecosystems that are unique.

Geography

Although shallower than the trenches north of it, the Hikurangi Trough reaches depths of as close as from shore. The southern trough structure is wide off the coast of northern

Canterbury Canterbury (, ) is a City status in the United Kingdom, city and UNESCO World Heritage Site, in the county of Kent, England; it was a county borough until 1974. It lies on the River Stour, Kent, River Stour. The city has a mild oceanic climat ...

with an initial local depth of about , and towards its northern portions it has structures more like those found in oceanic trenches. The trough widens about the latitude of Cook Strait to between up to the Māhia Peninsula. There are various mappings of its northern limits as the Ruatōria debris avalanche was a poorly understood structure. The limits of the trough are not defined by standard geographical works and various geological works have different mappings. A meandering depression within the trough like structure, the Hikurangi Channel (also known as Hikurangi Sea Channel or Hikurangi Seachannel), commences in the south and trends to the eastern side of the trough. By the latitude of Cook Strait the channel is about below the general level of the trough. The seafloor Hikurangi Channel continues along its eastern side, until at the Māhia Peninsula it breaks off towards the east through the Hikurangi Plateau and discharges into the south-west Pacific abyssal plains. The channel has a total length between , but only the initial portion is in the trough. The trough to the south has related structures such as the undersea Pūkākī, Okains and Pegasus canyons at the latitude of Pegasus Bay, the Hurunui Canyon which is beyond the Hurunui River mouth, and the sudden coastal shelving to just south-east of

Kaikōura Kaikōura (; ) is a town on the east coast of the South Island of New Zealand, located on New Zealand State Highway 1, State Highway 1, north of Christchurch. The town has an estimated permanent resident population of as of . Kaikōura is th ...

of the Kaikōura Canyon, where the sea floor drops to the depth of the trough. Extending from the hilly coast north of Kaikōura the undersea Kowhai canyons are to the west of the trough. Towards the north eastern part of Cook Strait is the Cook Strait Canyon with part of the trough being deep. Off Cape Palliser, a part of the trough is deep, less than the Hikurangi Plateau's on the eastern side of the trough at this latitude. The Hikurangi Plateau extends east of the trough between the Chatham Rise to the south and a point north of New Zealand's East Cape. On the east coast of the North Island between the Pāhaoa and Māhia canyons there is not direct drainage from the coast into the trough as the seafloor has a number of intermediate ridges and basins. In terms of being a definite trench with two sides, this is found on the sea floor north of the Māhia Peninsula but is broken by the Tuaheni submarine landslide at about 38°50'S, and the submarine Ruatōria debris avalanche at about 38°S. Beyond the Ruatōria debris avalanche the Hikurangi Margin joins the Ruatōria Scarp and then drops away from the undersea East Cape Ridge and commences trench–like structures to depths of assigned by many authorities to the Kermadec Trench which further north has a distinct transition to a much greater depth.

Geology

The Hikurangi Trough is sediment filled as a result of being a key part of the eastern New Zealand oceanic sedimentary system for several million years. The present North Island subduction and accretion that began in the middle and late

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 ...

, caused thick sedimentary sequences to form in the then trench with enhanced abyssal erosion in the late

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 ...

. More recently it has sediment from the erosion of the uplifting mountains of the South Island of New Zealand such as the Southern Alps, which formed from 6.4 million years ago. This system currently contributes about 0.5% of the total sediment input to the world oceans. The sediments in the trench are up to deep in the south and where they exit the Kaikōura Canyon have acoustic characteristics of gravel

s. The turbidites thin to about in the north. Gas hydrates have been identified in the sediments and there are widespread

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

seeps. Radiodating analysis of the carbonate rocks formed at such seeps show that the carbonate formation has been going on for periods between 2,360±70 years BP to 12,400±160 years BP. The active turbidite channel represented by the Hikurangi Channel, guides turbidity currents into the path of the Pacific deep western boundary current to the north. The Hikurangi Channel is known to be less than 3.5 million years old. Initially the channel developed during the late

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.582016 Kaikōura earthquake precipitated submarine mudslides and sediment flows that displaced about of sediment into the trench from the Kaikōura Canyon, with a turbidity current that travelled more than along the Hikurangi Channel. The furtherest marine core sampled in the channel so far revealed more than of fresh sediment. The full analysis is nuanced, with flows from ten turbide triggering catchments but also flows that may be due to submarine land sliding from shaking associated with ground-motion amplitude peaks that produce failure in muddy sediments. The catchment of the Cook Strait Canyon was a large contributor to the turbidite deposits from the earthquake.

Tectonics

The Hikurangi Margin subduction zone is where the thick oceanic Hikurangi Plateau has been subducting for about 25 million years beneath

continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as '' continental shelves''. This layer is sometimes called '' si ...

of the old Indo-Australian plate possibly without an intermediate continental crust microplate, although there are rotation features. The relative motion at the north is at East Cape and is down to at Cape Turnagain with regard to convergence. The net vector is to with a vector direction of 266° and in a direction of 259° at the Canterbury end. This is split into convergence near the trench, strike-slip motion around the top of the forearc ridge, and extension in the Taupō Rift. There is not a continuous trench with two sides at the margin and some complexity in the trench or trough like structures is due to the complex transitional tectonics and old subducting seamounts. By contrast, the Kermadec and

Tonga trench 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 occurri ...

es represent the parts of the Kermadec-Tonga subduction zone where the

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramaf ...

of the Pacific plate is subducting beneath oceanic crust of the Kermadec and

Tonga Tonga, officially the Kingdom of Tonga, is an island country in Polynesia, part of Oceania. The country has 171 islands, of which 45 are inhabited. Its total surface area is about , scattered over in the southern Pacific Ocean. accordin ...

microplates which also abut oceanic crust of the current Australian plate.

Ecology

Because of the delivery of waters by the Lower Circumpolar Deep Waters and current and coastal drainage and surface currents such as the East Cape Current a rich marine ecosystem exists in the coastal waters over the trench. The trench ecosystem itself has been much less studied as it is even deeper than the economically important deep marine fisheries. The middle and southern Hikurangi Margin includes fisheries for hoki, alfonsino and orange roughy. The far south area of the trough near the Kaikōura Canyon, the Subtropical Front forms a convergence zone between the subtropical and subantarctic water masses represented by the Southland Current down to about . This region of ocean has been determined to be one of the most productive marine ecosystems on the planet. At this end of the trench, many deep water species are found close to the shore. This food source attracts the

whales Whales are a widely distributed and diverse group of fully Aquatic animal, aquatic placental mammal, placental marine mammals. As an informal and Colloquialism, colloquial grouping, they correspond to large members of the infraorder Cetacea ...

for which the town of

is famous. The 2016 Kaikōura earthquake profoundly disturbed this ecosystem, but there is evidence of recovery. In one study, biological sampling of the walls of the trough took place at six sites between deep. Compared to similar sampling in the more northern

Bay of Plenty The Bay of Plenty () is a large bight (geography), bight along the northern coast of New Zealand's North Island. It stretches from the Coromandel Peninsula in the west to Cape Runaway in the east. Called ''Te Moana-a-Toitehuatahi'' (the Ocean ...

there were higher abundances in the Hikurangi Margin of many

crustaceans Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of Arthropod, arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquat ...

and

worm Worms are many different distantly related bilateria, bilateral animals that typically have a long cylindrical tube-like body, no limb (anatomy), limbs, and usually no eyes. Worms vary in size from microscopic to over in length for marine ...

s such as mud dragons,

ostracod Ostracods, or ostracodes, are a Class (biology), class of the crustacean, Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 33,000 species (only 13,000 of which are extant taxon, extant) have been identified,Brandão, S.N.; Antoni ...

nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...

worms,

copepod Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (living in the water column), some are benthos, benthic (living on the sedimen ...

s, tanaidacea, segmented worms, isopods and

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 ...

Methane seeps

In the trough itself, deep-sea chemosynthetic ecosystems are associated with methane cold seeps that with

authigenesis Authigenesis is the process whereby a mineral or sedimentary rock deposit is generated where it is found or observed. Such deposits are described as authigenic. Authigenic sedimentary Mineral, minerals form during or after sedimentation by Precipita ...

usually create

carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...

mounds. The single cell organisms responsible for these ecosystems are similar to those found at methane seeps worldwide. Unlike other areas of the deep ocean no methane–associated mud volcanoes have been found on the margin. Bacterial mats, often white, are seen in video imaging. Some of these systems differ from any others so far described worldwide. For example, there are dense populations of bristle worms in dark sulphide-rich soft sediment microhabitats where there is a high flux of both methane and sulphides. These dark patches have distinctive pitting associated with the worm burrows. Because bristle worms are found at cold seeps worldwide it is possible that the role of aerobic methanotrophy may have been underestimated in seafloor methane chemosynthetic ecosystems, as over 25% of the

escaping from the Hikurangi Margin cold seeps is metabolised aerobically. In general such methane seeps have anaerobic single cell organisms in the subsurface anoxic layers, with anaerobic methanotrophic

archaea Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...

and sulfate-reducing myxococcota. In highly reduced seep habitats the surface is dominated by bacterial mats, while where partial

oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

has happened the bristle worm ecosystem previously mentioned is found associated with aerobic methanotrophic Gammaproteobacteria. Genetically, many of these have been characterised as related to the chemosynthetic

endosymbiont An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...

s of marine invertebrates. Sulfide is exploited as an energy resource by both the tubeworms and the clams. With more advanced oxidation the ecosystem becomes dominated by chemosynthetic frenulate tubeworms of the genus '' Lamellibrachia'' closely related to those found 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-Colvill ...

, and clams, mussels, and various ''

Demospongiae Demosponges or common sponges are sponges of the class Demospongiae (from + ), the most diverse group in the phylum Porifera which include greater than 90% of all extant sponges with nearly 8,800 species worldwide (according to the World P ...

'' sponges. The saltwater clams are of the genus ''Calyptogena'', the

mussels Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, whic ...

are in the deep-sea sub-family ''Bathymodiolinae''.

Notes

References

Sources

* * * * * * * * * * * * * * {{Oceanic features of Zealandia, state=collapsed Zealandia Geography of the New Zealand seabed Oceanic trenches of the Pacific Ocean Subduction zones