The Hikurangi Trough (previously known as the Hikurangi Trench) is a sea floor feature of the Pacific Ocean off the north-east South Island and the east coast of the North Island of New Zealand. It has been forming for about 25 million years and is turbidite-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 3,000 metres (9,800 feet) as close as 80 kilometres (50 miles) from shore. The southern trough structure is 6–10 km (3.7–6.2 mi) wide off the coast of northern Canterbury with an initial local depth of about 2,000 m (6,600 ft), 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 50–80 km (31–50 mi) 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 80 m (260 ft) 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 1,400–2,000 km (870–1,240 mi), but only the initial 800 km (500 mi) 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 823 m (2,700 ft) just south-east of Kaikōura 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 2,250 m (7,380 ft) deep. Off Cape Palliser, a part of the trough is 2,236 m (7,336 ft) deep, less than the Hikurangi Plateau's 2,588 m (8,491 ft) 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 4,580 m (15,030 ft) assigned by many authorities to the Kermadec Trench which further north has a distinct transition to a much greater 7,436 m (24,396 ft) 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, caused thick sedimentary sequences to form in the then trench with enhanced abyssal erosion in the late Miocene. 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 5 km (3.1 mi) deep in the south and where they exit the Kaikōura Canyon have acoustic characteristics of gravel turbidites. The turbidites thin to about 1 km (0.62 mi) in the north. Gas hydrates have been identified in the sediments and there are widespread methane 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 and extended along the Hikurangi Trench northwards. The sediments are predominantly delivered by submarine canyons and slope gullies that cut across or circumvent obstructions to flow, and the Kaikōura Canyon is known to be the dominant current active and longterm contributor in the case of the Hikurangi Trench. Exceptionally, the 2016 Kaikōura earthquake precipitated submarine mudslides and sediment flows that displaced about 850×106 t (8.4×108 long tons; 9.4×108 short tons) of sediment into the trench from the Kaikōura Canyon, with a turbidity current that travelled more than 680 km (420 mi) along the Hikurangi Channel. The furtherest marine core sampled in the channel so far revealed more than 16 cm (6.3 in) 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.
… excerpt ends here. Continue reading the full article.
