Hikurangi Trough
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.
Extending from the hilly coast north of Kaikōura the undersea Kowhai canyons are to the west of the trough.
[20] 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.
[4][b] 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.
[22] 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.
[13] 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.
[3] 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,[29][30] with a turbidity current that travelled more than 680 km (420 mi) along the Hikurangi Channel.
[6] This is split into convergence near the trench, strike-slip motion around the top of the forearc ridge, and extension in the Taupō Rift.
[34] The middle and southern Hikurangi Margin includes fisheries for hoki, alfonsino and orange roughy.
[37] In the trough itself, deep-sea chemosynthetic ecosystems are associated with methane cold seeps that with authigenesis usually create carbonate mounds.
[41] 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.
[44] In highly reduced seep habitats the surface is dominated by bacterial mats, while where partial oxidation has happened the bristle worm ecosystem previously mentioned is found associated with aerobic methanotrophic Gammaproteobacteria.
[25] 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,[42] and clams, mussels, and various Demospongiae sponges.
