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 occurring at this location, as the Pacific plate is being subducted westward in the trench.

Horizon Deep

The deepest point of the Tonga Trench, the Horizon Deep at 23.25833°S 174.726667°W / -23.25833; -174.726667, is 10,800 ± 10 m (35,433 ± 33 ft) deep, making it the deepest point in the Southern Hemisphere and the second deepest on Earth after the Challenger Deep in the Mariana Trench. It is named for the research vessel Horizon of the Scripps Institution of Oceanography, the crew of which found the deep in December 1952. As one of the deepest hadal trenches, the sediment of the Horizon Deep harbours a community of roundworms. A 2016 study found that the abundance of individuals in this community is six times greater than it is at a site on the trench edge at approximately 6,250 m (20,510 ft) near the deep and that the difference in biomass between these locations is even bigger. Species diversity, on the other hand, is twice as big on the trench slope, probably because of a small number of opportunistic species in the trench. Figures for abundance and biomass are similar for the deeps of the Mariana Trench but considerably lower in the Peru–Chile Trench.

Crewed descent

The Tonga Trench and the operating area was surveyed by the support ship, the Deep Submersible Support Vessel DSSV Pressure Drop, with a Kongsberg SIMRAD EM124 multibeam echosounder system. The gathered data will be donated to the GEBCO Seabed 2030 initiative. The dive was part of the Five Deeps Expedition. The objective of this expedition is to thoroughly map and visit the deepest points of all five of the world's oceans by the end of September 2019. Horizon Deep was visited by Victor Vescovo on the first crewed descent to its bottom on 10 June 2019 in the Deep-Submergence Vehicle DSV Limiting Factor, a Triton 36000/2 model submersible. A depth of 10,823 m (35,509 ft) ±10 m (33 ft) was obtained by direct CTD pressure measurements, hence confirming its status as the second deepest point on the planet and the deepest in the Southern Hemisphere.

Geology

Tonga-Kermadec arc system The region between the Tonga trench and the Lau back-arc basin, the Tonga-Kermadec Ridge, moves independently from the Australian and Pacific plates and is subdivided into several small plates, the Tonga, Kermadec, and Niuafo'ou plates. The Tonga plate is facing the Tonga Trench. The Tonga Trench-Arc system is an extension-dominated, non-accretionary convergent margin. The Pacific plate is being subducted westward in the trench. The convergence rate has been estimated to 15 cm/year (5.9 in/year) but GPS measurements in the northern trench indicate a convergence rate of 24 cm/year (9.4 in/year) there. This is the fastest plate velocity on Earth, a result is the earth's most active zone of mantle seismicity. Subduction rates decrease southward along the Tonga-Kermadec Arc, from 24 cm/year (9.4 in/year) in the north to 6 cm/year (2.4 in/year) in the south and also become more oblique southward. The high rate in the Tonga Trench is largely due to a reduction in extension in the Lau Basin. Crustal extension in the Miocene Lau-Colville Ridge began at 6 Ma which initiated the opening of the Lau Basin-Havre Trough. This extension has propagated southward since and has developed into a spreading centre in the Lau Basin in front of the Tonga Trench. New crust is thus produced in front of the Tonga-Kermadec trenches while old crust is consumed behind it in the Tonga Trench.

Pacific slab avalanche While most of the large earthquakes occur at the contact zone between both tectonic plates, related to the friction during subduction, others are produced in the Pacific plate due to its bending. The Pacific crust that descends into the trench is old, 100–140 Ma, and relatively cold and it can therefore store a lot of elastic energy. As it reaches deep into the mantle, more than 600 km (370 mi), and encounters barriers, it is being contorted, which produces deep mantle earthquakes. About 500 km (310 mi) beneath the North Fiji Basin, a detached segment of the subducted Australian plate has collided with the subducted Pacific plate which produces many large-scale earthquakes. The subducted Pacific plate is also being deformed in the collision as both slabs settle on the 660 km discontinuity. This slab collision probably occurred 5–4 Ma when the Lau Basin started to open. Oceanic trenches are important sites for the formation of what will become continental crust and for recycling of material back into the mantle. Along the Tonga Trench mantle-derived melts are transferred to the island arc systems, and abyssal oceanic sediments and fragments of oceanic crust are collected.

Tonga Trench–Lau Basin transition

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