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Oceanic trenches are prominent, long, narrow topography depressions of the seabed. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about of oceanic trenches worldwide, mostly around the Pacific Ocean, but also in the eastern Indian Ocean and a few other locations. The greatest ocean depth measured is in the Challenger Deep of the Mariana Trench, at a depth of below sea level.
Oceanic trenches are a feature of the Earth's distinctive plate tectonics. They mark the locations of convergent plate boundaries, along which lithosphere plates move towards each other at rates that vary from a few millimeters to over ten centimeters per year. Oceanic lithosphere moves into trenches at a global rate of about per year. A trench marks the position at which the flexed, subduction slab begins to descend beneath another lithospheric slab. Trenches are generally parallel to and about from a volcanic arc.
Much of the fluid trapped in of the subducting slab returns to the surface at the oceanic trench, producing and . These support unique based on chemotrophic microorganisms. There is concern that plastic debris is accumulating in trenches and threatening these communities.
There are approximately of convergent plate margins worldwide. These are mostly located around the Pacific Ocean, but are also found in the eastern Indian Ocean, with a few shorter convergent margin segments in other parts of the Indian Ocean, in the Atlantic Ocean, and in the Mediterranean. They are found on the oceanward side of island arcs and Andean-type . Globally, there are over 50 major ocean trenches covering an area of 1.9 million km2 or about 0.5% of the oceans.
Trenches are geomorphologically distinct from troughs. Troughs are elongated depressions of the sea floor with steep sides and flat bottoms, while trenches are characterized by a V-shaped profile. Trenches that are partially infilled are sometimes described as troughs, for example the Makran Trough. Some trenches are completely buried and lack bathymetric expression as in the Cascadia subduction zone, which is completely filled with sediments. Despite their appearance, in these instances the fundamental plate tectonics structure is still an oceanic trench. Some troughs look similar to oceanic trenches but possess other tectonic structures. One example is the Lesser Antilles Trough, which is the forearc basin of the Lesser Antilles subduction zone. Also not a trench is the New Caledonia trough, which is an extensional sedimentary basin related to the Tonga-Kermadec subduction zone. Additionally, the Cayman Trough, which is a pull-apart basin within a transform fault zone, is not an oceanic trench.
Trenches, along with and Wadati–Benioff zones (zones of under a volcanic arc) are diagnostic of convergent plate boundaries and their deeper manifestations, subduction zones. Here, two tectonic plates are drifting into each other at a rate of a few millimeters to over per year. At least one of the plates is oceanic lithosphere, which plunges under the other plate to be recycled in the Earth's mantle.
Trenches are related to, but distinct from, continental collision zones, such as the Himalayas. Unlike in trenches, in continental collision zones continental crust enters a subduction zone. When buoyant continental crust enters a trench, subduction comes to a halt and the area becomes a zone of continental collision. Features analogous to trenches are associated with . One such feature is the peripheral foreland basin, a sediment-filled foredeep. Examples of peripheral foreland basins include the floodplains of the Ganges River and the Tigris-Euphrates river system.
During the 1920s and 1930s, Felix Andries Vening Meinesz measured gravity over trenches using a newly developed gravimeter that could measure gravity from aboard a submarine. He proposed the tectogene hypothesis to explain the belts of negative gravity anomalies that were found near island arcs. According to this hypothesis, the belts were zones of downwelling of light crustal rock arising from subcrustal convection currents. The tectogene hypothesis was further developed by Griggs in 1939, using an analogue model based on a pair of rotating drums. Harry Hammond Hess substantially revised the theory based on his geological analysis.
World War II in the Pacific led to great improvements of bathymetry, particularly in the western Pacific. In light of these new measurements, the linear nature of the deeps became clear. There was a rapid growth of deep sea research efforts, especially the widespread use of Echo sounding in the 1950s and 1960s. These efforts confirmed the morphological utility of the term "trench." Important trenches were identified, sampled, and mapped via sonar.
The early phase of trench exploration reached its peak with the 1960 descent of the Bathyscaphe Trieste to the bottom of the Challenger Deep. Following Robert S. Dietz' and Harry Hess' promulgation of the seafloor spreading hypothesis in the early 1960s and the plate tectonic revolution in the late 1960s, the oceanic trench became an important concept in plate tectonics theory.
Though narrow, oceanic trenches are remarkably long and continuous, forming the largest linear depressions on earth. An individual trench can be thousands of kilometers long. Most trenches are convex towards the subducting slab, which is attributed to the spherical geometry of the Earth.
The trench asymmetry reflects the different physical mechanisms that determine the inner and outer slope angle. The outer slope angle of the trench is determined by the bending radius of the subducting slab, as determined by its elastic thickness. Since oceanic lithosphere thickens with age, the outer slope angle is ultimately determined by the age of the subducting slab. The inner slope angle is determined by the angle of repose of the overriding plate edge. This reflects frequent earthquakes along the trench that prevent oversteepening of the inner slope.
As the subducting plate approaches the trench, it bends slightly upwards before beginning its plunge into the depths. As a result, the outer trench slope is bounded by an outer trench high. This is subtle, often only tens of meters high, and is typically located a few tens of kilometers from the trench axis. On the outer slope itself, where the plate begins to bend downwards into the trench, the upper part of the subducting slab is broken by bending faults that give the outer trench slope a horst and graben topography. The formation of these bending faults is suppressed where oceanic ridges or large seamounts are subducting into the trench, but the bending faults cut right across smaller seamounts. Where the subducting slab is only thinly veneered with sediments, the outer slope will often show seafloor spreading ridges oblique to the horst and graben ridges.
Sediments are sometimes transported along the axis of an oceanic trench. The central Chile trench experiences transport of sediments from source fans along an axial channel. Similar transport of sediments has been documented in the Aleutian trench.
In addition to sedimentation from rivers draining into a trench, sedimentation also takes place from landslides on the tectonically steepened inner slope, often driven by megathrust earthquakes. The Reloca Slide of the central Chile trench is an example of this process.
Accretionary margins, such as the southern Peru-Chile, Cascadia, and Aleutians, are associated with moderately to heavily sedimented trenches. As the slab subducts, sediments are "bulldozed" onto the edge of the overriding plate, producing an accretionary wedge or accretionary prism. This builds the overriding plate outwards. Because the sediments lack strength, their angle of repose is gentler than the rock making up the inner slope of erosive margin trenches. The inner slope is underlain by imbricated thrust sheets of sediments. The inner slope topography is roughened by localized mass wasting. Cascadia has practically no bathymetric expression of the outer rise and trench, due to complete sediment filling, but the inner trench slope is complex, with many thrust ridges. These compete with canyon formation by rivers draining into the trench. Inner trench slopes of erosive margins rarely show thrust ridges.
Accretionary prisms grow in two ways. The first is by frontal accretion, in which sediments are scraped off the downgoing plate and emplaced at the front of the accretionary prism. As the accretionary wedge grows, older sediments further from the trench become increasingly lithification, and faults and other structural features are steepened by rotation towards the trench. The other mechanism for accretionary prism growth is underplating (also known as basal accretion) of subducted sediments, together with some oceanic crust, along the shallow parts of the subduction decollement. The Franciscan Group of California is interpreted as an ancient accretionary prism in which underplating is recorded as tectonic mélanges and duplex structures.
Subduction of seamounts and aseismic ridge into the trench may increase aseismic creep and reduce the severity of earthquakes. Contrariwise, subduction of large amounts of sediments may allow ruptures along the subduction décollement to propagate for great distances to produce megathrust earthquakes.
Forces perpendicular to the slab (the portion of the subducting plate within the mantle) are responsible for steepening of the slab and, ultimately, the movement of the hinge and trench at the surface. These forces arise from the negative buoyancy of the slab with respect to the mantle modified by the geometry of the slab itself. The extension in the overriding plate, in response to the subsequent subhorizontal mantle flow from the displacement of the slab, can result in formation of a back-arc basin.
Seismic tomography provides evidence for slab rollback. Results demonstrate high temperature anomalies within the mantle suggesting subducted material is present in the mantle. Ophiolites are viewed as evidence for such mechanisms as high pressure and temperature rocks are rapidly brought to the surface through the processes of slab rollback, which provides space for the exhumation of ophiolites.
Slab rollback is not always a continuous process suggesting an episodic nature. The episodic nature of the rollback is explained by a change in the density of the subducting plate, such as the arrival of buoyant lithosphere (a continent, arc, ridge, or plateau), a change in the subduction dynamics, or a change in the plate kinematics. The age of the subducting plates does not have any effect on slab rollback. Nearby continental collisions have an effect on slab rollback. Continental collisions induce mantle flow and extrusion of mantle material, which causes stretching and arc-trench rollback. In the area of the Southeast Pacific, there have been several rollback events resulting in the formation of numerous back-arc basins.
The fluids released at mud volcanoes and cold seeps are rich in methane and hydrogen sulfide, providing chemical energy for Chemotroph that form the base of a unique trench biome. Cold seep communities have been identified in the inner trench slopes of the western Pacific (especially Japan), South America, Barbados, the Mediterranean, Makran, and the Sunda trench. These are found at depths as great as . The genome of the extremophile Deinococcus from Challenger Deep has sequenced for its ecological insights and potential industrial uses.
Because trenches are the lowest points in the ocean floor, there is concern that plastic debris may accumulate in trenches and endanger the fragile trench biomes.
| Mariana Trench | Pacific Ocean | Challenger Deep | ||
| Tonga Trench | Pacific Ocean | Horizon Deep | ||
| Philippine Trench | Pacific Ocean | Emden Deep | ||
| Kuril–Kamchatka Trench | Pacific Ocean | |||
| Kermadec Trench | Pacific Ocean | |||
| Izu–Bonin Trench (Izu–Ogasawara Trench) | Pacific Ocean | |||
| New Britain Trench | Pacific Ocean (Solomon Sea) | Planet Deep | ||
| Puerto Rico Trench | Atlantic Ocean | Milwaukee Deep | ||
| South Sandwich Trench | Atlantic Ocean | Meteor Deep | ||
| Peru–Chile Trench or Atacama Trench | Pacific Ocean | Richards Deep | ||
| Japan Trench | Pacific Ocean | 8,412 m (27,498 ft) | ||
| Cayman Trench | Atlantic Ocean | Caribbean Deep | ||
| South Sandwich Trench | Southern Ocean | Factorian Deep | ||
| Sunda Trench | Indian Ocean | Java Deep | ||
| Mauritius Trench | Indian Ocean | Mauritius Point | ||
| India Trench | Indian Ocean | Between India & Maldives | ||
| Ceylon Trench | Indian Ocean | Sri Lanka Deep | ||
| Somalia Trench | Indian Ocean | Somali Deep | ||
| Madagascar Trench | Indian Ocean | Madagascar Deep | ||
| Puerto Rico Trench | Atlantic Ocean | Rio Bermuda Deep | ||
| Mid-Atlantic Ridge | Arctic Ocean | Molloy Deep |
| Aleutian Trench | South of the Aleutian Islands, west of Alaska |
| Bougainville Trench | South of New Guinea |
| Cayman Trench | Western Caribbean |
| Cedros Trench (inactive) | Pacific coast of Baja California |
| Hikurangi Trough | East of New Zealand |
| Hjort Trench | Southwest of New Zealand |
| Izu–Ogasawara Trench | Near Izu Islands and Bonin Islands islands |
| Japan Trench | East of Japan |
| Kermadec Trench * | Northeast of New Zealand |
| Kuril–Kamchatka Trench * | Near Kuril Islands |
| Manila Trench | West of Luzon, Philippines |
| Mariana Trench * | Western Pacific Ocean; east of Mariana Islands |
| Middle America Trench | Eastern Pacific Ocean; off coast of Mexico, Guatemala, El Salvador, Nicaragua, Costa Rica |
| New Hebrides Trench | West of Vanuatu (New Hebrides Islands). |
| Peru–Chile Trench | Eastern Pacific Ocean; off coast of Peru & Chile |
| Philippine Trench * | East of the Philippines |
| Puerto Rico Trench | Boundary of Caribbean and Atlantic Ocean |
| Puysegur trench | Southwest of New Zealand |
| Ryukyu Trench | Eastern edge of Japan's Ryukyu Islands |
| South Sandwich Trench | East of the South Sandwich Islands |
| Sunda Trench | Curves from south of Java to west of Sumatra and the Andaman and Nicobar Islands |
| Tonga Trench * | Near Tonga |
| Yap Trench | Western Pacific Ocean; between Palau Islands and Mariana Trench |
(*) The five deepest trenches in the world
| Intermontane Trench | Western North America; between the Intermontane Islands and North America |
| Insular Trench | Western North America; between the Insular Islands and the Intermontane Islands |
| Farallon Trench | Western North America |
| Tethys Trench | South of Turkey, Iran, Tibet and Southeast Asia |
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