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Oceanic trench

Oceanic trenches are prominent, long, narrow topographic depressions of the ocean floor. They are typically 50 to 100 kilometers (30 to 60 mi) wide and 3 to 4 km (1.9 to 2.5 mi) below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about 50,000 km (31,000 mi) 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 10,920 m (35,830 ft) below sea level.

Oceanic crust is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at trenches

Oceanic trenches are a feature of the Earth's distinctive plate tectonics. They mark the locations of convergent plate boundaries, along which lithospheric 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 3 km2 (1.2 sq mi) per year.[1] A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab. Trenches are generally parallel to and about 200 km (120 mi) from a volcanic arc.

Much of the fluid trapped in sediments of the subducting slab returns to the surface at the oceanic trench, producing mud volcanoes and cold seeps. These support unique biomes based on chemotrophic microorganisms. There is concern that plastic debris is accumulating in trenches and threatening these communities.

Geographic distribution

 
Major Pacific trenches (1–10) and fracture zones (11–20): 1. Kermadec 2. Tonga 3. Bougainville 4. Mariana 5. Izu–Ogasawara 6. Japan 7. Kuril–Kamchatka 8. Aleutian 9. Middle America 10. Peru–Chile 11. Mendocino 12. Murray 13. Molokai 14. Clarion 15. Clipperton 16. Challenger 17. Eltanin 18. Udintsev 19. East Pacific Rise (S-shaped) 20. Nazca Ridge

There are approximately 50,000 km (31,000 mi) 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.[2] They are found on the oceanward side of island arcs and Andean-type orogens.[3] Globally, there are over 50 major ocean trenches covering an area of 1.9 million km2 or about 0.5% of the oceans.[4]

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.[4] Trenches that are partially infilled are sometimes described as troughs, for example the Makran Trough.[5] Some trenches are completely buried and lack bathymetric expression as in the Cascadia subduction zone,[6] which is completely filled with sediments.[7] Despite their appearance, in these instances the fundamental plate-tectonic 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.[8] Also not a trench is the New Caledonia trough, which is an extensional sedimentary basin related to the Tonga-Kermadec subduction zone.[9] Additionally, the Cayman Trough, which is a pull-apart basin within a transform fault zone,[10] is not an oceanic trench.

Trenches, along with volcanic arcs and Wadati-Benioff zones (zones of earthquakes under a volcanic arc) are diagnostic of convergent plate boundaries and their deeper manifestations, subduction zones.[2][3][11] Here, two tectonic plates are drifting into each other at a rate of a few millimeters to over 10 centimeters (4 in) 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 collision zones. 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.[2]

History of the term "trench"

Trenches were not clearly defined until the late 1940s and 1950s. The bathymetry of the ocean was poorly known prior to the Challenger expedition of 1872–1876,[12] which took 492 soundings of the deep ocean.[13] At station #225, the expedition discovered Challenger Deep,[14] now known to be the southern end of the Mariana Trench. The laying of transatlantic telegraph cables on the seafloor between the continents during the late 19th and early 20th centuries provided further motivation for improved bathymetry.[15] The term trench, in its modern sense of a prominent elongated depression of the sea bottom, was first used by Johnstone in his 1923 textbook An Introduction to Oceanography.[16][2]

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.[11] 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.[17]

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 echosounders 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 tectonic theory.[11]

Morphology

 
Cross section of an oceanic trench formed along an oceanic-oceanic convergent boundary
 
The Peru–Chile Trench is located just left of the sharp line between the blue deep ocean (on the left) and the light blue continental shelf, along the west coast of South America. It runs along an oceanic-continental boundary, where the oceanic Nazca Plate subducts beneath the continental South American Plate

Oceanic trenches are 50 to 100 kilometers (30 to 60 mi) wide and have an asymmetric V-shape, with the steeper slope (8 to 20 degrees) on the inner (overriding) side of the trench and the gentler slope (around 5 degrees) on the outer (subducting) side of the trench.[18][19] The bottom of the trench marks the boundary between the subducting and overriding plates, known as the basal plate boundary shear[20] or the subduction décollement.[2] The depth of the trench depends on the starting depth of the oceanic lithosphere as it begins its plunge into the trench, the angle at which the slab plunges, and the amount of sedimentation in the trench. Both starting depth and subduction angle are greater for older oceanic lithosphere, which is reflected in the deep trenches of the western Pacific. Here the bottoms of the Marianas and the Tonga-Kermadec trenches are up to 10–11 kilometers (6.2–6.8 mi) below sea level. In the eastern Pacific, where the subducting oceanic lithosphere is much younger, the depth of the Peru-Chile trench is around 7 to 8 kilometers (4.3 to 5.0 mi).[21]

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.[3] Most trenches are convex towards the subducting slab, which is attributed to the spherical geometry of the Earth.[22]

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.[23][20] The inner slope angle is determined by the angle of repose of the overriding plate edge.[20] This reflects frequent earthquakes along the trench that prevent oversteepening of the inner slope.[2]

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.[20]

Sedimentation

Trench morphology is strongly modified by the amount of sedimentation in the trench. This varies from practically no sedimentation, as in the Tonga-Kermadec trench, to almost completely filled with sediments, as with the southern Lesser Antilles trench or the eastern Alaskan trench. Sedimentation is largely controlled by whether the trench is near a continental sediment source.[22] The range of sedimentation is well illustrated by the Chilean trench. The north Chile portion of the trench, which lies along the Atacama Desert with its very slow rate of weathering, is sediment-starved, with from 20 to a few hundred meters of sediments on the trench floor. The tectonic morphology of this trench segment is fully exposed on the ocean bottom. The central Chile segment of the trench is moderately sedimented, with sediments onlapping onto pelagic sediments or ocean basement of the subducting slab, but the trench morphology is still clearly discernible. The southern Chile segment of the trench is fully sedimented, to the point where the outer rise and slope are no longer discernible. Other fully sedimented trenches include the Makran Trough, where sediments are up to 7.5 kilometers (4.7 mi) thick; the Cascadia subduction zone, which is completed buried by 3 to 4 kilometers (1.9 to 2.5 mi) of sediments; and the northernmost Sumatra subduction zone, which is buried under 6 kilometers (3.7 mi) of sediments.[24]

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.[25] Similar transport of sediments has been documented in the Aleutian trench.[2]

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.[26]

Erosive versus accretionary margins

Convergent margins are classified as erosive or accretionary, and this has a strong influence on the morphology of the inner slope of the trench. Erosive margins, such as the northern Peru-Chile, Tonga-Kermadec, and Mariana trenches, correspond to sediment-starved trenches.[3] The subducting slab erodes material from the lower part of the overriding slab, reducing its volume. The edge of the slab experiences subsidence and steepening, with normal faulting. The slope is underlain by relative strong igneous and metamorphic rock, which maintains a high angle of repose.[27] Over half of all convergent margins are erosive margins.[2]

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.[27] 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.[19]

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.[2] As the accretionary wedge grows, older sediments further from the trench become increasingly lithified, and faults and other structural features are steepened by rotation towards the trench.[28] The other mechanism for accretionary prism growth is underplating[2] (also known as basal accretion[29]) 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.[2]

 
Oceanic trench formed along an oceanic-oceanic convergent boundary
 
The Mariana trench contains the deepest part of the world's oceans, and runs along an oceanic-oceanic convergent boundary. It is the result of the oceanic Pacific plate subducting beneath the oceanic Mariana plate.

Earthquakes

Frequent megathrust earthquakes modify the inner slope of the trench by triggering massive landslides. These leave semicircular landslide scarps with slopes of up to 20 degrees on the headwalls and sidewalls.[30]

Subduction of seamounts and aseismic ridges 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.[31]

Trench rollback

Trenches seem positionally stable over time, but scientists believe that some trenches—particularly those associated with subduction zones where two oceanic plates converge—move backward into the subducting plate.[32][33] This is called trench rollback or hinge retreat (also hinge rollback) and is one explanation for the existence of back-arc basins.

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.[34] These forces arise from the negative buoyancy of the slab with respect to the mantle[35] modified by the geometry of the slab itself.[36] 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.[37]

Processes involved

Several forces are involved in the process of slab rollback. Two forces acting against each other at the interface of the two subducting plates exert forces against one another. The subducting plate exerts a bending force (FPB) that supplies pressure during subduction, while the overriding plate exerts a force against the subducting plate (FTS). The slab pull force (FSP) is caused by the negative buoyancy of the plate driving the plate to greater depths. The resisting force from the surrounding mantle opposes the slab pull forces. Interactions with the 660-km discontinuity cause a deflection due to the buoyancy at the phase transition (F660).[36] The unique interplay of these forces is what generates slab rollback. When the deep slab section obstructs the down-going motion of the shallow slab section, slab rollback occurs. The subducting slab undergoes backward sinking due to the negative buoyancy forces causing a retrogradation of the trench hinge along the surface. Upwelling of the mantle around the slab can create favorable conditions for the formation of a back-arc basin.[37]

Seismic tomography provides evidence for slab rollback. Results demonstrate high temperature anomalies within the mantle suggesting subducted material is present in the mantle.[38] 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.[35] 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.[36] 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.[37] In the area of the Southeast Pacific, there have been several rollback events resulting in the formation of numerous back-arc basins.[35]

Mantle interactions

Interactions with the mantle discontinuities play a significant role in slab rollback. Stagnation at the 660-km discontinuity causes retrograde slab motion due to the suction forces acting at the surface.[36] Slab rollback induces mantle return flow, which causes extension from the shear stresses at the base of the overriding plate. As slab rollback velocities increase, circular mantle flow velocities also increase, accelerating extension rates.[34] Extension rates are altered when the slab interacts with the discontinuities within the mantle at 410 km and 660 km depth. Slabs can either penetrate directly into the lower mantle, or can be retarded due to the phase transition at 660 km depth creating a difference in buoyancy. An increase in retrograde trench migration (slab rollback) (2–4 cm/yr) is a result of flattened slabs at the 660-km discontinuity where the slab does not penetrate into the lower mantle.[39] This is the case for the Japan, Java and Izu–Bonin trenches. These flattened slabs are only temporarily arrested in the transition zone. The subsequent displacement into the lower mantle is caused by slab pull forces, or the destabilization of the slab from warming and broadening due to thermal diffusion. Slabs that penetrate directly into the lower mantle result in slower slab rollback rates (~1–3 cm/yr) such as the Mariana arc, Tonga arcs.[39]

Hydrothermal activity and associated biomes

As sediments are subducted at the bottom of trenches, much of their fluid content is expelled and moves back along the subduction décollement to emerge on the inner slope as mud volcanoes and cold seeps. Methane clathrates and gas hydrates also accumulate in the inner slope, and there is concern that their breakdown could contribute to global warming.[2]

The fluids released at mud volcanoes and cold seeps are rich in methane and hydrogen sulfide, providing chemical energy for chemotrophic microorganisms 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[40]), South America, Barbados, the Mediterranean, Makran, and the Sunda trench. These are found at depths as great as 6,000 meters (20,000 ft).[2] The genome of the extremophile Deinococcus from Challenger Deep has sequenced for its ecological insights and potential industrial uses.[41]

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.[42]

Deepest oceanic trenches

Recent measurements, where the salinity and temperature of the water was measured throughout the dive, have uncertainties of about 15 m (49 ft).[43] Older measurements may be off by hundreds of meters.

Trench Ocean Lowest Point Maximum Depth Source
Mariana Trench Pacific Ocean Challenger Deep 10,920 m (35,830 ft) [43]
Tonga Trench Pacific Ocean Horizon Deep 10,820 m (35,500 ft) [43]
Philippine Trench Pacific Ocean Emden Deep 10,540 m (34,580 ft) [44]
Kuril–Kamchatka Trench Pacific Ocean 10,542 m (34,587 ft) [44]
Kermadec Trench Pacific Ocean 10,047 m (32,963 ft) [44]
Izu–Bonin Trench (Izu–Ogasawara Trench) Pacific Ocean 9,810 m (32,190 ft) [44]
New Britain Trench Pacific Ocean (Solomon Sea) Planet Deep 9,140 m (29,990 ft) [45]
Puerto Rico Trench Atlantic Ocean Brownson Deep 8,380 m (27,490 ft) [43]
South Sandwich Trench Atlantic Ocean Meteor Deep 8,265 m (27,116 ft) [43]
Peru–Chile Trench or Atacama Trench Pacific Ocean Richards Deep 8,055 m (26,427 ft) [44]
Japan Trench Pacific Ocean 8,412 m (27,598 ft) [44]

Notable oceanic trenches

Trench Location
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 Trench East of New Zealand
Hjort Trench Southwest of New Zealand
Izu–Ogasawara Trench Near Izu and Bonin 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

Ancient oceanic trenches

Trench Location
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

See also

References

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External links

  • "HADEX: Research project to explore ocean trenches". Woods Hole Oceanographic Institution.
  • "Ocean Trenches". Woods Hole Oceanographic Institution.

oceanic, trench, prominent, long, narrow, topographic, depressions, ocean, floor, they, typically, kilometers, wide, below, level, surrounding, oceanic, floor, thousands, kilometers, length, there, about, oceanic, trenches, worldwide, mostly, around, pacific, . Oceanic trenches are prominent long narrow topographic depressions of the ocean floor They are typically 50 to 100 kilometers 30 to 60 mi wide and 3 to 4 km 1 9 to 2 5 mi below the level of the surrounding oceanic floor but can be thousands of kilometers in length There are about 50 000 km 31 000 mi 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 10 920 m 35 830 ft below sea level Oceanic crust is formed at an oceanic ridge while the lithosphere is subducted back into the asthenosphere at trenches Oceanic trenches are a feature of the Earth s distinctive plate tectonics They mark the locations of convergent plate boundaries along which lithospheric 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 3 km2 1 2 sq mi per year 1 A trench marks the position at which the flexed subducting slab begins to descend beneath another lithospheric slab Trenches are generally parallel to and about 200 km 120 mi from a volcanic arc Much of the fluid trapped in sediments of the subducting slab returns to the surface at the oceanic trench producing mud volcanoes and cold seeps These support unique biomes based on chemotrophic microorganisms There is concern that plastic debris is accumulating in trenches and threatening these communities Contents 1 Geographic distribution 2 History of the term trench 3 Morphology 3 1 Sedimentation 3 2 Erosive versus accretionary margins 3 3 Earthquakes 4 Trench rollback 4 1 Processes involved 4 2 Mantle interactions 5 Hydrothermal activity and associated biomes 6 Deepest oceanic trenches 7 Notable oceanic trenches 8 Ancient oceanic trenches 9 See also 10 References 11 Bibliography 12 External linksGeographic distribution Edit Major Pacific trenches 1 10 and fracture zones 11 20 1 Kermadec 2 Tonga 3 Bougainville 4 Mariana 5 Izu Ogasawara 6 Japan 7 Kuril Kamchatka 8 Aleutian 9 Middle America 10 Peru Chile 11 Mendocino 12 Murray 13 Molokai 14 Clarion 15 Clipperton 16 Challenger 17 Eltanin 18 Udintsev 19 East Pacific Rise S shaped 20 Nazca Ridge There are approximately 50 000 km 31 000 mi 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 2 They are found on the oceanward side of island arcs and Andean type orogens 3 Globally there are over 50 major ocean trenches covering an area of 1 9 million km2 or about 0 5 of the oceans 4 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 4 Trenches that are partially infilled are sometimes described as troughs for example the Makran Trough 5 Some trenches are completely buried and lack bathymetric expression as in the Cascadia subduction zone 6 which is completely filled with sediments 7 Despite their appearance in these instances the fundamental plate tectonic 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 8 Also not a trench is the New Caledonia trough which is an extensional sedimentary basin related to the Tonga Kermadec subduction zone 9 Additionally the Cayman Trough which is a pull apart basin within a transform fault zone 10 is not an oceanic trench Trenches along with volcanic arcs and Wadati Benioff zones zones of earthquakes under a volcanic arc are diagnostic of convergent plate boundaries and their deeper manifestations subduction zones 2 3 11 Here two tectonic plates are drifting into each other at a rate of a few millimeters to over 10 centimeters 4 in 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 collision zones 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 2 History of the term trench EditTrenches were not clearly defined until the late 1940s and 1950s The bathymetry of the ocean was poorly known prior to the Challenger expedition of 1872 1876 12 which took 492 soundings of the deep ocean 13 At station 225 the expedition discovered Challenger Deep 14 now known to be the southern end of the Mariana Trench The laying of transatlantic telegraph cables on the seafloor between the continents during the late 19th and early 20th centuries provided further motivation for improved bathymetry 15 The term trench in its modern sense of a prominent elongated depression of the sea bottom was first used by Johnstone in his 1923 textbook An Introduction to Oceanography 16 2 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 11 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 17 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 echosounders 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 tectonic theory 11 Morphology Edit Cross section of an oceanic trench formed along an oceanic oceanic convergent boundary The Peru Chile Trench is located just left of the sharp line between the blue deep ocean on the left and the light blue continental shelf along the west coast of South America It runs along an oceanic continental boundary where the oceanic Nazca Plate subducts beneath the continental South American Plate Oceanic trenches are 50 to 100 kilometers 30 to 60 mi wide and have an asymmetric V shape with the steeper slope 8 to 20 degrees on the inner overriding side of the trench and the gentler slope around 5 degrees on the outer subducting side of the trench 18 19 The bottom of the trench marks the boundary between the subducting and overriding plates known as the basal plate boundary shear 20 or the subduction decollement 2 The depth of the trench depends on the starting depth of the oceanic lithosphere as it begins its plunge into the trench the angle at which the slab plunges and the amount of sedimentation in the trench Both starting depth and subduction angle are greater for older oceanic lithosphere which is reflected in the deep trenches of the western Pacific Here the bottoms of the Marianas and the Tonga Kermadec trenches are up to 10 11 kilometers 6 2 6 8 mi below sea level In the eastern Pacific where the subducting oceanic lithosphere is much younger the depth of the Peru Chile trench is around 7 to 8 kilometers 4 3 to 5 0 mi 21 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 3 Most trenches are convex towards the subducting slab which is attributed to the spherical geometry of the Earth 22 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 23 20 The inner slope angle is determined by the angle of repose of the overriding plate edge 20 This reflects frequent earthquakes along the trench that prevent oversteepening of the inner slope 2 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 20 Sedimentation Edit Trench morphology is strongly modified by the amount of sedimentation in the trench This varies from practically no sedimentation as in the Tonga Kermadec trench to almost completely filled with sediments as with the southern Lesser Antilles trench or the eastern Alaskan trench Sedimentation is largely controlled by whether the trench is near a continental sediment source 22 The range of sedimentation is well illustrated by the Chilean trench The north Chile portion of the trench which lies along the Atacama Desert with its very slow rate of weathering is sediment starved with from 20 to a few hundred meters of sediments on the trench floor The tectonic morphology of this trench segment is fully exposed on the ocean bottom The central Chile segment of the trench is moderately sedimented with sediments onlapping onto pelagic sediments or ocean basement of the subducting slab but the trench morphology is still clearly discernible The southern Chile segment of the trench is fully sedimented to the point where the outer rise and slope are no longer discernible Other fully sedimented trenches include the Makran Trough where sediments are up to 7 5 kilometers 4 7 mi thick the Cascadia subduction zone which is completed buried by 3 to 4 kilometers 1 9 to 2 5 mi of sediments and the northernmost Sumatra subduction zone which is buried under 6 kilometers 3 7 mi of sediments 24 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 25 Similar transport of sediments has been documented in the Aleutian trench 2 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 26 Erosive versus accretionary margins Edit Convergent margins are classified as erosive or accretionary and this has a strong influence on the morphology of the inner slope of the trench Erosive margins such as the northern Peru Chile Tonga Kermadec and Mariana trenches correspond to sediment starved trenches 3 The subducting slab erodes material from the lower part of the overriding slab reducing its volume The edge of the slab experiences subsidence and steepening with normal faulting The slope is underlain by relative strong igneous and metamorphic rock which maintains a high angle of repose 27 Over half of all convergent margins are erosive margins 2 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 27 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 19 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 2 As the accretionary wedge grows older sediments further from the trench become increasingly lithified and faults and other structural features are steepened by rotation towards the trench 28 The other mechanism for accretionary prism growth is underplating 2 also known as basal accretion 29 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 melanges and duplex structures 2 Oceanic trench formed along an oceanic oceanic convergent boundary The Mariana trench contains the deepest part of the world s oceans and runs along an oceanic oceanic convergent boundary It is the result of the oceanic Pacific plate subducting beneath the oceanic Mariana plate Earthquakes Edit Frequent megathrust earthquakes modify the inner slope of the trench by triggering massive landslides These leave semicircular landslide scarps with slopes of up to 20 degrees on the headwalls and sidewalls 30 Subduction of seamounts and aseismic ridges 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 decollement to propagate for great distances to produce megathrust earthquakes 31 Trench rollback EditTrenches seem positionally stable over time but scientists believe that some trenches particularly those associated with subduction zones where two oceanic plates converge move backward into the subducting plate 32 33 This is called trench rollback or hinge retreat also hinge rollback and is one explanation for the existence of back arc basins 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 34 These forces arise from the negative buoyancy of the slab with respect to the mantle 35 modified by the geometry of the slab itself 36 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 37 Processes involved Edit Several forces are involved in the process of slab rollback Two forces acting against each other at the interface of the two subducting plates exert forces against one another The subducting plate exerts a bending force FPB that supplies pressure during subduction while the overriding plate exerts a force against the subducting plate FTS The slab pull force FSP is caused by the negative buoyancy of the plate driving the plate to greater depths The resisting force from the surrounding mantle opposes the slab pull forces Interactions with the 660 km discontinuity cause a deflection due to the buoyancy at the phase transition F660 36 The unique interplay of these forces is what generates slab rollback When the deep slab section obstructs the down going motion of the shallow slab section slab rollback occurs The subducting slab undergoes backward sinking due to the negative buoyancy forces causing a retrogradation of the trench hinge along the surface Upwelling of the mantle around the slab can create favorable conditions for the formation of a back arc basin 37 Seismic tomography provides evidence for slab rollback Results demonstrate high temperature anomalies within the mantle suggesting subducted material is present in the mantle 38 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 35 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 36 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 37 In the area of the Southeast Pacific there have been several rollback events resulting in the formation of numerous back arc basins 35 Mantle interactions Edit Interactions with the mantle discontinuities play a significant role in slab rollback Stagnation at the 660 km discontinuity causes retrograde slab motion due to the suction forces acting at the surface 36 Slab rollback induces mantle return flow which causes extension from the shear stresses at the base of the overriding plate As slab rollback velocities increase circular mantle flow velocities also increase accelerating extension rates 34 Extension rates are altered when the slab interacts with the discontinuities within the mantle at 410 km and 660 km depth Slabs can either penetrate directly into the lower mantle or can be retarded due to the phase transition at 660 km depth creating a difference in buoyancy An increase in retrograde trench migration slab rollback 2 4 cm yr is a result of flattened slabs at the 660 km discontinuity where the slab does not penetrate into the lower mantle 39 This is the case for the Japan Java and Izu Bonin trenches These flattened slabs are only temporarily arrested in the transition zone The subsequent displacement into the lower mantle is caused by slab pull forces or the destabilization of the slab from warming and broadening due to thermal diffusion Slabs that penetrate directly into the lower mantle result in slower slab rollback rates 1 3 cm yr such as the Mariana arc Tonga arcs 39 The Puerto Rico TrenchHydrothermal activity and associated biomes EditAs sediments are subducted at the bottom of trenches much of their fluid content is expelled and moves back along the subduction decollement to emerge on the inner slope as mud volcanoes and cold seeps Methane clathrates and gas hydrates also accumulate in the inner slope and there is concern that their breakdown could contribute to global warming 2 The fluids released at mud volcanoes and cold seeps are rich in methane and hydrogen sulfide providing chemical energy for chemotrophic microorganisms 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 40 South America Barbados the Mediterranean Makran and the Sunda trench These are found at depths as great as 6 000 meters 20 000 ft 2 The genome of the extremophile Deinococcus from Challenger Deep has sequenced for its ecological insights and potential industrial uses 41 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 42 Deepest oceanic trenches EditRecent measurements where the salinity and temperature of the water was measured throughout the dive have uncertainties of about 15 m 49 ft 43 Older measurements may be off by hundreds of meters Trench Ocean Lowest Point Maximum Depth SourceMariana Trench Pacific Ocean Challenger Deep 10 920 m 35 830 ft 43 Tonga Trench Pacific Ocean Horizon Deep 10 820 m 35 500 ft 43 Philippine Trench Pacific Ocean Emden Deep 10 540 m 34 580 ft 44 Kuril Kamchatka Trench Pacific Ocean 10 542 m 34 587 ft 44 Kermadec Trench Pacific Ocean 10 047 m 32 963 ft 44 Izu Bonin Trench Izu Ogasawara Trench Pacific Ocean 9 810 m 32 190 ft 44 New Britain Trench Pacific Ocean Solomon Sea Planet Deep 9 140 m 29 990 ft 45 Puerto Rico Trench Atlantic Ocean Brownson Deep 8 380 m 27 490 ft 43 South Sandwich Trench Atlantic Ocean Meteor Deep 8 265 m 27 116 ft 43 Peru Chile Trench or Atacama Trench Pacific Ocean Richards Deep 8 055 m 26 427 ft 44 Japan Trench Pacific Ocean 8 412 m 27 598 ft 44 Notable oceanic trenches EditTrench LocationAleutian Trench South of the Aleutian Islands west of AlaskaBougainville Trench South of New GuineaCayman Trench Western CaribbeanCedros Trench inactive Pacific coast of Baja CaliforniaHikurangi Trench East of New ZealandHjort Trench Southwest of New ZealandIzu Ogasawara Trench Near Izu and Bonin islandsJapan Trench East of JapanKermadec Trench Northeast of New ZealandKuril Kamchatka Trench Near Kuril islandsManila Trench West of Luzon PhilippinesMariana Trench Western Pacific Ocean east of Mariana IslandsMiddle America Trench Eastern Pacific Ocean off coast of Mexico Guatemala El Salvador Nicaragua Costa RicaNew Hebrides Trench West of Vanuatu New Hebrides Islands Peru Chile Trench Eastern Pacific Ocean off coast of Peru amp ChilePhilippine Trench East of the PhilippinesPuerto Rico Trench Boundary of Caribbean and Atlantic oceanPuysegur trench Southwest of New ZealandRyukyu Trench Eastern edge of Japan s Ryukyu IslandsSouth Sandwich Trench East of the South Sandwich IslandsSunda Trench Curves from south of Java to west of Sumatra and the Andaman and Nicobar IslandsTonga Trench Near TongaYap Trench Western Pacific Ocean between Palau Islands and Mariana Trench The five deepest trenches in the worldAncient oceanic trenches EditTrench LocationIntermontane Trench Western North America between the Intermontane Islands and North AmericaInsular Trench Western North America between the Insular Islands and the Intermontane IslandsFarallon Trench Western North AmericaTethys Trench South of Turkey Iran Tibet and Southeast AsiaSee also Edit Oceans portalList of landforms List of submarine topographical features Mid ocean ridge Physical oceanography Ring of FireReferences Edit Rowley 2002 a b c d e f g h i j k l m Stern 2005 a b c d Kearey Klepeis amp Vine 2009 p 250 a b Harris et al 2014 Dastanpour 1996 Thomas Burbidge amp Cummins 2007 Goldfinger et al 2012 Westbrook Mascle amp Biju Duval 1984 Hackney Sutherland amp Collot 2012 Einsele 2000 a b c Geersen Voelker amp Behrmann 2018 Eiseley 1946 Weyl 1969 p 49 Thomson amp Murray 1895 McConnell 1990 Johnstone 1923 Allwrardt 1993 Kearey Klepeis amp Vine 2009 pp 250 251 a b Geersen Voelker amp Behrmann 2018 p 420 a b c d Geersen Voelker amp Behrmann 2018 pp 411 412 Kearey Klepeis amp Vine 2009 pp 250 251 a b Kearey Klepeis amp Vine 2009 p 251 Bodine amp Watts 1979 Geersen Voelker amp Behrmann 2018 pp 412 416 Volker et al 2013 Volker et al 2009 a b Geersen Voelker amp Behrmann 2018 p 416 Kearey Klepeis amp Vine 2009 pp 264 266 Bangs et al 2020 Volker et al 2014 Geersen Voelker amp Behrmann 2018 p 421 Dvorkin et al 1993 Garfunkel Anderson amp Schubert 1986 a b Schellart amp Moresi 2013 a b c Schellart Lister amp Toy 2006 a b c d Nakakuki amp Mura 2013 a b c Flower amp Dilek 2003 Hall amp Spakman 2002 a b Christensen 1996 Fujikura et al 2010 Zhang et al 2021 Peng et al 2020 a b c d e Amos 2021 a b c d e f Jamieson et al Gallo et al 2015 Bibliography EditAllwrardt Allan O 1993 Evolution of the tectogene concept 1930 1965 PDF Proceedings of the Fifth International Congress on the History of Oceanography Retrieved 29 September 2021 Amos Jonathan 11 May 2021 Oceans extreme depths measured in precise detail News BBC Retrieved 2 October 2021 Bangs N L Morgan J K Trehu A M Contreras Reyes E Arnulf A F Han S Olsen K M Zhang E November 2020 Basal Accretion Along the South Central Chilean Margin and Its Relationship to Great Earthquakes Journal of Geophysical Research Solid Earth 125 11 Bibcode 2020JGRB 12519861B doi 10 1029 2020JB019861 S2CID 225154312 Bodine J H Watts A B gt 1979 On lithospheric flexure seaward of the Bonin and Mariana trenches Earth and Planetary Science Letters 43 1 132 148 Bibcode 1979E amp PSL 43 132B doi 10 1016 0012 821X 79 90162 6 Christensen UR 1996 The Influence of Trench Migration on Slab Penetration into the Lower Mantle Earth and Planetary Science Letters 140 1 4 27 39 Bibcode 1996E amp PSL 140 27C doi 10 1016 0012 821x 96 00023 4 Dastanpour Mohammad March 1996 The Devonian System in Iran a review Geological Magazine 133 2 159 170 Bibcode 1996GeoM 133 159D doi 10 1017 S0016756800008670 S2CID 129199671 Dvorkin Jack Nur Amos Mavko Gary Ben Avraham Zvi 1993 Narrow subducting slabs and the origin of backarc basins Tectonophysics 227 1 4 63 79 Bibcode 1993Tectp 227 63D doi 10 1016 0040 1951 93 90087 Z Einsele Gerhard 2000 Sedimentary Basins Evolution Facies and Sediment Budget 2nd ed Springer p 630 ISBN 978 3 540 66193 1 Eiseley Loren 1946 The Great Deeps The Immense Journey 1959 ed United States Vintage Books p 38 41 ISBN 0394701577 Ellouz Zimmermann N Deville E Muller C Lallemant S Subhani A B Tabreez A R 2007 Impact of Sedimentation on Convergent Margin Tectonics Example of the Makran Accretionary Prism Pakistan Thrust Belts and Foreland Basins Frontiers in Earth Sciences 327 350 doi 10 1007 978 3 540 69426 7 17 ISBN 978 3 540 69425 0 Fujikura K Lindsay D Kitazato H Nishida S Shirayama Y 2010 Marine Biodiversity in Japanese Waters PLoS ONE 5 8 e11836 Bibcode 2010PLoSO 511836F doi 10 1371 journal pone 0011836 PMC 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doi 10 1016 S0967 0645 97 00074 X Smith W H F Sandwell D T 1997 Global sea floor topography from satellite altimetry and ship depth soundings Science 277 5334 1956 1962 doi 10 1126 science 277 5334 1956 Stern R J 2002 Subduction Zones Reviews of Geophysics 40 4 1012 1049 Bibcode 2002RvGeo 40 1012S doi 10 1029 2001RG000108 S2CID 247695067 Stern R J 2005 TECTONICS Ocean Trenches Encyclopedia of Geology 428 437 doi 10 1016 B0 12 369396 9 00141 6 ISBN 9780123693969 Thomas C Burbidge D Cummins P 2007 A preliminary study into the tsunami hazard faced by southwest Pacific nations Risk and Impact Analysis Group Geoscience Australia Retrieved 26 September 2021 Thomson C W Murray J 1895 Report on the scientific results of the voyage of H M S Challenger during the years of 1872 76 page 877 19thcenturyscience org Archived from the original on 17 April 2012 Retrieved 26 March 2012 Volker David Geersen Jacob Contreras Reyes Eduardo Sellanes Javier Pantoja Silvio Rabbel Wolfgang Thorwart Martin Reichert Christian Block Martin Weinrebe Wilhelm Reimer October 2014 Morphology and geology of the continental shelf and upper slope of southern Central Chile 33 S 43 S PDF International Journal of Earth Sciences 103 7 1765 1787 Bibcode 2014IJEaS 103 1765V doi 10 1007 s00531 012 0795 y S2CID 129460412 Volker D Weinrebe W Behrmann J H Bialas J Klaeschen D 2009 Mass wasting at the base of the south central Chilean continental margin The Reloca Slide Advances in Geosciences 22 155 167 Bibcode 2009AdG 22 155V doi 10 5194 adgeo 22 155 2009 Volker David Geersen Jacob Contreras Reyes Eduardo Reichert Christian 2013 Sedimentary fill of the Chile Trench 32 46 S Volumetric distribution and causal factors Journal of the Geological Society 170 5 723 736 Bibcode 2013JGSoc 170 723V doi 10 1144 jgs2012 119 S2CID 128432525 Watts A B 2001 Isostasy and Flexure of the Lithosphere Cambridge University Press 458p Weyl Peter K 1969 Oceanography an introduction to the marine environment New York Wiley ISBN 978 0471937449 lt ref gt Westbrook G K Mascle A Biju Duval B 1984 Geophysics and the structure of the Lesser Antilles forearc PDF Initial Reports of the Deep Sea Drilling Project 78 23 38 Retrieved 26 September 2021 Wright D J Bloomer S H MacLeod C J Taylor B Goodlife A M 2000 Bathymetry of the Tonga Trench and Forearc a map series Marine Geophysical Researches 21 489 511 2000 Bibcode 2000MarGR 21 489W doi 10 1023 A 1026514914220 S2CID 6072675 Zhang Ru Yi Huang Ying Qin Wen Jing Quan Zhe Xue June 2021 The complete genome of extracellular protease producing Deinococcus sp D7000 isolated from the hadal region of Mariana Trench Challenger Deep Marine Genomics 57 100832 doi 10 1016 j margen 2020 100832 PMID 33867118 S2CID 229392459 External links Edit HADEX Research project to explore ocean trenches Woods Hole Oceanographic Institution Ocean Trenches Woods Hole Oceanographic Institution Retrieved from https en wikipedia org w index php title Oceanic trench amp oldid 1136429741, wikipedia, 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