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109-633: In the classical theory of its origin the HT is an oceanic trench containing the Hellenic subduction zone , directly related to the subduction of the African plate under the Eurasian plate . Alternate views developed later on additional data question the classical view postulating that the HT may be the result wholly or partially of back-arc extension and slab rollback . The "partial" view hypothesizes that

218-476: 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. There are approximately 50,000 km (31,000 mi) of convergent plate margins worldwide. These are mostly located around

327-457: 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. 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

436-607: 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 km (1.2 sq mi) per year. 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

545-426: A high angle of repose. Over half of all convergent margins are erosive margins. 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

654-531: A normal fault). It goes under the highlands raised by the collision, in this case the Hellenic arc . The two plates moving across each other (a dip-slip movement) generate earthquakes, so the subducted part of the plate is basically a seismic zone, called the Wadati–Benioff zone . As it turns out, further research on the Hellenic Trench revealed that the concept of a subductive trench, where subduction

763-475: A prominent elongated depression of the sea bottom, was first used by Johnstone in his 1923 textbook An Introduction to Oceanography . 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,

872-524: A separate forearc from the previously docked coastal ridge, consisting of strips of the Outer Hellenides in the Ionian and some other zones. Slab rollback moved the subduction zone away from, but not parallel to, the continental coastline. A bathymetric view of the current configuration suggests that an angle was generated on the west by rotating the subduction zone away from the original strike of

981-699: A single ancestor. The Adriatic plate and the Ionian Plate (under the Ionian Sea) were one. Zakynthos was in the line of islands at the edge of the future border between the two plates. Greece lacked its current projection into the Aegean; in fact, the Aegean was not there. At this stage, as early as 30 MYA in the Oligocene , the mainland of the Balkans had been formed by successive waves of subduction of

1090-765: A submarine range thought to be the subduction arc of the African plate under the Anatolian plate. The Strabo Trench does not connect with it. Instead there is a gap, the Rhodian Basin. On its north boundary is the Rhodian Fault, trending NNE, and making the final connection to the Anatolian Fault. The linear distance of around the trough depends on its definition. Various estimates are available. The main requirements for definition are two end points and

1199-558: A top one moving from inner to outer. The extension of the top layer required for this excursion of the arc and the trench comes from thinning of the back-arc ("in back of the arc"), weakening the crust there. There was already a mountain chain north of the arc, a legacy from the Alpine Orogeny , called the "inner arc." Its tops are the Cyclades . In addition, a chain of volcanos has appeared across it, due to magma breaking through

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1308-422: A vigorous motion in the opposite direction, a theoretical paradox requiring additional geological theory to explain. The final solutions were back-arc extension and slab rollback. As the subducting plate, or slab , rolls under the overriding plate, an arc of highlands is pushed up on the margin of the overriding plate. For reasons still not entirely understood, the back of the arc begins to thin and extend, pushing

1417-528: 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 . Trenches were not clearly defined until the late 1940s and 1950s. The bathymetry of

1526-665: Is again deferred until more definitive evidence can be obtained. The Hellenic Trench from the intersection with the KFZ to south of Crete consists of a line of deep-sea basins named after surface features and divided from each other by gravity rises. The three major parts of the western trench are as follows. The KFZ is on the outer border of an archipelago termed (by some) the Southern Ionian Island Chain. The four main islands are Lefkada , Ithaki , Kefalonia , and Zakynthos . The geographical custom in designating

1635-548: Is also the southern peripheral distance around the Hellenic Arc. The Arc is arcuate; the angle is straight lines, another paradox, if one assumes a single subduction. The general geologic answer is that the subduction due to the compression of Africa against Eurasia is a different movement from the southward thrusting of the Aegean Plate. There are two different resultants of all the small motion vectors. The subduction

1744-497: Is an arcuate mountain chain of the southern Aegean Sea located on the southern margin of the Aegean Sea plate . Geologically it results from the subduction of the African plate under it along the Hellenic subduction zone . The Hellenic Trench trends parallel to its southern side. The Aegean Sea plate, a microplate, is often considered part of the Eurasian plate from which it is in the process of diverging. The arc itself

1853-476: 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. 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

1962-422: 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

2071-416: 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

2180-729: Is distributed into three ENE lines, the Ptolemy Trench, the Pliny Trench outside of and parallel to it, and the outer Strabo Trench, parallel to the other two. The overall appearance resembles an arc inscribed in a vertex angle, except for the asymmetry. The three trenches fall short of Rhodes, the Strabo Trench going the farthest east. Between it and the Cyprus Trench are the Anaximander Mountains,

2289-489: 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

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2398-422: 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

2507-609: Is mainly marine, the mountaintops appearing as islands in the Ionian Sea, Crete and its environs, or in the Dodecanese group. It encroaches on mainland terrain in the Peloponnesus, on Crete, on Rhodes, and on the southern coast of Anatolia, thus being encompassed by both Greece and Turkey. The direction of subduction is northward. Locations on the arc or near it on the north side are therefore called "outer" as they are at

2616-460: Is not at 90° to the NW-bearing scarp, but is at 70°–75°. The scarp is believed to be rotating CW away from perpendicularity. Initially the trench was considered the surface expression of African and Eurasian plate collision. Such a view could not be verified because the trench was full of obscuring sediment, and because the arc-shaped Mediterranean Ridge seemed part of the subduction complex. If

2725-409: Is occurring now (if that is what it is), would only strictly apply to the west side; moreover, not all the subduction, wherever it does occur, is due to plate collision. The east side of the trench is not a trench but is a series of ascending scarps of faults where strike slippage is the main movement, due to further complexities discovered later (see this article, below). However, the term "trench" and

2834-484: Is recorded as tectonic mélanges and duplex structures. 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. 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

2943-431: Is termed the subduction, or more rarely subductive, zone. It features an upper plate and a lower plate. The initial line of the subduction, traditionally believed to be located in the trench, and to be at the foot of the margin of the overriding plate, has a direction, the strike. The plate diving down does so at an angle, the dip. The direction of dip is roughly perpendicular, or normal, to the strike (not to be confused with

3052-400: Is that the slab flexes down ("deformation front") further and further back, a phenomenon called " slab roll back ." In geologic terminology, the part of the plate rolling under is termed "negatively buoyant," meaning the segment of combined overriding and overridden plates have not found the depth at which they float over the mantle. One study notes that the rollback of the HT is so severe that

3161-580: Is that throughout these successive subductions there was only one subduction zone acting continuously to convey (as on a conveyor belt) and emplace (obduct) microcontinents broken from the African slab. Between each microcontinent was a local ocean, which was subducted and closed in turn: in the Cenozoic the Vardar , 1,000 kilometres (620 mi) subducted; the Pindos, 500 kilometres (310 mi) subducted; and

3270-638: Is the Kephallenia Fault Zone (KFZ), or Kephallenia Transform Fault (KTF), or Cephalonia–Lefkada Transform Fault Zone (CTF). The Aegean Plate slips along the side of the Adriatic plate in a SSW direction. A second leg trends N60E, which is ENE, to the island of Rhodes , where it ceases. There is not a singular vertex. Prior to reaching its end point the ESE leg has two more vertices, so that the ENE leg

3379-412: 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

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3488-515: 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. Seismic tomography provides evidence for slab rollback. Results demonstrate high temperature anomalies within

3597-563: The Adriatic or Apulian Plate subducts under the Balkans. More recently and rarely the terms "North Hellenic Subduction" and "North Hellenic Trench" have been applied there, rendering the HT and HS into the "South HT" and "South HS." The distinction is based on a differentation of North Hellenides from South Hellenides. The dividing feature is the Gulfs of Patras and Corinth . From their vicinity and southward an extensional regime prevails, while

3706-479: The Global Positioning System , which can detect changes of position in millimeters; i.e., geologic movement, good for measuring geologic velocities. The work done so far indicates that the appearance of symmetry is an illusion based on the shape of the forearc; that is, on the raised arc of the margin of the overriding plate. Bathymetric representations of the Hellenic Trench to the south of

3815-579: The Ionian Islands in the west to just east of the island of Rhodes in the east, where it links to the Cyprus arc . The current geometry of the Hellenic arc is a result of the southwards migration of the subduction zone. This has led to extension both along the line of the arc as it bulged out and extension perpendicular to the arc, which is the current tectonic state. The Hellenic arc is one of

3924-444: The shear stresses at the base of the overriding plate. As slab rollback velocities increase, circular mantle flow velocities also increase, accelerating extension rates. 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

4033-572: 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. Oceanic trenches are 50 to 100 kilometers (30 to 60 mi) wide and have an asymmetric V-shape, with

4142-613: The Abyssal Plain of the Ionian Sea at an angle to the strike of the previously known KFZ. The Plain is the site of the Mesozoic basement that further east is subducted. It is believed the KFZ may extend into it to a depth of as much as 15 kilometres (49,000 ft). As the KFZ may terminate both the HT and the MR on the north, either may be the location of the subduction. The location of the border between Aegean Plate and Ionian Sea Plain

4251-532: The Adriatic on maps, the subduction does not actually do so. The stress of the rotation was too great for the rock. The subducting plate broke along the KTF and also along the Plato–Strabo trench area, forming a parallelogram that slipped outward between the two strike-slip cross-faults. More than one fault was required to release the stress to the east because the velocity of the rotating subduction increases outward along

4360-550: The Adriatic, or Apulian, Plate under the edge of the Balkans was continuous with the Hellenic Trench. One might conclude that the Trench is the location of the subduction and the border of the Aegean Plate, as some have. As it turns out, the Mediterranean Ridge (MR), also arcuate, curves a little more to the north to intersect the KFZ a little further out than the HT. There is evidence that the KFZ projects further into

4469-540: The African plate had surrounded the Aegean Sea and was compressing it inward toward a point in the north Aegean, and that one might expect a mountain range to arise there. The western side of the trench has the appropriate faulting , a destructive convergent border in a reverse fault with a dip under the Hellenic Arc perpendicular to the strike. Further investigation in the second half of the 20th century soon quelled any such speculation. A plate-compressive velocity of

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4578-600: The African plate under the Eurasian, called " thrusts " from their thrusting of the Eurasian plate to the NE. The various forearcs, or "thrust sheets," created by this thrusting had moved to the north and had docked against the preceding, closing the ancient seas between them. Each forearc was a complex of folds, or " nappes ," raised by compression (or "shortening of the crust"), which had a tendency to fall over, creating tilted layers exposed later in highlands. The general hypothesis

4687-470: 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. Convergent margins are classified as erosive or accretionary, and this has a strong influence on the morphology of

4796-626: The Cascadia subduction zone. Sedimentation is largely controlled by whether the trench is near a continental sediment source. 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

4905-534: The Cayman Trough, which is a pull-apart basin within a transform fault zone, 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 . 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

5014-502: The EMNT as a baseline in the CW direction about a vertex, or pole, on the coast of Apulia, Italy. A triangle was formed of the base line, the subduction line, and a chord across the arc of the subtended angle. Currently the vertex opposite the base line does not extend as far as the chord. The east leg curves, shortening the west leg. The curvature demonstrates that the east leg is not as rigid as

5123-412: 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

5232-442: The Hellenic Arc and other related features are lineaments important to the geology primarily of Greece and secondarily of Turkey . Morphology or geomorphology studies the "shapes" (morphai) of the lineaments, while kinesiology studies their "motions" (kineseis). Both topics as used typically in geology articles do not go beyond plane geometry , trigonometry , elementary algebra , and elementary statistics , which are taught at

5341-429: The Hellenic Arc ought to have been in evidence, given the precision with which GPS can measure geological movement. Instead all investigations began to report a closure of the Hellenic Arc on the coast of Africa (or Nubia as is currently said) at various estimated rates that were far larger than the small rate of convergence of Africa on Eurasia. The expected closure of the Hellenic Arc on the north Aegean turned out to be

5450-529: The Hellenic Arc; rather, the line (approximated by the method of small straight segments on the map) to achieve 1200 km must follow the outer edge of the foredeep zone, located toward the midline of the Hellenic Trough. Being further out on the radius of the Arc as a segment of a circle, it has a longer peripheral distance. In this definition "the arc" is both the Hellenic Arc and its foredeep, measured on

5559-400: The Hellenic Trench decoupled from it. However, it cannot be seen under the ridge. Moreover, the Hellenic Arc would not be the forearc, the edge of the Aegean Plate, but this edge would be hidden under the ridge. It would now be necessary to find a reason for the trench. Opinions vary. The search goes on. Historical geology offers reasons for hypothesizing that, in its earlier development, there

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5668-596: 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. 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

5777-578: The Oligocene, evidenced in the zone structure of Greece, was compressional. The subduction was in the Trench and its forearc was the edge of the overriding plate (the classical model). Subsequently, a superimposed extensional regime moved the subduction and the Trench back, but not necessarily at the same rate, nor did they always necessarily coincide. The former reverse faults were converted to normal, and many new extensional lineaments (tectonic features), such as pull-apart basins, appeared. The start line of

5886-744: 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 orogens . Globally, there are over 50 major ocean trenches covering an area of 1.9 million km or about 0.5% of the oceans. Trenches are geomorphologically distinct from troughs . Troughs are elongated depressions of

5995-649: The Zakynthos-Strofades System. The Matapan Deep or Matapan–Vavilov Deep is roughly 5,120 meters (16,797 feet) . The Calypso Deep , located in the Matapan–Vavilov Deep, is roughly 5,267 metres (17,280 feet) deep and is the deepest point in the Mediterranean Sea . The Kithera–Antikithera deep is 4,615 metres (15,141 feet) . The trench and the arc to the north of it, including a strip of southern Anatolia, are home to some of

6104-522: The arc depict a different shape. As far as the major parameters are concerned: fault type, dip, depth, velocity, seismicity, etc., the subduction zone in the trench is asymmetric, which some consider a unique distinction. The zone begins near the Gulf of Corinth and trends ESE in an arc approximating a straight line. It terminates to the south of Crete in an angular vertex. This leg of the HT contains mainly dip-slip faults (a hanging wall slips up or down over

6213-448: The arc in a "back" direction projectively across the foredeep . This extension may or may not happen in a subduction, but if it does, the spread is like the expansion of space, applicable everywhere, but only in a given direction. The entire Aegean Plate comes from this extension behind the Hellenic Arc. Circles on the early plate would eventually have become ellipses pointing in the direction of expansion. The Aegean Plate stretches out to

6322-479: The area of the Southeast Pacific, there have been several rollback events resulting in the formation of numerous back-arc basins. 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. Slab rollback induces mantle return flow, which causes extension from

6431-407: The asymmetry is an area of active research. If the problem is in part a matter of definition of terms, then the answer as far as it goes is a matter of redefinition. One redefinition distinguishes the Hellenic Trench from the Hellenic Trough, or Hellenic Subduction Trough. The Trench is only the foredeep of the Hellenic arc on the west side. It is possibly the location of the line of subduction, but

6540-446: The behaviour of various aquatic species. This is the trench where several earthquakes, including the 365 Crete earthquake , occurred. [REDACTED] Media related to Hellenic Trench at Wikimedia Commons 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

6649-460: 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

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6758-497: 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. 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 completely filled with sediments, as with

6867-423: The concept of "subductive zone" continue to be used of the whole arc, sometimes in questioning quotes, by some sort of analogy, perhaps because the zone once was or could be a convergent subductive plate border. The basis of the analogy is the Hellenic arc, the raised border. It could not have been raised all the way around without a subduction zone all the way around. The search for data revealing possible reasons for

6976-421: The contradiction of the Hellenic Trench not going far enough around the arc to account for the eastern side. The Trough and the zone go all the way around. The Hellenic Arc seen on a map or in high-altitude photographs appears to be, if not actually is, an amphitheater, at least a bilaterally symmetrical arc about a N–S axis, with vertex on Crete, opening to the north. The wings of the arc are somewhat flatter than

7085-590: The current being Oceanic. This Hellenic orogeny to this point was part of the Alpine orogeny . The newly formed Alps connected to the Dinaric Alps , which were continuous with a chain called the Outer Hellenides, the last to form. Each former forearc was its own type of rock, or facies . Mainland Greece thus consists geologically of strips, or isopic zones ("same facies"), or "tectono-stratigraphic units" of distinct rock trending from NW to SE. The regime through

7194-417: The dip of a floor wall). North of its end on the west another subduction zone is created by the Adriatic plate diving under the Balkans, which are in the Eurasian plate proper, and not the Aegean Plate. The subduction line between the two is not continuous; there is a gap of about 100 kilometres (62 mi). Between the south end of the Adriatic plate subduction and the north end of the Aegean Plate subduction

7303-579: The eastern Mediterranean, still being suducted. Between the Vardar and the Pindos was the Pelagian microcontinent; between the Pindos and the Mediterranean was the Apulian (or Adriatic) microcontinent, with 900 kilometres (560 mi) subducted for the two, amounting to a closure of 2,400 kilometres (1,500 mi) between Africa and Eurasia. Individual subductions thus varied between Oceanic and Continental,

7412-426: 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. 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

7521-558: The extension was a transform fault that has been called the Eastern Mediterranean North Transform (EMNT). It trended from the SW corner of Anatolia in a NW direction through the future center of the forearc across Central Greece well north of the future Gulf of Corinth. At some point the new forces began to pull apart the former strike-slip fault north of Anatolia merging it with the subduction, and pulling out

7630-503: The forearc (the raised chain of highlands and islands), or perhaps it was part of a wrinkle in the foredeep produced by compressional motion of the Aegean Plate against the " backstop " of the Mediterranean Ridge. Or, perhaps it was a normal fault, a " half-graben " produced by extension of the Aegean Plate. In these other theories, the subducting plate would start its subduction under the Mediterranean Ridge , and pass under

7739-690: 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 . Also not a trench is the New Caledonia trough, which is an extensional sedimentary basin related to the Tonga-Kermadec subduction zone . Additionally,

7848-410: The high school level. More daunting are the geologic special terms, which are numerous, and continue to be innovated. This article assumes basic knowledge of mathematics and science, but includes parenthetical clues as to the meaning of the special terms as well as links to articles explaining them. In subduction one plate dives under another at a convergent plate boundary and the band across this line

7957-621: The homes of a number of marine mammals, such as Cetaceans, some of which are endangered species threatened by maritime traffic in the Eastern Mediterranean. The study of the overall features of the surface of the Earth has been the concern of plate tectonics since the Plate Tectonics Revolution of the 1970s. It was a development of the continental drift theory of Alfred Wegener . These features are often called lineaments . The Hellenic Trench along with

8066-462: 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 . 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

8175-431: The inner slope of the trench. Erosive margins, such as the northern Peru-Chile, Tonga-Kermadec, and Mariana trenches, correspond to sediment-starved trenches. 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

8284-451: 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 6,000 meters (20,000 ft). 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

8393-515: The larger marine mammals, some of which are endangered species. Accordingly, the ACCOBAMS , an organization based on an international agreement to work for the conservation of these animals, has declared the trench and arc an IMMA, International Marine Mammal Area, and an MPA, Marine Protected Area. For example, the animals are at risk of, and suffer decimation and mutilation from, being run down inadvertently by ships. The ACCOBAMS keeps in contact with

8502-632: 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 10,994 m (36,070 ft) below sea level . Oceanic trenches are

8611-481: The line of subduction, which is an angle at the intersection of two roughly straight lines (see article above). The vertex is to the south of Crete. A leg bears to the NW from there and is 600 kilometres (370 mi) long. The line is a scarp, though not visible because the trench has filled with sediment. A second leg bears to the NE and is 400 kilometres (250 mi) long, for a total of 1,000 kilometres (620 mi), which

8720-411: 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

8829-461: The navies of its members to try to avoid adverse encounters. Sometimes it conducts rescues of animals, and polices against hunting. The ACCOBAMS's Scientific Committee conducts investigations, manages data, and makes recommendations to member countries. Those currently include every state that borders on the Mediterranean. The Hellenic trench region is an ecosystem to sperm whales and other aquatic life and has been used by marine biologists to study

8938-424: The negative buoyancy is the major cause of subduction; that is, northward thrusting of the African plate still is present, but the slab has already started to flex long before it gets to the point where thrusting makes a difference. But there are other complexities as well. A number of mapping techniques have been applied to research the arc zone, such as seafloor mapping , reflection seismology , and application of

9047-433: The north remains in a compressional. The Hellenides are the mountains of Greece, divided into an inner and outer range. The extensional regime cuts across them transversely, producing four quarters. The South Hellenic Subduction Zone, and the Hellenic Trench, if different (many still consider them not to be so) are located in the southern outer Hellenides. Meanwhile, the deep basins of the Trench and their marine ecologies are

9156-437: The ocean floor, there is concern that plastic debris may accumulate in trenches and endanger the fragile trench biomes. Recent measurements, where the salinity and temperature of the water was measured throughout the dive, have uncertainties of about 15 m (49 ft). Older measurements may be off by hundreds of meters. (*) The five deepest trenches in the world Hellenic arc The Hellenic arc or Aegean arc

9265-530: The ocean was poorly known prior to the Challenger expedition of 1872–1876, which took 492 soundings of the deep ocean. At station #225, the expedition discovered Challenger Deep , 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. The term trench , in its modern sense of

9374-400: The outer margin of the plate. Locations further north are "inner." Generally the motion of subduction is from outer to inner. It so happens that, due to back-arc extension, the Hellenic Arc and Trench are moving in the reverse direction, from inner to outer, accounting for the severe arcuate form. There are in essence two layers at the subduction zone, a bottom one moving from outer to inner, and

9483-557: The outer periphery. The northern end point is more solid, being located on or near the Cephalonia–Lefkada Transform Fault Zone, generally agreed to be the northern edge of the subduction zone. The southern end point is placed arbitrarily in the Rhodes Basin at the end of the Hellenic subduction. No point chosen there would cause significant variation in the 1200 km length. Another source concentrates on

9592-415: 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). The unique interplay of these forces

9701-512: 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

9810-510: The radius of rotation. The surface expression of the KFZ appears to come to an end on the west at 37°48′N 20°00′E  /  37.8°N 20.0°E  / 37.8; 20.0 . It is generally sgreed that the fault represents the offsetting of the Hellenic Arc from the Hellenides north of the Gulf of Corinth due to Aegean Plate extension. Prior to the offset, the subduction zone of

9919-525: 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

10028-416: 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

10137-460: The shape of the path between them. One source specifying end points of " 37°30′N 20°00′E  /  37.5°N 20.0°E  / 37.5; 20.0 offshore the island of Zakynthos" and " 36°00′N 29°00′E  /  36.0°N 29.0°E  / 36.0; 29.0 offshore of the island of Rhodes" offers an arcuate distance of 1,200 kilometres (750 mi) for "the arc," here used loosely. Neither coordinates are on or next to

10246-416: The south, becoming thin and shallow, allowing a volcanic arc to break out 200 kilometres (120 mi) to the north of the Hellenic Arc, which is moving to the south on the edge of the extension. There are two layers on the overriding plate margin: the contact surface with the subducting plate, and a thinned surface layer moving "back." As it does the trench must move back, "consuming" more plate. The mechanism

10355-444: 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. The bottom of the trench marks the boundary between the subducting and overriding plates, known as the basal plate boundary shear or the subduction décollement . The depth of the trench depends on the starting depth of the oceanic lithosphere as it begins its plunge into

10464-399: The strike of the subducting plate is in the Hellenic Trench (often termed "the classical view"), then it is far distant from the accretionary ridge supposed to have been accreted there. Subsequent data, especially earthquake, made possible other theories. Perhaps the bottom of the trench did not connect with (was decoupled from) the subducting plate at all but was a "pull apart" fault basin in

10573-477: 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). 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

10682-516: The subduction begins with the first flexure of the African plate downward (deformation front), which at least one source places at the Libyan continental margin. The Mediterranean Ridge is in this theory an accretionary complex associated with the subduction; that is, a collection of loose material left over from previous subduction. The term "subduction zone" also includes the slab of the overridden Wadati–Benioff zone . These definitions appear to solve

10791-416: The subduction décollement to propagate for great distances to produce megathrust earthquakes. 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. This is called trench rollback or hinge retreat (also hinge rollback ) and is one explanation for

10900-399: The subsequent subhorizontal mantle flow from the displacement of the slab, can result in formation of a back-arc basin. 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

11009-455: The trench become increasingly lithified , 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

11118-513: 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

11227-458: The vertex. The radius has been calculated at 400 kilometres (250 mi), which places the center at about 38°30′00″N 25°30′00″E  /  38.50000°N 25.50000°E  / 38.50000; 25.50000 , in the middle of the north Aegean Sea. The parallel trend of the volcanic arc at a radius of 200 kilometres (120 mi) seems to give some approximate verification. One might suppose at first glance that some anomalous curvature of

11336-690: The waters between an island and the mainland is to call it a basin: the Zakynthos Basin (ZB), etc. The Southern Ionians also include the diminutive islands around the larger, including the two small islands to the south of Zakynthos, the Strofades . They and Zakynthos are joined by the submarine Zakynthos–Strofades Ridge. The waters around Zakynthos are the ZB; around the Strofades, the SB. The two together are

11445-483: The weakened crust; hence, this "inner arc" is termed the South Aegean Volcanic Arc . The two arcs are considered distinct, being from different orogenies . The term "Hellenic Arc" most often refers to the marginal, or "non-volcanic" arc, also called the Aegean forearc in the direction from outer to inner, which is consonant with the Hellenic Trench being the foredeep. The Hellenic arc extends from

11554-429: The west. Plate consumption varies slightly over the west leg but falls off sharply over the east. It is hypothesized that the consumption on the east is expressed by short segments cutting across the scarps, which nevertheless have slip vectors aligned with the western vectors over the entire arc in a wheel-spoke pattern; that is, the azimuths of the vectors decrease regularly from west to east. Though often shown crossing

11663-429: 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

11772-428: The western leg of the HT, Ionian Sea east to eastern Crete, exhibits the line of subduction and therefore is an oceanic trench. The "not at all" view, relying on the theory that the subduction line is under or south of the Mediterranean Ridge , questions whether any of the HT is currently subductional. If not, it is merely a legacy, a remnant of a previous subduction zone that has gone elsewhere. North of this subduction

11881-555: Was one trench traversing what is now the Aegean, and that it contained the subduction zone and the edge of the Eurasian Continent. If one imagines all the geologic changes brought about by extension to be reversed, then all the islands descend from an ancestral Hellenic Arc traversing the North Aegean. The Gulf of Patras was closed, as well as the Gulf of Corinth . Lefkadi, Ithaki, and Kefalonia were telescoped into

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