Long narrow depression bordering the Aegean Sea to the south
Hellenic Trench
Bathymetric view of the Mediterranean. The light blue region between mainland Greece and Anatolia represents the Aegean Sea and most of the Aegean Plate on which it is situated. These waters are relatively shallow. The southern margin is occupied by the Hellenic arc, a range of mountains and islands. Parallel to its southern edge is the deep-water Hellenic Trench, at the bottom of which is the line of subduction, a convergent consuming boundary, which is gradually pushed to the south by back-arc extension.
middle arc
Hellas and the Aegean Sea
Show map of Mediterranean
middle arc
middle arc (Greece)
Show map of Greece
Etymology
Republic of Hellas (Greece)
Location
Off the south coast of Crete is at about the center of the trench's arc
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 the western leg of the HT, Ionian Sea east to eastern Crete, exhibits the line of subduction and therefore is an oceanic trench.[3] 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.[4] If not, it is merely a legacy, a remnant of a previous subduction zone that has gone elsewhere.[lower-alpha 1]
North of this subduction 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."[5] 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 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 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 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 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 is termed the subduction, or more rarely subductive, zone. It features an upper plate and a lower plate.[6] 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 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.[7]
As it turns out, further research on the Hellenic Trench revealed that the concept of a subductive trench, where subduction 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).[8]
However, the term "trench" and 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.[9] 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.[10] The search for data revealing possible reasons for 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.[11][lower-alpha 2] It is possibly the location of the line of subduction, but 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 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.[12] The wings of the arc are somewhat flatter than the vertex. The radius has been calculated at 400 kilometres (250mi),[13] which places the center at about 38°30′00″N25°30′00″E / 38.50000°N 25.50000°E / 38.50000; 25.50000, in the middle of the north Aegean Sea.[14] The parallel trend of the volcanic arc at a radius of 200 kilometres (120mi) seems to give some approximate verification.
One might suppose at first glance that some anomalous curvature of 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 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.[15]
The expected closure of the Hellenic Arc on the north Aegean turned out to be 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 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.[16] Circles on the early plate would eventually have become ellipses pointing in the direction of expansion.
The Aegean Plate stretches out to the south, becoming thin and shallow, allowing a volcanic arc to break out 200 kilometres (120mi) 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."[17] As it does the trench must move back, "consuming" more plate. The mechanism 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 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.[18] But there are other complexities as well.
Hellenic Trough morphology
A number of mapping techniques have been applied to research the arc zone, such as seafloor mapping, reflection seismology, and application of 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 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 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 (62mi). Between the south end of the Adriatic plate subduction and the north end of the Aegean Plate subduction is the Kephallenia Fault Zone (KFZ), or Kephallenia Transform Fault (KTF), or Cephalonia–Lefkada Transform Fault Zone (CTF).[lower-alpha 3] The Aegean Plate slips along the side of the Adriatic plate in a SSW direction.[19]
A second leg trends N60E,[lower-alpha 4] 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 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.[20] 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, a submarine range thought to be the subduction arc of the African plate under the Anatolian plate.[lower-alpha 5] 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.
Length of the Hellenic trough
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 the shape of the path between them. One source specifying end points of "37°30′N20°00′E / 37.5°N 20.0°E / 37.5; 20.0 offshore the island of Zakynthos" and "36°00′N29°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 (750mi) for "the arc,"[21] here used loosely. Neither coordinates are on or next to the Hellenic Arc; rather, the line (approximated by the method of small straight segments on the map) to achieve 1200km 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 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 1200km length.
Another source concentrates on 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 (370mi) 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 (250mi) long, for a total of 1,000 kilometres (620mi), which 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.[22] 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 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.[23]
Geologic history of the current regime
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 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 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.[24]
In these other theories, the subducting plate would start its subduction under the Mediterranean Ridge, and pass under the Hellenic Trench decoupled from it. However, it cannot be seen under the ridge.[25] 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.[lower-alpha 6]
Historical geology offers reasons for hypothesizing that, in its earlier development, there was one trench traversing what is now the Aegean, and that it contained the subduction zone and the edge of the Eurasian Continent.
The compressional regime
Isopic zones of Greece. Aqua: "Adriatic", or Ionian, zone. The last to be docked, in place N of the KTF, extended into the islands S of it. Beige: Pindos zone, docked prior to the Ionian. Appears in Central Greece, Peloponnesus, Crete. The Corinthian Gulf, a pull-apart, transects it. Yellow: "Neogene" (extensional) regions.
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 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.[26]
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 the African plate under the Eurasian, called "thrusts"[lower-alpha 7] 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.[27] 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 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 (620mi) subducted; the Pindos, 500 kilometres (310mi) subducted; and 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 (560mi) subducted for the two, amounting to a closure of 2,400 kilometres (1,500mi) between Africa and Eurasia.[28] Individual subductions thus varied between Oceanic and Continental, the current being Oceanic.[29]
This Hellenic orogeny to this point was part of the Alpine orogeny.[30] The newly formed Alps connected to the Dinaric Alps, which were continuous with a chain called the Outer Hellenides, the last to form.[lower-alpha 8] 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.[31][lower-alpha 9]
The regime through the Oligocene, evidenced in the zone structure of Greece, was compressional.[lower-alpha 10] 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 extensional regime
The start line of the extension was a transform fault that has been called the Eastern Mediterranean North Transform (EMNT).[32] 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 a separate forearc from the previously docked coastal ridge, consisting of strips of the Outer Hellenides in the Ionian and some other zones.
CW rotation of the subduction zone
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 the EMNT as a baseline in the CW direction about a vertex, or pole, on the coast of Apulia, Italy.[33] 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 the west.[34] 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,[35] 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.[36][lower-alpha 11]
Though often shown crossing 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 the radius of rotation.
Subduction zone structure
The western trough
The surface expression of the KFZ appears to come to an end on the west at 37°48′N20°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 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 the Abyssal Plain of the Ionian Sea at an angle to the strike of the previously known KFZ.[37] 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 (49,000ft). 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 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.[38] The three major parts of the western trench are as follows.
The Zakynthos-Strophades basins
View of Zakynthos from Alykos, a port on the inner side. The highlands in the background are a remnant of the compressional regime from 30 MYA and before; i.e., part of the forearc. The lowlands in the foreground are an extensional feature; i.e., the back of the arc.
The KFZ is on the outer border of an archipelago termed (by some) the Southern Ionian Island Chain.[39] The four main islands are Lefkada, Ithaki, Kefalonia, and Zakynthos.[lower-alpha 12] The geographical custom in designating 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.[lower-alpha 13] 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 the Zakynthos-Strofades System.[40]
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.
Kithera–Antikithera deep
The Kithera–Antikithera deep is 4,615 metres (15,141 feet).
The eastern trough
Hellenic arc ecology
The trench and the arc to the north of it, including a strip of southern Anatolia, are home to some of 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 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 the behaviour of various aquatic species.
This is the trench where several earthquakes, including the 365 Crete earthquake, occurred.
↑ The morphology of the Hellenic Trench is currently a topic of active research. The traditional view is that the trench extended to the east of the entire Hellenic Arc. That some evidence of subduction did so is not in question. Whether the current trench does so is in question. The best proper terminology is being debated in various articles. This article follows the convention generally but relates questions where relevant.
↑ Most recently geologic researchers have found a way to summarize complex bathymetry, or "measurement of the deep," by referencing the strength of the gravitational field in the atmosphere at the oceanic surface. Highlands, which have more mass, have a slightly stronger gravitational field, while depressions have a weaker, hence the "gravity trough."
↑ The transform fault is a strike-slip fault (a hanging wall slips along the strike of a floor wall).
↑ An azimuth in the US army system of bearings. It gives the number of degrees to the east of north on a 360° compass with N at 0°.
↑ Writers trying to summarize the complex geological circumstances sometimes bundle the Cyprus Trench with the Hellenic Trench. More technically, they are not considered the same, as different plates and different parameters are involved.
↑ Most geological or quasi-geological maps of the feature show a toothed line to represent the subduction, with the teeth pointing to the overriding plate. The strikes of any of these lines are entirely hypothetical. Another device is the profile, a cross-section of the subduction zone. It may show the trench reaching the slab or not, but the depiction is entirely hypothetical. Different depictions depend on different types of data. A total morphology has yet to be acquired.
↑ A thrust fault is a reverse fault with an angle less than 45 degrees, which most subductions are.
↑ "Hellenides" is a geologic term, not a geographic one. It refers to all the mountains of Greece, for which there is no universal geographic name. The designations "inner" and "outer" descend from the obsolete Geosyncline Theory, when the inner was considered to be the center of the Greek geosyncline, while the outer was on the margin. In today's theories, there was no geosyncline (not to be confused with ordinary synclines, which abounded). Now the terms mean roughly the west (outer) and the east (inner) part of Greece. A recent development is the division into northern and southern Hellenides north and south of the Gulf of Corinth. The Hellenic Arc and Trench are in the OH and also the SH. Rather than try to name all the mountains, the geologists assign zones to the inner and outer.
↑ The expression of these zones appears irregular in many places and is fragmented by the many geologic events; nevertheless, the patterns are discernable by geologists, who have published them in numerous representations, such as color maps or labeled drawings. All the fragments cannot be included except on a large geological map. The decision of which to show is arbitrary. Alternative names may be used.
↑ The geologic concept of "regime"," or "tectonic regime," or "stress regime," refers to the type of stress prevalent in a plate or section of a plate, resulting in a specific kind of plate interaction associated with a specific kind of faulting: the compressional regime, resulting in reverse faults, the extensional (normal faults), and the strike-slip. In and around the Aegean Plate, multiple regimes add complexity to the morphology and kinematics.
↑ A motion vector is an arrow representing quantity and direction of motion. Slip vectors in a compressional regime signify the dip-slip of the slab. They are thus the direction of compression. Strictly speaking the dip-slip vector is 3-dimensional but this presentation following the sources uses the projection of the 3-D vector on the surface, while the angle of dip and depth of the locations are stated separately.
↑ Geographically the full set of Ionian Islands include Corfu and Paxi to the north as well as Kythira to the south of the Peloponnesus, based on their proximity to the Ionian Sea.
↑ There are various ways to Anglicize the Greek by varying the f with ph or the d with dh.
↑ Sheppard, Charles, ed. (2018). World Seas: An Environmental Evaluation. Vol.I: Europe, The Americas and West Africa (2nded.). London: Academic Press. p.227. ISBN978-0-12-805068-2. The Hellenic Trench is a major element in the seafloor morphology of the Hellenic Seas. It extends from the northern Ionian Sea to south of Crete in the Libyan Sea to the Rhodes Basin in the Levantine Sea and hosts the deepest basins and troughs of the Mediterranean Sea
↑ A.D. Ansell; R.N. Gibson; Margaret Barnes, eds. (1997). Oceanography And Marine Biology. Annual Review, Volume 35. CRC Press. p.439. ISBN978-0-203-50172-6. The Hellenic part of the Ionian Sea (Eastern Ionian Sea) is characterized by the existence of the deep Hellenic Trench, lying along the western and southwestern Hellenic coast, and the islands of the Cretan Arc (Fig. 3). At the Hellenic Trench, the depths generally exceed 4000m, with a maximum depth of 5121m, just southwest of Peloponnisos ..., which is also the maximum depth of the entire Mediterranean Sea.
↑ Meier, T.; etal. (2007). "A model for the Hellenic subduction zone in the area of Crete based on seismological investigations". In Taymaz, T.; Yilmaz, Y.; Dilek, Y. (eds.). The Geodynamics of the Aegean and Anatolia. Geological Society, Special Publications 291. Vol.291. p.184. doi:10.1144/SP291.9. ISBN9781862392397. S2CID129674811. The widely used term 'Hellenic Trench' commonly used to refer to the distinct topographic feature marking the site where the subducted plate disappears beneath the forearc at a convergent plate boundary may be misleading here.
↑ "subduction zone". Earthquake Glossary. The USGS Earthquake Hazards Program of the U.S. Geological Survey (USGS). Retrieved 31 December 2020.
↑ Prashant Katti (2017). "Slide 18". Island Arcs. "A Wadati–Benioff zone (also Benioff–Wadati zone or Benioff zone or Benioff seismic zone) is a planar zone of seismicity corresponding with the down-going slab in a subduction zone."
↑ Shaw & Jackson 2010, Introduction "A ... south-facing bathymetric scarp runs in an arc between the Peloponnese and Crete, splitting into at least three branches south of Crete, .... Although this scarp is referred to by us, and others, as the Hellenic Trench, it is clearly not the expression of an oceanic trench in the usual sense...."
↑ An example of a recent map continuing to label both west and east sides the "subduction zone" can be viewed in Prashant Katti (2017). "Slide 36". Island Arcs.
↑ Royden & Papanikolaou 2011, pp.2–6 "... the actual trench length of the Hellenides probably varied considerably with time, and was probably much longer when the system extended from the northern Adriatic (Dinarides) through western Turkey..."
↑ Le Pichon 2019, p.1147 "the Hellenic subduction trough ... is best defined by the gravity trough resulting from the bending of the African plate as it is subducted below Aegea. Although we call it the Hellenic subduction zone, this should not be confused with the Hellenic trench. This so-called trench is a forearc structure ...."
↑ For example, Taymaz 1990, p.695 gives "at least 60 mm per year," which takes into consideration the small "north-south convergence between Africa and Europe, known to be about 10 mm ...."
↑ Peterson 1993, p.86 "The Aegean Sea basin is a back-arc extensional island block and trough complex located on the stretched and thinned continental crust of the European plate."
↑ Le Pichon 2019, p.1147 "This demonstrates complete decoupling between the slab and the overlying Aegea that is in extension, instead of compression, parallel to the arc."
↑ Royden & Papanikolaou 2011, p.9 "This leaves slab buoyancy as the dominant process driving subduction despite the concomitant convergence between Africa and Eurasia."
↑ Le Pichon & Angelier 1979, p.3 "Whereas the external Aegean arc as well as the inner volcanic arc approximately follow small circles centered near 38.5°N 25.5°E, the trench system is subangular."
↑ Hieke 2003, p.280 "The detailed internal structure of the MR is still mainly unknown, due to the poor penetration."
↑ Royden & Papanikolaou 2011, p.19, Figure 15. The figure shows successive positions of the Hellenic Trench from the beginning of the extension, including also the development of the Aegean Sea.
↑ Le Pichon & Angelier 1979, p.10 "... there is some systematic difference between measured and computed slip vectors, ... which indicates that this distribution ... might be due to either errors in determining the slip vectors, or to the non-perfect rigidity of the consuming boundary, or both."
↑ Le Pichon & Angelier 1979, p.6 "... NE–SW transform faults join a number of short segments of NW–SE consuming boundaries there (see Figs. 1 and 4)."
↑ Le Pichon & Angelier 1979, p.8 "Table II and Fig. 2 show that the mean azimuth decreases systematically from west to east."
Oceanic trenches are prominent, long, narrow topographic depressions of the ocean floor. They are typically 50 to 100 kilometers wide and 3 to 4 km 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 10,994 m (36,070 ft) below sea level.
The Ryukyu Trench, also called Nansei-Shotō Trench, is a 1398 km (868 mi) long oceanic trench located along the southeastern edge of Japan's Ryukyu Islands in the Philippine Sea in the Pacific Ocean, between northeastern Taiwan and southern Japan. The trench has a maximum depth of 7460 m (24,476 ft). The trench is the result of oceanic crust of the Philippine Plate obliquely subducting beneath the continental crust of the Eurasian Plate at a rate of approximately 52 mm/yr. In conjunction with the adjacent Nankai Trough to the northeast, subduction of the Philippine plate has produced 34 volcanoes. The largest earthquake to have been recorded along the Ryukyu Trench, the 1968 Hyūga-nada earthquake, was magnitude 7.5 and occurred along the northernmost part of the trench on 1 April 1968. This earthquake also produced a tsunami.
The Hunter-Bowen Orogeny was a significant arc accretion event in the Permian and Triassic periods affecting approximately 2,500 km of the Australian continental margin.
The Mornos is a river in Phocis and Aetolia-Acarnania in Greece. It is 70 km (43 mi) long. Its source is in the southwestern part of the Oiti mountains, near the village Mavrolithari, Phocis. It flows towards the south, and enters the Mornos Reservoir near the village Lefkaditi. The dam was completed in 1979. It leaves the reservoir towards the west, near Perivoli. The river continues through a deep, sparsely populated valley, and turns south near Trikorfo. The lower course of the Mornos forms the boundary between Phocis and Aetolia-Acarnania. The Mornos empties into the Gulf of Corinth about 3 km southeast of Nafpaktos.
The Pleistos is a river in central Greece. It drains the Pleistos valley, named after it, a relatively recent rift valley north of the Gulf of Corinth, and parallel to it. They have the same geologic causes. Being situated in karst topography, much of the river runs or seeps through underground channels. The surface stream is intermittent. However, the limestone riverbed reflecting the light gives the appearance of a stream of water.
The geology of Turkey is the product of a wide variety of tectonic processes that have shaped Anatolia over millions of years, a process which continues today as evidenced by frequent earthquakes and occasional volcanic eruptions.
The Izu–Bonin–Mariana (IBM) arc system is a tectonic plate convergent boundary in Micronesia. The IBM arc system extends over 2800 km south from Tokyo, Japan, to beyond Guam, and includes the Izu Islands, the Bonin Islands, and the Mariana Islands; much more of the IBM arc system is submerged below sealevel. The IBM arc system lies along the eastern margin of the Philippine Sea Plate in the Western Pacific Ocean. It is the site of the deepest gash in Earth's solid surface, the Challenger Deep in the Mariana Trench.
An accretionary wedge or accretionary prism forms from sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the downgoing slab of oceanic crust, but in some cases the wedge includes the erosional products of volcanic island arcs formed on the overriding plate.
The South Aegean Volcanic Arc is a volcanic arc in the South Aegean Sea formed by plate tectonics. The prior cause was the subduction of the African Plate beneath the Eurasian Plate, raising the Aegean arc across what is now the North Aegean Sea. It was not yet the sea, nor an arc, or at least not the one it is today, nor was there a chain of volcanoes. In the Holocene, the process of back-arc extension began, probably stimulated by pressure from the Arabian Plate compressing the region behind the arc. The extension deformed the region into its current configuration. First, the arc moved to the south and assumed its arcuate configuration. Second, the Aegean Sea opened behind the arc because the crust was thinned and weakened there. Third, magma broke through the thinned crust to form a second arc composed of a volcanic chain. And finally, the Aegean Sea Plate broke away from Eurasia in the new fault zone to the north.
The Aegean Sea plate is a small tectonic plate located in the eastern Mediterranean Sea under southern Greece and western Turkey. Its southern edge is the Hellenic subduction zone south of Crete, where the African plate is being swept under the Aegean Sea plate. Its northern margin is a divergent boundary with the Eurasian plate.
The Hellenic arc or Aegean arc 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 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 back-arc region is the area behind a volcanic arc. In island volcanic arcs, it consists of back-arc basins of oceanic crust with abyssal depths, which may be separated by remnant arcs, similar to island arcs. In continental arcs, the back-arc region is part of the continental platform, either dry land (subaerial) or forming shallow marine basins.
The 1956 Amorgos earthquake occurred at 03:11 UTC on July 9. It had a magnitude of 7.7 on the moment magnitude scale and a maximum perceived intensity of IX on the Mercalli intensity scale. The epicentre was to the south of the island of Amorgos, the easternmost island of the Cyclades in the Aegean Sea. There was significant damage on Amorgos and the neighbouring island of Santorini. It was the largest earthquake in Greece in the 20th century. It was followed 13 minutes later by a magnitude 7.2 earthquake near Santorini. It triggered a major tsunami with a maximum run-up of 30 m. The combined effects of the earthquake shaking and the tsunami caused the deaths of 53 people with a further 100 injured.
The geology of Sicily records the collision of the Eurasian and the African plates during westward-dipping subduction of the African slab since late Oligocene. Major tectonic units are the Hyblean foreland, the Gela foredeep, the Apenninic-Maghrebian orogen, and the Calabrian Arc. The orogen represents a fold-thrust belt that folds Mesozoic carbonates, while a major volcanic unit is found in an eastern portion of the island. The collision of Africa and Eurasia is a retreating subduction system, such that the descending Africa is falling away from Eurasia, and Eurasia extends and fills the space as the African plate falls into the mantle, resulting in volcanic activity in Sicily and the formation of Tyrrhenian slab to the north.
The geology of Greece is highly structurally complex due to its position at the junction between the European and African tectonic plates.
The subduction tectonics of the Philippines is the control of geology over the Philippine archipelago. The Philippine region is seismically active and has been progressively constructed by plates converging towards each other in multiple directions. The region is also known as the Philippine Mobile Belt due to its complex tectonic setting.
The Hellenic subduction zone (HSZ) is the convergent boundary between the African plate and the Aegean Sea plate, where oceanic crust of the African is being subducted north–northeastwards beneath the Aegean. The southernmost and shallowest part of the zone is obscured beneath the deformed thick sedimentary sequence that forms the Mediterranean Ridge accretionary complex. It has a well-defined Wadati–Benioff zone of seismicity, which demonstrates the relatively shallow dip of its southern part, which increases markedly to the north of the non-volcanic part of the Hellenic arc. The descending slab has been imaged using seismic tomography down to the top of the mantle transition zone at 410 km depth.
Oblique subduction is a form of subduction for which the convergence direction differs from 90° to the plate boundary. Most convergent boundaries involve oblique subduction, particularly in the Ring of Fire including the Ryukyu, Aleutian, Central America and Chile subduction zones. In general, the obliquity angle is between 15° and 30°. Subduction zones with high obliquity angles include Sunda trench and Ryukyu arc.
The Chile Ridge, also known as the Chile Rise, is a submarine oceanic ridge formed by the divergent plate boundary between the Nazca plate and the Antarctic plate. It extends from the triple junction of the Nazca, Pacific, and Antarctic plates to the Southern coast of Chile. The Chile Ridge is easy to recognize on the map, as the ridge is divided into several segmented fracture zones which are perpendicular to the ridge segments, showing an orthogonal shape toward the spreading direction. The total length of the ridge segments is about 550–600 km.
The North Luzon Trough is a major geological feature located off the northern coast of Luzon Island between the Manila Trench and the Vigan-Agao Fault in the Philippines. It is a well-developed forearc basin formed in front of the Luzon Volcanic Arc, an island arc system. The trough is a result of the active subduction of the Philippine Sea Plate beneath the Eurasian Plate.
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