The Hellenic Trench (HT) is an oceanic trough located in the forearc of the Hellenic arc, an arcuate archipelago on the southern margin of the Aegean Sea plate, or Aegean Plate, also called Aegea, the basement of the Aegean Sea. The HT begins in the Ionian Sea near the mouth of the Gulf of Corinth and curves to the south, following the margin of the Aegean Sea. It passes close to the south shore of Crete and ends near the island of Rhodes just offshore Anatolia.
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. 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 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 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.
Hellenic subduction zone
Subduction applied to the trench 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. 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. "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."
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). "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...."
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.An example of a recent map continuing to label both west and east sides the "subduction zone" can be viewed in 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 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..." 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. "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 ...." 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.
Subduction zone geometry Hellenic Arc amphitheater
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 the vertex. The radius has been calculated at , which places the center at about , in the middle of the north Aegean Sea. The parallel trend of the volcanic arc at a radius of 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.For example, 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 ...."
Aegean Plate extension
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. "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." 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 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." "This demonstrates complete decoupling between the slab and the overlying Aegea that is in extension, instead of compression, parallel to the arc." 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. "This leaves slab buoyancy as the dominant process driving subduction despite the concomitant convergence between Africa and Eurasia." 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.