The Hellenic subduction zone (HSZ) is the

convergent boundary A convergent boundary (also known as a destructive boundary) is an area on Earth where two or more Plate tectonics, lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can ...

between the

African Plate The African Plate is a major tectonic plate that includes much of the continent of Africa (except for its easternmost part) and the adjacent oceanic crust to the west and south. It is bounded by the North American Plate and South American Plat ...

and the

Aegean Sea Plate The Aegean Sea Plate (also called the Hellenic Plate or Aegean 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

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumu ...

of the African is being

subducted Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

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 The Mediterranean Ridge is a wide ridge in the bed of the Mediterranean Sea, running along a rough quarter circle from Calabria, south of Crete, to the southwest corner of Turkey. It is an accretionary wedge caused by the African Plate subductin ...

accretionary complex 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 d ...

. It has a well-defined

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. Differential motion along the zone produces numerous earthqu ...

seismicity Seismicity is a measure encompassing earthquake occurrences, mechanisms, and magnitude at a given geographical location. As such, it summarizes a region's seismic activity. The term was coined by Beno Gutenberg and Charles Francis Richter in 19 ...

, 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 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 subductio ...

. The descending

slab Slab or SLAB may refer to: Physical materials * Concrete slab, a flat concrete plate used in construction * Stone slab, a flat stone used in construction * Slab (casting), a length of metal * Slab (geology), that portion of a tectonic plate tha ...

has been imaged using

seismic tomography Seismic tomography or seismotomography is a technique for imaging the subsurface of the Earth with seismic waves produced by earthquakes or explosions. P-, S-, and surface waves can be used for tomographic models of different resolutions based on ...

down to the top of the

mantle transition zone The transition zone is part of the Earth's mantle, and is located between the lower mantle and the upper mantle, between a depth of 410 and 660 km (250 to 400 mi). The Earth's mantle, including the transition zone, consists primarily o ...

at 410 km depth.

Surface expression

When the subduction zone was originally identified in the 1970s, it was thought that the

Hellenic trench 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 i ...

was the surface expression of the HSZ. Once the Mediterranean Ridge was recognised as an accretionary complex, the majority of geoscientists regarded the Hellenic trenches as features within the

forearc Forearc is a plate tectonic term referring to a region between an oceanic trench, also known as a subduction zone, and the associated volcanic arc. Forearc regions are present along a convergent margins and eponymously form 'in front of' the vol ...

region of the Hellenic arc, created by some combination of extension, strike-slip or thrusting within the crust of the Aegean Sea Plate. Some geoscientists continue to refer to the Hellenic trench as the surface expression of the subduction zone. The 2,000 km long Mediterranean Ridge is the fastest growing accretionary complex in the world. The size of the complex is a result of a combination of fast rate of convergence combined with the unusually thick sequence of sediments deposited on the Neotethyan oceanic crust, which is thought to be of Jurassic age. The northern edge of the ridge is generally interpreted as a northward-moving backthrust. The degree to which the observed thrust faults within the ridge connect directly with the subduction interface remains unclear, due to poor seismic imaging caused by the presence of a thick layer of Messinian salt.

Slab geometry

Tomographic data indicate that there is no link between the descending HSZ slab and those associated with either the Calabrian arc to the west or the

Cyprus arc The Cyprus arc is part of the plate boundary zone that accommodates the motion of the African Plate relative to the Anatolian Plate. It is an arcuate depression located in the southern reaches of Cyprus. The Cyprus arc is considered to be in col ...

to the east. However, a study of earthquake hypocentres suggests that the shallower part of the zone is continuous with the subduction zone west of Cyprus, with a developing slab tear further to the north in its deeper parts. The HSZ slab is divided into two main segments, the western and eastern, with the division between them running roughly north–south through central

Crete Crete ( el, Κρήτη, translit=, Modern: , Ancient: ) is the largest and most populous of the Greek islands, the 88th largest island in the world and the fifth largest island in the Mediterranean Sea, after Sicily, Sardinia, Cyprus, and ...

. The Wadati–Benioff zone for the western segment dips at about 30° over a depth range of 20–100 km and 45° from 100 to 150 km. The boundary between the two is interpreted to be a slab tear.

Magmatism

The subduction of the African Plate slab has led to the development of a

volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc lo ...

, known as the

South Aegean Volcanic Arc The South Aegean Volcanic Arc is a volcanic arc (chain of volcanoes) 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 n ...

(SAVA). Magmatism began in the early

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58

–

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...

volcanism, stretching from the

Saronic Gulf The Saronic Gulf (Greek: Σαρωνικός κόλπος, ''Saronikós kólpos'') or Gulf of Aegina in Greece is formed between the peninsulas of Attica and Argolis and forms part of the Aegean Sea. It defines the eastern side of the isthmus of Co ...

in the west to

Santorini Santorini ( el, Σαντορίνη, ), officially Thira (Greek: Θήρα ) and classical Greek Thera (English pronunciation ), is an island in the southern Aegean Sea, about 200 km (120 mi) southeast from the Greek mainland. It is the ...

in the east. In the mid to late

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...

, the area of active volcanism spread into the eastern part of the SAVA, with a more varied chemistry, including

tholeiitic The tholeiitic magma series is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma i ...

and calk-alkaline basalts, large amounts of dacite with some

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

. This change in chemistry is thought to represent the effects of regional extension.

Development

There is evidence that more than 1500 km of Neotethyan oceanic crust has been subducted along this structure or earlier versions of it. Broadly northward subduction of Neotethys beneath Eurasia was already established in the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...

. This proceeded by the progressive closing up of different parts of Neotethys with the accretion of intervening continental areas to Eurasia. This involves backstepping of the subduction zone southwards across each microcontinent, so maintaining a continuous slab, as suggested by the tomographic results. The onset of subduction of southern Neotethys was diachronous, starting in the east in the latest

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

(ca. 35 Ma), beneath Crete in the early

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

and in the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Ionian Islands. The initially near planar slab began to fold in the latest

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

(25–23 Ma), associated with differential slab rollback and trench retreat, causing major clockwise rotation of western Greece. From mid-Miocene times (ca. 15 Ma) the slab curvature became more pronounced and southwestern Turkey began to rotate anticlockwise. During the period a major tear developed between the main part of the HSZ and the Western Cyprus zone, forming a

slab window In geology, a slab window is a gap that forms in a subducted oceanic plate when a mid-ocean ridge meets with a subduction zone and plate divergence at the ridge and convergence at the subduction zone continue, causing the ridge to be subducted ...

in the deeper part of the slab. Currently, the rate of movement along the HSZ is estimated to be about 35 mm per year. However, the overall convergence between the African and Eurasian plates is only about 5 mm per year. This discrepancy is consistent with continuing slab rollback and relatively fast southward movement of the Aegean Sea Plate, accompanied by ongoing extension within that plate.

References

{{Reflist Subduction zones Geology of Greece