Hellenic subduction zone
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The Hellenic subduction zone (HSZ) is the convergent boundary between the African plate and the Aegean Sea plate, where oceanic crust of the African continent 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.[1] 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.[2]
When the subduction zone was originally identified in the 1970s, it was thought that the Hellenic trench was the surface expression of the HSZ.[3] Once the Mediterranean Ridge was recognised as an accretionary complex, the majority of geoscientists regarded the Hellenic trenches as features within the forearc 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.[4]
The 2,000 km long Mediterranean Ridge is the fastest growing accretionary complex in the world.[5] 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 to the east.[2] 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.[6] 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.[1] 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.[7] The boundary between the two is interpreted to be a slab tear.[1]