User:Mikenorton/Hellenic Trench

Main bathymetric features within the Hellenic Trench

The Hellenic Trench is a major bathymetric trough in the eastern part of the Mediterranean Sea. It has an overall arcuate form, but in detail consists of several separate linear depressions, including the Ptolemy, Pliny and Strabo trenches in its eastern part. These lows lie just south of the Hellenic arc. The origin of these troughs remains a matter of debate, with different groups of geoscientists arguing for it being the surface expression of the Hellenic subduction zone, as an oceanic trench, or basins within the forearc to this plate boundary, created by either thrust faulting, strike-slip faulting, normal faulting or a combination of these.

Extent

The Hellenic Trench (HT) in the broad sense extends from the Ionian Islands in the west, to Rhodes in the east, passing south of the island of Crete in its central section. The western part of the HT is sometimes known as the Hellenic Trench (in the narrow sense), but the names Ionian Trench and Matapan Trench are also used. In the eastern part of the HT there are three main linear troughs with an overall en echelon geometry, named the Ptolemy, Pliny and Strabo trenches, before ending at the Rhodes Basin.

Tectonic setting

The Hellenic Trench is part of the larger Hellenic arc structure related to the ongoing subduction of oceanic crust of the African Plate beneath the Aegean Sea Plate. The pronounced arcuate shape of the trench and the Hellenic Arc to the north of it are thought to be a result of southward migration of the subduction zone as a result of trench rollback. This has led to large amounts of extension, particularly in the back-arc region. The Hellenic arc has also been extended as it bowed out, with active extension both along the length of the arc and perpendicular to it.

Origin

When it was first identified as a bathymetric trough, the Hellenic Trench was interpreted to be an oceanic trench, the surface expression of the Hellenic subduction zone. Although this interpretation still appears in many books and scholarly publications about the area, an alternative explanation for the formation of the trench was proposed in 1982. This followed recognition of active folding and thrusting within the Mediterranean Ridge to the south of the trench. In this model, the ridge is interpreted to be an accretionary complex, with the plate boundary lying to the south, making the trench a basin within the southern part of the forearc of the plate boundary, entirely within the Aegean Sea Plate.

In the forearc basin model, the style of faulting responsible for the form of the various trenches within the overall Hellenic Trench is also debated. Thrust faulting along a major splay fault, linking back to the plate interface, has been interpreted to control the form of the Ionian Trench. A similar geometry has been assumed for the fault that gave rise to the 365 Crete earthquake. The presence of a major splay thrust fault beneath Crete provides an explanation of the continuing uplift of the island. The trenches in the eastern part of the overall Hellenic Trench have been attributed to dominant left lateral strike-slip faulting, formed as Riedel shears within a zone of overall left lateral shear. All of the observed active faults on the islands of Rhodes and Crete, however, show dominant extension and this has led to the hypothesis that many of the trenches are a result of extensional faulting. An alternative explanation for the 365 earthquake as rupturing known extensional faults supports this view. If extensional faulting is the main mechanism for formation of the trenches, the uplift of Crete and other parts of the arc must be due to another process, possibly underplating by sediments from the subducting slab. Faulting in the Mediterranean ridge does not affect the full thickness of the sequence, meaning that at least a few kilometres of sediment are being subducted, but beyond Crete there is no evidence for a significant sedimentary sequence above the slab, supporting the underplating model.