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= Gulf of Corinth Basin =

Geological Setting


The Gulf of Corinth basin is a rapidly subsiding, half-graben, extensional marine sedimentary basin dating back to the late Miocene-early Pleistocene (1-2 Ma). The basin separates the Peloponnese from the western mainland of Greece. The gulf is 105 km long, 30 km wide, and has a maximum depth of 900 m. The current rifting rate with respect to the Eurasia Plate is 10-15 mm/yr. The basin is bound in the east by the Isthmus of Corinth and in the west by the Strait of Rion. It is bound in the north by the Anatolian Fault and in the south by Peloponnese highlands. The northern side of the basin is Mesozoic-aged limestone, and the southern side of the basin is Pliocene-aged marine and lacustrine strata overlain by Pleistocene fluvial and lacostrene sediments. Tectonic movement of the gulf is similar to that in parts of Iceland and Turkey. Seismically, it is a very active area. The active and inactive faults of the basin result in a syn-rift sediment fill, which provides a great opportunity for the study of the tectonic and stratigraphic development of a rift. This helps us better understand how a basin is actually made.

A combination of factors contribute to the crustal extension of this basin. The North Anatolian Fault moved westward, and the thickened Hellenide crust went through gravitational collapse. As the African plate subducted beneath the Aegean Plate it pulled the over-riding plate with it. This creates extension. Rifting then occurs when the plate stretches and weakens the thickened crust. This caused it to collapse on itself, and the basin is created.

Western Fault Architecture (Aigion to Akrata)
The Western Gulf of Corinth has a complex fault structure. The North Eratini and South Eratini faults, each approximately 15 km long, overlap each other completely. This results in the uplift of a prominent basement horst. The south-dipping West Channel fault controls a prominent axial channel that drains the western rift. The basin floor widens at the eastern tip of the West Channel fault. The northern margin then becomes controlled by the south-dipping East Channel fault. The East Channel fault steps 2 km to the north. The East and West Eliki faults control the southern coastline position and its onshore topography. An earlier phase of extension is believed to have been caused by a series of largely inactive faults landward of the south coast.

Central Fault Architecture (Akrata to Xylokastro)
The main fault in the center of the basin is a fault is a north-dipping fault named the Corinth fault. The fault is the southern margin boundary. This fault is actually broken up into two segments: the Derveni fault (the western segment) and the Likoporia fault (the eastern segment). It is a north-dipping fault with a length of about 30 to 40 km. According to available seismic data and coastal morphology, the separation point between the two segments lies at the apex of the coastline at Likoporia. The south-dipping West Antikyra fault and the eastern extension of the East Channel fault act as the northern basin boundary.

Eastern Fault Architecture (Xylokastro to Perachora Peninsula)
The northern basin margin is defined by minor fault segments such as the southward dipping Eastern Antikyra fault. Several inactive buried faults (N. and S. Corinth) produce localized subsidence. The eastern southern margin is controlled by two 12-km-long northwestward dipping Perachora faults and the northward dipping Xylokastro fault.

Alkyonides Gulf
Offshore, the north-dipping West and East Alkyonides faults are the most important. Onshore, the north-dipping Pisia and Skinos faults are the most important. It is believed that the West and East Alkyonides faults are not linked at the surface and the East Alkyonides fault most likely overlaps the western section of the West Alkyonides fault.

Stratigraphy
There is an easily-identified unconformity which acts as a barrier between the younger unit (Unit A) and the older unit (Unit B). This can be seen in the top right of the figure below. Chronostratigraphic interpretation shows that the age of the A/B unconformity is approximately 0.4 Ma. Unit B is poorly stratified and non-reflective, while Unit A is cyclical and well-stratified. The change in sediment character between Units A and B is related to the change in the intensity of glacial and inter-glacial cycles which occurred 0.4Ma. Alternations between highstand marine and lowstand lacustrine conditions mark the cycles seen in Unit A. The marine highstand layers are thin, high-amplitude, low-frequency bands. The lacustrine lowstand layers are thicker, lower-amplitude, and higher-frequency. The distinction between the highstand and lowstand layers becomes less clear more eastward in the basin.

Basin Evolution
The evolution of the basin can be broken up into 3 stages. Stage 1 is pre-2 Ma. Stage 2 is from 2 Ma to 0.4 Ma. Stage 3 is from 0.4 Ma to recent.

Stage 1: pre-2 Ma
The oldest syn-rift sediments in the basin were deposited at this time. The location of the northern rift boundary is not known for this time. The western rift was believed to be wide, caused by a number of faults. The modern offshore basin saw little subsidence due to a lack of deposition. At the eastern rift, onshore extension resulted from the Megara Basin, located south of the modern Alkyonides Gulf.

Stage 2: 2 Ma to 0.4 Ma
During this stage, distributed deformation in the western rift ended. The oldest sediments in the basin were deposited here, and their estimated age is 1-2 Ma. These sediments are broken into 3 main depocentres. In the west, sediments are deposited in a north-thickening half graben. In the center, the sediments thicken southward. In the east, sediments thicken southward.

Stage 3: 0.4 Ma to Recent
Deposition in this stage created a single depocentre. The thickest accumulation here coincides with the thinnest deposition from Stage 1. The Derveni and Likoporia faults control the region of greatest deposition, therefore they experience the greatest displacement. The slip rates of the dominant Derveni and Likoporia faults in the center exceed those of the Eliki fault in the west and the Xylokastro fault in the east.

Earthquakes
There is a well-defined seismogenic zone with a depth between 5.5 and 10 km. It is located under the north Peloponnese, and deepens to 12 km under the northern shore of the gulf. When an earthquake occurs under the norther shore of the gulf, it indicates extensional slip. This extensional slip occurs on a E-W striking, 20-40 degree north-dipping plane. Numerous earthquakes occur in this area. For example, there were 232 earthquakes recorded in the summer of 1993 alone. It is believed that extensional slip on E-W striking, 20 to 40 degree north-dipping planes are caused by earthquakes beneath the northern shore. Fault planes in the Aigion area (like the Helike fault) are much steeper, with dip values between 55 and 70 degrees. Contrasting interpretations are indeed possible, as the geometric connections between outcropping faults and the seismogenic source at depth are still an open debate. An alternative hypothesis is that the steep-dipping faults undergo a progressive downdip curvature and merge into low-angle detachments.