Unraveling the geological and tectonic setting of Crete island along a north-south transect in central Crete, Greece

Charalambos Fassulas, Adamantios Kilias, Aikaterini Pavlaki, Sean Gallen, Jeffrey Rahl, and Demosthenis Mountrakis

The island of Crete is located at the centre of the Hellenic arc and just over the active Hellenic subduction zone, holding thus a key role in the understanding of the development of the recent orogenic processes and the active geodynamic regime in south Aegean. At least seven main nappes built up the island of Crete, all exposed at its central part, mainly around the Psiloritis mountains, which host the highest summit of the island at 2456 m. Most of these rocks have their equivalence or are the direct continuation of the mainland Greece geotectonic units. The backbone of the whole area of Crete forms the Plattenkalk or Kriti-Mani unit, which is considered as the metamorphic equivalent of the Ionian zone. It forms a continuous carbonate platform of more than 5 kms in thickness, developed from the Permian to Oligocene times. The whole stratigraphy is displayed in excellent outcrops in the area of Talea Ori mountains, the northern extension of Psiloritis; in several areas even in inverted form. Over the Plattenkalk and in direct tectonic contact are to be found either the group of rocks that constitute the Phyllite-quartzites nappe, which exposes a similar to Plattenkalk, high pressure metamorphism of Late Oligocene/Early Miocene times, or due to its disappearance, the overlying Tripolitsa carbonate nappe. As in mainland Greece, the typical Pindos nappe occurs over the Tripolitsa in several places in Psiloritis mountains, but mainly at the southern ridge of the Asteroussia mountains.
All these external Hellenides rocks are overthrusted in Crete by three more nappes that could be considered as part of the Internal Hellenides. Over Pindos a composite group of rocks or according to some researches, distinct tectonic units, form a peculiar mélange which includes a variety of rock types like blueschists (Preveli Group), medium temperature metamorphics and metabasalts (Spili group), pillow lavas and radiolarites (Arvi group) and unmetamorphosed marine sediments (Vatos group). The majority of these groups crops out in central Psiloritis area, at its southwestern margins and mainly in Asteroussia Mts. On top of these mélange rests a crystalline basement called Asteroussia nappe with high temperature metamorphosed rocks of Upper Cretaceous age. The uppermost tectonic unit of Crete is constituted by the upper Jurassic Ophiolites.
This complicated tectonic setting serves to study the late orogenic development of the Hellenides, but also the early, Eocene/Oligocene mountain building process that emplaced the Internal Hellenide rocks over the Pindos and Tripolitsa Units. Well preserved thrusts, reverse faults and intense folding, like the famous Agios Pavlos folds southwest of Psiloritis, are preserved mainly in Pindos rocks, in some tectonic units of the mélange group and in quite a few tectonic contacts of the uppermost nappes, as these have been reactivated in later tectonic phases.
The most outstanding geological process though, that can be studied in central Crete is related with the unroofing and the exhumation of the lowermost Plattenkalk and Phyllites-quartzite, high pressure metamorphic rocks in late Miocene times. The earlier Late Oligocene/Early Miocene underplatting process is documented by a residual paragenesis with mainly carpholite, which can be found in the Phyllite-quartzites and Plattenkalk rocks but also, by the intense folding of the Plattenkalk series which exposes in all scales intense recumbent, tight and even isoclinals folds with east-west trending b-axis, that are associated with thrusts, reverse faults, and boudinage of the competent silica layers in the platy marble. The Vossakos Section in Talea Ori offers a marvelous outcrop of all these structures and their relation with the metamorphic processes. According to zircon datings the unroofing of the lower, high pressure metamorphosed rocks was a very fast process. Most researchers agree that it took place during a north-south crustal extension which resulted in low-angle detachment faulting at the lower crust and high angle, listric normal faulting on the upper crust, which prevailed in all nappes at the latest exhumation stages. Due to this crustal extension all nappes have been reemplaced, the Phyllite-quartzite nappe has been boudinaged and totally squeezed between Plattenkalk and Tripolitsa carbonates (like in Psiloritis mountains), and all contacts of the upper nappes appear now as normal faults. A typical succession of rocks from the Tripolitsa till the uppermost Ophiolites nappes, that have been affected by the Late Miocene extension can be seen along the Gonies Section of the Psiloritis UNESCO Global Geopark.
The neotectonic activity in Crete can be considered as the direct continuation of the Late Miocene exhumation process. The first tectonic basins were formed by the lystric normal faults which fragmented the nappe pile and resulted in the grabens of Mylopotamos, Rethimnon and Messara along the east-west orientation. Such east-west trending normal faults dominate at the southern flanks of Psiloritis mountains constituting the northern borders of the Messara basin. Detailed studies in the Heraklion basin though, have shown that this regime did not last for long as in Late Miocene/early Pliocene, large north-south trending faults fragmented again the area, due to an arc parallel extension, forming large depressions and deep basins. The Herakion basin which rests between Psiloritis and Lassithi mountains is an example. Such north-south trending faults appear active in our days but only in western and eastern Crete. In Central Crete though, younger northwest-southeast faults (like the Spili fault in Amari), as well as northeast-southwest faults (like the Assimi and Klima faults in Messara) appear to be active, indicating probably a universal extension for the area due to its geographical location just at the centre of the island or due to a transtentional regime developed in the island as a result of the lateral plate convergence south of Crete.
Nowadays many studies have documented that the island of Crete is tectonically uplifted with rates at about 1 – 1,2 mm/year, with the highest rates observed at the western and eastern parts. Studies in the southern coast of Asteroussia mountains have attested that this tectonic uplift is related to offshore normal faulting. It has been proposed that continuous underplatting of continental sediments bellow Crete may result in the crustal extension and tectonic uplift of Crete, on a similar way that it had happened in Late Miocene times. Focal mechanism solutions support such an interpretation as only the medium depth earthquakes nearby and below Crete are related with compressional tectonics, whereas as swallow seismicity is mainly related with normal or lateral faulting.

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