The objective of this article is to use the geology and tectonics of a critical part of the Tethyan orogen, represented by Greece and Albania, to shed light on the tectonic development of Tethys on a regional to global scale. A review of existing Tethyan reconstructions reveals little consensus concerning key aspects, such as the timing and direction of subduction, arc magmatism, ophiolite genesis, and continental collision. The regional to local-scale geology of individual regions, therefore, has to be considered in detail to test existing models and to develop a viable tectonic reconstruction. For Carboniferous time, much evidence suggests that the Korabi–Pelagonian crustal unit as exposed in Albania and Greece formed above a northward-dipping subduction zone along the Eurasian continental margin, with Palaeotethys to the south. However, there is also evidence of southward subduction beneath Gondwana, especially from southern Greece and central-southern Turkey. Palaeotethys is inferred to have closed in Europe as far to the east as the longitude of Libya, although remaining open beyond this region. Uncertainty still exists regarding the Pangaea A-type reconstruction that would restore all the present units in the area within the Eastern Mediterranean region versus the Pangaea B-type reconstruction that would require right-lateral displacement of exotic terranes by up to 3500 km eastwards. In either reconstruction, fragments of the Variscan collisional orogen are likely to have been displaced variable distances eastwards in the Balkan region prior to Late Permian–Early Triassic time. From the ∼Late Permian, the Greece–Albania crustal units were located in their present relative position within Tethys as a whole. From the mid-Permian onwards, the northern margin of Gondwana was affected by crustal extension. A Mesozoic ocean (i.e. the Pindos–Mirdita Ocean) then rifted during Early–Middle Triassic time, culminating in final continental break-up and sea-floor spreading during the Late Triassic (Carnian–Norian). Subduction-influenced volcanics of mainly Early–Middle Triassic age probably reflect the extraction of magma from sub-continental lithosphere that was enriched in subduction-related fluids and volatiles during an earlier Variscan(?) subduction event. The existence of Late Triassic mid-ocean ridge-type igneous rocks, known locally in Albania and Greece, points to rifting of a Red Sea-type oceanic basin rather than a back-arc basin related to contemporaneous subduction. After initial, inferred slow spreading at a Late Triassic, rifted ocean ridge and spreading during the Early Jurassic, the ocean basin underwent regional convergence. Subduction was initiated at, or near, a spreading axis perhaps adjacent to an oceanic fracture zone. The Jurassic supra-subduction zone-type ophiolites of both Greece and Albania largely relate to melting of rising asthenosphere in the presence of aqueous volatiles derived from subducting oceanic lithosphere. High-magnesian boninite-type magmas that are present both in the Albanian and Greece ophiolites and in some underlying melanges reflect remelting of previously depleted oceanic upper mantle. Localized MOR-type ophiolites of Late–Middle Jurassic age, mainly exposed in northeast Albania, were created at a rifted spreading axis. The amphibolite-facies metamorphic sole of the ophiolites was mainly derived from the oceanic crust (including within-plate-type seamounts), whereas the underlying lower-grade, greenschist-facies sole was mainly sourced from the rifted continental margin. The melange, dismembered thrust sheets, and polymict debris flows (olistostromes) beneath the ophiolites formed by accretion and gravity reworking of continental margin units. The in situ radiolarian chert cover of the ophiolites in northern Albania is overlain by polymict debris flows (olistostromes). Pelagic carbonate deposition followed during Tithonian–Berriasian time, and then a regional carbonate platform was restored during the Cretaceous. Exhumation of deeply buried parts of the overridden continental margin probably took place during the Early Cretaceous. Structural evidence, mainly from northern Greece (Vourinos, Pindos, and Othris areas), indicates that the ophiolites, the metamorphic sole, the accretionary melange, and the underlying continental margin units were all deformed by top-to-the-northeast thrusting during late Middle–early Late Jurassic time. However, such kinematic evidence is not obviously replicated in Albania, characterized by reports of ∼southwest-directed (or variable) emplacement. The remaining Pindos–Mirdita oceanic crust subducted ∼northeastwards during Late Cretaceous–Eocene time, whereas oceanic crust continued to form in the south Aegean region at least locally during Late Cretaceous time. During early Cenozoic time, the Pindos–Mirdita Ocean closed progressively southwards, triggering mainly southward progradation of turbidites derived from the overriding Korabi–Pelagonian microcontinent. Smaller volumes of sediment were also derived from the Apulia (Adria) continent. The Mesohellenic Trough of Greece and its counterpart in Albania evolved from an Eocene fore-arc-type basin above subducting oceanic lithosphere to a thrust-top basin as continental crust continued to underthrust during the Oligocene after final closure of the Pindos–Mirdita Ocean. Miocene and Plio-Quaternary successor flexural fore-deeps developed in response to continuing regional plate convergence. The preferred tectonic alternatives are assembled into a new overall tectonic model, which in turn needs to be tested and developed in the light of future studies.
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Dec 1, 2011
Hathaithip Thassanapak + 3
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