Tethyan evolution and continental collision in Georgia
Published:December 21, 2017
Shota Adamia, A. Chabukiani, T. Chkhotua, O. Enukidze, N. Sadradze, G. Zakariadze, 2017. "Tethyan evolution and continental collision in Georgia", Tectonic Evolution, Collision, and Seismicity of Southwest Asia: In Honor of Manuel Berberian’s Forty-Five Years of Research Contributions, Rasoul Sorkhabi
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Georgia, the westernmost part of the southern Caucasus, is located at the junction of the European and Asiatic branches of the Alpine-Himalayan orogenic belt and represents a unique area where the Tethys Ocean was completely closed only in the late Cenozoic as a result of prolonged convergence between the Eurasian and Africa-Arabian plates. During the Neoproterozoic–early Cenozoic, the territory of Georgia and the adjacent area of the Caucasus were parts of the Tethys Ocean and its northern margin. The Tethys was not a single continuous oceanic plate, but rather developed in branches separating continental terranes of different sizes, which rifted and drifted away from the Gondwana margin and eventually collided with Laurasia. Prior to the final collision in the late Cenozoic, the region hosted systems of islandarc, intra-arc, and back-arc basins located between the East European continent and the oceanic basins of the Tethys. Integrative geological and paleogeographical studies show a collage of several tectonic units (terranes) in Georgia and adjoining areas that have distinctive geological histories with marine Tethyan, Eurasian, or Gondwanan affinities. These include the Scythian platform, the Great Caucasus, the Transcaucasus-Pontides, and the Lesser Caucasus–Alborz–West Iran regions. Their position between the Africa-Arabian and Eurasiatic continents provides a reason for grouping them into the Northern Tethyan (Eurasian) and Southern Tethyan (Gondwanan) domains. The Scythian platform, Great Caucasus, and Transcaucasus-Pontian belts are of North Tethyan origin, while Anatolia, Taurus, Iran, and the southern Lesser Caucasus belong to the South Tethys. The Arabia-Nubian Shield, at the end of the Proterozoic, experienced basement consolidation related to the final stages of the Pan-African cycle of tectogenesis. In contrast to the southern Lesser Caucasus (Daralagöz), the Transcaucasus did not undergo this process because it broke away from the Arabia-Nubia Shield and, during Cambrian–Devonian times, drifted deep into the Prototethys toward the northern (Baltica) continent. During the early–middle Paleozoic, in the wake of northward-migrating Gondwanan fragments, the Paleotethyan basin formed, and, in the Ordovician, along its border with the Transcaucasus, subduction of oceanic crust occurred, accompanied by suprasubduction volcanic eruptions. Northward migration of the Transcaucasus throughout the Paleozoic caused narrowing of the Prototethys and its transformation into an oceanic back-arc (Dizi) basin. Fragments of Paleotethyan crust are found along the southern border of the Transcaucasus, within accretionary complexes of the Lesser Caucasus ophiolite suture, and in the Pontides. Relicts of Paleotethyan crust crop out in Iranian Garadagh. During the late Paleozoic–early Mesozoic, the oceanic basin separating the Africa-Arabian continent from the Taurus-Anatolian-Iranian platformal domain gradually extended. During this phase, only the Central Iranian terrane separated from Gondwana, drifted northward, and collided with the Eurasian continent in the Late Triassic. The Taurus-Anatolian terranes separated from Gondwana later, in the Early–Middle Jurassic. During the Mesozoic–Cenozoic, Daralagöz represented the northwesternmost margin of the Central Iranian platform and was separated from the North Anatolian platform by an oceanic or back-arc basin (Khoy basin), which within the modern structure is represented by Mesozoic–Cenozoic ophiolites of Urumieh-Khoy and Van. The Paleozoic–Paleogene evolution of the North Tethyan domain was marked by major magmatic events corresponding to the Pacific-type and Mediterranean stages of Tethyan development. The precollisional magmatic assemblages reflect a variety of paleogeographic environments. They are indicative of a west Pacific-type oceanic setting in which a mature, Andean-type continental arc developed. There were several episodes of oceanic lithospheric obduction onto the continental terranes of the region; these included (1) the middle–late Paleozoic, during which basite-ultrabasite complexes were thrust over the island-arc system of the Transcaucasus and the Main Range zone of the Great Caucasus; (2) pre–Late Triassic obduction in the Lesser Caucasus; and (3) pre–Late Jurassic obduction during which ultrabasic rocks were thrust over the continental block of the Artvin-Bolnisi zone. The metabasites apparently represent Paleotethyan fragments. Final closing of the oceanic and back-arc basins, head-on continent-continent collision, topographic inversion, and formation of the present-day structures in Georgia and adjacent countries took place in the late Cenozoic.