Geology of the Caucasus: a Review
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Turkish Journal of Earth Sciences (Turkish J. Earth Sci.), Vol.S. ADAMIA 20, 2011, ET pp. AL. 489–544. Copyright ©TÜBİTAK doi:10.3906/yer-1005-11 First published online 11 April 2011 Geology of the Caucasus: A Review SHOTA ADAMIA1, GURAM ZAKARIADZE 2, TAMAR CHKHOTUA3, NINO SADRADZE1,3, NINO TSERETELI1, ALEKSANDRE CHABUKIANI1 & ALEKSANDRE GVENTSADZE1 1 M. Nodia Institute of Geophysics, 1/1 M. Alexidze str., 0171, Tbilisi, Georgia (E-mail: [email protected]) 2 Vernadsky Institute of Geochemistry and Analytical Chemistry, RAS, 119991, Moscow, Russia 3 Al. Janelidze Institute of Geology, 1/9 M. Alexidze str., 0193, Tbilisi, Georgia Received 15 May 2010; revised typescripts receipt 30 January 2011 & 14 January 2011; accepted 11 April 2011 Abstract: Th e structure and geological history of the Caucasus are largely determined by its position between the still- converging Eurasian and Africa-Arabian lithospheric plates, within a wide zone of continental collision. During the Late Proterozoic–Early Cenozoic, the region belonged to the Tethys Ocean and its Eurasian and Africa-Arabian margins where there existed a system of island arcs, intra-arc rift s, back-arc basins characteristic of the pre-collisional stage of its evolution of the region. Th e region, along with other fragments that are now exposed in the Upper Precambrian– Cambrian crystalline basement of the Alpine orogenic belt, was separated from western Gondwana during the Early Palaeozoic as a result of back-arc rift ing above a south-dipping subduction zone. Continued rift ing and seafl oor spreading produced the Palaeotethys Ocean in the wake of northward migrating peri-Gondwanan terranes. Th e displacement of the Caucasian and other peri-Gondwanan terranes to the southern margin of Eurasia was completed by ~350 Ma. Widespread emplacement of microcline granite plutons along the active continental margin of southern Eurasia during 330–280 Ma occurred above a north-dipping Palaeotethyan subduction zone. However, Variscan and Eo-Cimmerian–Early Alpine events did not lead to the complete closing of the Palaeozoic Ocean. Th e Mesozoic Tethys in the Caucasus was inherited from the Palaeotethys. In the Mesozoic and Early Cenozoic, the Great Caucasus and Transcaucasus represented the Northtethyan realm – the southern active margin of the Eurasiatic lithospheric plate. Th e Oligocene–Neogene and Quaternary basins situated within the Transcaucasian intermontane depression mark the syn- and post-collisional evolution of the region; these basins represented a part of Paratethys and accumulated sediments of closed and semiclosed type. Th e fi nal collision of the Africa-Arabian and Eurasian plates and formation of the present-day intracontinental mountainous edifi ce of the Caucasus occurred in the Neogene–Quaternary period. From the Late Miocene (c. 9–7 Ma) to the end of the Pleistocene, in the central part of the region, volcanic eruptions in subaerial conditions occurred simultaneously with the formation of molasse troughs. Th e geometry of tectonic deformations in the Transcaucasus is largely determined by the wedge-shaped rigid Arabian block intensively indenting into the Asia Minor-Caucasian region. All structural-morphological lines have a clearly-expressed arcuate northward-convex confi guration refl ecting the contours of the Arabian block. However, farther north, the geometry of the fold-thrust belts is somewhat diff erent – the Achara-Trialeti fold-thrust belt is, on the whole, W–E-trending; the Greater Caucasian fold-thrust belt extends in a WNW–ESE direction. Key Words: Caucasus, convergence, collision, Eurasia, Gondwana, volcanism Kafk asların Jeolojisi Özet: Kafk asların yapısını ve jeolojik tarihini denetleyen ana unsur birbirine yaklaşan Avrasya ve Afrika-Arabistan levhaları arasındaki konumudur. Geç Proterozoyik ile Tersiyer arasında Kafk aslar, Tetis okyanusu ve bu okyanusun Avrasya ve Afrika-Arabistan kıta kenarları içermekteydi; bu sistem içerisinde yer alan ada yayları, yay-içi rift ler, yay-ardı havzalar Kafk asların çarpışma öncesi jeoloji tarihinin bir parçasını teşkil eder. Erken Paleozoyik’te batı Gondwana’nın altına güneye doğru dalan bir dalma-batma zonu üzerinde gelişen yay-ardı rift leşme ile Kafk aslar, ve Alpin orojenik kuşak içinde yer alan diğer üst Prekambriyen–Kambriyen kristalen temel parçaları, Gondwana’dan ayrılmıştır. Kuzeye hareket eden bu Gondwana-çevresi (peri-Gondwana) mıntıkalarının güneyinde Paleotetis okyanusu açılmıştır. Kafk asya ve diğer Gondwana-çevresi mıntıkalarının Avrasya güney kenarını eklenmesi ~350 Ma’de tamamlanmıştır. Avrasya kıta kenarının altına kuzeye doğru dalan bir dalma batma zonu üzerinde yaygın mikroklinli granitoid plutonlarının yerleşimi 320–280 Ma aralığında gerçekleşmiştir. Tüm bu Variskan, Eo-Kimmeriyen ve erken Alpin olaylara rağmen Kafk asların güneyindeki Paleozoyik okyanusunun tamamen kapanmamış, ve Mesozoyik Tetis Paleotetis’ten miras 489 THE GEOLOGY OF THE CAUCASUS kalmıştır. Mesozoyik ve erken Tersiyer’de, Büyük Kafk aslar ve Transkafk asya, Avrasya’nın levhasının güney aktif kıta kenarını, bir diğer ifade ile kuzey Tetis bölgesini temsil ediyordu. Transkafk aya’nın dağ arası çöküntü bölgelerinde gelişen Oligosen–Neojen ve Kuvaterner havzalar bölgenin çarpışma ve çarpışma sonrası evrimini temsil eder. Bu havzalar Paratetisin bir kesimini temsil eder ve sedimanları kapalı veya yarı-kapalı havzalarda çökelmiştir. Afrika-Arabistan ve Avrasya levhalarının nihai çarpışması ve bugünkü kıtalararası Kafk aya dağ kuşağının oluşumu Neojen–Kuvaterner’de meydana gelmiştir. Geç Miyosen’den (9–7 Ma) Pleistosen’in sonuna kadar geçen zamanda Kafk afl arın merkezi kesimlerinde volkanik faaliyetler meydana gelmiş ve molas havzaları oluşmuştur. Transkafk asya’daki tektonik deformasyonun geometrisini kontrol eden ana etken kama şeklinde sert Arabistan blokunun Anadolu-Kafk asya bölgesine saplanmasıdır. Buna bağlı olarak tüm yapısal-morfolojik çizgilerin, Arabistan levhasının kuzey sınırını yansıtan bir şekilde, kuzeye doğru içbükey bir geometri gösterir. Buna karşın daha kuzeyde kıvrım-bindirme kuşaklarının geometrisi farklıdır – Acara-Trialeti kuşağının yönü doğu-batı, Kafk aslar kıvrım- bindirme kuşağının uzanımı ise BKB-DGD’dur. Anahtar Sözcükler: Kafk aslar, yakınlaşan, çarpışma, Avrasya, Gondwana, volkanizma Introduction Azizbekov 1972; Geology of the USSR 1977; Jones & Th e structure and geological evolution of the Simons 1977; Eastern Paratethys 1985; Vincent et al. Caucasian segment of the Black Sea-Caspian Sea 2007; Adamia et al. 2008; Okay et al. 2010). region (Figure 1) are largely determined by its position between the still converging Eurasian and Main Tectonic Units Africa-Arabian lithosphere plates, within a wide zone of continent-continent collision. Problems of Th e Caucasus is divided into several main tectonic Late Proterozoic–Phanerozoic development of this units or terrains (Figure 2). Th ere are platform area have been considered and discussed during (sub-platform, quasi-platform) and fold-thrust the past decades in a great number of publications. units, which from north to south are: the Scythian According to some authors (Khain 1975; Adamia (pre-Caucasus) young platform, the fold-thrust 1975; Adamia et al. 1977, 1981, 2008; Giorgobiani mountain belt of the Great Caucasus including & Zakaraia 1989; Zakariadze et al. 2007), the region zones of the Fore Range, Main Range, and Southern in the Late Proterozoic, Palaeozoic, Mesozoic, and Slope, the Transcaucasian intermontane depression Early Cenozoic belonged to the now-vanished Tethys superimposed mainly on the rigid platform zone Ocean (Prototethys, Palaeotethys, Tethys) and its (Georgian massif), the Achara-Trialeti and the Eurasian and Gondwanan/Africa-Arabian margins. Talysh fold-thrust mountain belts, the Artvin-Bolnisi Within this ocean-continent convergence zone, rigid massif, the Loki (Bayburt)-Karabagh-Kaphan there existed a system of island arcs, intra-arc rift s, fold-thrust mountain belt, the Lesser Caucasus and back-arc basins etc. characteristic of the Late ophiolitic suture, the Lesser Caucasian part of the Proterozoic–Early Cenozoic pre-collisional stage Taurus-Anatolia-Central Iranian platform, and of evolution of the region. During syn-collisional the Aras intermontane depression at the extreme (Oligocene–Middle Miocene) and post-collisional south of the Caucasus. Th e youngest structural unit (Late Miocene–Quaternary) stages of the Late Alpine is composed of Neogene–Quaternary continental tectonic cycle, as a result of Africa-Arabia and Eurasia volcanic formations of the Armenian and Javakheti collision back-arc basins were inverted to form plateaus (highlands) and extinct volcanoes of the fold-thrust belts in the Great and Lesser Caucasus Great Caucasus – Elbrus, Chegem, Keli, and Kazbegi. and, in between, the Transcaucasian intermontane Within the region, Upper Proterozoic– depression. Normal marine basins were replaced by Phanerozoic sedimentary, magmatic, and semi-closed basins of euxinic type (Paratethys) and metamorphic complexes are developed. Th eir later on (Late Miocene) by continental basins with formation occurred under various palaeogeographic subaerial conditions of sedimentation (Milanovsky and geodynamic environments: oceanic and small & Khain 1963; Gamkrelidze 1964; Andruschuk 1968; oceanic basins, intercontinental areas, active and 490 S. ADAMIA ET AL. P R E C A U C A S U S G R E A T C A U C A S U S B L A C K S E A T R A N S C A U C A S U S LESSER CAUCASUS EASTERN P O N T I D E S L. SEVAN ANATOLIA L. VAN CAS