Abstract Metamorphic and igneous rocks exposed in NW-vergent thrust sheets and their autocthonous basement in the NE Pontides were dated by the U–Pb method using zircons, supported by geochemical data for granitic rocks. Two meta-sedimentary units (Narlık schist and Karadağ paragneiss) yielded detrital zircon populations of 0.50–0.65 and 0.9–1.1 Ga, suggesting an affinity with NE Africa (part of Gondwana). The youngest concordant zircon age is Ediacaran for the schist but Devonian for the paragneiss, bracketing the paragneiss depositional age as Mid-Devonian to Early Carboniferous. Metamorphic rims of zircon cores in the paragneiss gave Carboniferous ages (345–310 Ma). The zircon rim data indicate two Variscan metamorphic events (334 and 314 Ma) separated by a hiatus (320–325 Ma). Granite emplacement took place during early Carboniferous, Early Jurassic and Late Jurassic phases. The crystallization age of the early Carboniferous granites ( c. 325 Ma) corresponds to a hiatus in the zircon age data that could reflect subduction slab break-off. The Variscan granitic rocks intruded a Gondwana-derived continental terrane that was loosely accreted to Eurasia during early–late Carboniferous time but remained isolated from Eurasian-derived terrigenous sediment. In contrast, the Jurassic granitic magmatism relates to later back-arc extension along the southern margin of Eurasia. Supplementary material: Full isotope data (8 tables) are available at http://www.geolsoc.org.uk/SUP18558

Abstract Early Carboniferous–Eocene units exposed in the Arvin area document the development of the southerly, active continental margin of Eurasia. The oldest rocks exposed in the area are Early Carboniferous granites that regionally intrude schists and gneisses. The continental terrane rifted along the entire length of the Pontides (>1000 km east–west) during the Early–Middle Jurassic. Subsidence of the rift basin in the Artvin area was accompanied by terrigenous debris flows, turbidites and deep-sea radiolarian muds, and was associated with local extrusion of chemically ‘enriched’ basalts. Swarms of subduction-influenced basic, intermediate, to locally silicic dykes, intruded high-grade metamorphic basement within the rift. A basement horst within the rift was covered by condensed pink ammonite-bearing pelagic facies. Large volumes of subduction-influenced basalts erupted during the later stages of extensional basin development (Mid-Jurassic), associated with volcaniclastic sedimentation. The Artvin Basin is interpreted as a supra-subduction rift associated with incipient arc magmatism. The basin was stratigraphically inverted in response to Late Middle Jurassic ‘Neo-Cimmerian’ deformation. It was then partially eroded and covered by Upper Jurassic continental, to shallow-marine sediments, together with localized eruption of ‘enriched’ (non-subduction-influenced) basalts. The margin collapsed during the Late Jurassic–Early Cretaceous, initiating deposition of pelagic carbonates and mixed terrigenous, biogenic and volcaniclastic gravity flows. Subduction during the Late Cretaceous then constructed the east Pontide magmatic arc and a thick volcaniclastic fore-arc apron to the south. Supra-subduction-type ophiolites and accretionary melange formed within Neotethys to the south during the Late Cretaceous and were emplaced regionally northwards onto the leading edge of the Pontide active continental margin during the latest Cretaceous. Continental collision during the Mid-Eocene telescoped the distal part of the active margin which was emplaced northwards onto the east Pontide continental basement. The geological evolution of Artvin area correlates with the Pontides further west and with the southern and northern Transcaucasus to the east. Our favoured tectonic model involves long-lived, episodic, northward subduction of Tethys. Finally, there is no evidence of ‘Palaeotethyan’ ophiolites in the eastern Pontides region.

Abstract The eastern Pontides (northeastern Turkey) and Transcaucasus (Georgia) belong to the same geological belt representing an active margin of the Eurasian continent. According to palaeotectonic–palaeogeographic reconstructions, based on regional geological, palaeomagnetic, palaeobiogeographical and petrological data, the eastern Pontides and the major part of the Transcaucasus, situated to the north of the North Anatolian–Lesser Caucasian ophiolitic suture, comprise island arc, forearc, back and interarc basins. The eastern Pontide segment of the belt consists of three structural units which, from north to south, are the northern, central and southern units. The northern unit, the southeastern Black Sea coast–Adjara–Trialeti Unit, represents a juvenile back arc basin formed during the Late Cretaceous (pre-Maastrichtian). This unit separates the southern and northern Transcaucasus zones. The central Artvin–Bolnisi Unit is also known as the northern part of the southern Transcaucasus and is characterized by Hercynian basement, unconformably overlying the Upper Carboniferous–Lower Permian molasse and Upper Jurassic–Cretaceous arc association. The southern unit is the imbricated Bayburt–Karabakh Unit and is known as the southern part of the southern Transcaucasus. This unit has a similar basement to the Artvin–Bolnisi Unit and also includes a chaotic assemblage; it unconformably overlies the Upper Jurassic–Cretaceous forearc association. The eastern Pontide system is interpreted as the product of interference between a spreading ridge and subduction zone during Late Jurassic–Cretaceous times. The North Anatolian–Lesser Caucasus Suture, comprising ophiolites, mélanges and an ensimatic arc association, separates the overlying system from the Anatolian–Iranian Platform in the south. Maastrichtian–Lower Eocene cover rocks in the region unconformably overlie all the other units. Middle Eocene rifting resulted in the formation of new basins, some of which closed during an Oligocene–Early Miocene regression. Others, such as the Black Sea and Caspian Basins, have survived to the present day as relict basins.

ABSTRACT 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.

Abstract The geology of the Western Caucasus can be divided into seven zones. From north to south they are (1) the Scythian Epihercynian plate; (2) the anticlinorium of the main range of the Great Caucasus; (3) the folded system of the southern slope of the Great Caucasus; (4) the Georgian block; (5) the Adzharo-Trialetskiy folded system; (6) the Artvin-Bolnissian block; and (7) the Loki- Karabakh gently folded zone. The tectonic zonation is based on the lability and relative stability of the area during the formation of alpine structures. The types of rocks, their distribution and thicknesses, and the character of tectonic structures are also determined by these properties. Basement is exposed in the anticlinorium of the main range and is crystalline schist and gneiss of Protero- zoic(?)-early Paleozoic age. This geosyncline underwent folding during the Caledonian and Hercynian orogenies. The Scythian plate was formed by Hercynian orogenesis. A period of denudation is indicated prior to post-orogenic deposition. Deep-seated and basement faults subdivide the region. These faults have controlled volcanism between orogenic phases. Although the faults were initiated at different times, all were active during the Alpine cycle, which began with Liassic transgression. The earliest geosynclinal development was in the Albian-Late Cretaceous. The critical period of geosynclinal development began during the late Eocene. The Attic phase (Miocene) was the first mountain-building period. During the Pliocene and Quaternary, the most intensive tilting and uplift occurred; subsidence of molasse basins also took place during this interval. All these tectonic phases probably had significance in the formation of the Black Sea depression.