Th e structure and geological history of the Caucasus are largely determined by its position between the stillconverging 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. The 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. The 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.