468 RUSSIA Palaeozoic pre-granitic terranes strike obliquely relative to its western and eastern fault limits These are two Precambrain metamorphic terranes and one Palaeozoic island arc terrane inbetween The latter is principally similar to the Magnitogorsk Terrane, but it is offset sinistrally for 300 km This suggests that the boundary faults are likely to be strike-slip faults Seismic studies revealed that they dip to the west, producing an apparent divergent structure of the orogen Further to the east is the Trans-Uralian Terrane, consisting of Ordovician to Devonian accretionary wedge rocks The easternmost structure in the southern Urals is the poorly exposed Carboniferous Valerianovka Volcanic Arc It has been considered as an accreted arc, but regional airborne magnetic maps suggest that it can be traced to the Kurama Arc of Uzbekistan This indicates that the Uralides can be interpreted as the result of arc-arc-continent collision This collision was accompanied by the formation of flysch in the Famennian to Early Carboniferous, which continued on into the Middle Carboniferous This collision progressed northward, because in the northern Urals the accumulation of flysch and thrust deformations started in the Early Visean and ended by Kungurian time, when accumulation of salt and molasse took place The collisional deformations terminated at the end of the Permian In the Early Triassic, graben structures filled with clastic rocks and basalts formed in the Urals They were deformed in Early Jurassic times The northern continuation of the Urals is a longlasting debate Some researchers suggested that PaiKhoi and Novaya Zemlya represent its ‘degraded continuation’ However, airborne magnetic data clearly suggest that magmatic arcs in the Polar Urals turn to the south-east under Mesozoic-Cenozoic sediments and might also be exposed in eastern Kazakhstan, forming a giant orocline In the core of this orocline are Early and Middle Palaeozoic accretionary, magmatic arc and 1.0 Ga metamorphic terranes, identified on the basis of geophysical and drilling data They are well exposed in the Kazakh uplands The bulk of the Palaeozoic orogenic structures in Russia lie under the West Siberian sedimentary basin Mesozoic-Cenozoic Sedimentary Basins of Western Siberia The West Siberian Basin is one of the three world’s largest supergiant oil and gas reservoirs It occurs between the Urals Orogen and the Siberian Craton (Figure 1) Its basement consists of various Neoproterozoic to Palaeozoic terranes that were amalgamated in the Permian It has steep eastern and western borders and a flat bottom that dips to the north At the Permo-Triassic transition, all western Siberia experienced orogenic collapse which was followed by Triassic rifting and voluminous basalt magmatism of the Siberian Superplume Rifts formed between the Urals and the Siberian Craton and in the western Kara Sea Basin Urengoi is the largest rift, which was drilled down to a depth of more than km At the Triassic-Jurassic transition was a weak inversion and uplift, followed by post-rift subsidence for 4–7 km in the Jurassic-Cenozoic In Early-Middle Jurassic times, continental sedimentation dominated in the south, whereas there was a shallow-water marine environment in the north The shoreline frequently migrated north or south The dominant rocks are clastic alluvial, limnic, and marine sediments In the Late Jurassic, the proportion of marine sandy and clayey sediments increased At the Jurassic-Cretaceous transition, West Siberia subsided for 500 m, accompanied by the accumulation of the Bazhenovo oil-bearing facies which are enriched in organic matter This subsidence was possibly facilitated by synchronous dextral strike-slip faulting along the western flank of the Siberian Craton In the Neocomian it became a relatively deep basin, which was quickly filled with sand-clay clinoforms The main flux of clastic rocks was from the Siberian Craton and the Altai-Sayan Orogen In the AptianAlbian, the sedimentation environment became shallow-water In the Late Cretaceous, Paleocene, and Eocene, the typical rocks are clay and sand with some cherty sediments At the end of the Eocene, the northern part of the basin was uplifted and its link with the Arctic Seas was terminated The OligoceneNeogene in the central and southern parts of the basin is represented by shallow-water marine, limnic, and alluvial facies Quaternary sediments are widely distributed These are various sandy-clay facies which accumulated during several episodes of glacial and interglacial events and development of limnic-alluvial systems Circum-Pacific Orogenic Collages The Circum-Pacific orogenic collages started to form in the Palaeozoic and their growth has continued until the present time They occupy the Russian North-east and Far East In the Russian North-east is the Verkhoyansk-Chukotka Orogenic Collage, bordered by the Nipponides Verkhoyansk-Chukotka Orogenic Collage The Verkhoyansk-Chukotka Orogenic Collage (Figure 10)