Journal of the Geological Society, London, Vol. 152, 1995, pp. 903-906, 5 figs. Printed in Northern Ireland

compression in the Early Devonian led to the inversion of the Timan and Varandey-Adz'va rifts to form ridges, which Tectonic evolution of the northern Ural acted as the source for the Middle Devonian clastic rocks Orogen found in the Timan-Pechora Basin. However, by the mid-Frasnian, shallow marine conditions were re- established, with carbonates deposited on highs and S. C. OTTO 1 & R. J. BAILEY 2 'Domanik' facies organic-rich shales laid down in the lPetroconsultants (UK) Ltd, 266 Upper Richmond intervening lows. Road, London SW15 6TQ, UK Closure of the ocean began in the Tournaisian, and 2Victoria Villa, 5 Station Road, Southwell, eastward subduction under the Siberian craton is made Nottinghamshire NG25 OET, UK evident by basement of volcanic island arc affinity found only to the east of the Urals (Churkin et al. 1981). In the Timan-Pechora Basin, the transpressional reactivation of faults resulted in the complete inversion of the earlier rifts. Local highs were formed, such as the Usa and Vozey highs, The closure of the Uralian Ocean occurred in Early Permian-Early which were a source of clastics during the Late Triassic time. In the northern Ural fold belt, overthrusting to the Tournaisian-Early Visean. Throughout the remainder of west produced a major foreland basin to the west of the mountain chain. In contrast, in the northern extension of the Ural Orogen, the Carboniferous and into the earliest Permian, the basin the Taymyr fold belt, thrusting was directed to the SE. It is formed part of the western passive margin of the Uralian proposed that Novaya Zemlya, at the interface of these two zones, Ocean, on which sequences of shallow marine carbonates acted as a thin-skinned allochthonous nappe emplaced by gravity and shales were deposited. Similar Carboniferous-Lower tectonics into a basin produced by rapid Permo-Triassic rifting in Permian carbonate passive margin sequences are found the eastern Barents Sea. throughout the region, in the South Barents Sea Basin (Johansen et al. 1993), the Urals and Novaya Zemlya (Churkin et al. 1981) and the West Siberia Basin (Peterson Keywords: Russia, Urals, Triassic, tectonics. & Clarke 1991; Ostisty & Cheredeev 1993).

In mainland Russia, the Ural Mountains trend broadly N-S, Uralian Orogeny. Continental collision between East extending from the Arctic Ocean to the Aral Sea. The Europe and Siberia is first recorded in central and eastern northern part of the orogen is more complex and comprises West Siberia, where Late Carboniferous deformation three NE-SW-trending segments (Fig. 1): the Polar Urals, generated 'Early Hercynide' basement. In western West the island of Novaya Zemlya and the Taymyr fold belt. Siberia, the basement is younger (termed Late Hercynian) Novaya Zemlya is offset by some 600 km to the NW and is and contains Upper Palaeozoic granites (Clarke et al. 1978). linked to the remainder of the orogen by two NW-trending Initial effects of the collision are recorded in the highs: the Pay-Khoy fold belt in the SW, and the North Timan-Pechora Basin in the Early Permian (Sakmarian- Siberia Swell and Sverdrup Monocline that lie to the NE Artinskian) and suturing took place in the Kungurian. under the Kara Sea. Subsequent E-W compression led to the uplift and Previous studies, which have included reconstructions of deformation of the carbonate platform sequences, and the the northern Ural Orogen (Green et al. 1986; Ziegler 1989; obduction of ophiolitic suites onto the eastern edge of the Zonenshain 1990; Johansen et al. 1993), have shown the East European continent (Efimov et al. 1978). Ophiolites island of Novaya Zemlya as being 'fixed', retaining its offset are also found in the basement of the West Siberia Basin throughout time. Our study of the surrounding sedimentary (Clarke et al. 1978). basins and their interrelationships has led to the The Early Permian E-W convergence produced development of a new model for the tectonic evolution of overthrusting to the west, which, as the Ural mountain belt the northern Ural Orogen during the Permo-Triassic, which grew, in turn produced tectonic loading and flexing of the is discussed in detail and illustrated by a series of schematic crust, generating a foreland basin to the west of the Urals diagrams below. (Fig. 2) that filled with 2000m of Artinskian clastic sediments. The asymmetric foreland basin continued to develop, migrating to the west throughout the Late Permian Pre-Hercynian evolution. Evidence from the Timan- and Early Triassic, and was filled with over 4000 m of coarse Pechora Basin shows that a rifting event in the Early clastic sediments shed from the eroding mountains to the Riphean (Milanovsky 1981) produced a series of half- east. Farther to the west, the reactivation of the Timan grabens and grabens (including the Timan, Pechora-Kolva Ridge produced a further phase of uplift and inversion. and Varandey-Adz'va rifts) bounded by normal faults with Compression in the western parts of the Ural Orogen a NW-SE (Baikalian) trend (Fig. 1). This was the continued into the Late Triassic, but in the east at this time precursor to E-W extension in the Cambrian, which was post-orogenic collapse began, initiating a series of grabens followed by sea-floor spreading and the opening of the which filled with Triassic extrusive rocks (Figs 1 & 3). The Uralian Ocean in Ordovician-Silurian times. Passive graben, which reactivated pre-existing basement faults, have margin sequences developed on both sides of this ocean: an overall N-S trend (Rigassi 1986). thick Upper Ordovician to Lower Devonian carbonates are widespread throughout the Timan-Pechora Basin (Fig. 2), and deformed carbonate platform sequences are found in Taymyr fold belt. In the central part of the Taymyr fold belt the basement of the West Siberia Basin. Localized (Fig. 1), a sequence of Palaeozoic carbonates is identified

903

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80oN

75oN

70°N

ural framework of northern Ural Orogen, :ion of Figs 2, 3 and 4. Note the alignment of 65°N lashed lines), parts of the orogen (Pay-Khoy Monocline) and some northerly rifts. Inset map 400E 50°E 60°E 70°E shows geographical location of the area.

that is similar to that described above. Structurally, this belt, loading and flexing the Siberian plate to form the sector of the fold belt comprises complex, linear folds cut by Yenisey-Khatanga foreland basin. Collision continued in numerous thrusts that parallel the regional NE trend, and this area into the Jurassic (Ziegler 1989). intruded by small Upper Palaeozoic plutons (Churkin et al. 1981), confirming that this belt is also part of the Ural New model for Novaya Zemlya and the eastern Barents Orogen. The Yenisey-Khatanga foredeep, to the SE of the Sea. In the intervening portion of the fold belt between fold belt, developed (like the Uralian foredeep) in Late Taymyr and the Polar Urals, Novaya Zemlya shows similar Permian-Early Triassic time, as is shown by the large regional development of Late Palaeozoic carbonate passive thickness of Permo-Triassic sediments (up to 6200 m) which margin sequences, in this case grading eastwards into pass from Lower Permian marine shales through Upper fine-grained clastics (Churkin et al. 1981). These sediments Permian continental coal measures to Triassic clastic were compressively deformed in the Late Permian to Early molasse sediments. However, in the archipelagos of Triassic, the limits of deformation being significantly offset Severnaya Zemlya and Franz Josef Land to the N and NW, with respect to the Polar Urals (Fig. 1). the basement is of Caledonian origin and the overlying However, offshore the Permo-Triassic history of the sedimentary sequence is markedly different: for example, South Barents Sea Basin cannot be reconciled with its Devonian rocks are of continental facies and virtually no conventional depiction as a foreland basin to the west of Late Palaeozoic sediments are found (Churkin et al. 1981). Novaya Zemlya (e.g. Rider 1988; Ziegler 1989). Regional This suggests that this northern area was part of a separate seismic profiles (Johansen et al. 1993; Baturin et al. 1991) do terrain (Smith 1986) which was also involved in the Uralian not show the asymmetry expected of a Permo-Triassic collision, causing thrusting to the SE in the Taymyr fold foredeep, but instead show that significant extension and thinning of the continental crust occurred in Late Permian time. This extension led to the development of the South URALS Barents Sea Basin, which filled with up to 7000 m of Upper SW NE

2,ooo NW SE

4,000 i + 4

6,00O +

s,ooo + +++ .% ** ++.~ L__ + 6,0(X)~ + + Jt + + 1o,ooo + i+ +++ Triassic 8,0oot ~ + + + Permian lo,ooo ~ Palaeozoic Carbonates [-~ Jurassic - Tertiary ~ PalaeozoicCarbonates Trtasstc [~ Basement Fig. 2. Cross-section of Timan-Pechora Basin (after Meyerhoff ~oo~ 1980). Note thickening of Permian section to form foreland basin of the Urals and evidence of repeated reactivation of basement faults. Fig. 3. Cross-section of Kara Sea, from a depth converted seismic (See Fig. 1 for location). interpretation. (See Fig. 1 for location).

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PRINOVOZEMELSK NOVAYA HIGH ZEMLYA seen in Mid-Triassic time, when left-lateral transpressional NW SE reactivation of a pre-existing Baikalian fault trend uplifted the Ludlov High, dividing the depression into two major sinistrally offset depocentres (Fig. 5b). The development of the northwesterly-directed salient in the orogen is thought to date from this episode. Seismic profiles also provide clear evidence of the developing Late Triassic inversion of the eastern margin of the South Barents Sea Basin, with the thick Permian-Triassic succession arched into regional-scale anticlines (the Prinovozemelsk, Admiraltey and Mys- Zheleniya highs, Figs 1, 4 and 5c). The segmentation of the marginal inversion to create this string of highs again suggests the reactivation of Baikalian trends, but apparently has no counterpart in the structure of Novaya Zemlya. Fig. 4. W-E geoseismic of eastern Barents Sea (after Baturin et al. There is also.a marked contrast between the broad-scale 1991). Note lack of thickening of Permian or Triassic sections inversion of the offshore Permian and the tightly folded and towards the Urals, and major inversion anticline to west of frontal metamorphosed Permian on the island. thrusts. (See Fig. 1 for location). Reconciliation of Permo-Triassic extension in the eastern Barents Sea, coeval compressive deformation in Novaya Permian and Lower Triassic clastic rocks, derived from the Zemlya, and Late Triassic extension in the Kara Sea and east (Figs 4 and 5a). West Siberia can be achieved, and structural contrasts The influence of Uralian deformation on the basin is between Novaya Zemlya and the offshore areas tO the west

C "~ ~

Uplifted area Foredeep Continental sedimentation Marine sedimentation ~ Extension <~ Compression ..~ Antiformal trends J Lineaments / faults J Direction of sediment supply Extent of rift-related subsidence in Barents Sea

Fig. 5. Schematic reconstructions of the northern Ural Orogen during Uralian collision: (a) Late Permian-Early Triassic. (b) Mid-Triassic. (e) Late Triassic.

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can be explained, if Novaya Zemlya is considered to be an publish these results of the regional studies of the CIS, and Basin allochthonous body thrust over the margins of the South Monitor Group colleagues including D.G. Smith and M. Barents Sea Basin from an orogen to the SE. It is proposed Modelevsky and his correspondents in Russia, on whose work some that this movement took place in the Late Triassic (Fig. 5c). of our regional synthesis has drawn. John Barnard drafted the The northerly thickening of the Triassic and Jurassic diagrams. sediments in the Timan-Pechora Basin provides evidence of the widening influence of the post-rift thermal subsidence of the eastern South Barents Sea Basin (Fig. 5a-c). It is References suggested that in the Late Triassic, the final pulse of Uralian compression combined with the widening influence of BATURIN, D., VINOGRADOV, A. & YUNOV, A. 1991. Tectonics and hydrocarbon thermal subsidence in the Barents Sea provided the potential of the Barents Megatrough. Multiclient Report, Laboratory of gravitational instability which caused the Novaya Zemlya Regional Geodynamics, LARGE International, Moscow. Abstract allochthon to detach from its orogenic roots along a catalogued in: American Association of Petroleum Geologists Bulletin, pre-existing basal thrust plane and slide northwestwards 75/8, 1404. CHURKIN, M. JR., SOLEIMANI, G., CARTER, C. & ROBINSON, R. 1981. Geology down slope as a thin-skinned nappe. The nappe overrode of the Soviet Arctic: Kola Peninsula to Lena River. In: NAmN, A.E.M., the eastern margin of the South Barents Sea Basin and its CHURKtN, M. JR., STEHLt, F.G. (eds) The ocean basins and margins, emplacement caused down-slope compression, generating Volume 5; The Arctic Ocean. Plenum Press, New York. 331-375. (or accentuating) the giant inversion anticlines that CLARKE, J.W., GIRARD, O.W. JR., PETERSON, J.A. & RACHLIN, J. 1978. Petroleum geology of West Siberian Basin and Samotlor oil field. Oil & characterize that part of the basin. Simultaneously, up-slope Gas Journal, May 8, 1978, 311-328. extension enhanced the initial collapse-related rifting in the EFIMOV, A.A., LENNYKH, V.l., PUCHKOV, V.N., SAVELYEV, A.A., SAVELYEVA, eastern part of the orogen, seen in the Kara Sea and West G.N. & YAZEVA, R.G. 1978. Ophiolites of Polar Urals. Fourth field Siberia Basin. ophiolite conference, IGCP, Guide-book, Moscow. GREEN, A.R., KAPLAN, A.A. & VIERBUCHEN, R.C. 1986. Circum-Arctic This interpretation implies that there is considerable petroleum potential. In: HALaOUTY, M.T. (ed.) Future petroleum strike-slip displacement on the southwestern and northeast- provinces of the world. American Association of Petroleum Geologists ern margins of the Novaya Zemlya block, again exploiting Memoirs, 40, 101-130. features which are interpreted as reactivated Baikalian JOHANSEN, S.E., OSTISTY, B.K., BIRKELAND, O., FEDEROVSKY, Y.F., MARTIROSJAN, V.N., CHRISTENSEN, O.B., CHEREDEEV, 5.1., IGNATENKO, planes of weakness. E.A. & MARGULIS, L.S. 1993. Hydrocarbon potential in the Barents Sea: play distribution and potential. In: VORREN, T.O. ET AL. (eds) Arctic Post-orogenic basin development. In the Kara Sea and geology and petroleum potential. Norwegian Petroleum Society Special West Siberia basins, Upper Triassic syn-rift volcanic rocks Publications, 2, 273-320. (Fig. 5c) were unconformably overlain by Lower Jurassic MEYERHOFF, A.A. 1980. Petroleum basins of the Soviet Arctic. Geological Magazine, 117, 101-210. shallow marine clastics, derived from the eroding Urals. MILANOVSKY, E.E. 1981. Aulacogens of ancient platforms: problems of their Post-rift thermal subsidence led to widespread sedimenta- origin and tectonic development. Tectonophysics, 73, 213-248. tion over the whole basin by the Callovian, with deep OSTISTV, B.K. & CHEREDEEV, S.I. 1993. Main factors controlling regional oil marine shales deposited in all areas except for the extreme and gas potential in the west Arctic, former USSR. In: DORE, A.G., ET AL. (eds) Basin modelling: advances and applications. Norwegian west of the basin, where shallow-water clastic rocks were Petroleum Society Special Publications, 3, 591-597. sourced from the Urals. Throughout the Cretaceous and PETERSON, J.A. & CLARKE,J.W. 1991. Geology and hydrocarbon habitat of the Early Tertiary the basin continued to fill with sediments, West Siberian Basin. American Association of Petroleum Geologists with long phases of subsidence and sedimentation Studies in Geology, 32. interspersed with short periods of uplift and erosion which RIDER, M. 1988. Play potentials of the Barents shelf. Oil & Gas Journal, January 25, 1988, 87. are thought to be governed by oblique-slip reactivation of R1GASSI, D.A. 1986. Wrench faults as a factor controlling petroleum basement faults (Rigassi 1986). Post-rift subsidence in the occurrences in West Siberia. In: HALBOUTY,M.T. (ed.) Future petroleum South Barents Sea Basin led to the deposition of Jurassic provinces of the world. American Association of Petroleum Geologists and Cretaceous sediments up to 3500m thick. In the Memoirs, 40, 529-544. SMITH, D.G. 1986. Late Paleozoic to Cenozoic reconstructions of the Arctic. Timan-Pechora Basin, slight subsidence led to deposition of In: TAILLEUR, I. t~ WEIMER, P. (eds) Alaskan North Slope geology. a thin Jurassic-Cretaceous cover, which is thickest in the Special Publication of the American Association of Petroleum Geologists north. A period of mild compression in the Oligocene and Society of Economic Paleontologists and Mineralogists, 785-795. produced limited inversion on pre-existing faults and led to ZIEGLER, P.A, 1989. Evolution of Laurussia: a study in Late Palaeozoic plate tectonics. Kluwer Academic Publishers, Dordrecht. a further phase of uplift in the Urals. ZONENSHAIN, L.P., KUZMIN, M.I & NATAPOV, L.M. 1990. Geology of the USSR: a plate-tectonic synthesis. Geodynamics Series, 21. American The authors thank Petroconsultants (UK) Ltd for permission to Geophysical Union, Washington DC.

Received 28 April 1995; revised typescript accepted 20 June 1995.

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