Home – Home – Jornals – Russian Geology and Geophysics 2003 number 1-2

On the basis of generalized new data on regional and thematic researches, the main problems involved with the evolution of the Paleoasian ocean and structure of the huge Ural-Mongolian fold belt are considered.
Some segments of the Ural-Mongolian fold belt formed at different stages of the evolution of the Paleoasian ocean developed in a specific fashion. Hence, Altaides, Kazakhstanides, Tien-Shanides, Uralides, Salairides, and Baikalides may be recognized within its limits. At the same time, the main stages of development of separate structures are well correlated with each other, and the paleomagnetic data and palinspastic reconstructions confirm the common evolution of the Paleoasian ocean in the range of 950-250 Ma.
The large stages of reconstruction and closure of the Paleoasian ocean are involved with superplumes, which appeared in the Permian-Triassic (Siberian), in the Devonian (European), and, possibly, in the Early-Middle Ordovician. These plumes also favored the synchronization of events in separate parts of the Paleoasian ocean. The best synchronization is observed in the periods of superplume appearance at about 485, 360, and 245 Ma, and about 120 Ma in the Mesozoic, at intervals of 120 myr. The presence of these superplumes is hypothesized in the Late Precambrian as well, in a lapse of 120 myr, thus reaching the best synchronization at around 610, 730, and 850 Ma. In the periods between these major plumes, smaller local plumes appear at an interval of about 30 myr and, correspondingly, local synchronization of tectonic processes occurs (rearrangement of island arcs and related local collisional processes, exhumation of eclogites and glaucophane schists, reflected in the maxima of isotope ages).

We substantiate theoretically that the Paleozoic-Early Mesozoic Ural-Pai-Khoi-Novaya Zemlya fold belt recognized as Uralides are not analogous to Hercynides (Variscides) and the preceding Late Vendian Timan folded structures and lower structural units of the Urals recognized as Timanides are not analogous to Baikalides. Uralides and underlying Timanides are noticeably distinguished from the other rocks of the Ural-Mongolian fold belt. The Ordovician rifting and subsequent drift of continents, resulted in the Paleouralian ocean, seriously disturbed the initially intimate relationship between Timanides and European Cadomides and brought the former into a closer proximity to Baikalides, which formed at a different time. The subsequent evolution of Uralides also proceeded mainly in antiphase with more eastern, initially remote parts of the Ural-Mongolian belt. However, from Carboniferous to Middle Jurassic, during the formation of Pangea, the East European, Kazakhstanian, and Siberian continents underwent collision, and new intimate structural relationships were established within the Ural-Mongolian fold belt.

Geological and geophysical evidence reveals a tectonic (rift) origin of the junction of the Arctic Uralian orogen with pre-Jurassic complexes of the West Siberian Plate. The available data allow an idea of the relationship between the Ural and Pai-Khoi structures. The Hercynian Uralian complexes are bounded by an arcuate deep fault in the north at the termination of Baidaratskaya Bay and contact the Kimmeridgian (?) Pai-Khoi fold com- plexes in the west. The study territory may store oil and gas in Permian (Permian-Triassic) reservoirs.

Correlation and synthesis of published and new structural, paleomagnetic, geochronological, and paleogeographic data from the Altai-Sayan and East Kazakhstan orogenic areas in Central Asia show an important role of strike-slip faulting in their evolution.
The pattern of major strike-slip faults outlines a terrane collage produced by a Late Devonian-Early Carboniferous collision of Gondwanian terranes (Altai-Mongolia and others) with Siberia and a Late Carboniferous-Permian collision of East Europe, Kazakhstan, and Siberia. The accreted continental margins were cut by strike-slip faults and conjugate thrusts into numerous terranes, which mixed with one another and disturbed the previous structural and facies framework.
The first collision stage was accompanied by the formation of the Charysh-Terekta system of right-lateral transforms, followed by left-lateral strike-slip faulting in the Kurai and Kuznetsk-Teletsk-Bashkaus zones. The Siberia/Kazakhstan collision involved left-lateral motion along the Char ophiolite belt and the Irtysh-Kurchum and North East shear zones. Deformations associated with each collision stage are progressively younger toward the center of Siberia; the amount of horizontal displacement decreases in the same direction from a few thousands to hundreds of kilometers. East Europe had reached its present position and welded with the Siberia-Kazakhstan continent by the earliest Jurassic, and strike-slip faulting in the composite continent lasted as long as the Early Jurassic. The Triassic-Jurassic deformations acted mostly upon continental margins. The final evolution stage of the Central Asian orogen is marked by Early Jurassic coaliferous molasse and granite magmatism in the Kazakhstan and Altai-Sayan regions.

Analysis of accumulated information and new geological and geophysical data permitted us to improve the model for the structure of the Yenisei Range and its multistep thrust over adjacent regional structures. Quantitative parameters of the dislocation of the Riphean-Cambrian horizons, interpretation of seismotomographic sections, and comparison of their geoelectric characteristics to depths of 10-15 km show a principal difference between the areas of the West Siberian Plate and Siberian Platform adjacent to the Yenisei Range, though the plate has much in common with the range.

We consider the lithology of Permo-Triassic buried volcanic rocks of the West Siberian Plate. By now, three types of volcanic rocks have been recognized: basalts, rocks of shoshonite-latite series, and acid effusions (rhyolites and rhyodacites). Basalts, amounting to >80% of all effusions, are referred to subalkaline and tholeiitic series and are enriched in mobile trace elements relative to N-MORB. The rocks of shoshonite-latite series are subdivided into moderate- and high-potassium. Their common feature is enrichment in easily mobile elements (Rb, K, Ba, Sr, Nb, etc.). Acid effusions are referred to typical rhyolites and rhyodacites of calc-alkalic series.
We have shown that all the studied rocks are the products of rifting magmatism caused by the action of a superplume beneath the Siberian craton. Though the rock series differ significantly in mineral and chemical compositions, they show a similar distribution of trace elements, which confirms the influence of plume smoothing the compositions of volcanic rocks.
The available geochemical, including isotope, data evidence that the parental magmas for the three series had different sources. Basalts and moderate-potassium shoshonites formed from undepleted mantle; high-potassium shoshonites, from enriched mantle; and acid effusions, from crustal substratum.

The deep structure of the West Siberian Plate and its folded surroundings has been imaged in geoscience transects based on geophysical surveys (DSS, ECWM, CMP profiling, seismic tomography, gravity, magnetic, geothermal, MTS, VES, etc.) along regional profiles. The resulting interpretative cross sections obtained by original geological-geophysical and geodynamic modeling show the contours of the Ural, Kazakhstan, Central West Siberian, Altai-Sayan, and Yenisei orogenic systems beneath the sedimentary cover of the West Siberian Plate, as well as the 3D distribution and the Phanerozoic geodynamic environments of paleoplates, sutures, regional shear zones, and relict basins.

New geological and geophysical models of pre-Jurassic stages in West Siberia based on log data, CMP reflection and refraction profiling, and DSS shed more light on the Paleozoic and Triassic evolution of the region. The structure of Paleozoic and Triassic formations, reinterpreted on the basis of new evidence, suggests their high petroleum potential.

The paper considers the Neogean evolution of the North American and North Asian cratons. Ophiolite zones and major faults on the western margin of North Asia and the southeastern margin of North America are interpreted as zigzag outlines of Riphean-Paleozoic stable blocks that rifted off the Rodinia supercontinent and were offset along transform faults. The original boundaries of the blocks were smoothed out by later tectonism and are poorly pronounced in the present-day framework.
Reentrants of any type, age, and facies are zones favorable for oil and gas accumulation as their folded sedimentary complexes may be both source and reservoir rocks.

The geologic structure, geochemistry, lithology, and paleontology of the Late Cambrian-Early Ordovician Zasur'ya volcanosedimentary Group (Gorny Altai) are considered. The geochemical parameters, lithological features, and biomarkers of this unit are indicative of its oceanic genesis. The regularities of the spatial distribution of various facies types of Zasur'ya Group sections are revealed. Eight conodont zones are recognized there. They allow precise dating of the unit. The Gorny Altai sector of the Paleoasian ocean has been proven to exist for about 40 myr. The stratigraphic position of the metamorphic and volcanosedimentary deposits of Rudny Altai is considered. In the Char sector of the Paleoasian ocean, the age positions of the volcanosedimentary units are refined and the stratigraphic levels of oceanic sediments are outlined.

A Late Devonian paleogeographic reconstruction for the territory of the West Siberian Plate and the surrounding regions of Altai-Sayan orogenic area and Siberian Platform is based on lithology and facies of Upper Devonian deposits. Zones of continental and shallow and deep water marine deposition are interpreted as continental, shelf, and oceanic-type offshore environments. As a result of correlation of revised regional stratigraphic charts for the Upper Devonian, the Kuznetsk basin is proposed as a reference region for the West Siberian Plate, the Siberian Platform, and Siberia as a whole. The reconstruction includes reliably timed volcanism in the West Siberian Plate. The Kolyvan'-Tom' fold belt has not been outlined as it evolved into an independent structural element during the final stage of the Hercynian orogeny.