On the Discussion of the Boundaries of the Siberian Platform

T. V. Yankauskas

(Presented by Prof., Dr. A. G. Sivov)

Since the time when the Siberian Platform was distinguished as an independent structural unit of the Earth’s crust of the first order (the end of the last century—the beginning of the present one), an acute discussion has continued concerning the position and type of its boundaries. The differing understanding of this question is not accidental and is due to the still insufficient study of the geology of this region, as well as to the immaturity of existing geotectonic hypotheses. This allows each investigator to solve this complex geological problem on the basis of his own theoretical views. And although recently an evolution of views has become quite clearly outlined in the direction of a rapprochement of points of view, and a unity of opinions is gradually beginning to crystallize among an ever larger number of investigators, the problem of the boundaries of the Siberian Platform is still far from final resolution.

Differences of opinion on the problem under consideration are determined chiefly by different understandings of the age and structural position of the areas of Baikalian folding that frame the epicratonic part of the platform, and by different interpretations of the geological role and structure of their boundary structures.

N. S. Shatskii [17], who first distinguished the Baikalides as structures homologous to the Caledonides, Hercynides, etc., regarded the areas of Baikalian folding as folded edifices that soldered separate granite-gneiss Archean and Proterozoic blocks into a single ancient platform (craton) of the central part of northern Eurasia. At the same time, the Baikal mountain region, the Eastern Sayan, and the Yenisei Ridge were considered by him as part of the Siberian Platform and were interpreted as projections of its basement. The point of view of this authoritative researcher was inherited by a whole galaxy of his followers and found reflection, with various modifications, in the works of M. M. Odintsov [8] and others, A. A. Bogdanov [2] and others, as well as in the Explanatory Note to the Tectonic Map of Eurasia [15].

Subsequently, the mountain-folded edifices of southern Siberia began to be excluded from the platform by many geologists. Such an opinion was expressed in their works by S. P. Sitnikov [11], T. N. Spizharskii [12], L. I. Salop [10], V. T. Mordovskii [7], I. P. Karasev and O. I. Karasev [4], A. K. Bobrov [1], and others.

In contrast to them, Yu. A. Kosygin [5], L. M. Parfenov [9], and a number

others included within the platform only part of the areas of Baikalian folding exposed at the surface.

Let us consider the question of the boundaries of the Siberian Platform from the point of view of the differing positions of the Baikalides in the structure of the Earth’s crust. Proceeding from the basic principle of tectonic regionalization according to the age of the concluding folding and from the formal principle that “ancient platforms are areas of Precambrian folding,” it would seem that all the areas of development of Baikalides adjoining them should be included within the ancient platforms. However, such a formulation of the question comes into conflict with the concept of “region” accepted in geology. On the Tectonic Map of Eurasia [15] the western boundary of the platform was drawn by its compilers in such a way that the entire eastern part of the West Siberian Plate proved to be included within the Siberian Platform. The basis for this was the idea of the Precambrian age of the plate basement in this part of it. However, no one disputes the fact that the West Siberian Plate constitutes a single region, characterized throughout its territory by common features of geological structure and by a single history of development beginning with the Mesozoic. Is there any basis, on the grounds solely of the differing age of the basement of this single structure, for artificially dividing it in half? This contradicts even the principle of tectonic regionalization as a method of subdividing the Earth’s crust into separate regions distinguished by geological features. And if all areas of Precambrian fold belts adjoining the epicratonic parts of ancient platforms are to be regarded as parts of these platforms, then why not extend the limits of the Siberian Platform to the Urals and Novaya Zemlya? After all, the basement of the West Siberian Plate in its northern and eastern parts is composed of Precambrian formations [3, 6, 14, 16, etc.] directly adjoining the Siberian Platform.

The author believes that the more correct view is that the western boundary of the Siberian Platform lies in the zone of transition of its structures to the folded structures of the Yenisei Ridge and the Turukhan uplift, and to the structures of the young West Siberian Plate. Such strict regional delimitation should promote greater clarity and specificity of the tectonic map.

In the south, the boundary of the platform should be drawn along the base of the platform cover. The basis for this is the absence of elements of the cover within the mountain-folded structures that frame the platform; the presence within them of geosynclinal deposits of the Lower Cambrian, Caledonian granitoid intrusions, and also the existence of the Pre-Baikal and Pre-Sayan marginal troughs, a marginal volcanic belt, and a marginal structural suture [13, 4, etc.].

In addition, it has now become evident that the presently accepted division of the principal structural elements of the Earth’s crust into geosynclines and platforms is far from satisfying the principles of tectonic regionalization. The Baikalian mountain structures of Transbaikalia, the Eastern Sayan, and the Yenisei Ridge, which formed the folded frame of the Siberian Platform in the south and west at the very beginning of the Paleozoic, fully correspond to the concept of an orogenic belt (an area of completed folding—according to Yu. M. Sheinmann [18]), which is being accepted by an ever larger number of geologists as an independent major structural element of the Earth’s crust, equivalent to geosynclines and platforms.

Thus, areas of Baikalian folding that completed geosynclinal development at the end of the Proterozoic—beginning of the Paleozoic, depending on their subsequent geological history, became part of

different structural elements of the Earth’s crust. In our opinion [19], Baikalides of different structural positions should be distinguished: 1) within the epipaleozoic orogenic folded belts—Transbaikalia, the Eastern Sayan, the Yenisei Ridge; 2) within the basements of young platforms (plates)—the West Siberian, Turan, etc.; 3) within the basements of Mesozoic folded structures and their median massifs—the Mesozoides of the northeastern USSR; 4) within the basements of ancient platforms—the Irkineeva and Urinsky aulacogens of the Siberian Platform, the exogonal syneclises of the Russian Platform, etc.

In this connection, the boundaries of ancient platforms in each particular interval must be drawn with allowance for the structural position of the Baikalides that frame them and with allowance for the accepted concepts of “geological regions.”

LITERATURE

1. A. K. Bobrov. Geology of the Cis-Baikal Marginal Trough. “Nauka,” 1964.

2. A. A. Bogdanov. Basic regularities in the development of Paleozoic folding on the territory of the USSR. Caledonian orogeny. International Geological Congress, XXI Session. Reports of Soviet Geologists, Moscow, Publishing House of the Academy of Sciences of the USSR, 1960.

3. S. M. Zamarayev. Marginal Structures of the Southern Part of the Siberian Platform. Moscow, “Nauka,” 1967.

4. I. P. Karasev, O. I. Karasev. Tectonic structure of the Irkutsk amphitheater. In: Geological Structure and Oil-and-Gas Potential of the Irkutsk Amphitheater. Moscow, Gostoptekhizdat, 1960.

5. Yu. A. Kosygin. Development of the Siberian Platform in the Precambrian. Geology and Geophysics, No. 7, 1962.

6. V. P. Markevich. History of Geological Development and Oil-and-Gas Potential of the West Siberian Lowland. Moscow, “Nauka,” 1966.

7. V. T. Mordovsky. Tectonics and Oil-and-Gas Potential of the Southern Part of the Siberian Platform. Moscow, Publishing House of the Academy of Sciences of the USSR, 1959.

8. M. M. Odintsov, V. A. Tverdokhlebov, B. M. Vladimirov. Structure, Volcanism, and Diamond Potential of the Irkutsk Amphitheater. Moscow, Publishing House of the Academy of Sciences of the USSR, 1962.

9. L. M. Parfenov. Principal Features of the Precambrian Structure of the Eastern Sayan. Moscow, “Nauka,” 1967.

10. L. I. Salop. Geology of the Baikal Mountain Region. Moscow, “Nedra,” 1967.

11. S. P. Sitnikov. Scheme of the tectonics of the southern part of the Siberian Platform and prospects for its oil-and-gas potential. Proceedings of the Academy of Sciences of the USSR, Geological Series, No. 6, 1943.

12. T. N. Spizharsky. The Siberian Platform, its origin and history of development. In: Problems of Comparative Tectonics of Ancient Platforms. Moscow, “Nauka,” 1964.

13. I. N. Sulimov, N. V. Vorobyev, V. V. Koptev, I. G. Safronova, I. E. Sulimova, S. L. Shvartsev. Geology and Prospects for Oil-and-Gas Potential of the Southwest of the Siberian Platform. Leningrad, “Nedra,” 1966.

14. V. S. Surkov. Geotectonic zoning of the basement of the West Siberian Plate. Soviet Geology, No. 8, 1968.

15. Tectonics of Eurasia. Explanatory Note to the Tectonic Map of Eurasia. Moscow, “Nauka,” 1966.

16. E. E. Fotiadi, V. S. Surkov. Structure of the folded basement of the West Siberian Plate. Soviet Geology, No. 2, 1967.

17. N. S. Shatsky. Principal features of the tectonics of the Siberian Platform. Bulletin of MOIP, Geological Section, 10, Nos. 3, 4, 1932.

18. Yu. M. Sheinmann. Platforms, folded belts, and the development of structures of the Earth. Transactions of VNII-1, Geological Series, issue 49, Magadan, 1959.

19. G. V. Yatskovskaya. On the question of the position of Baikalides in the structure of the Earth’s crust. Abstracts of reports, conference of young scientists. Tomsk, 1968.