GEOPHYSICS AND SOME ISSUES OF THE PALEOTECTONICS OF ALTAI

F. B. Baksht; Yu. I. Tveritinov

(Presented by Prof. D. S. Mikov)

Geophysical and geological data suggest that, in the Late Precambrian and Lower Cambrian, the formation of the Altai mountain region was governed by near-latitudinal (east–northeast) trends. Materials from gravimetric and magnetic surveys (F. I. Rubakh, A. M. Komarov, I. V. Shestak, F. B. Baksht, and others) indicate that geological structures of northwestern trend are later than the near-latitudinal ones. The latter dominate within many areas composed of formations of the Lower Paleozoic (Salairian) structural stage. Associated with them are faults of approximately meridional strike. This is also attested by data from structural-facies analysis.

Is the near-latitudinal orientation of the Sayan–Altai geosynclinal system, which was established as early as Sinian time, accidental? Let us use the data of R. M. Demenitskaya (1960). She showed that in the Eastern Hemisphere of the Earth, along the equator, there extends a belt of the most complex and prolonged development of the Earth’s crust. It stretches in a near-latitudinal direction from the Black Sea in the west to the Sea of Okhotsk in the east, encompassing the entire territory of the mountain systems of Central and Southern Asia.

A sharp increase in the thickness of the Earth’s crust within the territory described by us is observed south of the Zmeinogorsk—Biysk line. This line, as a first approximation, coincides with the axis of an ancient throughgoing deep fault, clearly recorded by gravity prospecting and distinguished under the name of the “Altai–Sayan throughgoing tectonic zone” by G. L. Pospelov (1959). In the modern relief this line is expressed by the so-called “face of Altai,” whose neotectonic nature was already pointed out by V. A. Obruchev. On gravimetric maps, this zone corresponds to an extended zone of relative minima of gravity, which testifies to the great mobility of this part of the Earth’s crust. Thus, the near-latitudinal orientation of the Sinian Gorno-Altai geosynclinal system is not accidental and is possibly determined by causes of planetary scale.

It may be assumed that the orientation of the principal structures throughout the entire history of Altai’s development was governed by the same regularity: the stress pattern at each separate moment was close to latitudinal. In other words, the inception of ancient geosynclines was determined by deep faults oriented along paleo-paral—

Table 1

latitudinal. Near-latitudinal faults are associated with nearly meridional ones.

We find confirmation of this by comparing the data of structural-facies analysis of the territory of Gorny Altai with the results of paleomagnetic observations. The latter were carried out in 1962–1966 by L. V. Shirokov. L. V. Shirokov’s data agree well with, and are partly supplemented by, observations made in the eastern part of the Altai–Sayan system and in the south of the Siberian Platform (Vlasov and Aparina, 1963; Goncharov, 1963; Popova, 1963; Sidorova, 1963; Khramov, 1967, and others). Averaging these data, it is not difficult to establish the change in the directions of paleoparallels in Altai from the Proterozoic to the Anthropogene. Their comparison with the dominant strikes of the ancient structures of Gorny Altai makes it possible to state the following.

A counterclockwise rotation of the plan of tectonic stresses is accompanied by a rotation of the direction of paleoparallels, that is, by a displacement of the magnetic pole. In the Middle and Upper Paleozoic the displacement of the pole lags somewhat behind the rotation of the stress plan. The rates of rotation of structures and paleoparallels in all intervals of time, and on the whole over the entire period, nearly coincide. The rate of rotation of structures generally amounts to \(0.2^\circ\) per 1 million years, at times reaching \(0.8^\circ\) per 1 million years. These values are comparable with the worldwide paleomagnetic data on the rates of relative rotation of continents (\(0.16^\circ\)—\(0.22^\circ\) to \(0.83^\circ\) per 1 million years), cited by T. Nagata (1965).

A sharp change in the orientation of structures and the pole displacement associated with it occurred synchronously with the main phases of tectonic activity, which possibly indicates a causal connection between these phenomena.

The northwestern orientation of the oldest structures in the Altai–Sayan region (the junction with the Siberian Platform), assigned to the Lower Proterozoic (Baikalian) cycle of tectogenesis, also coincides with the paleolatitudinal direction for that time, reconstructed from the data of A. P. Khramov (1961). Fragmentary information confirming the proposition we have advanced is available for the Urals, Kazakhstan, Tuva, Transbaikalia, and other regions. However, the validity of the assertion that the orientation of the tectonic-stress plan remained constant relative to the equator—an assertion important for understanding the history of the Earth—must be tested in other regions.

Without going into consideration of the causes leading to such parallelism between the orientation of regional geological structures and the rotation of the Earth’s axis, let us only say that the very fact of the existence of such an interrelationship makes more substantiated those paleotectonic and metallogenic constructions that are based on the assumption of a sublatitudinal orientation of the oldest structures of Gorny Altai.

LITERATURE

1. A. Ya. Vlasov, V. P. Aparin. On the paleomagnetism of the Late Precambrian according to the data of study of the Sinian deposits of the Yenisei Ridge. Izv. AN SSSR, geophys. ser., 3, 1963.

2. G. I. Goncharov. On the results of paleomagnetic studies of the Paleozoic of the northwestern part of the Siberian Platform. In the collection Magnetism of Rocks and Paleomagnetism. Siberian Branch, Academy of Sciences of the USSR, Krasnoyarsk, 1963.

3. R. M. Demenitskaya. Structure of the crystalline part of the Earth’s shell according to geophysical data. International Geological Congress, XXI session. Reports of Soviet geologists. Problem 2. Section 2. Gosgeoltekhizdat, Moscow, 1960.

4. T. Nagata. Magnetism of Rocks. Mir, Moscow, 1965.

5. A. V. Popova. Paleomagnetism and paleoclimates of the south of Krasnoyarsk Krai. In the collection Magnetism of Rocks and Paleomagnetism. Siberian Branch, Academy of Sciences of the USSR, Krasnoyarsk, 1963.

1. G. L. Pospelov. On the regularities in the distribution of magmatogenic iron-ore deposits of the Altai–Sayan folded region. In the collection Regularities in the Distribution of Mineral Resources, vol. 2. Moscow, 1959.

2. E. P. Sidorova. Results of paleomagnetic studies of Lower and Middle Cambrian deposits of the Lena and Olekma Rivers. In the collection Magnetism of Rocks and Paleomagnetism. Siberian Branch of the Academy of Sciences of the USSR, Krasnoyarsk, 1963.

3. A. N. Khramov, G. N. Petrova, A. G. Komarov, V. V. Kochegura. Methods of Paleomagnetic Studies. Gostoptekhizdat, Leningrad, 1961.

4. A. N. Khramov, L. E. Sholpo. Paleomagnetism. Nedra, Leningrad, 1967.

5. L. V. Shirokov. Paleomagnetic studies of rocks of Altai. In the collection New Data on the Geology and Mineral Resources of the Altai Territory. Novosibirsk, 1963.