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RECENT PROCESSES AT THE BOUNDARIESOF THE MICROPLATES OF SOUTH SIBERIAAND NORTH MONGOLIAN. V. Lukin aa Institute of Geology , Russian Academy of Sciences , MoscowPublished online: 06 Jul 2010.

To cite this article: N. V. Lukin (1993) RECENT PROCESSES AT THE BOUNDARIES OF THE MICROPLATESOF SOUTH SIBERIA AND NORTH MONGOLIA, International Geology Review, 35:7, 650-657, DOI:10.1080/00206819309465549

To link to this article: http://dx.doi.org/10.1080/00206819309465549

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RECENT PROCESSES AT THE BOUNDARIES OF THE MICROPLATES OF SOUTH SIBERIA AND

NORTH MONGOLIA

N. V. Lukina1

The south Siberia-north Mongolia region is clearly a complex of micro-plates interacting in the general stress field of the Indian-Eurasian conti­nental convergence. Microplate boundaries are to some extent delineated by neotectonic and historic seismic features, with displacements controlled by the local trend of the boundaries. Considerable faulting occurs, never­theless, within microplates, making the delineation of their boundaries somewhat arbitrary.

Plate boundaries, including those between continental microplates, consist of very complex active fault zones. As a rule, they exhibit Late Pleistocene-Holocene and historical movements of various intensities and orientations [20]. The epicenters of large earthquakes, both as recorded instrumentally and indicated by paleoseismic dislocations, occur along these boundaries.

As an example, we may consider faults in southern Siberia and northern Mongolia that have been active in late Cenozoic time, particularly in the Late Pleistocene-Holocene, which the author has studied in the field for 8 years.

The major structural units distinguished in this region are (Fig. 1): the Tuva and Mongolian plates, separated from the Dzhungarian and Alashan' plates by the Altay block-folded region; and the Amur plate, buried on the north and northwest by the Baikal rift zone. The boundaries of these continental microplates are large fault zones which have been active at various stages of neotectonic history and at present.

The Tuva plate is bounded on the east and north by the old Main Sayan and Sayan-Minusinsk faults, rejuvenated in the late Cenozoic [15]. Its western boundary is formed by the Kuray-Teletskoye, Shapshal', and the northern segment of the Tsagan-Shibety faults, rejuvenated and very active in the late Cenozoic [7, 8]. On the south, the boundary is drawn along the sublatitu-dinal Khantay and Tsetserleg fault zones, which were substantially rejuvenated in the Bolnay and Tsetserleg earthquakes of 1905 [15], and on the east it is drawn along the submeridional Lake Hobsogol fault and the sublatitudinal Baikal-Mondy tectonic suture [13].

The Mongolian plate is contiguous to the Tuva plate on the south and is bounded by the structures of the Mongolian and Gobi Altay. Bounding them on the west is the Kobdo fault, active in Late Pleistocene-Holocene time, and on the south, the Gobi Altay fault system, which underwent great deformation at the time of the Gobi-Altay earthquake of 1957 [3, 19, 21, etc.].

Institute of Geology, Russian Academy of Sciences, Moscow.

650

International Geology Review, 1993, 35, No. 7, pp. 650-657. Copyright © 1993 by V. H. Winston & Son, Inc. All rights reserved.

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FIGURE 1. Sketch map of the microplates of Central Asia (from data of [6], [21], P. Molnar and P. Tapponier, and others): 1) plate boundaries (a—reliable, b—inferred); 2) direction of relative movements along microplate boundaries (a—tensional, b—compressional, c—strike-slip); 3) folded regions of com­pression; 4) earthquake epicenters (after [18], [5]): 1) Tas-Yuryakh—1967, Nyukzha and Olekma—1958; 2) Great East Siberian (China-Vakat)—1725; 3) Muya—1957; 4) North Baikal—1917; 5) Muyakan— 1962; 6) Lower Angara—1931; 7) Tsagan—1962, Central Baikal—1959; 8) Mondy—1950; 9) Tsetser-leg—1905; 10) Bolnay—1905; 11) Saykhan—1958; 12) Mogod—1967; 13) Gobi-Altay—1957, Bayan-Tsagan—1958; 14) Unegety—1903; 15) Tashiynshar—1974; 5) orientation of regional compression.

On the east, the Tuva and Mongolian plates are in contact with the Amur plate, a substantial part of the north and northeast boundaries of which coincide with the seismically active Baikal rift zone and its northeast flank. South of Lake Baikal, the boundary of the Amur plate is well defined on space photographs, along the fault of the Snezhnaya River valley [15], and still farther south it apparently coincides with seismic faults active during the Saykhan (1958), Mogod (1967), and Unegetey (1903) earthquakes [5].

The morphology of the late Cenozoic and active fault zones at the boundaries of these microplates was predetermined by the old tectonic structure of these zones, and the kinematics of movement along them is intimately related to their configuration, simply speaking, to the strike of the faults. With a change in strike there are changes in the direction, rate, and amplitude of movements in the fault zones.

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652 INTERNATIONAL GEOLOGY REVIEW

As an example, let us consider the boundary faults of the Tuva plate. In the late Cenozoic, the northwest-striking Main Sayan fault (az. 320°) was an upthrust with a left-lateral component of movement on its eastern segment, striking 300° [1,15]. On its sublatitudinally-striking parts, the Sayan-Minusinsk fault is an overthrust with complex block structure and a slight component of left-lateral displacement [15].

The submeridional Kuray-Teletskoye fault runs almost at a right angle to the Sayan-Minus­insk. It is a typical pull-apart structure, constituting the boundary faults of the long (up to 80 km), narrow (3-5 km) graben of Lake Teletskoye [11, 15].

On its west-northwest and northwest-striking stretch, the Shapshal' fault is a complex zone up to 20 km wide (Fig. 3). According to our data, it was formed by an en-echelon system of right-lateral reverse + strike slip and normal + strike-slip counter-faults [14, 15].

The sublatitudinal Khangay fault system which meets the Tsagan-Shibety fault on the east is 485 km long. Considerable rupture occurred during the Bolnay earthquake of 1905. Most investigators interpret it as the result of left-lateral movement of 5.5 ± 0.5 m with a minor vertical component [5]. V. G. Trifonov demonstrated periodic seismic movements of similar magnitude along this fault every 600 years [22].

The boundary of the Tuva plate farther east can be drawn along the late Cenozoic Tsetserleg fault, striking east-northeast and east; its western part was rejuvenated at the time of the Tsetserleg earthquake of 1905. According to the description by Trifonova, Khil'ko et al. [5, 15, 22, etc.], this fault is a left-lateral reverse + strike-slip fault. It is connected to the submeridional late Cenozoic normal faults which bound the west shore of Lake Hobsogol and the graben of the valley of the Egiyn-Gol River which flows out of it.

The sublatitudinal Baikal-Mondy tectonic suture, active at the present time, abuts the north end of Lake Hobsogol [9, 10, 14]. This feature was established a long time ago, is very deep (as indicated by the high values of mantle helium in numerous thermal-mineral springs and young volcanoes), is clearly expressed in the relief, and is highly seismic. The suture is a left-lateral strike-slip with a vertical component of movement, manifested by a series of tectonic steps.

The morphology and kinematics of the faults bounding the Mongolian plate vary in similar fashion. For example, we find the Kobdo fault, oriented north-northwest, is a right-lateral thrust + strike-slip fault in accordance with the strike (Fig. 4) [14, 15, 19, 22]. Khil'ko [5] described the seismogenic Chikhteyn structure in this fault zone.

The sublatitudinal faults of the Gobi-Altay system which bound the Mongolian plate on the south have left-lateral strike-slip displacement with a very small vertical component of the movements. Most of these dislocations arose at the time of the Gobi-Altay earthquake of 1957 [3,5, 19, 21, etc.].

Thus the Tuva and Mongolian plates are bounded by active fault zones whose morphology and kinematics are closely related to their strike: northwest-oriented faults are thrusts; sublatitu­dinal ones are left-lateral strike-slips, often with a considerable thrust component of the move­ment; submeridional sutures bear signs of pull-apart stresses, and faults oriented north-northwest have right-lateral slip in addition to a thrust component.

The rate and amplitude of late Cenozoic and historical movements also depends on the orientation of the boundary faults. Maximum horizontal displacement is recorded on the sublati­tudinal sutures. Rates of horizontal displacement may reach about 3 mm/yr, 5-7 times greater than vertical movements in these same sectors [10]. Maximum vertical displacement is typical of faults striking northwest.

Typically, the horizontal component of late Cenozoic and historical movements on the faults increases as the vertical component decreases. Thus, on the Kobdo right-lateral reverse + strike-slip fault, the 1000-m vertical amplitude of displacement corresponds to a horizontal

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N. V. LUKINA 653

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654 INTERNATIONAL GEOLOGY REVIEW

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FIGURE 3. The Tsagan-Shibety fault. For sym- FIGURE 4. The Kobdo fault. For symbols, see bols, see Figure 2. Figure 2.

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N. V. LUKINA 655

FIGURE 5. Intraplate terrane. Late Cenozoic and recent network of faults in the western part of the West Sayans (based on space image interpretation): Black) lakes: Teletskoye (at left), Kara-Khol' (at right).

displacement of 100-200 m, and as the vertical amplitude decreases to 500 m, the horizontal component of movements increases to 500-600 m. The same relationship is typical of the Tsagan-Shibety fault: the 1700-m maximum vertical amplitude of displacement corresponds to not more than 100 m of horizontal movement; at the same time, the latter increases (as the vertical amplitude of movement decreases) to 300 m on the south and to 500 m on the north.

These patterns of variation of the morphology and kinematics of the faults as a function of their strike indicates that the late Cenozoic and historic tectonic stress field throughout the territory has been constant. It is dominated by northeast-oriented regional compression, produced by convergence of the Hindustan and Eurasian continental plates [9-11, 13] (see Fig. 1).

Surface discharges of thermal-mineral waters (as a rule with high contents of abyssal helium), high heat flow, and sometimes young volcanism and eruptions of Quaternary basalts, etc., may be related to the fault zones of the continental microplate boundaries in the region.

It also has been found that the fault zones bounding these microplates not only were very active in the late Cenozoic and historic stage of evolution, but also are very dangerous seismic-ally. It is precisely along these fault zones that epicenters of catastrophic are located.

For instance, on the north and northwest boundaries of the Amur plate are found the epicenters of the magnitude 7.0 Takh-Yuryakh earthquake (1967), the magnitude 6.5 Nyukzha and Olekma (1958), the 8.3 Great East Siberian (China-Vakat) (1725), the 7.9 Muya (1957), the 6.6 North Baikal (1917), the 6.0 Muyakan (1962), the 5.9 Lower Angara (1931), the 7.1 Tsagan (1862), the

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6.8 Central Baikal (1959), the 6.2 Saykhan (1958), the 7.8 Mogod (1967), and the 7.5 Unegetey (1903) [2, 5, 18, etc.].

Paleoseismic dislocations of intensity 9 to 11 earthquakes have also been described in this same zone: the Snezhnaya, Osinovo-Kedrovaya, Makhturikha, Posol'sk, Istoksk, Nesterikha, Malochivyrkuya, Svyatoye Nos, Dzelinda, Kovoktinsk, Muyakan, Ul'to, Usmun, Spitsyno, Koyra, Namarakit, Syni, Dovachan, Talaya, Etyrko, Medved', and others [2, 4, 15-18, etc.].

However, the continental microplates themselves are not stable regions. Active faults are common within them. For instance, the Sayan-Tuva, South Tannu-Ola, Ka-Khem, and many other faults in the Altay-Sayan region, and the Belino-Busiyngol, Darkhat, and others in northern Mongolia, have been very active in late Cenozoic and historic time. In general, the "intraplate space" is cut by numerous late Cenozoic faults and faults active in Late Pleistocene-Holocene and historic time, with very different orientations (Fig. 5). Often earthquake epicenters and paleoseis­mic dislocations of thermal-mineral springs, young volcanoes, eruptions of Quaternary basalts, and the like are confined to them [11, 12, 15]. It is very difficult to judge which orientations of tectonic sutures are more active and which less, as they all lie in a single structural association, produced by the late Cenozoic and active stress field.

Thus, the concept of continental microplate boundaries seems to us to be very arbitrary at the present time. It appears that ranks should be distinguished among the late Cenozoic and active faults (in particular, in the parts of southern Siberia and northern Mongolia that we have considered), cutting the territory into blocks of different sizes. This is necessary not only for correct theoretical ideas concerning the neotectonics, but also for practical purposes, primarily detailed seismic regionalization.

References

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2. Geologiya i seysmichnost' zony BAM. Seysmogeologiya i seysmicheskoye rayonirovaniye (Geology and Seismicity of the Zone of the Baikal-Amur Trunkline. Seismogeology and Seismic Regionalization), 1985: Nauka Press, Novosibirsk.

3. Gobi-Altayskoye zemletryaseniye (The Gobi-Altay Earthquake), 1963: AN SSSR Press, Moscow.

4. Zhivaya tektonika, vulkany i seysmichnost' Stanovogo nagor'ya (Active Tectonics, Volca­noes and Seismicitiy of the Stanovoy Upland), 1966: Nauka Press, Moscow.

5. Zemletryaseniya i osnovy seysmicheskogo rayonirovaniya Mongolii (Earthquakes and Basis of the Seismic Regionalization of Mongolia), 1985: Nauka Press, Moscow.

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9. Lukina, N. V., 1988, Strike-slip component of Quaternary movements on some faults in the Baikal rift zone (geomorphological analysis). In Tes. dokl. I Vsesoyuz. soveshch. po sdvigo-voy tektonike "Sdvigovyye tektonicheskiye narusheniya i ikh rol' v obrazovanii mestorozh-deniyi" (Abstracts of Papers at the First All-Union Conference on Strike-Slip Tectonics

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11. Lukina, N. V., 1991, The young grabens of Lake Teletskoye: Priroda, No. 2, pp. 56-64. 12. Lukina, N. V., 1988, Morphology of active faults in the Altay-Sayan region, based on data

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19. Trifonov, V. G., 1976, Late Quaternary faults in Western and Central Asia according to interpretation of aerospace photos and ground observations: Geologiya i razvedka, No. 11, pp. 54-64.

20. Trifonov, V. G., 1979, Interaction of continental plates under conditions of compression. In Tektonicheskoye razvitiye zemnoy kory i razlomy (Tectonic Evolution of the Earth's Crust and Faults) (pp. 185-201): Nauka Press, Moscow.

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22. Trifonov, V. G., 1985, Evolution of active faults: Geotektonika, No. 2, pp. 16-26.

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