a mountainous region lying to the east of Lake Baikal, stretching nearly 1,000 km southward from the Patom and Northern Baikal uplands to the Soviet border and over 1,000 km eastward from Lake Baikal to the meridian of the confluence of the Shilka and Argun’ rivers. Mountains of medium elevation (1,200–1,800 m) predominate, ringed on the north and west by a belt of high mountains (up to 3,000 m). The Buriat ASSR and Chita and part of Irkutsk oblasts of the RSFSR are in Transbaikalia.
Geological structure and mineral resources. Folded systems of various ages extending from north to east form the structure of Transbaikalia. About two-thirds of the region is composed of granites. Southeast of the tectonic geosuture stretching from the upper reaches of the Chikoi River to the Shilka River is a Hercynian folded region, consisting of terrigenous strata of the Cambrian, Ordovician, and Silurian systems and of terrigenous-carbonate-volcanic deposits of the Devonian, Carboniferous, and Permian systems.
The central zone of Transbaikalia (Dzhida and Iablonovyi ranges) is assigned to the early Caledonides. Its structure includes Lower Paleozoic granites, among which are found outcrops of Cambrian terrigenous-carbonate-volcanic strata. The northwestern part of Transbaikalia (Vitim Plateau) is part of the Baikal folded region. It is formed of Riphean granites as well as of gneisses, marbles, and shales of the Riphean and Lower Proterozoic. In the northeastern part of Transbaikalia (Olekma Stanovik) lies a zone of Lower Proterozoic folding, consisting of deeply metamorphosed rock of the Archean and Lower Proterozoic. Narrow depressions created by continental terrigenous-volcanic strata of the Jurassic and Cretaceous systems occur above the structures of the Hercynian and Caledonian regions. In the central part of Transbaikalia are mantles of Cenozoic plateau basalts. In the tectonics of Transbaikalia old and youthful fractures, extending from north to east, are very important.
The Hercynian region is rich in mineral deposits—tin (Sherlovaia Gora and Khapcheranga deposits), wolfram, and molybdenum, genetically related to Paleozoic granitic intrusions. There are gold ore deposits associated with minor intrusions at Balei and polymetallic deposits at Akatui. In the Caledonian zone are found deposits of chromium, titanium, iron, zinc, wolfram, molybdenum (Amanan-Makit), and gold (Darasun). In the Baikal region are deposits of ore and placer gold. Nonmetallic mineral deposits include fluorite, semi-precious stones, magnesite, dolomite, graphite, and mineral paint. In the Mesozoic depressions are lignite and hard coal (Tarbagatai and Tugnui deposits). In the Hercynian region are carbonated, radonic-carbonated, and nitric mineral springs. In the Baikal region are thermal nitric, methane, and carbonated waters.
K. A. KLITIN
Terrain. The relief is characterized by the alternation of mountain ranges with intermontane basins, stretching in a north-east direction. In the formation of the basic features of this relief, Mesozoic and Cenozoic structures, complicated by fractures, played the major role. The northwestern part of Transbaikalia is part of a vaulted uplift with a high degree of neotectonic activity and seismicity (up to numbers 9–10). Here are found the high (up to 2,500–3,000 m) ranges of the Baikal Region (the Khamar-Daban, Barguzin, Ikatskii) and of Transbaikalia (Severnaia-Muia, Iuzhnaia-Muia, Kodar, Udokan) and the large, deep basins of the Baikal type (the Barguzin, Verkhniaia Angara, Muia-Kuandy, Verkhniaia Chara, and Baunt basins). In the medium- and low-altitude mountain regions and on the plateaus located farther east, seismicity is lower (up to numbers 6–7); the elevations of mountain ranges decrease to 1,800–800 m; and basins are neither as vast nor as deep (Transbaikalian type). The Selenga medium-altitude mountain region is the most dissected and the Vitim Plateau the least, particularly in areas of basalt mantles with preserved cones of Holocene volcanoes. In southern Transbaikalia lies the extremely dissected Khentei Upland (range), rising to an elevation of 2,500 m, and in the extreme southeast is the Torei Plain (500–800 m).
Climate. The climate is continental. Transbaikalia is located in a region influenced by western and eastern air masses and by the prolonged action of winter anticyclones. Winters are long, cold, and severe. In the basins the weather is marked by few clouds and little wind; the mean January temperature is below -20°C and may drop to -35°C. On the slopes of mountain ranges, temperatures increase as a result of inversion, dropping again, however, above an altitude of 1,500–2,000 m. The snow cover, only 10–15 cm on the basin floors and in the southern plains, may attain a thickness of 150 cm or more in the high ranges. Summers are moderately warm; in the high mountain regions they are cool. In basins the mean July temperature may range from 10° to 20°C or from 12° to 15°C; at an altitude of 1,800 m to 2,000 m it is 8°-10°C; and at an altitude of 2,500 m it is 5° to 7°C. Precipitation is relatively low and unevenly distributed according to locality and season. Moving from the southeast to the northwest and from the basin floors to the higher elevations of the mountain ranges, the amount of precipitation increases from 250–300 mm to 700–900 mm and even to 1,000 mm. Maximum precipitation (50–55 percent of the annual amount) occurs in the second half of the summer, during the monsoon rains. Permafrost is characteristic of the region, continuous throughout most of the territory but only sporadic in the south. More than 30 contemporary glaciers are known in the Kodar range with a total area of about 15 sq km.
Rivers and lakes. Part of the main watershed between the Arctic and Pacific oceans passes through Transbaikalia. The largest rivers are the Vitim, Olekma, Selenga, Shilka, and Argun’. The density of the river network is 0.7–0.8 km per sq km; in the south it is 0.3–0.4 km per sq km. Most are mountain rivers, fed mainly by rain. The seasonal flow is uneven, with 50–80 percent of the total annual flow occurring during the summer rains and causing frequent heavy floods. Many rivers freeze completely and do not flow at all during the winter. The appearance of water above ice (naled1) is characteristic. The few lakes were formed during the last glaciation and are associated with the floors of intermontane trough basins. The largest lakes—Baikal, Torei, Baunt, Eravnoe, and Gusinoe—are located in basins. There are thermalkarst lakes and, in southern regions without drainage, salt lakes.
Natural regions. Transbaikalia lies in a taiga zone that in the south gives way to forest steppes and steppes, extending north from Mongolia. The mountain-basin relief of the region results in a complex intermingling of latitudinal and vertical zonality. Steppe covers the southeastern plains and lowaltitude mountain regions and may also occur in some large basins. There are crested hairgrass, feather grass, and tansy steppes on chestnut soils and gramineous mixed-grass and mixed-grass steppes on chernozems. In the steppe region are patches of frozen ground. Forest steppe is widespread in the Selenga and Verkhnii Amur medium-altitude ranges to an elevation of 1,000–1,200 m. The climate of the forest-steppe region is moderately warm, and the permafrost is of the valley type. Birch, larch-birch, and aspen forests on dark gray forest soils alternate with tansy mixed-grass and mixed grass steppes on chernozem soils. In addition to the typical steppe rodents, other forest fauna is represented, including roe, elk, squirrel, and the Siberian chipmunk.
Mountain taiga predominates in Transbaikalia. In the south the taiga occupies the middle and upper parts of mountain slopes, rising to 1,700–1,900 m; in the central belt the taiga covers all mountain slopes and summits to an altitude of 1,500 m; and in the northern high mountain belt it covers the lower and middle parts of slopes to an altitude of 1,100–1,400 m. The climate is cool and humid. Permafrost occurs everywhere. Taiga forests of dahurian larch predominate, and cedars may be found in the Khentei Upland (range) and in the Khamar-Daban Range. The soil is of the mountain permafrost-taiga ferruginous type, with traces of podzolization in the east. In the south larch and birch forests grow on soddy forest nonpodzolized soils. In sandy places and on steep slopes are pine forests growing on podzolic soils. The floors of intermontane depressions are usually swampy, un-forested, and occupied by dwarf arctic birch or sometimes by meadows; in the east the floors are covered by man (sedgesphagnum bogs with shrubs and sparse larch). The mountain taiga is inhabited by elk, musk deer, and bears; animals of this region that are especially valuable commercially are squirrel and sable.
Sparse forests lying below mountain balds occur in Transbaikalia on the upper slopes of the high ranges and on the flat summits of the taiga medium-altitude mountains at an altitude of 1,200–1,700 m. The balds are treeless summits and mountain slopes rising above 2,000 m (above 1,600 m in the north); balds are particularly widespread in northern Transbaikalia. The climate of this region is less continental than in the other regions; there is comparatively heavy snowfall in winter, accompanied by strong winds, and avalanches are common. Rock streams are covered with high-mountain lichen tundra on which thickets of dwarf cedar and rhododendron grow. The fauna is represented by few species, predominantly pika and high mountain voles and some reindeer, ibex, and mountain sheep.
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S. S. VOSKRESENSKII and L. I. MUKHINA