Tibetan Highlands

a highland region in China, in Central Asia. Area approximately 2 million sq km; average elevation, 4,000–5,000 m. The Tibetan Highlands are bounded by the Karakoram in the west, the Kunluns in the north, and the Tansiie Shan in the east. (Sometimes all these mountain systems are included in the Tibetan Highlands.) The Himalayas form the southern border.

Topography. The northern and central parts of the Tibetan Highlands (known locally as Chang Tang) consist of flat or rolling plains (elevation 4,600–5,200 m) alternating with relatively short mountain ranges. The ranges extend for the most part in a latitudinal or sublatitudinal direction (elevations to 6,000 m) and have broad, flat watersheds, gentle slopes only slightly affected by erosional processes, and numerous bodies of rock fragments. Despite its enormous altitude, Chang Tang has the appearance of a region with middle-mountain topography; only individual peaks that rise above the snow line have glaciers and alpine characteristics. Cirques, troughs, and moraine are found at elevations above 4,500 m. In the outlying areas of the Tibetan Highlands, especially the areas adjacent to the Tansiie Shan and the Himalayas, the elevation of the plains decreases to 3,500 m, and the plains frequently assume the form of intermontane basins (the largest being occupied by the upper courses of the Indus and Brahmaputra rivers); local relief here is 2,500–3,000 m. The slopes of the peripheral ranges are precipitous and strongly dissected, the river valleys (especially in the east) forming deep gorges.

Geological structure and mineral resources. The Tibetan Highlands are located within the Mediterranean geosynclinal belt, constituting a special formation with the features of a median mass. Regions with various histories of geological development can be distinguished. The region of northern Tibet has carbonaceous rocks of the Upper Paleozoic era, and large areas are covered by Cretaceous red rocks; the Karakoram-Tanglha region has a broad distribution of marine deposits of Permian and Triassic age; the region where the larger lakes (Nam Tso, Selling Tso, Tangra Yum Tso) are located has vast areas of Jurassic continental deposits and Cretaceous marine deposits; the region of the Trans-Himalayas is composed of sandstone and shale strata of the Carboniferous and Permian periods and volcanic rocks and granites of the Cretaceous period; and the region of the upper courses of the Tsangpo and Indus rivers is characterized by the development of flysch of Cretaceous-paleogene age.

Deposits of the Upper Paleozoic and Mesozoic eras have been displaced in an extremely irregular fashion. Over most of the highlands, they form wide, gently sloping doubly plunging anticlines, which become narrow and steep in the fault zones. Up to the middle of the Cenozoic era, the Tibetan Highlands experienced primarily downwarping; over most of the highlands, sediment accumulated and marine conditions persisted. Upheavals occurred on a large scale in the late Cenozoic era; in the period since the end of the Pleistocene glaciations alone, the highlands have been uplifted 300–500 m. Evidence of continuing tectonic activity is provided by recent movements along faults, high seismicity, and the existence of young volcanic cones and thermal springs.

Placers of gold and cassiterite are among the mineral deposits in the region of northern Tibet. The Karakoram-Tanglha region has deposits of complex ores, and in the southern regions deposits of anthracite occur in association with the Jurassic continental deposits. The highlands’ closed lakes (primarily in Chang Tang) contain large reserves of borax and soda.

Climate. The climate is severe and dry, a consequence of the highlands’ continentality, very high elevation, and isolation from humid air masses. In the northern and central parts of the highlands (Chang Tang), the average annual temperature is 0°–5°C. Winters are long in this region, with temperatures dropping to –32°C; there is little snow. Summers in Chang Tang are cool, with temperatures averaging 10°–15°C and only occasionally rising above 20°C; frosts occur even in July. Temperatures are considerably higher in the valleys of the south (– 2° to – 4°C in January and 14°–18°C in July). The air of the Tibetan Highlands is extremely rarefied, which is conducive to sharp daily fluctuations of temperature, pronounced drops in temperature at night, and local winds, which cause frequent dust storms. Precipitation totals 100–200 mm a year over most of the highlands, with a considerable portion in the form of snow; in the outlying areas, the total can exceed 500 mm, while the southern regions, influenced by the summer monsoon, receive 700–1,000 mm. Owing to the aridity of the climate, the snow line in Chang Tang is at 6,000 m, the highest on earth; in the highlands’ outlying areas, it falls to 5,000–5,500 m. However, because of the great elevations, nearly all the main mountain ranges have snowfields and glaciers. The largest area of glaciation is in the south (Kailas and other ranges), which receives greater amounts of precipitation.

Rivers and lakes. Most of the highlands has internal drainage. The territory of Chang Tang and adjacent regions is divided into a number of closed basins with relatively short rivers. The sources of the Huang Ho and Yangtze, Mekong, Salween, Brahmaputra (Tsangpo), and Indus rivers are in the outlying areas of the Tibetan Highlands that are influenced by the monsoons. The rivers are typical of those found in plains; both the volume of water carried and the speed of the current rise sharply in the peripheral mountain ranges, and the valleys deepen into gorges. The rivers are fed primarily by snow and glaciers in the northern and central regions and by rain in the south. High water is in summer, and the rivers freeze in winter; Aufeis is formed in certain areas of the plains. The rivers’ vast hydroelectric potential is virtually untapped. There is boat traffic on the Tsangpo and its tributaries.

Numerous lakes have formed in the tectonic basins at elevations of 4,500–5,300 m. The largest of these are Nam Tso, Selling Tso, and Tangra Yum Tso. The lakes are usually shallow and are often salty or brackish; the shores are low-lying and, in some places, swampy; salt marshes are widespread. Owing to the high concentration and varied content of salts, the lakes often have turquoise, brown or other hues. They are usually frozen over from November to May.

Soils and vegetation. The soils most common in the Tibetan Highlands are typical of those found in steppes and deserts at great elevations. The soils of the highlands, however, are distinguished by a primitive profile, a high content of detritus, and a low content of humus. Many areas are without a soil and vegetative cover; here, the surface is a shield of pebbles and broken stone. Fertile mountain, meadow-steppe, and mountain meadow soils are prevalent in the peripheral regions of the highlands.

High-altitude (cold) deserts and semideserts predominate in the Tibetan Highlands. They are characterized by low-growing (usually less than 5 cm; more rarely up to 15 cm), sparse herbaceous and subshrubby vegetation; that is, there are parterre and pillow-like forms (Ceratoides, Tanacetum, ephedra, Myricaria) and herbaceous plants (wormwood, Astragalus, Acantholimon, and Saussurea). Plants of the grass family are encountered (Ptilagrostis). Mosses, liverworts, and lichens predominate in the northern part of Chang Tang; in areas close to groundwater, there are meadow biotic communities of sedge, Kobresia, cotton grass, and rush. Altitudinal zonality is seen in the outlying areas to the east and south of the Tibetan Highlands, where precipitation is greater and differences in elevation are more pronounced. With decreasing elevation, cold deserts and semideserts give way to mountain steppes (fescue, Stipa, meadow grass), over which the vegetative cover is continuous. There are meadow areas near the sources of the Yangtze. The peripheral regions and the valleys of the larger rivers have shrub thickets (rhododendron, pea tree, juniper) and areas of gallery forests of willow and Euphrates poplar.

Fauna. Wild hoofed animals are typical of the entire northern portion of the Tibetan Highlands. Found here are the yak, chiru, addax, kiang, bharal, and argali; mammals of the families Leporidae and Lagomyidae and of the subfamily Microtinae are also common. Predators include the Asiatic black bear, wolf, true fox, and golden jackal. The numerous birds include the snow pheasant, Pallas’s sandgrouse, and the brambling, as well as such predators as the vulture Gyps hymalayensis and Pallas’s sea eagle. The rivers and lakes abound in fishes of the families Salmonidae and Cyprinidae. The desert faunal complex gives way to the meadow-steppe complex in the southern and eastern peripheral regions; here, the musk deer and, in the mountains, the leopard are encountered, as are pheasants, pigeons, and birds of the genera Milvus and Falco.