The Tibetan Plateau is the largest high area of thickened continental crust on Earth, with an average height of 16,000 feet (4,880 m) over 470,000 square miles (1,220,000 km2). Bordered on the south by the Himalayan Mountains, the Kunlun Mountains in the north, the Karakoram on the west, and the Hengduan Shan on the east, Tibet is the source of many of the largest rivers in Asia. The Yangtze, Mekong, Indus, Salween, and Brahmaputra Rivers all rise in Tibet, and flow through Asia, forming the most important source of water and navigation for huge regions.
Southern Tibet merges into the foothills of the northern side of the main ranges of the Himalaya, but are separated from the mountains by the deeply incised river gorges of the Indus, Sutlej, and Yarlung
Zangbo (Brahmaputra) Rivers. Central and northern Tibet consists of plains and steppes that are about 3,000 feet (1,000 m) higher in the south than the north. Eastern Tibet includes the Transverse ranges (the Hengduan Shan) that are dissected by major faults in the river valleys of the northwest-southeast-flowing Mekong, Salween, and Yangtze Rivers.
Tibet has a high plateau climate, with large diurnal and monthly temperature variations. The center of the plateau has an average January temperature of 32°F (0°C), and an average June temperature of 62°F (17°C). The southeastern part of the plateau is affected by the Bay of Bengal summer monsoons, whereas other parts of the plateau experience severe storms in fall and winter months.
Geologically, the Tibetan Plateau is divided into four terranes, including the Himalayan terrane in the south, and the Lahasa terrane, the Qiangtang ter-rane, and Songban-Ganzi composite terrane in the north. The Songban-Ganzi terrane includes Trias-sic flysch and Carboniferous-Permian sedimentary rocks, and a peridotite-gabbro-diabase sill complex that may be an ophiolite, overlain by Triassic flysch. Another fault-bounded section includes Paleozoic limestone and marine clastics, probably deposited in an extensional basin. South of the Jinsha suture, the Qiangtang terrane contains Precambrian basement overlain by Early Paleozoic sediments that are up to 12 miles (20 km) thick. Western parts of the Qiangtang terrane contain Gondwanan tillites, and Triassic-Jurassic coastal swamp and shallow marine sedimentary rocks. Late Jurassic-Early Cretaceous deformation uplifted these rocks, before they were unconformably overlain by Cretaceous strata.
The Lhasa terrane collided with the Qiangtang terrane in the Late Jurassic and formed the Bangong suture, containing flysch and ophiolitic slices, that now separates the two terranes. It is a composite terrane containing various pieces that rifted from Gondwana in the Late Permian. Southern parts of the Lhasa terrane contain abundant upper Cretaceous to Paleocene granitic plutons and volcanics, as well as Paleozoic carbonates, and Triassic-Jurassic shallow marine deposits. The center of the Lhasa ter-rane is similar to the south but with fewer magmatic rocks, whereas the north contains upper Cretaceous shallow marine rocks that onlap the upper Jurassic-Cretaceous suture.
The Himalayan terrane collided with the Lhasa terrane in the Middle Eocene, forming the ophio-lite-decorated Yarlungzangbo suture. Precambrian metamorphic basement is thrust over Sinian through Tertiary strata, including Lower Paleozoic carbonates and Devonian clastics, overlain unconformably by Permo-Carboniferous carbonates. The Himalayan terrane contains Lower Permian Gondwanan flora,
An 80-mile (130-km) wide view of the Himalayas from International Space Station, January 28, 2004—Mount Everest is shown in the upper center, Makalu is at the top left. The view looks south: Tibet is at the bottom, and Nepal is the land beyond Everest. (NASA/Photo Researchers, Inc.)
and probably represents the northern passive margin of Mesozoic India, with carbonates and clastics in the south, thickening to an all clastic continental rise sequence in the north.
The Indian plate rifted from Gondwana and started its rapid (3.2-3.5 inches per year, 80-90 mm/ yr) northward movement about 120 million years ago. Subduction of the Indian plate beneath Eurasia until about 70 million years ago formed the Cretaceous Kangdese batholith belt, containing diorite, granodiorite, and granite. Collision of India with Eurasia at 50-30 million years ago formed the Lha-goi-Khangari belt of biotite and alkali granite, and the 20-10 million-year-old Himalayan belt of tourmaline-muscovite granites.
Tertiary faulting in Tibet is accompanied by vol-canism, and the plateau is presently undergoing east-west extension with the formation of north-south graben associated with hot springs, and probably deep magmatism. Seismic reflection profiling has detected some regions with unusual characteristics beneath some of these grabens, interpreted by some seismologists as regions of melt or partially molten crust.
Much research has focused on the timing of the uplift of the Tibetan Plateau and modeling the role this uplift has had on global climate. The plateau strongly affects atmospheric circulation, and many models suggest that the uplift may contribute to global cooling and the growth of large continental ice sheets in latest Tertiary and Quaternary times. In addition to immediate changes to air-flow patterns around the high plateau, the uplift of large amounts of carbonate platform and silicate rocks exposes them to erosion. The weathering of these rocks causes the exposed rocks to react with atmospheric carbon dioxide, which combines these ions to produce bicarbonate ions such as are used to form calcium carbonate (CaC03), drawing down the atmospheric carbon dioxide levels and contributing to global cooling.
The best estimates of the time of collision between India and Asia is between 54 and 49 million years ago. Since then convergence between India and Asia has continued, but at a slower rate of 1.6-2.0 inches per year (40-50 mm/yr), and this convergence has resulted in intense folding, thrusting, shortening, and uplift of the Tibetan Plateau. Timing the uplift to specific altitudes is difficult, and considerable debate has centered on how much earlier than 50 million years ago the plateau reached its current height of 16,404 feet (5 km). Most geologists would now agree that this height was attained by 13.5 million years ago, and that any additional height increase is unlikely since the strength of the rocks at depth has been exceeded, and the currently active east-west extensional faults are accommodating any additional height increase by allowing the crust to flow laterally.
When the plateau reached significant heights, it began to deflect regional air-flow currents that in turn deflect the jet streams, causing them to meander and change course. Global weather patterns were strongly changed. In particular, the cold polar jet stream is now at times deflected southward over North America, northwest Europe, and other places where ice sheets have developed. The uplift increased aridity in Central Asia by blocking moist airflow across the plateau, leading to higher summer and cooler winter temperatures. The uplift also intensified the Indian Ocean monsoon over what it was before the uplift, because the height of the plateau intensifies temperature-driven atmospheric flow as higher and lower pressure systems develop over the plateau
Map of Tibet showing different terranes that make up the during winter and summer. This has increased the amount of rainfall along the front of the Himalayan Mountains, where some of the world's heaviest rainfalls have been reported, as the Indian monsoons are forced over the high plateau. The cooler temperatures on the plateau led to the growth of glaciers, which in turn reflect back more sunlight, further adding to the cooling effect.
Paleoclimate records show that the Indian Ocean monsoon underwent strong intensification 7-8 million years ago, in agreement with some estimates of the time of uplift, but younger than other estimates. The effects of the uplift would be different if the uplift had occurred rapidly in the Late Pliocene-Pleistocene (as suggested by analysis of geomorphology, paleokarst, and mammal fauna), or if the uplift had occurred gradually since the Eocene (based on lake sediment analysis). Most geologists accept analysis of data that suggest that uplift began about 25 million years ago, with the plateau reaching its current height by 14 or 15 million years ago. These estimates are based on the timing of the start of extensional deformation that accommodated the exceptional height of the plateau, sedimentological records, and uplift histories based on geothermometry and fission track data.
Map of Tibet showing different terranes that make up the high plateau
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