THE TIBETAN PLATEAU / THE LARGEST HIGHEST PLACE ON EARTH

THE TIBETAN PLATEAU / THE LARGEST HIGHEST PLACE ON EARTH

Postby salsinawi » Tue Mar 06, 2007 9:05 pm

THE TIBETAN PLATEAU / THE LARGEST HIGHEST PLACE ON EARTH


The Tibetan Plateau

http://geology.about.com/library/weekly/aa112501a.htm

The largest highest place on Earth is a showcase of plate tectonics.

In all the world's continents, there are five exceptional places that have strong, outsized effects on the whole globe. Two of them, the Antarctic and Greenland ice caps, act as great cooling engines for the ocean and atmosphere. The other three are high places, including the North American mountain belt and the Altiplano of South America. But the third is the greatest by far: the Tibetan Plateau.

The Tibetan Plateau is an immense upland, some 3500 by 1500 kilometers in size, that averages more than 5000 meters in elevation. It includes almost all of the world's territory higher than 4000 meters. Its southern rim, the Himalaya-Karakoram complex, contains not just Mount Everest and all 13 other peaks higher than 8000 meters, but hundreds of 7000-meter peaks each higher than anywhere else on Earth.

The Tibetan Plateau is not just the largest, highest area in the world today; it may be the largest and highest in all of geologic history. That's because the set of events that formed it, and continue to build it, appears to be unique: a full-speed collision of two continental plates.

Nearly 100 million years ago, India was a separate continent much like Australia is today. It had just separated from southern Africa as part of the breakup of the supercontinent Gondwanaland. From there the Indian plate moved to the north at speeds of around 150 millimeters per year—much faster than any plate is moving today.

Topography(left) and plate-tectonic setting (right) of the Tibetan Plateau. The plateau is the large red topographic high north of the Indian subcontinent. The white square shows the location of the 14 November 2001 earthquake (M7.8). The plateau is part of a broad zone of Asia that is being deformed by the northward movement of the Indian plate. The full versions of both of these maps are here.

What made the plate move so fast? Our best guess is that the Indian plate was being pulled from the north as the oceanic crust making up that part of it was being subducted beneath the Asian plate. The oceanic crust was old, which means that it was cold and denser than the mantle beneath it. Once you start subducting this kind of crust, it wants to sink fast. The "slab pull" that results is especially strong. Today, the Pacific plate is moving relatively fast for the same reason .

Another reason the Indian plate moved so fast may have been "ridge push." This force comes from the crustal spreading zone—the midocean ridge—on the other edge of the plate, where new hot crust is created. The new crust in spreading zones stands higher in elevation than old ocean crust, and the difference in elevation results in a downhill gradient. In India's case, the mantle beneath Gondwanaland may have been especially hot. Thus the ridge push may have been more significant than usual too.
Whatever the reasons, India was moving north and, beginning around 55 million years ago, began to plow directly into the Asian continent. (See an animation here.) Now when two continents meet, neither one can be subducted under the other. Continental rocks are too light. Instead, they pile up. The continental crust beneath the Tibetan Plateau is the thickest on Earth, some 70 kilometers thick on average. And under the Pamir mountains, at the northwest end of the plateau, the thickness reaches nearly 100 km.

Besides piling up, continental rocks can also be shoved aside. This is what's happening to the north of the Tibetan Plateau: great chunks of Asian rock are being pushed eastward. This is why the large Chinese earthquake in November 2001 was a strike-slip event, like those on California's San Andreas fault, and not a thrust quake. A French team including Paul Tapponnier, who first described this process 30 years ago, has put up a preliminary analysis of that earthquake.

This is just a bare outline of the Tibetan Plateau. Today's researchers are finding this region a showcase of collision-related geology, and they're asking questions that may pay off everywhere else on our planet. Part 2 will get into some of the specifics: things like Nanga Parbat, Pleistocene granites, super erosion, and eduction.


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http://oak.ucc.nau.edu/wittke/Tibet/Plateau.html

Geology of the Tibetan Plateau

Tibet has often been called the "Roof of the World." The plateau is probably the largest and highest area ever to exist in Earth history, with an average elevation exceeding 5000 m (16,400'). In the image, high elevations are shown in gray and red, and low elevations are shown in blue. The Tibetan Plateau covers an area about half that of the lower 48 United States and is bounded by the deserts of the Tarim and Qaidam Basins to the north and the Himalayan, Karakoram, and Pamir mountain chains to its south and west. Its eastern margin is more diffuse and consists of a series of alternating deep forested valleys and high mountain ranges that run approximately north-south, bounded by the lowlands of the Sichuan Basin of China. Recent research was presented at the eleventh workshop on the Himalaya-Karakoram-Tibet region. An excellent site about the geology of Tibet and the Himalayas is at:

http://www.geo.arizona.edu/~ozacar/frontp~1.htm.

The Tibetan Plateau is a collage of continental fragments that were added successively to the Eurasian plate during the Paleozoic and Mesozoic eras. Paleomagnetic data indicate that these fragments were at southern latitudes during the Paleozoic. The sutures between these microplates are marked by scattered occurrences of ophiolitic material caught up between the crustal blocks during accretion. From north to south, the main Tibetan crustal blocks are the Kunlun, Songban-Ganzi, Qiangtang, and Lhasa terranes. It is underlain by continental crust about 65 km thick, compared with more usually thicknesses of about 30 km. Uplift of the plateau began in the early Miocene and it probably reached its present elevation by about 8 Ma (million years). Three major theories have been proposed for the origin of this immense thickness with many additional minor variations upon them.

The first proposal, basement reactivation, involves distributed shortening of the Plateau by folding and thrusting of its rocks. Crust is thickened by the faulting and subsequent movement of large masses of rock, which are stacked one on top of another like cordwood. The process is like squeezing a block of clay by its ends: what happens is controlled by the rate of squeezing and mechanical behavior of the clay. At sufficiently high rates of deformation the clay will break and the resulting multitude of fractures will cause it to thicken in the middle. At slower rates of squeezing, the clay flows plastically, thickening by folding without fractures. This model when applied to the Tibetan Plateau predicts that there will be abundant evidence of recent compressional deformation.
The second theory, continental subduction, entails the wholesale underthrusting of the Indian continental crust beneath the Tibetan Plateau and subsequent uplift. This process is reminiscent of taking a block of ice and pushing it beneath another ice slab, causing the latter to rise upwards. However, it is difficult to imagine how the buoyant Indian crust could be kept deep enough to get far beneath the plateau before bobbing to the surface. Perhaps the great speed at which India is colliding to Eurasia allowed this to happen.

The third proposal, continental injection, involves the introduction of Indian crust beneath Tibet as melted rock, called magma. Granitic melts derived from the subducting Indian crust rise into the overlying Eurasian and transfer heat into the base of the Tibetan Plateau. The resulting thickened crust is heated by radioactive decay of the element potassium, uranium, and thorium, which are preferentially concentrated in the magmas. Like a hot-air balloon, the heated crust is buoyant and rises with the addition of light granitic material at the bottom of the Eurasian crust increasing the height of the Plateau. The presence of a partially molten zone at the base of the Tibetan Plateau has been documented by seismic experiments. In addition to providing heat to cause uplift, the partially molten zone at the base of Tibet also inhibits the rise of basaltic melts.
The ascent of these magmas is driven by differences in density between basaltic magma and the surrounding rocks. While basaltic magmas are lighter than the upper mantle in which they are produced and rise like droplets of oil in water, they tend to stall out when the density difference becomes too small. Usually lower crust is cold and dense, promoting the ascent of basaltic magmas, but the hot Tibetan crust acts as a density "filter," stopping the rise of these mafic melts. This mechanism may explain the high heat flow observed on the plateau and relative dearth of mafic volcanic rocks.

Going from north to south, the blocks comprising Tibet are the Kunlun Terrane, Songban-Ganzi Complex, Qiangtang Terrane, and Lhasa Terrane. All, save the Songban-Ganzi Complex, are true continental fragments, underlain by ancient Precambrian basement.
Readings

•Fielding et al., 1994, How flat is Tibet?: Geology, v. 22, p. 163-167.

•Harrison et al., 1992, Raising Tibet: Science, v. 255, p. 1663-1670.

•Kong et al., 1997, Evaluating the role of preexisting weaknesses and topographic distributions in the Indo-Asian collision by use of a thin-shell numerical model: Geology, v. 25, p. 527-530.

•Molnar, 1989, The geological evolution of the Tibetan Plateau: American Scientist, v. 77, p. 350-359.

•Rothery & Drury, 1984, The neotectonics of the Tibetan Plateau: Tectonics, v. 3, p. 19-26.

•Tapponier et al., 1982, Propogating extrusion tectonics in Asia: New insights from simple experiments with plasticine: Geology, v. 10, p. 611-616.

•Zhao & Morgan, 1985, Uplift of Tibetan Plateau: Tectonics, v. 4, p. 359-369.

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http://en.wikipedia.org/wiki/Tibetan_Plateau

Tibetan Plateau

The Tibetan Plateau or Chang Tang, also known as the Qinghai-Tibetan (Qingzang) Plateau, is a vast, elevated plateau in East Asia covering most of the Tibet Autonomous Region and Qinghai Province in China. It occupies an area of around 1,000 by 2,500 kilometers, and has an average elevation of over 4,500 meters. Called "the roof of the world," it is the highest and biggest plateau in the world, with an area of 2.5 million square kilometers (about four times the size of Texas or France) [1]. The plateau was formed by the collision of the Indo-Australian and Eurasian tectonic plates in the Cenozoic period (approximately 55 million years ago), in a process that is still ongoing.

The plateau is a high-altitude arid steppe interspersed with mountain ranges and large brackish lakes. Annual precipitation ranges from 100mm to 300mm and falls mainly as hailstorms. [2] The southern and eastern edges of the steppe have grasslands which can sustainably support populations of nomadic herdsmen, although frost occurs for six months of the year. Proceeding to the north and northwest, the plateau becomes progressively higher, colder and drier, until reaching the remote Kekexili region in the northwestern part of the plateau. Here the average altitude exceeds 5,000 meters (16,500 feet), the air contains only 60% of the oxygen of sea level, and year-round temperatures average -4°C, dipping to -40°C in winter. [3] As a result of this extremely inhospitable environment, the Kekexili region is the least populated region in Asia, and the third least populated area in the world after Antarctica and northern Greenland.


The Chang Tang is bordered to the northwest by the Kunlun Range which separate it from the Tarim Basin, and to the northeast by the Qilian Range which separates the plateau from the Gobi Desert. In the south the plateau is delineated by the Brahmaputra river valley which flows along the base of the Himalayas, and by the vast Indo-Gangetic Plain. To the east and southeast the plateau gives way to the forested gorge and ridge geography of the mountain headwaters of the Salween, Mekong, and Yangtze rivers in western Sichuan. In the west it is embraced by the curve of the rugged Karakoram range of northern Kashmir.


The tectonic uplift of the plateau is thought to have had a significant effect on climate change, and it is believed to affect the Asian monsoon. In the Indian monsoon season (June to October) when the winds bring humid, tropical air from the south, the Himalayas create a rain shadow which makes northern India very wet and keeps the Tibetan Plateau very dry. As the winds continue over the plateau, they drop what little moisture remains in the air, becoming drier as they move northwards and creating deserts such as the Taklamakan and the Gobi Desert.
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