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. 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.