As plateaux go, that forming Tibet is by far the highest and the largest. Sitting at an average elevation above 5 km and spanning about 3500 x 1500 km, it dwarfs the next in the list, the Andean Altiplano (mean elevation 3.8 km). The position of the Tibetan Plateau, ahead of the Indian subcontinent’s northward collision with Eurasia marks it obviously as being of tectonic origin. Some plateaux are possibly buoyed up by underlying thermal anomalies in the mantle (the Colorado Plateau of North America, underpinned by a subducted spreading centre), while others, such as that of northern Ethiopia, result partly from vast outpourings of flood basalts and partly from thermal effects of active mantle plumes and rebound associated with massive crustal extension.
There are two basic models for Tibet. It may have formed as a result of a near doubling of crustal thickness as Indian crust was driven beneath that of Asia, low density of the thickened continental crust acting to buoy up its vast area. If that is so, then as soon as India collided with Asia, around 40-50 Ma ago, Tibet would have steadily risen and its plateau would have grown in extent. There are however signs of sudden changes in thermal structure, marked by large-scale magmatism of roughly Late Miocene (8-10 Ma) age. That may have been induced by an extraordinary event, the detachment and foundering (delamination) of a large mass of underlying mantle, whose loss resulted in rapid uplift of the whole overlying region. Because Tibet is known to play a central role in the mechanism that drives the South Asian monsoon, assessing the timing of its formation is crucial to understanding the onset of the monsoon and the many phenomena of accelerated weathering and erosion associated with it. Cores from the floor of the Indian Ocean suggest that the monsoon suddenly increased in intensity at around 8 Ma. Both as a sink for carbon dioxide as a result of weathering of the continental crust, and as a means of obstructing and redirecting continental wind patterns, the growth of the Tibetan Plateau and the Himalaya in front of it have been assigned a major role in the decline of global mean temperatures that resulted in northern hemisphere glaciations. So establishing the timing of their formation makes or breaks two major geoscientific hypotheses of recent decades. The key is some form of proxy for past elevations in the area. One such proxy, the stomatal index of plant leaves found in Tibetan sediments of Miocene age, showed that 15 Ma ago the southern Plateau was just as high as today (see When did southern Tibet get so high? in March 2003 EPN). That cast doubt on a later cause of uplift, but remained unconfirmed.
Sediments deposited in lakes that periodically fill Tibet’s many basins form a record that goes back at least 35 Ma. Carbonates in such lacustrine sediments offer a geochemical means of charting changes in elevation (Rowley, D.B. & Currie, B.S. 2006. Palaeo-altimetry of the late Eocene to Miocene Lunpola basin, Central Tibet. Nature, v. 439, p. 677-681). That depends on the proportion of 18O to the lighter 16O isotope of oxygen (δ18O) in carbonate, which is believed to be inherited from rainwater that originally drained into the basins. The higher the elevation at which water falls as rain or snow, the less of the heavier oxygen isotope it contains, so δ18O is a potential means of measuring the evolution of surface elevation. For central Tibet, this shows that the topography was at least 4 km high as early as 35 Ma ago. Results from other basins that span the Tibetan Plateau clearly suggest that 4 km elevation was achieved progressively later from south to north, anging from 40 to 10 Ma ago. So the delamination model for a sudden springing-up of the Plateau seems now to be a less plausible mechanism for the uplift than the simpler model of progressive crustal thickening following the collision of India. That does not entirely rule out an episode of delamination in the Miocene, for which geochemical evidence is fairly convincing. The implication of the new results is that if Tibet has been a major influence over climate, then it was one that developed progressively from the late Eocene.
See also: Mulch, A and Page Chamberlain, C. 2006. The rise and growth of Tibet. Nature, v. 439, p. 670-671. Kerr, R.A. 2006. An early date for aising the roof of the world. Science, v. 311, p. 758.