Geology of the Tibetan Plateau

Geology of the Tibetan Plateau The Tibetan Plateau is an immense land, about 3,500 by 1,500 kilometers in size, averaging more than 5,000 meters in elevation. Its southern rim, the Himalaya-Karakoram complex, contains not just Mount Everest and all 13 other peaks higher than 8,000 meters, but hundreds of 7,000-meter peaks that are 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. Thats because the set of events that formed it appears to be unique: a full-speed collision of two continental plates. Raising the Tibetan Plateau Nearly 100 million years ago, India separated from Africa as the supercontinent Gondwanaland broke up. From there the Indian plate moved north at speeds of around 150 millimeters per year- much faster than any plate is moving today. The Indian plate moved so quickly because it was being pulled from the north as the cold, dense oceanic crust making up that part of it was being subducted beneath the Asian plate. Once you start subducting this kind of crust, it wants to sink fast (see its present-day motion on this map). In Indias case, this slab pull was extra strong. Another reason may have been ridge push from the other edge of the plate, where the new, hot crust is created. New crust stands higher than old ocean crust, and the difference in elevation results in a downhill gradient. In Indias case, the mantle beneath Gondwanaland may have been especially hot and the ridge pushed stronger than usual too. About 55 million years ago, India began to plow directly into the Asian continent. 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 on average and 100 kilometers in places. The Tibetan Plateau is a natural laboratory for studying how the crust behaves during the extremes of plate tectonics. For example, the Indian plate has pushed more than 2000 kilometers into Asia, and its still moving north at a good clip. What happens in this collision zone? Consequences of a Superthick Crust Because the crust of the Tibetan Plateau is twice its normal thickness, this mass of lightweight rock sits several kilometers higher than average through simple buoyancy and other mechanisms. Remember that the granitic rocks of the continents retain uranium and potassium, which are incompatible heat-producing radioactive elements that dont mix in the mantle beneath. Thus the thick crust of the Tibetan Plateau is unusually hot. This heat expands the rocks and helps the plateau float even higher. Another result is that the plateau is rather flat. The deeper crust appears to be so hot and soft that it flows easily, leaving the surface above its level. Theres evidence of a lot of outright melting inside the crust, which is unusual because high pressure tends to prevent rocks from melting. Action at the Edges, Education in the Middle On the Tibetan Plateaus north side, where the continental collision reaches farthest, the crust is being pushed aside to the east. This is why the large earthquakes there are strike-slip events, like those on Californias San Andreas fault, and not thrust quakes like those on the plateaus south side. That kind of deformation happens here at a uniquely large scale. The southern edge is a dramatic zone of underthrusting where a wedge of continental rock is being shoved more than 200 kilometers deep under the Himalaya. As the Indian plate is bent down, the Asian side is pushed up into the highest mountains on Earth. They continue to rise at about 3 millimeters per year. Gravity pushes the mountains down as the deeply subducted rocks push up, and the crust responds in different ways. Down in the middle layers, the crust spreads sideways along large faults, like wet fish in a pile, exposing deep-seated rocks. On top where the rocks are solid and brittle, landslides and erosion attack the heights. The Himalaya is so high and the monsoon rainfall upon it so great that erosion is a ferocious force. Some of the worlds largest rivers carry Himalayan sediment into the seas that flank India, building the worlds largest dirt piles in submarine fans. Uprisings from the Deep All this activity brings deep rocks to the surface unusually fast. Some have been buried deeper than 100 kilometers, yet surfaced fast enough to preserve rare metastable minerals like diamonds and coesite (high-pressure quartz). Bodies of granite formed tens of kilometers deep in the crust have been exposed after only two million years. The most extreme places in the Tibetan Plateau are its east and west ends- or syntaxes- where the mountain belts are bent almost double. The geometry of collision concentrates erosion there, in the form of the Indus River in the western syntaxis and the Yarlung Zangbo in the eastern syntaxis. These two mighty streams have removed nearly 20 kilometers of crust in the last three million years. The crust beneath responds to this unroofing by flowing upward and by melting. Thus  leading to the large mountain complexes rise in the Himalayan syntaxes- Nanga Parbat in the west and Namche Barwa in the east, which is rising 30 millimeters per year. A recent paper likened these two syntaxial upwellings to bulges in human blood vessels- tectonic aneurysms. These examples of feedback between erosion, uplift and continental collision may be the most wonderful marvel of the Tibetan Plateau.