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The Tibetan Plateau


nanga parbat

Nanga Parbat, a tectonic aneurysm in the Tibetan Plateau

Courtesy Embassy of Pakistan
india plate tectonics

Plate-tectonic setting of the Tibetan Plateau

US Geological Survey
tibetan plateau

Digital elevation model of the Tibetan Plateau

Courtesy Chris Duncan, UMass

The Tibetan Plateau is an immense upland, some 3500 by 1500 kilometers in size, averaging more than 5000 meters in elevation. It includes almost all 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 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 fast 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 India's case, this "slab pull" was extra strong.

Another reason may have been "ridge push" from the other edge of the plate, where new hot crust is created. New crust stands higher 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 and the ridge push stronger than usual too.

About 55 million years ago India 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 on average and 100 km 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 km into Asia, and it's 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, "incompatible" heat-producing radioactive elements that don't 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 it level. There's 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, Eduction in the Middle

On the Tibetan Plateau's 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 California's San Andreas fault, and not thrust quakes like those on the plateau's 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 km 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 in a process called eduction. On top where the rocks are solid and brittle, landslides and erosion—mass wasting—attacks the heights.

The Himalaya is so high and the monsoon rainfall upon it so great that erosion is a ferocious force. Some of the world's largest rivers carry Himalayan sediment into the seas that flank India, building the world's 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 km, 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 2 million years.

The most extreme places in the Tibetan Plateau are its east and west ends, known as 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 km of crust in the last 3 million years.

The crust beneath responds to this unroofing by flowing upward and by melting. Thus 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 marvelous marvel of the Tibetan Plateau.

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