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Extreme Tectonics |
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Amazing things go on beneath the surface crust of the Tibetan Plateau.
When India began to collide with Asia some 50 million years ago, it set up a natural laboratory for studying the behavior of the crust during the extremes of plate tectonics. For example, the Indian plate has pushed more than 2000 kilometers into Asia and it's still moving north at a good clip. What happens to the rocks in this collision zone?
As I said in Part 1, the continental crust has been compressed into twice its normal thickness, and the region has risen several kilometers through simple buoyancy and other mechanisms. You have to remember that the granitic rocks of the continents tend to accumulate uranium and other heat-producing radioactive elements that don't mix in the mantle.
As a result, the middle and lower crust of the Tibetan Plateau is unusually hot. This heat expands the crustal rocks and helps the plateau float even higher. But another clue can be seen in the shape of the plateau, as shown in the digital elevation map below.
The main body of the plateau is actually rather flat. One theory holds that the deeper crust is so hot and soft that it has flowed rather than been folded, leaving the brittle surface layer above it level. There's evidence of a lot of outright melting inside the crust, which is unusual because high pressure tends to prevent melting.
That applies to the central part of the plateau. Around the edges, other interesting things are happening. On the north side, as explained in Part 1, the rocks are being pushed aside, largely to the east, as a set of long narrow slices bounded by great strike-slip faults. That kind of deformation happens here at a uniquely large scale.
Along the southern edge is a dramatic zone of underthrusting. Seismic studies have detected continental rocks being shoved more than 200 kilometers deep under the collision zone in the Himalaya. As the Indian side is pushed down, the Asian side is pushed up into the highest mountains on Earth. They continue to rise as a whole at about 3 millimeters per year.
Gravity works against this state of affairs. The deeply subducted rocks push up as the mountains weigh down, and the crust responds in different ways. Down in the middle layers, the crust spreads sideways along large faults, like fish in a pile, exposing deep-seated rocks in a process called eduction. On top where the rocks are solid and brittle, mass wasting
landslides and erosionattacks the heights.The Himalaya is so high and the monsoon rainfall upon it is so great that erosion is a ferocious force. Several of the world's largest riversthe Indus, the Ganges, and the Yarlung Zangbo-Brahmaputracarry Himalayan sediment into the seas on either side of India, building the world's largest dirt piles in submarine fans.
All this activity brings deep rock to the surface unusually fast. Thus, for instance, some rocks are found that have been buried more than 100 kilometers deep, yet they have reached the surface fast enough to preserve
metastable minerals like diamonds and coesite that almost never survive the upward trip. Bodies of granite, which form tens of kilometers deep in the crust, have been exposed only 2 million years after their birth.The most extreme places in the Tibetan Plateau, from a geologist's viewpoint, are at the east and west ends. If you compare the shape of the Himalaya to a smile, these are the corners, also known as syntaxes. Here 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 carve deep canyons and carry off huge amounts of rock, removing nearly 20 kilometers in the last 3 million years.
The crust beneath responds to this unroofing by flowing upward and melting. Thus large mountain complexes rise in the Himalayan syntaxes. In the west is the Nanga Parbat massif, with the gorge of the Indus wrapped around it (see the
digital map at peakware.com). In the east, nestled in a hairpin turn of the Yarlung Zangbo, which courses down a canyon with walls 5000 meters high, is Namche Barwa. This great peak is rising 30 millimeters per year.
A recent article in GSA Today 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.
