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"The subduction of continental crust to this depth has never been reported in the Himalayas and is also extremely rare in the rest of world," said Dr Anju Pandey of the National Oceanography Centre in Southampton, who led the research.
Pandey and her colleagues used sophisticated analytical techniques to demonstrate the occurrence of relict majorite, a variety of mineral garnet, in rocks collected from the Himalayas. Majorite is stable only under ultra-high pressure conditions, meaning that they must have been formed very deep down in the Earth's crust, before the subducted material was exhumed millions of years later.
"Our findings are significant because researchers have disagreed about the depth of subduction of the Indian plate beneath Asia," said Pandey. In fact, the previous depth estimates conflicted with estimates based on computer models. The new results suggest that the leading edge of the Indian plate sank to a depth around double that of previous estimates.
Originally posted by sandri_90
Thank you both!
extremely rare in the rest of world
Majorite is stable only under ultra-high pressure conditions, meaning that they must have been formed very deep down in the Earth's crust, before the subducted material was exhumed millions of years later.
The researchers discovered that as the Indian and Eurasian tectonic plates collide, the Indian lower crust slides under the Tibetan crust, while the upper mantle peels away from the crust and drops down in a diffuse manner.
"The building of Tibet is not a simple process," said John Nabelek, an Oregon State University geophysicist and lead author on the Science study. "In part, the mountain building is similar to pushing dirt with a bulldozer except in this case, the Indian sediments pile up into a wedge that is the lesser Himalayan mountains.
In fact, the previous depth estimates conflicted with estimates based on computer models. The new results suggest that the leading edge of the Indian plate sank to a depth around double that of previous estimates.
About 55 million years ago, the Indian plate crashed into the Eurasian plate, forcing the land to slowly buckle and rise. Containing nearly one-tenth the area of the continental U.S., and averaging 16,000 feet in elevation, the Tibetan Plateau is the world's largest and highest plateau.
Tectonic models of Tibet vary greatly, including ideas such as subduction of the Eurasian plate, subduction of the Indian plate, and thickening of the Eurasian lithosphere. According to this last model, the thickened lithosphere became unstable, and a piece broke off and sank into the deep mantle.
"While attached, this immense piece of mantle lithosphere under Tibet acted as an anchor, holding the land above in place," said Wang-Ping Chen, a professor of geophysics at the U. of I. "Then, about 15 million years ago, the chain broke and the land rose, further raising the high plateau."
These models suggest that plate tectonics is primarily driven by subduction and that supercontinents break up and migrate from sites of mantle upwelling to reassemble at sites of mantle downwelling where subduction zones exist.
Such models would predict that the young oceans created by the breakup of a supercontinent some 600 million years ago would have continued to expand as the continental fragments migrated toward sites of mantle downwelling that existed in the older ancestral Pacific Ocean. Instead, Pangea assembled as a result of the closure of the young oceans.
The geologic record suggests that there are geodynamic linkages between the younger and older oceans that deserve more detailed study; it also suggests that, in the case of Pangea, the reversal in continental motion may have coincided with emergence of a superplume 460–400 million years ago that produced mantle upwelling in the ancestral Pacific.
If so, the top-down geodynamics driven by subduction, which accounts for the assembly of the supercontinent that broke up 600 million years ago, may have been overpowered by bottom-up geodynamics involving large-scale mantle upwelling that led to the amalgamation of Pangea.