The collision of the Indian and Asian landmasses some 90 million years ago forced the Indian tectonic plate down under the Asian plate to a depth of 200 km - around double of previous estimates - in the earth's mantle, according to a new geological study.
“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 Anju Pandey of the National Oceanography Centre (NOC) in Southampton (Great Britain), who led the research. Subduction is a geological process in which one edge of a crustal plate is forced sideways and downward into the mantle below another.
Dr. 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 it must have been formed very deep down in the Earth's crust, before surfacing millions of years later.
“Our findings are significant because researchers have disagreed about the depth of subduction of the Indian plate beneath Asia,” said Dr. 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.
“Our results are backed up by computer modelling and will radically improve our understanding of the subduction of the Indian continental crust beneath the Himalayas,” said Pandey, according to an NOC release.
The new discovery is also set to modify several fundamental parameters of Himalayan tectonics, such as the rate of Himalayan uplift, angle, and subduction of the Indian plate.
The new research findings were published in Geology.