Rock study finds India-Asia dual collision

New Delhi, Feb. 6: Scientists studying rocks from a Himalayan zone named the Kohistan Ladakh Arc (KLA) have rewritten a key chapter in India's geological history, showing the collision between the Indian and Asian plates occurred in, not one, but two episodes.

The researchers at the Massachusetts Institute of Technology in the US have discovered evidence for what they say was a dual collision ' India first slammed into an arc of islands about 50 million years ago, pushed them northward, then hit Asia 40 million years ago.

Their study, accepted for publication in the journal Earth and Planetary Science Letters, challenges the current thinking in geological circles that India collided with Asia about 50 million years ago.

"We've got strong evidence for a dual collision," said Oliver Jagoutz, an Austrian geologist and an assistant professor at MIT, who led the study. "The evidence also explains a number of observations that have puzzled geologists over the past two decades," Jagoutz told The Telegraph.

Jagoutz and his colleagues studied chemical signatures of certain elements found in rocks from the KLA, a mountainous terrain shared by India and Pakistan that was once part of an oceanic island in the Tethys Sea lying between India and Asia.

The signatures have tell-tale signs of two great tectonic events. The scientists found that rocks younger than 50 million years along the southern side of the KLA show changes in the isotope ratios of certain elements, while similar changes occur along the northern boundary of the arc only in rocks that are about 40 million years old. "The KLA itself is a remnant of one of the islands in the Tethys Sea," Jagoutz said.

The changes in isotope ratios were associated with each collision.

Indian geophysicists who have been independently investigating the mechanisms of the India-Asia plate collision say the new results could change the value of "greater India" that is believed to have gone under Asia.

At the time of the collision, India was moving at about 10cm per year, which translates into about 1000km over 10 million years. "If the collision occurred 10 million years earlier, we will need to revise our estimate of how much of the crust of greater India has been consumed by Asia," said Shyam Sunder Rai, a senior scientist at the National Geophysical Research Institute (NGRI), Hyderabad.

"We might now need to redefine the northern limit of greater India which we had thought stretched up to beneath Lhasa in Tibet," Rai, who was not associated with the MIT study, told this newspaper.

Seven years ago, Rai and his colleagues at the NGRI and British collaborators had used seismic data to suggest that the northern edge of the Indian landmass extends up to Lhasa in the east and a split in the crust called the Altyn Tagh fault in western Tibet.

"This new study would mean that the area of India beneath Tibet would be much less than what was assumed earlier," Rai said. But, he said, earthquake potential across the Himalayas is unlikely to be altered significantly by these new results.

Jagoutz and his fellow-researchers have been studying the KLA rocks in the northwestern Himalayas over the past five years. The researchers have used sledgehammers to slice off pieces of rocks and pack mules to haul them, a media release from the MIT said. They have collected nearly three tonnes of rocks for chemical analysis, the release said.

Both Rai and Jagoutz said the idea of a dual collision has been around since the 1980s, but without adequate evidence to back it up.


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