India fused to Asia 10 million years later than previously thought

Planet
It is believed that a massive tectonic collision joined India with Asia tens of millions of years ago.

Previous estimates have suggested this collision occurred about 50 million years ago, as India, moving northward at a rapid pace, crushed up against Eurasia.

The crumple zone between the two plates gave rise to the Himalayas, which today bear geologic traces of both India and Asia. Geologists have sought to characterize the rocks of the Himalayas in order to retrace one of the planet's most dramatic tectonic collisions.

Now, researchers at MIT have found that the collision between India and Asia occurred only 40 million years ago 10 million years later than previously thought.

The scientists analyzed the composition of rocks from two regions in the Himalayas, and discovered evidence of two separate collisional events: As India crept steadily northward, it first collided with a string of islands 50 million years ago, before plowing into the Eurasian continental plate 10 million years later.

Oliver Jagoutz, assistant professor of geology in MIT's Department of Earth, Atmospheric and Planetary Sciences, said that the results change the timeline for a well-known tectonic story.

"India came running full speed at Asia and boom, they collided. But we actually don't think it was one collision this changes dramatically the way we think India works," Jagoutz, an author of the paper, stated.

In particular, Jagoutz said, the group's findings may change scientists' ideas about the size of India before it collided with Asia. At the time of collision, part of the ancient Indian plate known as "Greater India" slid underneath the Eurasian plate.

What we see of India's surface today is much smaller than it was 50 million years ago. It's not clear how much of India lies beneath Asia, but scientists believe the answer may come partly from knowing how fast the Indian plate migrates, and exactly when the continent collided with Asia.

By dating the Indian-Eurasian collision to 10 million years later than previous estimates, Jagoutz and his colleagues concluded that Greater India must have been much smaller than scientists have thought.

Their evidence supports a new timeline of collisional events: Fifty million years ago, India collided with a string of islands, pushing the island arc northward. Ten million years later, India collided with the Eurasian plate, sandwiching the string of islands, now known as the Kohistan-Ladakh Arc, between the massive continents.

"If you actually go back in the literature to the 1970s and '80s, people thought this was the right way. Then somehow the literature went in another direction, and people largely forgot this possibility. Now this opens up a lot of new ideas," Jagoutz said.

The results from the study will be published in Earth and Planetary Science Letters.

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