Unstirring the Himalayan Pot of Mashed Mountains

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Scientists have found a way to deconstruct the Himalayan Mountains and reconstruct the crust of India before it collided with Asia some 50 million years ago.

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Using neodymium isotopes for whole rocks and uranium-lead in zircon crystals in the rocks, the researchers have essentially un-stirred part of the pot of rocks that crashed together into the uneven, and extremely complicated, geological monstrosity that is now the highest mountain range on the planet.

Geologist Catherine Mottram and her colleagues looked specifically at the rocks in the Sikkim Himalaya around what's called the Main Central Thrust (MCT). This is a major fault along which the crust of India was shortened and thickened as it piled up against Asia.

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“As the rocks of the Indian plate were stuffed into the mountain belt, much of the movement of rock was along near-flat faults, known as thrusts,” explained geologist Simon Wellings in his Metageologist blog.

So the collision was less like a train wreck and more like shoving together a line of flat, crumbly dominoes. Some of the dominoes appear to have been lined with ancient faults that were part of an old rift zone. These faults may have been re-used in the continental collision to help shuffle materials of different ages when they were pushed against Asia -- making reconstruction of the pre-collision situation very complicated.

But that's only what happened near the surface.

“Thrusts near the surface may be a single fault plane, but at greater depths, rocks flow rather than snap and a thick thrust zone of deformed rocks is formed," Wellings said. "This makes drawing a line on a map and calling it the Main Central Thrust (MCT) rather difficult.”

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So although it might seem like geologists should be able to sort things out and reconstruct the pre-collision layout by just looking at rocks on either side of the MCT, that line is too blurry. To untangle the mess, Mottram and her colleagues used the isotopes to separate the ages of the rocks and very durable zircon grains, and in this way sorted out the rocks.

That's possible because the Indian rocks have long and varied histories from the time before they were piled into the Himalayas. What the isotopes confirmed is that the MCT is really more of a zone where rocks were shuffled together.

Naturally, the findings also make it possible to theoretically unshuffle the deck and build a picture of what India looked like hundreds of millions of years before the collision. This picture includes a northern Indian shoreline backed by granite mountains and high plains further south. All of that was ultimately crammed into a pile against Asia.

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“The Sikkim Himalaya exposes a window into a well-preserved mid-crustal thrust zone formed during the Himalayan (mountain building),” explained Mattram, who is affiliated with The Open University in the U.K. Their paper on the work appears in a recent issue of the Journal of the Geological Society of London.