Magnetism of Himalayan rocks reveals complex tectonic history of mountains

Quickly breathing the rarefied mountain air, my colleagues and I put down our gear. We are at the base of a jagged outcrop that protrudes from a steep gravel slope.

The muffled soundscape of the spectacular Himalayan wilderness is punctuated by a roaring military convoy along the Khardung-La road below. It’s a reminder how close we are to the long contested borders between India, Pakistan, and China that lie on the ridge lines a few miles away.

This area also contains a different type of boundary, a narrow winding geological structure that stretches along the Himalayan mountain range. Known as the suture zone, it is only a few miles wide and is made up of shards of different types of rock, all cut together by fault zones. It marks the limit where two tectonic plates have merged and an ancient ocean has disappeared.

Our team of geologists traveled here to collect rocks that erupted as lava over 60 million years ago. By decoding the magnetic recordings kept inside, we hoped to reconstruct the geography of ancient landmasses – and review the creation story of the Himalayas.

Sliding plates, growing mountains

Tectonic plates form the surface of the Earth, and they are constantly in motion — drifting at imperceptibly slow rhythm by a few centimeters each year. Oceanic plates are cooler and denser than the mantle beneath them, so they sink into subduction zones.

The descending edge of the oceanic plate pulls the ocean floor behind it like a conveyor belt, pulling the continents towards each other. When the whole oceanic plate disappears in the coat, the continents on either side intertwine with enough force to lift large belts of mountains, like the Himalayas.

Geologists generally believed that the Himalayas formed 55 million years ago in a single continental collision – when the Neotethys oceanic plate subducted under the southern edge of Eurasia and the Indian and Eurasian tectonic plates collided.

But by measuring the magnetism of rocks in the remote, mountainous region of Ladakh in northwest India, our team showed that the tectonic collision that formed the world’s largest mountain range was, in fact, a complex process. in several stages involving at least two subduction zones.

Magnetic messages, kept forever

The constant movement of the metallic outer core of our planet creates electric currents which in turn generate Earth’s magnetic field. It is oriented differently depending on where you are in the world. The magnetic field always points to the magnetic north or south, which is why your compass works, and on average over thousands of years, it points to the geographic pole. But it also descends into the ground at an angle that varies depending on how far you are from the equator.

When lava erupts and cools down to form rock, the magnetic minerals inside lock in the direction of that location’s magnetic field. So by measuring the magnetization of volcanic rocks, scientists like me can determine what latitude they come from. Essentially, this method allows us to unwind millions of years of plate tectonic motion and create maps of the world at different points in geological history.

During several expeditions to the Ladakh Himalayas, our team collected hundreds of 1 inch diameter rock core samples. These rocks originally formed on an active volcano between 66 and 61 million years ago, when the early stages of the collision began. We used a hand-held electric drill with a specially designed diamond core drill bit to drill about 10 centimeters into the bedrock. We then carefully marked these cylindrical cores with their original orientation before chiseling them out of the rock with non-magnetic tools.

The goal was to reconstruct where these rocks originally formed, before they were sandwiched between India and Eurasia and lifted into the high Himalayas. Keeping track of the orientation of samples as well as the rock layers from which they came is essential in calculating in which direction the old magnetic field was pointing relative to the ground surface as it was over 60 million years ago. ‘years.

We brought our samples back to MIT Paleomagnetism Laboratory and, inside a special room shielded from the modern magnetic field, we heated them in increments up to 1,256 degrees Fahrenheit (680 degrees Celsius) to slowly remove the magnetization.

Different mineral populations acquire their magnetization at different temperatures. Incremental heating and then measuring samples in this way allows us to extract the original magnetic direction by removing more recent overprints that might obscure it.

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• Martin et al 'Paleocene latitude of the Kohistan-Ladakh arc indicates multi-stage India-Eurasia collision,' PNAS 2020, CC BY-NC-SA">

  

Magnetic traces build a map

Using the average magnetic direction of all the samples, we can calculate their old latitude, which we call the paleolatitude.

The original one-stage collision model for the Himalayas predicts that these rocks would have formed near Eurasia at a latitude of about 20 degrees North, but our data shows that these rocks did not form on the continents. Indian or Eurasian. Instead, they formed on a chain of volcanic islands, in the open Neotethys Ocean at a latitude of about 8 degrees north, thousands of miles south of where the Eurasia at the time.

This result can only be explained if there was two subduction zones rapidly pulling India towards Eurasia, rather than just one.

During a geological period known as the Paleocene, India overtook the chain of volcanic islands and collided with it, scraping up rocks that we eventually sampled on the northern edge of India. India then continued north before entering Eurasia about 40 to 45 million years ago – 10 to 15 million years later than was generally thought.

This latest continental collision raised the volcanic islands from sea level over 4,000 meters to their present location, where they form jagged outcrops along a spectacular Himalayan pass.

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