Rock magnetism smooths out complex Himalayan collision zone | MIT News


With some of the tallest peaks in the world, Asia’s ‘home of snow’ region is a magnet for thrill seekers, worshipers and scientists. The imposing 1,400-mile Himalayan mountain range that separates the plains of the Indian subcontinent from the Tibetan plateau is the scene of an epic continent-continent collision that took place millions of years ago and changed the Earth, affecting its climate and weather conditions. The question of how the Indian and Eurasian tectonic plates collided and how the mountains came into being is a question scientists are still developing. Now, new search Posted in PNAS and led by MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS) confirms it’s more complicated than previously thought.

“The Himalayas are the classic example of continent-continent collision and an excellent laboratory for studying mountain building events and tectonics,” says an EAPS graduate student. Craig martin, the main author of the article.

The story begins about 135 million years ago, when the Neotethys Ocean separated the tectonic plates of India and Eurasia by 4,000 miles. The common opinion of geologists is that the Neotethys oceanic plate began to sink into the Earth’s mantle below Eurasia at its southern border, pulling India north and the tectonic plates above it to eventually form the Himalayas in a single collision event around 55 to 50 million years ago. since. However, geological evidence suggests that the observed high rate of subduction does not seem to quite match this hypothesis, and model reconstructions place the continental plates thousands of miles away at the time of this alleged collision. To account for the required time and force of subduction, IMTs Olivier Jagoutz, associate professor of geology and Leigh “Wiki” Royden, Professor Cecil and Ida Green of geology and geophysics, proposed that due to the high speed, orientation and location of the final continental collision, there must be another oceanic plate and area of subduction in the middle of the ocean, called the Kshiroda plate and the Trans-Tethysian Subduction Zone (TTSZ), which stretched from east to west. Additionally, EAPS geologists and others have postulated that an arc of volcanic islands, like the Marianas, existed in between, called the Kohistan-Ladakh arc. Located near the equator, they suffered the full brunt of the force coming from India before being crushed between the two continental crusts.

Tiny magnets point the way

This chain of events, its timing and geological configuration, were model-based speculation and geological evidence until EAPS researchers tested it – but first, they needed rocks. With planetary science teacher Ben weiss from MIT’s paleomagnetism lab, Martin, Jagoutz, Royden and their colleagues visited the region of Ladakh in northwest India bordering the Eurasian Plate. Over the course of several excursions, the team, which included Jade Fischer, an EAPS undergraduate student for a trip, rushed over outcrops and drilled rock cores, slightly larger than the size of a cork stopper. As they mined them, geologists and paleomagnetists marked the orientation of the samples in the bedrock and their location to determine when and where on Earth the rock formed. The team was looking for evidence showing whether a volcano, which was active around 66-61 million years ago, was part of a chain of volcanic islands in the ocean in southern Eurasia, or part of the Eurasian continent. It would also help determine the plausibility of a double subduction zone scenario.

Back in the lab, MIT researchers used rock dating and paleomagnetism to understand this ancient geological car crash. They took advantage of the fact that as lava cools and rock forms, it captures a signature of the Earth’s magnetic field, which runs north to south toward the Earth’s magnetic poles. If the rock forms near the equator, the magnetization (electron) spins in its ferrous minerals, like magnetite and hematite, will be oriented parallel to the ground. As you move away from the equator, the magnetization of the rock will tilt towards the Earth; however, subsequent heating and remagnetization may imprint on the original signature.

After checking this and correcting the bedrock tilt at the site, Martin and his colleagues were able to determine the latitude at which the rocks were created. The uranium and lead dating of the zircon minerals in the samples provided the other piece of the puzzle to constrain the timing of formation. If there had been a single collision, these rocks would have formed at a latitude somewhere around 20 degrees north, above the equator, near Eurasia; if the islands existed, they would have originated near the equator.

“It’s great to be able to piece together the atlas of the distant times of the world using the tiny magnets kept in the rocks,” says Martin.

A two-part system

With their measurements and models of time and latitude, the MIT researchers found the evidence they were looking for: the presence of an island chain and a double subduction system. 80 to probably 55-50 million years ago, the Neotethys Ocean was subducted in two places: along the southern edge of the Eurasian Plate (the Kshiroda Plate sank) and the mid-ocean TTSZ, just to the south of the Kshiroda plate and near the equator. Together these events closed the ocean, and tectonic activity worked with erosion and weathering to sequester and attract carbon, until the Paleocene era (66-23.03 million ago). years). “The presence of two subduction zones and the time of their destruction at low latitudes explain the cooling of the global climate in the Cenozoic (66 million years ago to the present day)”, explains Martin.

More importantly: “Our results mean that instead of India colliding directly with Eurasia to form the Himalayas, India first collided with a chain of volcanic islands (similar to the Mariana Islands today), then with Eurasia up to 10 million years later than is generally the case. accepted, ”says Martin. Indeed, the Kohistan-Ladakh arc and the collision with India slowed the rate of India-Eurasia convergence, which continued to decline until 45 to 40 million years ago, when the final collision s ‘is produced. “This finding is contrary to the long-held view that the India-Eurasia collision was a single-stage event that began 55 to 60 million years ago,” says Martin. “Our results strongly support the double subduction hypothesis of Oli and Wiki as to why India moved north so abnormally fast during the Cretaceous Period.”

Additionally, Martin, Jagoutz, Royden, and Weiss were able to determine the maximum extent of the Indian Plate before it was forced under Eurasia. The convergence between India and Eurasia for 50 to 55 million years was approximately 2,800 to 3,600 kilometers. This is largely due to the subduction of the Kshiroda Plate, which MIT researchers estimated to be about 1,450 kilometers wide, at the time of the first collision with the Island Arc, there are 55 to 50 million years. After the first stage of collision between the island chain and India, the Kshiroda plate continued to disappear under Eurasia. Then, 15 to 10 million years later, when the two continents moved closer together, the continental crust began to shorten, bend, and push the rocks upward, causing observable changes in composition. and the structure of rocks. “Our results also directly limit the size of the part of India ‘lost’ in the collision to less than 900 kilometers in the north-south direction, which is far less than the 2,000 kilometers previously required to account for the timing of the collision. collision.”

New knowledge gained about the mechanisms and geometry of such an archetypal mountain system has important implications for using the Himalayas to study continental collisions, Martin explains. Revising the number of subduction zones, the age of final collision, and the amount of continental crust involved in the formation of the Himalayas alters some key parameters needed to accurately model mountain belt growth, deformation continental crust and the relationship between plate tectonics and global climate.

Martin hopes to go further throughout his graduate studies by focusing on the intensely warped collision zone between the volcanic island chain and Eurasia. He hopes to understand the closure of the Kshiroda Ocean and the geological structures produced during the continental collision.

Not only is the find impressive, but as Martin notes, “I think it’s cool to imagine idyllic tropical volcanic islands, with dinosaurs roaming around them, having been sandwiched between two tectonic plates in collision and raised to form the roof of the world. “

This study was funded, in part, by NSF Tectonics Program and MISTI-India.


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