This finding can be explained only if there were two subduction zones pulling India rapidly toward Eurasia, rather than just one.ĭuring a geologic time period known as the Paleocene, India caught up with the volcanic island chain and collided with it, scraping up the rocks we eventually sampled onto the northern edge of India. Instead, they formed on a chain of volcanic islands, out in the open Neotethys Ocean at a latitude of about 8 degrees north, thousands of kilometers south of where Eurasia was located at the time. The original single-stage collision model for the Himalaya predicts that these rocks would have formed close to Eurasia at a latitude of around 20 degrees north, but our data shows that these rocks did not form on either the Indian or the Eurasian continents. Using the average magnetic direction of the whole suite of samples we can calculate their ancient latitude, which we refer to as the paleolatitude. Credit: Martin et al 'Paleocene latitude of the Kohistan-Ladakh arc indicates multi-stage India-Eurasia collision,' PNAS 2020, CC BY-NC-SA The Trans-Tethyan Subduction Zone is where the volcanic island chain formed before the Indian continent collided into it and pushed it into Eurasia, forming the Himalaya. The Trans-Tethyan Subduction Zone is the additional subduction zone not accounted for in the single-stage collision model. Black lines with triangular tick marks show subduction zones, with the direction of subduction. Credit: Craig Robert Martin, CC BY-NDīlack lines mark boundaries between tectonic plates. The magnetometer sits inside a magnetically shielded room at the MIT Paleomagnetism Laboratory. Incrementally heating and then measuring the samples in this way enables us to extract the original magnetic direction by removing more recent overprints that might hide it. We brought our samples back to the MIT Paleomagnetism Laboratory and, inside a special room that's shielded from the modern-day magnetic field, we heated them in increments up to 1,256 degrees Fahrenheit (680 degrees Celsius) to slowly remove the magnetization.ĭifferent mineral populations acquire their magnetization at different temperatures. Keeping track of the orientation of the samples as well as the rock layers they came from is essential to calculating which way the ancient magnetic field pointed relative to the surface of the ground as it was over 60 million years ago. The aim was to reconstruct where these rocks originally formed, before they were sandwiched between India and Eurasia and uplifted into the high Himalayas. We then carefully marked these cylindrical cores with their original orientation before chiseling them out of the rock with nonmagnetic tools. We used a hand-held electric drill with a specially designed diamond coring bit to drill approximately 10 centimeters down into the bedrock. These rocks originally formed on a volcano active between 66 and 61 million years ago, around the time that the first stages of collision began. Over multiple expeditions to the Ladakh Himalayas, our team collected hundreds of 1-inch diameter rock core samples. Essentially, this method allows us to unwind millions of years of plate tectonic motions and create maps of the world at different times throughout geologic history. So by measuring the magnetization of volcanic rocks, scientists like me can determine what latitude they came from. When lava erupts and cools to form rock, the magnetic minerals inside lock in the direction of the magnetic field of that location. Oceanic plates are colder and denser than the mantle beneath them, so they sink downward into it at subduction zones.Ī few rock core samples, with the sample orientation line marked on their sides. Tectonic plates make up the surface of Earth, and they're constantly in motion-drifting at the imperceptibly slow pace of just a few centimeters each year. By decoding the magnetic records preserved inside them, we hoped to reconstruct the geography of ancient landmasses-and revise the story of the creation of the Himalayas. Our team of geologists traveled here to collect rocks that erupted as lava more than 60 million years ago.

It marks the boundary where two tectonic plates fused together and an ancient ocean disappeared. Known as a suture zone, it's only a few kilometers wide and consists of slivers of different types of rocks all sliced together by fault zones. This area also contains a different type of boundary, a narrow sinuous geological structure that stretches along the length of the Himalayan mountain range. It's a reminder how close we are to the long-disputed borders between India, Pakistan and China which lie on the ridgelines just a few miles away. The muffled soundscape of the spectacular Himalayan wilderness is punctuated by a military convoy roaring along the Khardung-La road below.

Himalayas plate boundary