AsianScientist (Apr. 5, 2022)– The lofty Himalayas and the Tibetan plateau formed when the Eurasian and Indian continental plates collided. These plates are large independently moving tracts of the Earth’s surface. A new study reports that the Indian continental plate likely tilts downward under the Asian plate. Published in Proceedings of the National Academy of Sciences, these results, based on a helium signature found in more than 200 hot springs across the Tibetan plateau, oppose a widely believed model for mountain building, according to which the Indian plate simply lies flat under the Asian plate. Better idea about the arrangement of these plates will help scientists understand the continental collision from eons ago.
Some 50 million years ago, the Indian and Asian continental plates collided, leading to the formation of the Himalayas and not long after, the Tibetan plateau. But how the plates interface has long been debated. Beginning 1920s, several theories have been proposed but two came to be the most prominent. In the first, the Indian plate is thought to lie flat under Tibet, which is a part of the Asian plate. Whereas the other model says that the Indian plate tilts under the Asian plate, sloping downwards below Tibet.
Researchers from the United States and China, led by Simon Klemperer of the Department of Geophysics at Stanford University, wanted to see which of the two models accurately explain how the highest mountains came to be.
Over many years, Klemperer’s team sampled water and gas from 196 geothermal springs in the Tibetan plateau and included previously reported data from 29 other Tibetan springs. Ranging from hot to cool, the springs were dispersed across the plateau. They bring water from tens of kilometres underground to the surface. With the water, they carry elements such as helium. The researchers trapped water and gas from these springs in copper tubes, through which helium cannot escape.
Helium gas is a good “proxy to the process” of geological changes in Tibet, Shyam Rai, of the Indian Institute of Science Education and Research, Pune, told Asian Scientist. Rai who is an Emeritus Professor at the Department of Earth and Climate Science of the institute was not involved in the study.
Heavier only to hydrogen, helium has two naturally occurring forms, Helium-3 and Helium-4. Both forms move quickly through the Earth’s mantle and crust. “Most essentially, all the Helium-3 in Earth was created when the Earth condensed from the solar nebula,” Klemperer told Asian Scientist. “And that Helium-3 has slowly been leaking out of the Earth ever since.” If this form of the noble gas was found, it must have travelled from the mantle. On the contrary, the more abundant Helium-4 is a product of constant radioactivity in the crust of the planet. So, depending on which kind of Helium the researchers find in the hot spring sample, they can tell where it might have originated—Earth’s crust or mantle.
Klemperer and his colleagues noted a clear distinction in the helium signal from the hot springs—helium to the north came from the mantle and crustal helium to its south. This helium border mirrored the boundary between Tibetan mantle and Indian crust many kilometres under the Earth’s surface. From these signatures, the researchers interpreted that while the Indian plate is in contact with Tibet in the south, it angles away and downward from the Asian plate on the northern side of Tibet—how sharply remains to be discovered. “What’s new here is collecting such a vast data set in Tibet that allows us to map this boundary,” said Klemperer.
Such extensive data collection for the Tibetan region is certainly a first, Rai agreed. But the helium signature needs to be analysed in the context of other data on the geological processes in the region, added Rai. While the Indian plate certainly seems to slant down below the Asian plate in this stretch, whether this placement holds true to the east or west of this region remains to be seen, according to Rai.
Klemperer wants to follow this distinct boundary between the two continental places to the east and west to see what new data may reveal.
Source: Stanford Earth; Photo: Ping Zhao