Summary: New imagery reveals the causes of seismic activity deep beneath the Himalaya region, contributing to an ongoing debate over the continental collision process when two tectonic plates crash into each other.

New seismic data has provided the first west-to-east view of the subsurface where India and Asia collide, forming wonders like the Himalaya Mountains and the vast Tibetan Plateau.

The research contributes to an ongoing debate over the structure of the Himalaya collision zone, the only place on Earth where continental plates continue crashing today – and the source of catastrophes like the 2015 Gorkha earthquake that killed about 9,000 people and injured thousands more. The new seismic images reveal variations controlled by the curvature of the Himalayan arc that explains the fate of the Indian plate and the deepest earthquakes beneath Tibet. The observations suggest that two competing processes are simultaneously operating beneath the collision zone: movement of one tectonic plate under another, as well as thinning and collapse of the crust. The research, conducted by scientists at the Chinese Academy of Geological Sciences (CAGS) and Stanford University, was published in Proceedings of the National Academy of Sciences Sept. 21（https://www.pnas.org/content/early/2020/09/18/2000015117）.

“The physical inaccessibility of Tibet has limited scientific study, so most field experiments have either been too localized to understand the big picture or they've lacked sufficient resolution at depths to properly understand the processes,” said the lead author Shi Danian, a geophysics professor at CAGS. “This is really the first time that we have truly credible images of along-strike variation in the collision.” 

As the Indian plate collides with Asia it builds Tibet, the highest and largest mountain plateau on the planet. This process started very recently in geological history, about 57 million years ago. Researchers have proposed various explanations for its formation, including by thickening of the outermost layers of the Earth by widespread flow or by the Indian crust forcing its way under the entire Tibetan Plateau.

Because valleys across Tibet run from north to south, that is the direction in which roads were historically built – and where seismometers have been abundantly deployed. 

“We’ve treated everything from east to west as a single process of either underthrusting or subduction, but our detailed studies have been misleading at times because the seismometers have been in these rather special places,” said co-author Simon Klemperer, a geophysics professor at Stanford University.

       The researchers began installing new seismic recorders in 2011, eventually gaining the ability to resolve details that had previously been overlooked. The final images using 159 seismometers closely spaced along two 1000-km long profiles, reveal where the Indian crust has tears that are associated with the curvature of the Himalayan arc.

“We’re seeing at a much finer scale what we never saw before,” said co-author Wu Zhenhan, the vice president at CAGS. “It took a heroic effort to install closely spaced seismometers across the mountains, instead of along the valleys, to collect data in the west-east direction that made this research possible.” said co-author Academician Zhao Wenjin, a senior research scientist for the tectonics of Himalaya and the Tibetan plateau.

       Building and breaking

As the Indian tectonic plate moves from the south, the mantle, the strong part of the plate, is dipping beneath the Tibetan plateau. The analyses show that as that happens, small parts of the Indian plate break off beneath two of the surface rifts, revealing tears in the plate. The location of such tears can be critical for understanding how far a major earthquake like Gorkha will spread.

“These transitions, these jumps between the faults, are so important and they’re at a scale that we don't normally know about them until after an earthquake has happened,” Shi said.

An unusual aspect of Tibet involves the occurrence of very deep earthquakes, more than 40 miles below the surface. Using their seismic data, the researchers found associations between the plate tears and the occurrence of those deep quakes.

“Bits of India have dropped off – de-blobbed is what we’re calling it,” Shi said. “Now we know it’s not just bits dropping off, but bits dropping off and causing an earthquake. We think these deep earthquakes mark these tears at depth.”

The research also explains why the strength of gravity varies in different parts of the collision zone. The co-authors hypothesized that after the small pieces dropped off of the Indian plate, softer material from underneath came up, explaining a lack of mass balance in the India-Tibet collision zone.

       A natural laboratory

In addition to being the last horizon for adventurers and spiritual seekers, the Himalayas are known as a prime location for understanding geological processes. The region hosts world-class mineral deposits of copper, lead, zinc, gold and silver, as well as rarer elements lithium, antimony and chrome, essential to modern technology.  Even life itself depends on the continental crust and its material resources, and on the global climate change driven by plateau uplift.

The India-Tibet region also provides a glimpse into what may have formed areas like some other plateaus in the world, and the ancient eastern China plateau, which was created by continental collision but collapsed about 125 million years ago.

“The only way to understand what might have happened in other plateaus in the world today and the Mesozoic eastern China plateau is to come to Tibet,” Zhao said. “For geologists, this is the one big continental collision that is taking place on Earth today – it’s this natural labatory where we can study these processes.”

      The research was supported by the China National Natural Science Foundation, the Chinese Geological Survey and the U.S. National Science Foundation.

Fig. 1 Cutaway view of Tibet, looking south-southwest.  Blocks are ~500 miles south-north, 500 miles east-west,  and extend ~200 miles deep. The Indian mantle is pushing north beneath the Himalaya and southern Tibet (color-shaded relief map).  Green blobs mark  high-seismic-wavespeeds indicative of cold, strong material broken off the Indian crust by earthquake faulting, being replaced by upwelling of ‘asthenosphere’, softer and hotter material indicated by low seismic wave speeds in tan colors.

Fig.2 In a north-south rift above a tear in the Indian plate boiling springs bring fluids up 50 miles from the upwelling hot mantle, and the wide area of baked ground shows the high temperatures due to rifting.