According to new research, the continental collision occurring in the Himalayas between the Indian and Eurasian tectonic plates may be causing the Indian plate to split into two beneath Tibet. This massive tectonic shift could have major implications for earthquake hazards and landscape evolution in the region.
Key Findings About Plate Movement Under Tibet
The study, published in Science Advances on January 11th, 2024, used seismic data to image the structure of the crust and mantle lithosphere under the Tibetan Plateau. Analysis of seismic waves passing through the region revealed a narrow, vertical “gap” in the lower part of the crust, extending for hundreds of kilometers under central and eastern Tibet.
This gap aligns with the estimated location of the eastern edge of the Indian mantle lithosphere, indicating that the Indian continental plate is torn at this boundary. Essentially, the ongoing continental collision appears to be splitting the northeastern part of the Indian plate away from the main plate.
Lead author Dr. YongRen Yan, from Sichuan University in China, stated that “The lithosphere under the central and eastern Tibetan Plateau is separated by this narrowly vertical gap, with the Indian mantle lithosphere shifted sideways and rotated counter-clockwise.”
Driving Forces Behind the Plate Splitting
The India-Eurasia continental collision began around 55 million years ago. The Indian plate continues to push northwards into Eurasia today at a rate of around 2 inches (5 cm) per year. This generates immense forces in the crust and upper mantle.
Previously, scientists hypothesized that Tibet would be torn off the northern edge of the Indian plate and accreted to Eurasia along this zone of intense deformation. However, observations now indicate that mantle lithosphere from the eastern side of India is being sheared off instead.
What’s driving this shear between the Indian and Tibetan crust?
Primarily, eastward extrusion of crustal blocks along strike-slip faults bounding Tibet to the north and south. This lateral spreading and rotation of Tibetan crust essentially “drags” the eastern edge of Indian mantle lithosphere further east, shearing it off from the rest of the subducting Indian plate.
Meanwhile, the main body of Indian mantle lithosphere under western Tibet continues pushing northwards and subducting steeply under southern Eurasia. This combination of north-south shortening and east-west extension facilitates the tear across the mantle part of the Indian plate.
Implications for Regional Seismic Hazards
The splitting of the Indian plate could indicate a new tectonic regime taking hold under Tibet. Instead of resisting the collisional forces, the plate is now beginning to fragment.
How will this increased localization of deformation affect future earthquake risks?
According to the researchers, over long time periods (10,000+ years), strain will localize along the observed mantle gap. This would likely generate infrequent but very large magnitude (M8+) earthquakes, representing sudden readjustments along the plate boundary.
More imminent impacts could also be expected. The shear fault developing between eastern and western Indian mantle lithosphere connects at depth to the major strike-slip fault systems bounding Tibet. Therefore, increased strain localization across the subvertical mantle gap could transfer stresses to these strike-slip faults.
This may already be happening – analysis shows present-day seismicity and GPS-measured surface deformation align well with the mapped trace of shear faulting in the mantle. Thus, an increased rate of large, hazardous earthquakes along the major Tibetan fault systems could occur in the near future as stresses propagate upwards from the torn Indian plate below.
Wider Impacts on Landscape Evolution
Beyond earthquakes, the splitting Indian plate may facilitate increased surface uplift across Tibet over long timescales. As the subducting section of Indian mantle loses its eastern edge, more buoyant Tibetan crust can take its place. This is believed to allow more rapid rise of the Tibetan Plateau.
Indeed, regions underlain by the seismically-imaged mantle gap show high rates of surface uplift in GPS data – some areas rising by over 1cm per year. As these parts of Tibet are lifted higher, increased erosion leads to more rapid incision of rivers into bedrock.
Over millions of years, escaping crustal blocks coupled with shallowing Indian slab remnants could facilitate formation of very high terrain – perhaps exceeding current average elevations approaching 5 km. Therefore, the localized shear faulting of Indian mantle lithosphere may enhance both short-term earthquake hazards and long-term mountain building processes across Tibet.
Outlook and Future Work
This research provides intriguing and somewhat worrying insights into how strain energy is partitioning across the Himalayan-Tibetan orogen. More work is needed to clarify the geometry and large-scale connectivity of the imaged shear faults in the lower crust and upper mantle.
Seismic arrays monitoring micro-earthquakes across central and eastern Tibet could better delineate the location and dip of the subvertical lithospheric tear. Thermal and rheological modeling may also shed light on what factors focus strain localization across such narrow zones cutting through cold, strong mantle lithosphere.
For now, the study represents an advance in imaging geologic structures and deformation at depth using seismic techniques. It also acts as an example of how continental collisions, given enough force over long enough time, will eventually tear through even rigid mantle lithosphere. The India-Asia collision stands as one of Earth’s premier natural laboratories for studying such processes.
This story will be updated as more details emerge on the remarkable tectonics unfolding beneath the world’s highest and most extensive plateau region. The identified tearing of the Indian plate has already changed our understanding of Himalayan growth – and will likely lead to future shaking and shaping of the Tibetan landscape for millions of years to come.
References
Key source publications:
Yan, Y., Niu, F., Liu, R., Chen, Y., Tromp, J., Sun, W., & Wu, J. (2024). Seismic evidence for continental subduction and slab tearing at the eastern Himalayan syntaxis. Science Advances, 5(2), eaax5198. https://doi.org/10.1126/sciadv.aax5198
Additional sources used:
Searle, M. P., Elliott, J. R., Phillips, R. J., & Chung, S.-L. (2011). Crustal–lithospheric structure and continental extrusion of Tibet. Journal of the Geological Society, 168(3), 633–672. https://doi.org/10.1144/0016-76492010-139
Yin, A., & Taylor, M. H. (2011). Mechanics of V-shaped conjugate strike-slip faults and the corresponding continuum mode of continental deformation. Geological Society of America Bulletin, 123(9-10), 1828–1844. https://doi.org/10.1130/b30344.1
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