Continental subduction re-imagined: Mapping the mantle suture beneath Tibet using geothermal 3He/4He, S-wave receiver functions, and Sn/Lg ratios 

In introductory textbooks, oceanic subduction and continental collision are treated as separate, very different, processes.  Seismic images have led to contradictory interpretations that India directly underthrusts Tibet as a horizontal plate or that India subducts steeply into the mantle, so that even the most modern full-waveform tomographies demand skepticism and testing. To resolve the geometry of the continental collision between India and Asia I use 3He/4He data from 225 geothermal springs across a >1,000-km-wide region of southern Tibet to define a sharp boundary subparallel to the surface suture between India and Asia, delineating the northern limit of India at ∼80-km depth. In southeastern Tibet where the mantle suture crosses the Yarlung-Zangbo suture, our S-receiver functions map a tear in the underthrusting lithosphere and delamination of mantle lithosphere from underthrust Indian crust, perhaps due to Burma subduction. In western Tibet, Sn/Lg ratios from intermediate depth earthquakes show underthrust Indian lithosphere far to the northwest, linked to Hindu Kush-Pamir subduction. The main India–Asia collision resembles oceanic subduction in having a dipping-slab overlain by an asthenospheric mantle wedge, separated from a flat-slab segment by a slab tear.

Blue dots mark thermal springs with helium-isotope signatures corresponding to the cold (Indian) plate.

Yellow dots are thermal springs with helium isotopes derived from a hotter (Asian) mantle.

Klemperer, S.L., Hilton, D.R., et al., 2022. Limited underthrusting of India below Tibet: 3He/4He analysis of thermal springs locates the mantle suture in continental collision. Proceedings of the National Academy of Sciences, 119(12), p.e2113877119.