Limited underthrusting of India below Tibet: 3 He/ 4 He analysis of thermal springs locates the mantle suture in continental collision

Abstract

Significance Our regional-scale geochemical dataset ( 3 He/ 4 He) resolves the geometry of the continental collision between India and Asia. Geophysical images have led to contradictory interpretations that India directly underthrusts Tibet as a horizontal plate or India subducts steeply into the mantle. Helium transits from mantle depths to the surface within a few millennia, such that the ratio of mantle-derived 3 He to dominantly crust-derived 4 He provides a snapshot of the subsurface. 3 He/ 4 He data from 225 geothermal springs across a >1,000-km-wide region of southern Tibet define a sharp boundary subparallel to the surface suture between India and Asia, just north of the Himalaya, delineating the northern limit of India at ∼80-km depth. The India–Asia collision resembles oceanic subduction with an asthenospheric mantle wedge.