Physics and Geology Department

Joint Colloquium

Thursday, September 15, 2022

4:00pm-5:00pm

Gardiner Hall, Room 230

Host: Dr. James Ni and Dr. Michael Paolone

Refreshments served at 3:45pm

Simon Klemperer

Departments of Geophysics and of Geological Sciences,

Stanford University

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. Geophysical images (seismic tomography) have led to contradictory interpretations that India directly underthrusts Tibet as a horizontal plate or that 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 3He to dominantly crust-derived 4He provides a snapshot of the subsurface. S-wave receiver functions (SRF) image the lithosphere-asthenosphere boundary; and Sn/Lg amplitude ratios distinguish earthquakes in the continental upper mantle or eclogitized base-crustal layer. 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, just north of the Himalaya, delineating the northern limit of India at ∼80-km depth. I use S-wave receiver functions (SRF) to map out the region in southeastern Tibet where the mantle suture turns south across the Yarlung-Zangbo suture, requiring delamination of mantle lithosphere from underthrust Indian crust. I use Sn/Lg ratios to map the northern limit of eclogitized underthrust Indian crust and mantle lithosphere in far northwestern Tibet. The India–Asia collision resembles oceanic subduction in having a dipping-slab overlain by an asthenospheric mantle wedge, separated by a likely slab tear from a flat-slab segment. The gross along-strike geometry is controlled by the western transition to Hindu Kush-Pamir subduction and the eastern transition to Burma subduction.