West-East Variability in Indian Subduction Geometry: New Seismic Receiver Function Profile Across the Himalaya and Lhasa Terrane Along 92°E

Abstract:

Most interpretations of the Himalaya and southern Tibet have been made assuming large-scale west-east uniformity of the orogen co-axial with the surface trace of the Yarlung-Zangbo Suture (YZS), and assuming that the main structural elements can be captured with a single south-north cross-section. The HiCLIMB seismic transect is interpreted to show Indian lithospheric mantle attached to Indian lower crust and underthrusting horizontally to the northernmost extent of that crust at ~31°N at ~85°E. The older INDEPTH seismic transect had been interpreted to show Indian lithospheric mantle detaching from Indian crust (at the “mantle suture”) beneath the surface trace of the YZS, ~29.5°N at ~90°E.

We present P-wave and S-wave receiver-function CCP images along a new seismic transect, with about double the station density of the INDEPTH transect, along 92°E from the High Himalaya across the Tethyan Himalaya and Lhasa terrane, to the Bangong-Nujiang Suture. We image a prominent converter dipping c. 25°N from near-surface to c. 150 km depth that we interpret as the Yarlung-Zangbo Suture in the crust and the Tibetan lithosphere-asthenosphere boundary in the mantle. Sub-parallel and structurally deeper we interpret other converters as top and base of subducting Indian lithospheric mantle, detaching from underhtrusting Indian crust at the mantle suture >50 km south of the surface trace of the YZS. Based on this and the HiCLIMB image, as well as body-wave and surface-wave tomography, and potential-field and geochemical data, we believe the mantle suture is not parallel to the YZS at the surface, but rotated 25°CW.

In contrast, Indian crust seems to penetrate a fairly uniform distance beneath Tibet over a large sector of the orogen at least from 85° to 92°E, to the "Indian crustal front" >150 km north of the YZS. Thus underplating of Indian crust beneath the Lhasa Terrane may controlled by the geometry of collision as recorded by the Yarlung-Zangbo Suture at the surface, whereas progressive detachment of Indian mantle lithosphere from Indian crust may be related to stresses at the eastern syntaxis associated with ongoing subduction of the Burma plate. Irrespective of the cause, we infer that the relative geometry of subduction of Indian crust and Indian mantle lithosphere varies dramatically from west to east across Tibet.