Structural model of the lithosphere-asthenosphere system beneath the Qinghai-Tibet Plateau and its adjacent areas

The deep structure of the lithosphere–asthenosphere system, as imaged from geophysical data, of the Qinghai– Tibet Plateau, the highest on the Earth, provides important clues in understanding its orogenic processes. Here we reconstruct the main features of the structure of the crust and upper mantle from surface wave tomography in the Qinghai–Tibet Plateau and its adjacent areas, in order to understand the modality of the convergence and collision process between the Indian and Eurasian plates. Based on Rayleigh waves dispersion theory, we collect- ed long period and broad-band seismic data from the global and regional seismic networks surrounding the study area (20°N–50°N, 70°E–110°E). After applying instrument response calibration and filtering, group veloc- ities of the fundamental mode of Rayleigh waves are measured using the frequency–time analysis (FTAN). Com- bining the published dispersion data, a 2-D surface-wave tomography method is applied to calculate the lateral variations of group velocity distribution at different periods, in the range from 8 s to 150 s. The Hedgehog non- linear inversion method is performed to obtain shear wave velocity (Vs) versus depth models of the crust and upper mantle for 181 cells, with size 2° × 2°. In order to identify the cellular representative models, we applied the local smoothness optimization method (LSO). Fairly detailed structural models of the lithosphere–astheno- sphere system have been defined. The Vs models demonstrate the lateral variation of the thickness of the meta- somatic lid between the south and north of the Bangong–Nujiang Suture (BNS) and the west and east of Tibet. The variation in thickness of the metasomatic lid may suggest that the leading edge of the subducting Indian slab reaches up to BNS.