Structure of the mantle transition zone beneath the southeastern Tibetan plateau revealed by P-wave receiver functions

The deep structure of the southeastern Tibetan plateau is of great importance to a better understanding of the India-Eurasia collision as well as the evolution of the Tibetan plateau. In this study, we collected waveform data of 566 permanent and temporary seismic stations in southeastern Tibetan plateau, then extracted a total of 77853 high quality P-wave receiver functions using the maximum entropy deconvolution method. By the Common Conversion Point (CCP) stacking technique, we mapped the topography of the 410- and 660-km discontinuities ('410' and '660' hereinafter), and further investigated the lateral variation of the mantle transition zone (MTZ) beneath this region. The background velocity model deduced from H-kappa stacking results and a previous body-wave tomographic research were applied to the correction of the crustal and upper mantle heterogeneities beneath southeastern Tibetan plateau for CCP stacking. Our results reveal two significantly thickened MTZ anomalies aligned nearly in the south north direction. The magnitudes of both anomalies are similar to 30 km above the global average of 250 km. The southern anomaly located beneath the central-Yunnan sub-block and the Indo-China block is characterized by a slightly deeper '410' and a greater-than-normal '660', while the northern anomaly beneath western Sichuan has an uplifted '410' and a depressed '660'. Combining with previous studies in the adjacent regions, we suggest that after slab break-off between the Indian plate and Neo-Tethys oceanic plate, the remnant of the latter stagnating at the bottom of the MTZ may have caused the southern anomaly in our receiver function images. The anomaly in the north, on the other hand, might be the consequence of the delamination of the overlying lithosphere sinking into the MTZ. The origin of the Tengchong volcano is probably connected to the upwelling of the hot and wet asthenospheric material caused by the slab break-off process.