As the forefront of the plateau’s intracontinental expansion, the eastern margin of the Qinghai-Xizang Plateau, marked by significant topographical and geomorphological contrasts, is characterized by complex tectonic deformation and high seismicity. This region is critical for investigating the formation processes of continental strong earthquakes and serves as an ideal location for studying subsurface structural evolution and geodynamics. Therefore, the high resolution 3D P-wave velocity structures of the crust and upper mantle on the eastern margin of the Qinghai-Xizang Plateau is very important for understanding the uplift and deformation mechanisms of the plateau, as well as the crust-mantle structural evolution. However, there are obvious differences in the results of previous studies due to the different seismic stations data and methods used in these studies. For instance, debates continue regarding the subduction morphology, front location of the Indian lithospheric slab and the formation mechanisms of the Tengchong volcano.

In order to improve the accuracy of crust-mantle structure inversion in the eastern margin of the Qinghai-Xizang Plateau and its surrounding regions, the data used in this study are derived from two different sources, including 249 permanent broadband seismic stations of China Digital Seismograph Network, and 1 694 temporary broadband seismic stations deployed by international and domestic scientific expedition. Rigorous data screening was performed based on the rationality of arrival times, resulting in the selection of 200 737 P-wave arrival times from 4 377 local seismic events and 1 378 547 relative travel time residuals from 18 902 teleseismic events. A joint inversion of these local and teleseismic data was conducted using three-dimensional P-wave tomography, yielding a detailed P-wave velocity structure within a depth range of 0 to 900 km beneath the eastern margin of the Qinghai-Xizang Plateau. With the checkboard tests, the scale of anomalies 0.5°×0.5°×100 km could be recovered perfectly in most study area. The tomographic results with good resolution illuminate that the three-dimensional velocity structure is robust and reliable.

The three-dimensional P-wave velocity model reveals distinct deep structures within the study region. A high-v_P anomaly exists at 0−300 km in depth beneath the Sichuan Basin, representing the preserved cratonic lithosphere from early geodynamic processes of the Earth. Clear images of the Indian mantle lithosphere （IML） reveals the different subducted angles and northern limits between the west side and east side of the IML. The IML goes downdip to the Bangong-Nujiang suture zone at the west side, whereas the subduction of the IML extend much further at the east side, hinting the possible tearing of the IML, which corresponds to NS-trending rifts. In contrast, beneath the Myanmar Arc, a continuous high-velocity anomaly is observed in the upper mantle, extending significantly deeper than the Wadati-Benioff seismic zone and penetrating into the mantle transition zone. This anomaly reaches eastward up to approximately 100°E, potentially representing the eastward subduction of the Indian lithospheric plate. The observed discrepancy between the termination depth of seismicity and the penetration depth of the slab beneath Myanmar is hypothesized to be due to the failure of the physical mechanisms responsible for deep-focus earthquakes. A distinct high-velocity anomaly is identified at the base of the mantle transition zone beneath the southeastern margin of the Qinghai-Xizang Plateau. This anomaly is not connected with the high-velocity structure to the west, which represents the subducting Indian slab, and may represent delaminated lithosphere in the southeastern margin of the Qinghai-Xizang Plateau. Additionally, a distinct low-v_P anomaly exists beneath the Tengchong volcano at 0−300 km in depth, suggesting that the Tengchong volcano originates from slab dehydration and mantle convection induced by the subduction of the Indian plate.