Intracontinental lithospheric delamination: Constraints from imaging the mantle transition zone beneath the southwestern part of the Sichuan Basin

The southwestern part of the Sichuan Basin (SW-SCB) is adjacent to the eastern Himalayan syntaxis. Affected by the Indo-Eurasian collision and subsequent intrusion of the Indian plate into the Eurasian plate during the Cenozoic, this region is ideal for the study of the tectonic evolution of the intracontinental lithosphere and the dynamic processes of deep and shallow structures. In this study, we applied the receiver function technique to the data obtained from a recently deployed high-density broadband seismic array across the Sichuan Basin and Sichuan-Yunnan block (SCB-SYB). We conducted a multi-frequency and multi-model correction analysis to image the structure of the mantle transition zone beneath this region. The results showed the 660-km discontinuity gradually increasing in depth by 10-30 km beneath the western side of the Anninghe-Xiaojiang Fault, suggesting the presence of thermal anomalies caused by the subducted Indian plate from west to east. At the junction of the SCBSYB, the 410-km discontinuity exhibited a slight uplift of 5-10 km, while the 660-km discontinuity showed a significant depression of similar to 30 km over a lateral range of similar to 150 km. Based on previous surface GPS observation and dynamic numerical simulation studies, we suggest that the sharp lateral small-scale topography of this 660-km discontinuity beneath the SW-SCB may have resulted from dripping delamination of the lithosphere within the strain localization area. Furthermore, the aggregation of delaminated lithospheric material at the base of the 660-km discontinuity determines the regional topography of mantle transition zone discontinuities. In this study, we provided seismological evidence for the challenging detection of small-scale intracontinental lithosphere dripping delamination. Moreover, it provides a new view for studying deep and shallow dynamic processes in intracontinental regions with stress concentration resulting from plate/continental subduction and collision.