Two thin middle-crust low-velocity zones imaged in the Chuan-Dian region of southeastern Tibetan Plateau and their tectonic implications

Intracrustal low-velocity zones (LVZs) indicate a mechanically weak crust and are widely observed in the southeast margin of the Tibetan Plateau. However, their spatial distributions and formation mechanisms remain controversial. To investigate their distribution and detailed morphology of the LVZs in the southeastern Tibetan Plateau, here we used teleseismic events and continuous waveform data recorded by 40 broadband seismic stations newly deployed in the Sichuan-Yunnan region from December 2018 to October 2020. A total of 12,924 high-quality P-wave receiver functions and 5-40 s fundamental Rayleigh surface wave phase velocity dispersion curves from ambient noise cross-correlation functions were obtained. The S-wave velocity model at a depth interval of 0-100 km in the study area was inverted by using the trans-dimensional Markov chain Monte Carlo strategy to jointly invert the complementary data of the receiver function waveform and Rayleigh surface wave phase velocity dispersion. Our results show that there are two separate LVZs (similar to 3.5 km/s) surrounding the rigid Daliangshan subblock at crustal depths of approximately 30-40 km, providing new constraints on the geometry of the LVZs in our study region. The two LVZs obtained in this study may represent the middle crustal flow channels, through which the material in the center of the Tibetan Plateau extrudes to its southeast margin. Blocked by the rigid Sichuan Basin and the spindle-like Daliangshan subblock, the material continues to flow southward through the mechanically weak middle crustal channels surrounding the Daliangshan subblock. In addition, the existence of thin LVZs in the middle crust plays an important role in understanding the decoupling between the upper and lower crust in the study area. It also provides new constraint on the complex tectonic deformation process of the southeastern margin of the Tibetan Plateau caused by the collision and compression of the Indian and the Eurasian plates.