Crustal Structure of the Chuan‐Dian Block Revealed by Deep Seismic Soundings, and its Implications for the Outward Expansion of the East Tibetan Plateau

The Chuan-Dian Block (CDB) is located in the southeastern margin of the Tibetan Plateau, with a complex geological structure and active regional faults. The present tectonic condition with strong crustal deformation is closely related to the ongoing collision of the India and Eurasia plates since 65 Ma. The study of the crustal structure of this area is key to revealing the evolution and deep geodynamics of the lateral collision zone of the Tibetan Plateau. Deep seismic sounding is the most efficient method with which to unravel the velocity structure of the whole crust. Since the 1980s, 19 deep seismic sounding profiles have been captured within the CDB area. In this study, we systematically integrate the research results of the 19 profiles in this area, then image the 3D crustal velocity, by sampling with a 5 km spacing and 2D/3D Kriging interpolation. The results show the following. (1) The Moho depth in the study area deepens from 30 km in the south to 66 km in the north, whereas there is no apparent variation from west to east. The Pn wave velocity is higher in stable tectonic units, such as 7.95 km/s in the Lanping-Simao block and 7.94 km/s in the western margin of the Yangtze block, than in active or mobile tectonic units, such as 7.81 km/s in the Baoshan block, 7.72 km/s in the Tengchong block and 7.82 km/s in the Zhongdian block. (2) The crustal nature of the Tengchong block, the northern Lanping-Simao block and the Zhongdian block reflects a type of orogenic belt, having relatively strong tectonic activities, whereas the crustal nature of the central Lanping-Simao block and the western margin of the Yangtze block represents a type of platform. The different features of the upper-middle crust velocity, Moho depth and Pn wave velocity to both sides of the Red River fault zone and the Xianshuihe fault zone, reflect that they are clearly ultra-crustal. (3) Based on the distribution of the low velocity zones in the crust, the crustal material of the Tibetan Plateau is flowing in a NW-SE direction to the north of 26 degrees N and to the west of 101 degrees E, then diverting to flowing eastwards to the east of 101 degrees E.