Shear-wave velocity structures of the shallow crust beneath the Ordos and Sichuan Basins from multi-frequency direct P-wave amplitudes in receiver functions

As the two largest cratonic basins in China, the Ordos Basin and the Sichuan Basin are of key importance for understanding the evolutionary history of the Chinese continent. In this study, the shear-wave velocity ( V S ) structures of the shallow crust (depth up to 10 km) beneath the two basins are imaged based on the frequency-dependence of direct P-wave amplitudes in receiver functions. The teleseismic data used in the study came from 160 broadband seismic stations, including permanent and temporary stations. The results show that the V S and the thickness of the sediments in the Ordos Basin and the Sichuan Basin are respectively lower and thicker in the west than in the east. In the Ordos Basin, the shallow crustal V S increases gradually from 2.10 km s −1 in the northwest to 2.65 km s −1 in the southeast and the thickest sediments are 7–8 km in the northwest and 5 km in the east. In the Sichuan Basin, the shallow crustal V S increases from 2.4 km s −1 in the west to 2.7 km s −1 in the east and the thickness of the sediments decreases from >7 km in the west to 6 km in the east. The east-west difference of the shallow crustal structures of the two basins may have been controlled by the Cenozoic India-Eurasia collision. The western parts of the basins near the collision have a higher deposition rate, while in the parts inside the basins far from the collision, the V S slowly increases with depth, indicating that these areas have experienced a more uniform deposition process. In addition, both basins are characterized by velocity structures that are higher along the edges and lower inside of the basins. The edges of the basins suffered strong denudation due to the uplifting and deformation influenced by tectonic evolution. The downward gradient of the shear-wave velocity beneath the Ordos Basin is twice that of the Sichuan Basin, which may be caused by the different deposition and denudation rates of the two basins resulting from differences in structural evolution and thermal events. In addition, the northern Ordos Basin exhibits a strong structural horizontal stratification, while the southern part shows obvious lateral variations in the V S structure, both of which may have been affected by the Qilian orogenic event, the collision and assembly of the South China and the North China block, and the lateral extrusion of the Tibetan Plateau.