Abstract:

The geologic framework of the Qinling Orogen and adjacent region is developed as a result of the convergence of the North China Craton and Yangtze Craton during a long and complex evolutionary history. Detailed upper crustal structure connecting the superposed surface geological phenomena with geophysical images is a key for understanding the geodynamics of the convergence process. The Qinling wide angle reflection and refraction seismic profile extends from the Ordos basin, across the Weihe graben, Qinling orogeny, Dabashan thrusting nappe, to the Sichuan basin. The seismic waves from 15 explosions were recorded on 281 DFZ recorders, of which 690 arrivals of refracted waves from sediments and basement (Pg) were typically picked, and finite-difference tomography algorithm was used to obtain the upper crustal velocity structure. After 20 iteration, the RMSD for the resulting arrival time was 0.105 s, with good convergence. The final P-wave velocity model contains the following features of tectonic significance: (1) The Weihe graben is a typical down-faulted basin with the deepest sediments, as deep as 7 m, in the south, and its formation is controlled by the Qinling northern border fault, Weihe fault and Qianxian-Fuping fault. (2) The upper crustal structure of the Qinling orogen varies significantly with generally high velocity. (3) Sediments of 3-6 km thick beneath the Dabashan Mountain are preserved, but the velocity structure is not uniform, which reflects the thrusting induced reformation.

Key words: wide angle reflection and refraction, Qinling orogen, Weihe graben, Dabashan Mountain, seismic tomography