Study on crustal composition and geodynamics using seismic velocities in the northeastern margin of the Tibetan Plateau

Based on P-wave, S-wave velocity structures and Poisson's ratio of Maqin-Lanzhou-Jingbian seismic refraction/wide angle reflection profile, which crosses the northern Bayan Har block, the Qinling block, the Qilian block, the Haiyuan arcuate tectonic region, and the Ordos block, the crustal composition of the northeastern margin of the Tibetan plateau is studied and the geodynamic issues are discussed. At first, a set of feasible methods are systemically summed up, with which in situ P-wave velocities observed by deep seismic sounding can be corrected to the velocity under the special condition of temperature and pressure in the laboratory. It is the key to the methods to get temperatures in the crust using heat flow. Secondly, in situ P-wave velocities of the profile are corrected to the velocities under a standard pressure of 600 MPa and room temperature,the corrected velocities, combined with Poisson's ratio, are compared with laboratory measurements of ultrasonic velocities, and the crustal petrologic composition is determined in the study area. The results show that the average corrected velocity in the northeastern margin of Tibetan plateau is 6.43 km/s, and the bulk crustal composition is felsic like the composition of the upper crust. In the Bayan Har block and the southern part of Qinling block, there is a lack of V_p6.9 km/s mafic rock layer in the lowest crust, with felsic layer and intermediate layer alternating in the lower crust, and the bulk composition in the lower crust is felsic. In other blocks, there is a 2～10 km thick mafic layer of V_p6.9 km/s in the lowest crust and the bulk composition is intermediate in the lower crust. At last, on the basis of the crustal structure and composition in the northeastern margin of the Tibetan plateau, the opinion that crustal thickening occurred mainly in the lower crust is supported, and it is proposed that delamination possibly occurred in the lower crust of the Bayan Har block and southern Qinling block in the recent geologic time, which accelerated the uplift of the Tibetan plateau.