THE CRUSTAL STRUCTURE BENEATH THE EASTERN QINGHAI

A Sino\|French joint wide\|angle seismic reflection survey was conducted in eastern Qinghai Province of P. R. China in 1998. There are several geological units in this region, including Sanjiang Fold belt, Bayan Har fold belt, eastern Kunkun fold belt, western Qingling fold belt, Qaidam basin and Qilian fold belt. The seismic data acquired in the seismic experiment provides an abundant information and reveals the detailed crustal velocity structure and differences between all the tectonic units in this region. These results will help us to understand the evolution procedure and mechanism of the formation of Qinghai—Xizang plateau.According to the seismic data, several reflection phases were identified. After the phase correlation, the reflectivity method and Dynamic Ray Tracing were applied simultaneously to model the 1\|D velocity structure for different tectonic units, actually to determine the velocity distribution within the crust. Based on the 1\|D depth\|velocity function, keeping the layer\|velocities fixed and only adjusting the shape of the reflectors to fit the observed data by Dynamic Ray Tracing, the 2\|D crustal velocity structures were established for each tectonic unit. The results show that there are some obvious differences between Sanjiang and Bayan Har. The crust is about 76km thick with an average velocity of 6 33km/s in Sanjiang region, but about 64km with an average velocity of 6 26km/s in Bayan Har. On the other hand, the velocity of lower crust in Sanjian is lower (about 6 65km/s) than in Bayan Har (about 6 90km/s), and the thickness of the lower crust in Sanjiang is much thicker than in Bayan Har. A low velocity layer ( v p=5 80～5 85km/s) exists on both sides of Jinsha fault. These evidences indicate the existence of Jinsha suture.