INVESTIGATION ON DIVERSITY OF CRUSTAL STRUCTURES BENEATH THE BAYAN HAR BLOCK

Crustal lithology transformation, lithological variations with depth and tectonic deformation in the Tibetan plateau are the key to explore the crustal thickening and material motion in this region. Located in the central plateau, the Bayan Har block has a vast geographical territory. To make a further study on the fine crustal structure in the central and eastern Bayan Har block, we conduct a comprehensive analysis of the deep seismic wide-angle reflection/refraction phase data of different areas of this block, and then perform detailed simulation calculation of the travel-time and amplitude characteristics of different phases using the synthetic reflectivity seismograms. The results indicate that the thickness of the Bayan Har block varies from 50 km to 60 km, increasing westward gradually. The average velocity in the crystalline crust is 6.07∼6.18 km·s⁻¹, which is obviously reduced, and there are several strong reflection interfaces in the crust, which differ in different regions. In the east, the Zoigě basin has a low velocity within the crust and a clear crust-mantle boundary. In the central part, a high velocity structure (6.8 km·s⁻¹) is present beneath the Yushu-Madoi segment with an unclear crust-mantle boundary, i.e. the Moho interface transformed into a high velocity gradient layer with a thickness of 2∼4 km. These characteristics indicate the discrepancy of crustal thickening and lithology transformation within the Bayan Har block. Multi-group strong reflections in the crust and the low apparent velocity indicate the shattered, low-velocity, weakened, creeping, and possibly decoupling structures in the crust. High apparent velocity phase in the lower crust displays that there may be stable original crust residual or material exchanging with the upper mantle under the background of the crustal thickening and transformation in the Tibetan plateau. The diversity of different areas within the Bayan Har block involves the crustal thickening, lithology structure, crystalline basement and crust-mantle boundary nature transformation, which can provide new insights into the understanding of the crustal deformation and dynamical process in the Tibetan plateau.