巴颜喀拉块体地壳结构多样性探测

青藏高原内部地壳岩性的改造、岩性随深度变化及形变构造是探索研究地壳增厚、物质运动问题的关键.巴颜喀拉块体位于青藏高原中北部,地域广袤,通过对块体内中、东部不同区域的深地震广角反射/折射震相的综合分析,利用反射率理论地震图方法对不同性质震相走时及振幅特征进行细致的模拟计算,进一步研究巴颜喀拉块体内部不同区域地壳精细结构.结果显示:巴颜喀拉块体地壳厚度50~60 km、整体向西逐渐增厚,结晶地壳平均速度6.07~6.18 km·s~(-1)、岩层速度大幅降低,壳内多强反射界面结构、但不同区域差异明显;东部若尔盖盆地地壳介质速度整体低速、壳幔边界较为清晰;中部玉树一玛多段下部地壳发现约6.8 km·s~(-1)的"高速度"介质结构,壳幔边界不清、被改造为2~4 km厚的高速度梯度层,显示了巴颜喀拉块体内部地壳增厚、介质岩性结构被改造的差异性.地壳内部多组强反射、低视速度走时震相揭示了介质岩性的低速破碎、弱化蠕变以及可能的壳内解耦构造.局部地区下地壳的高视速度震相特征显示了青藏高原地壳改造增厚大背景下可能存在稳定的"原始地壳"结构残留或是与上地幔物质的浸入交流.巴颜喀拉块体内不同区域地壳增厚、岩性结构、结晶基底及壳幔边界性质被改造的多样性为深入认识青藏高原地壳形变及动力学过程带来新的启迪.

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 to the travel-time phase with different properties and amplitude characteristics using the synthetic reflectivity seismograms. Results indicate that the thickness of the Bayan Har block varies from 50 to 60 km, increasing westward gradually. The average velocity in the crystalline crust is 6.07~6.18 km·s^-1, which reduces obviously in rock layers, and there are several strong reflection interfaces in the crust, which differ in different regions. In the east, the Zoigê 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 the unsharp crust-mantle boundary, i.e. the Moho interface that 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 travel-time phase indicate the shattered low-velocity, weaken creep and possible decoupling structures in the crust. High apparent velocity phase in the lower crust displays that there may be stable original crust residual or the 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.