青藏高原东北缘地壳结构变化的地球动力学意义

Geodynamic significance of the crust structure beneath the northeastern margin of Tibet

由于印度板块和欧亚板块的碰撞作用,且扬子地块和鄂尔多斯地块的阻挡,位于青藏高原东北缘的甘东南地区成为地形变化极为剧烈的一个梯度带.该区域地壳发生了明显缩短,形成了复杂的地壳结构,因此,该地区地壳内部结构的探测对于青藏高原隆升机制的认识具有重要意义.本研究利用布设在该区39个固定宽频带台站远震接收函数,得到了该区域地壳底界面起伏形态以及波速比分布的详细结果.该结果表明,该区域地壳底界面存在着可达20km的起伏;接近青藏高原内部的研究区西部地壳较厚、波速比较大,该特性说明这一区域地壳中存在低速流变物质,是软地壳;位于鄂尔多斯和扬子地块的东边区域,地壳较薄、波速比较小,可能是硬地壳.在地形剧烈变化且位于软硬地壳之间的梯度带内,地壳厚度和波速比也呈现较为复杂分布,这可归结为长期复杂块体相互作用的结果.在历史上发生过海原巨大地震的局部区域,地壳起伏和波速比呈现较强烈的横向变化,这可能是该区域孕育巨大地震的一种深部结构条件.

Due to the collision of the Indian plate and Eurasian plate, obstacle of the Yangtze Block and the Ordos block, the terrain in the southeastern region in Gansu province become a gradient zone. The crustal shorten also occurred here, and led to the complicated crustal structure. So the detection of crustal structure in this region has important significance to understand the uplift of the Tibet Plateau. The teleseismic receiver functions of 39 permanent seismic stations are used to determine the crustal thickness and velocity ratio in the southeastern region in Gansu province. The results indicated that the crustal thickness fluctuates strongly with 20 km. The crust is thicker in the west region located in Tibet with high velocity ratio, which means the crust is there is soft with rheological material. The crust is thinner in the Ordos and Yangtze block with small velocity ratio, which means the crust is hard. The crustal structure is so complicated in the terrain gradient zone between the soft and hard crust, which can be attributed to the long and complicated interaction between different blocks. There are strong lateral changes in the crust structure beneath the region with active seismicity, especially in the region in which Haiyuan earthquake occurred.