青藏高原东北缘—鄂尔多斯地壳上地幔地震层析成像研究

SEISMIC TOMOGRAPHIC IMAGING OF THE CRUST AND UPPER MANTLE BENEATH THE NORTHEASTERN EDGE OF THE QINGHAI-XIZANG PLATEAU AND THE ORDOS AREA

根据横跨青藏高原东北缘—鄂尔多斯地区的流动宽频带地震台阵和区域地震台网的走时数据 ,利用地震层析成像方法研究了该地区 4 0 0km深度范围内地壳上地幔的P波速度结构 .结果表明 :青藏高原东北缘—鄂尔多斯地区地壳上地幔结构呈明显的横向不均匀性 .这种不均匀性体现在不同块体之间 ,同时也存在于块体内部 ;青藏高原东北缘上地幔P波平均速度低 ,而鄂尔多斯地块的P波平均速度高 ,这与两个地块的地质构造活动特征相吻合 .在青藏高原东北缘与鄂尔多斯地块之间的上地幔存在宽约 2 0 0km的过渡带 ,该过渡带在地表的界限在兰州—海原之间 ,在上地幔表现为向东 4 5°左右的倾斜条带 ,基本结构特征表现为高速与低速物质的混杂 ;青藏高原内部的柴达木地块平均速度偏低 ,而祁连地块的上地幔的平均速度偏高 ,两者相差约 8% ;在泽库、兰州和海原地区的上地幔顶部有明显的低速体侵入特征 .

Combining the data recorded by the portable broadband seismic array across the northeastern edge of the Qinghai-Xizang Plateau and the Ordos area and the travel time data of the regional seismic network, we investigate the P wave velocity structure of the crust and upper mantle down to 400km depth along a profile. Our results show that the structure of the crust and upper mantle along the profile has obviously laterally inhomogeneities, which occur not only between different blocks, but also within block. Along the profile can be identified obvious difference between the velocity structures of the upper mantle beneath the northeastern Qinghai-Xizang and Ordos plateau. The former has a lower averaged P wave velocity than the global model, and the latter is higher than the global model. This is consistent with tectonic activities in these two blocks. Between the northeastern Qinghai-Xizang and Ordos plateau there is a transitional zone of about 200km wide, which has indicators on the surface are Lanzhou and Haiyuan,?respectively.?In?the?upper?mantle?this?transitional zone appears as a sloping strip with about 45° toward the eastern direction. The main features of the transition zone appear as a mixture of high-velocity and low-velocity media. We can also identify the differences of the upper mantle velocity structures beneath the Qaidam basin and the Qilian block within the northeastern Qinghai-Xizang. This appears mainly as a low average velocity in the Qaidam block and a high average velocity in the Qilian block, respectively. The difference between them is up to about 8%. A low-velocity intrusive body is located on the top of the upper mantle beneath Zeko, Lanzhou and the Haiyuan area. This is in accord with the results by the receiver function method.