南北构造带北段S波分裂研究及其动力学意义

南北构造带北段位于青藏高原东北缘及其向北东方向扩展的区域，其岩石圈变形特征对于探讨青藏高原东北缘变形机制及其扩展范围具有非常关键的意义。地震波各向异性能很好地反映上地幔的变形特征。因此，本文对布设在南北构造带北段的流动地震台站记录的远震波形资料进行S波分裂研究，获得了研究区上地幔各向异性图像以及该区岩石圈地幔的变形特征信息。S波分裂研究结果表明，研究区地震波各向异性来自于上地幔，区内不同构造单元上地幔各向异性方向不尽相同。快波方向分布显示，青藏高原东北缘，鄂尔多斯西缘以及贺兰构造带北段的快波方向主要表现为NW-SE向，与前人在银川地堑和贺兰构造带中、北部得到的NW-SE向的上地幔各向异性方向一致，显示这些地区岩石圈地幔变形一致，该结果表明青藏高原东北缘向北东方向扩展的影响范围已到达贺兰构造带北段。阿拉善地块内部快波方向显示为NE-SW向，与阿拉善地块北部存在的北东向展布的晚古生代岩浆岩方向一致，表明该NE-SW向的快波方向可能代表地是“化石”各向异性，是晚古生代阿拉善地块受到古亚洲洋闭合作用的结果。此外，鄂尔多斯地块内也存在NE-SW向的各向异性方向，与区内中-晚侏罗世存在的NE-SW向逆冲推覆构造方向一致，因此该各向异性方向也代表了“化石”各向异性，是鄂尔多斯地块受到古特提斯构造域的块体碰撞、古太平洋板块北西向俯冲以及西伯利亚板块向南俯冲共同作用的结果。

The shear wave splitting beneath the northern segment of the north-south tectonic belt and its implications

The northern segment of the north-south tectonic belt is located in the northeastern margin of the Tibetan Plateau and its adjacent areas. The lithospheric deformation is very important for exploring the deformation mechanism and its extension of the northeastern margin of the Tibet Plateau. Seismic wave anisotropy can well reflect the upper mantle deformation. Thus, we analyze shear-wave splitting at temporary seismic stations in the northern segment of the north-south tectonic belt, and obtain the anisotropic image and deformation characteristics of the lithospheric mantle under the study area. The value of the delay time indicates that the anisotropy originates from the upper mantle beneath the study region. The shear-wave splitting results reveal obvious variations of the fast-wave directions in the different tectonic units. The dominant fast polarization direction（FPD）is NW-SE along the NE Tibetan Plateau, the western margin of the Ordos block and the northern section of the Helan Mountain, which is consistent with the FPDs in the Yinchuan graben and the Helan Mountain revealed by the previous studies. The results show the coherent deformation of the lithospheric mantle under these regions, suggesting that the northeastern extension of the Tibetan Plateau influenced the deformation of the lithospheric mantle beneath the northern segment of the Helan Mountain. Combined with geological research results, the NE-SW FPD in the Alashan block maybe represent the “fossil” anisotropy, which results from the deformation the lithospheric mantle related to the closure of the Paleo-Asian Ocean during the Late Paleozoic. The NE-SW FTD within the Ordos block may also represent the “fossil” anisotropy formed by the collision of the Tethys tectonic domain, the southward subduction of the Siberia block and the northwest moving of the Paleo-Pacific plate during the Middle-Late Jurassic, which result in the NE-SW trending thrust nappe structure.