丁志峰,
王椿镛
中国地震局地球物理研究所, 北京 100081
基金项目: 国家重点研发计划（2017FYC1500200）和国家自然科学基金（41774061，42074053，U1839209，41474088）资助
详细信息
作者简介: 常利军, 男, 1978年生, 研究员, 主要从事地震各向异性、深部构造和地球动力学方面的研究工作.E-mail:ljchang@cea-igp.ac.cn
中图分类号: P315 收稿日期:2020-08-17
修回日期:2020-10-30
上线日期:2021-01-10
Upper mantle anisotropy and implications beneath the central and western North China and the NE margin of Tibetan Plateau
CHANG LiJun,DING ZhiFeng,
WANG ChunYong
Institute of Geophysics, China Earthquake Administration, Beijing 100081, China
MSC: P315
--> Received Date: 17 August 2020
Revised Date: 30 October 2020
Available Online: 10 January 2021
摘要
摘要:基于华北中西部和青藏高原东北缘3个流动台阵共480个台站新得到的远震XKS（SKS、SKKS和PKS）波分裂结果，并结合研究区已得到的987个台站的分裂结果，获得了高分辨率的上地幔各向异性图像.分析表明，鄂尔多斯块体的时间延迟较小，反映了其稳定性和弱的各向异性变形特征，可能保留了古老克拉通根的"化石"各向异性，但其靠近边缘的局部区域表现出与相邻边缘相一致的各向异性特征，反映了其局部区域受到了与其相邻边缘的构造活动影响.青藏高原东北缘、阿拉善块体和鄂尔多斯块体西缘快波方向主要为NW-SE方向，鄂尔多斯块体北缘主要为NNW-SSE方向，反映了青藏高原沿NE方向推挤过程中岩石圈沿NW-SE方向和NNW-SSE方向发生了伸展变形；位于四川盆地和鄂尔多斯块体两个刚性块体间的秦岭造山带的快波方向为近E-W方向或NWW-SEE方向，时间延迟较大，推测岩石圈东向挤出和软流圈东流共同促进了观测的各向异性；在鄂尔多斯块体南部边缘，快波方向自西向东逆时针沿西南缘六盘山的NW-SE方向转到南缘渭河地堑的近E-W方向再到东南缘太行山的NEE-SWW方向，推断该区域可能存在一个绕刚性块体的逆时针软流圈绕流，与上覆岩石圈左旋简单剪切变形产生了观测的各向异性，并一起驱动了鄂尔多斯块体的逆时针旋转.作为华北克拉通东西部的过渡带，华北中部的各向异性相对复杂，其东部快波方向为近E-W方向或NWW-SEE方向，时间延迟较大，其各向异性主要反映了太平洋板块西向俯冲作用引起的地幔流；其西北部吕梁山的各向异性主要由岩石圈沿NNW-SSE到NW-SE的拉张变形导致，而西南部太行山的各向异性还反映了软流圈绕流作用.鄂尔多斯块体东北缘大同火山区存在一个快波方向顺时针快速旋转且时间延迟较小的区域，可能与火山群下地幔岩浆上涌形成的局部地幔对流相关.紧邻华北北部的中亚造山带中南部快波方向为近E-W方向，其各向异性不仅受到与构造走向一致的岩石圈变形作用，而且也受到太平洋板块西向俯冲引起的地幔流影响.
关键词: 地震台阵/
横波分裂/
岩石圈变形/
软流圈地幔流
Abstract:The central and western North China and the NE margin of the Tibetan Plateau connects east China and west China, which are dominated by horizontal extension and compression, respectively. This study area consists of a variety of tectonic units, including the Songpan-Garzê block and Qilian orogenic belt, which are terranes of the northeastern Tibetan Plateau; the Alxa block, the Ordos block and its surrounding grabens and orogenic belts-the central and western North China; and the mid-south Central Asian Orogenic Belt (CAOB). To the west of the study area, the ongoing convergence between India and Eurasia plate caused the uplift, crustal thickening and lateral escape of the Tibetan Plateau. To the east, the westward subduction of the Pacific plate to the Eurasian plate strongly affected the tectonic movement of the plate margin and interior. Here seismic activity is strong and tectonic structure is complex. Thus, the study area provides a unique nature laboratory for understanding continental interiors and lithosphere's deformation. Due to the intrinsic linking between anisotropy, finite strain and tectonic process, seismic anisotropy studies have been used widely to constrain on deformation pattern of the crust and lithosphere mantle during an orogenic process. In this study, Based on the polarization analysis of teleseismic XKS (SKS, SKKS and PKS) waveforms recorded at 480 stations from 3 temporary seismic array deployed in the central and western North China and the northeastern Tibetan Plateau, the fast-wave directions and delay times were measured by using the grid searching method of minimum tangential energy and stacking analysis, and we used the 480 new shear wave splitting observations from dense temporary seismic array and 987 published results to map out variations in the deformation of the study area. The comparative analysis of anisotropy with surface deformation, geological structure and lithospheric structure shows that, on the whole, the value of delay time in the Ordos block is less than that in its margins and other tectonic units, it shows the value of delay time in the stable units is less than that of the active units, it also shows weak anisotropy and lithospheric deformation, indicating the anisotropy of the stable Ordos block is possibly caused by "fossil" anisotropy frozen in the ancient North China Craton (NCC). However, the anisotropy of the parts of Ordos block near its margins is consistent with that of their adjacent margins, showing the lithospheric deformation in the parts of Ordos block is affected by the tectonic activity of its adjacent margins. The fast wave directions trend NW-SE in the northeastern Tibetan Plateau, and western margin of the Ordos block, and rotate to NNW-SSE in northern margin of the Ordos block, indicating the NE-trending push of the Tibetan Plateau has caused NW-SE-trending and NNW-SSE-trending lithospheric extension in the northeastern Tibetan Plateau, the Alxa block, the western and northern margins of the Ordos block. In the Qinling orogenic belt between the rigid Ordos block and the rigid Sichuan basin, the E-W or NWW-SEE trending fast wave directions are parallel to the tectonic strike of fault, orogenic belt, collision boundary and absolute plate movement, it indicates that the Qinling orogenic belt is not only the extrusion channel of the lithospheric material of the Tibetan Plateau, but also the eastward asthenospheric mantle flow channel. There is an overall counterclockwise rotation pattern from west to east of the fast wave directions around the southern Ordos block that are oriented NW-SE in the Liupan mountain of the southwestern margin of the Ordos block, to E-W in the Weihe graben of the southern margin, and then to NEE-SWW in the Taihang mountain of the southeastern margin, we conclude that there may be an asthenospheric flow around the southern Ordos block with thick-root, which produces the observed anisotropy with the simple left-lateral shear deformation in lithosphere, and they drive together the counterclockwise rotation of the Ordos block. The central North China is the transitional zone between east NCC and west NCC. Here the anisotropy is relatively complex. In the east part of the central North China, the fast wave directions are E-W or NWW-SEE, and the delay times are relatively large, the anisotropy mainly reflects the asthenospheric flow induced by the westward subduction of the Pacific plate; in the northwest, the anisotropy in the Lüliang mountain is mainly caused by NW-SE-trending and NNW-SSE-trending lithospheric extension; in the southwest, the anisotropy in the Taihang mountain also reflects the asthenospheric flow around the southeastern corner of the Ordos block. In the northeastern margin of the Ordos block, there exists a small area around the centre of N41° and E113°, where the fast wave directions rotate clockwise quickly and the delay times are small, the anisotropy may be related to the mantle upwelling beneath the Datong volcano group. The mid-south CAOB located in the northeast part of the study area, where the E-W trending fast wave directions are consistent with the tectonic strike of fault, collision boundary and absolute plate movement, its anisotropy is not only affected by the lithospheric deformation parallel to the tectonic strike, but also affected by the asthenospheric flow caused by the westward subduction of the Pacific plate.
Key words:Seismic array/
Shear wave splitting/
Lithospheric deformation/
Asthenospheric mantle flow
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