| 南北构造带北段上地幔各向异性特征 |
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| 引用本文： | 常利军,丁志峰,王椿镛.南北构造带北段上地幔各向异性特征[J].地球物理学报,2016,59(11):4035-4047. |
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| 作者姓名： | 常利军 丁志峰 王椿镛 |
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| 作者单位： | 中国地震局地球物理研究所, 北京 100081 |
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| 基金项目： | 国家自然科学基金（41474088），地震行业科研专项（201308011）和国家自然科学基金（41174070，41274063）资助. |
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| 摘 要： | 对布设在南北构造带北段的中国地震科学探测台阵项目二期674个宽频带流动台站和鄂尔多斯台阵21个宽频带流动台站记录的远震XKS(SKS、SKKS和PKS)波形资料作偏振分析,采用最小切向能量的网格搜索法和"叠加"分析方法求得每一个台站的XKS波的快波偏振方向和快、慢波的时间延迟,并结合该区域出版的122个固定台站的分裂结果,获得了南北构造带北段上地幔各向异性图像.快波方向分布显示青藏高原东北缘、阿拉善块体和鄂尔多斯块体西缘的快波方向主要表现为NW—SE方向,秦岭造山带的快波方向为近E—W方向,鄂尔多斯块体内部的快波方向在北部为近N—S方向,南部表现为近E—W方向.时间延迟分布来看,鄂尔多斯块体的时间延迟不仅明显小于其周缘地区,而且小于其他构造单元,特别是在高原东北缘、阿拉善块体和鄂尔多斯块体的交汇地区的时间延迟很大,反映了构造稳定单元的时间延迟小于构造活跃单元.通过比较快波方向的横波分裂测量值与地表变形场模拟的预测值,并结合研究区地质构造和岩石圈结构特征分析表明,在青藏高原东北缘、阿拉善块体和鄂尔多斯块体西缘各向异性主要由岩石圈变形引起,地表变形与地幔变形一致,地壳耦合于地幔,是一种垂直连贯变形模式;秦岭造山带的各向异性不仅来自于岩石圈,而且其岩石圈板块驱动的软流圈地幔流作用不可忽视;鄂尔多斯块体内部深浅变形不一致,具有弱的各向异性、厚的岩石圈和构造稳定的特征,我们认为其各向异性可能保留了古老克拉通的"化石"各向异性.
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| 关 键 词： | 南北构造带 地震台阵 横波分裂 岩石圈变形 软流圈地幔流 |
| 收稿时间： | 2016-03-31 |
Upper mantle anisotropy beneath the northern segment of the north-south tectonic belt in China |
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| CHANG Li-Jun,DING Zhi-Feng,WANG Chun-Yong.Upper mantle anisotropy beneath the northern segment of the north-south tectonic belt in China[J].Chinese Journal of Geophysics,2016,59(11):4035-4047. |
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| Authors: | CHANG Li-Jun DING Zhi-Feng WANG Chun-Yong |
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| Institution: | Institute of Geophysics, China Earthquake Administration, Beijing 100081, China |
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| Abstract: | The north-south tectonic belt (NSTB) is a north-south meridional tectonic boundary between the eastern and western Chinese mainland with a very complex structure, showing significant changes in geology, geomorphology, and geophysical field characteristics on both sides. Meanwhile, the NSTB is seismically active zone, so also named the well-known north-south seismic belt (NSSB). Thus, the NSTB is regarded as a unique natural laboratory for understanding continental interiors and lithospheric deformation. The study region of this paper is located in the northern NSTB, including the Songpan-Garzê and Kunlun-Qilian fold belts, which are terranes of the northeastern margin of the Tibetan Plateau, and the Alxa and Ordos blocks which are west portions of the North China craton (NCC). This work uses knowledge of seismic anisotropy to provide important constraints on deformation patterns of the crust and lithosphere mantle during an orgency process. It is based on 695 new shear-wave splitting observations from a dense temporary seismic array and 122 published results from permanent seismic stations to map variations in the deformation of the northern segment of the NSTB. The new XKS (SKS, SKKS, and PKS) shear wave splitting observations include 674 measurements from portable deployments in the NSTB (2013-2015, the ChinArray Phase Ⅱ) and 21 measurements from temporary stations deployed in the Ordos block (2010-2011, the Ordos Array). We determine the XKS fast wave polarization directions and delay times between fast and slow shear waves for 695 new seismic stations in the northern segment of the NSTB using both the grid searching method of minimum transverse energy and stacking analysis method. To obtain a reliable estimate of splitting parameters, the following criteria are taken as diagnostics for successful splitting parameter estimations:(1) Clear XKS arrivals and distinct tangential component. (2) The horizontal particle motion is elliptical when anisotropy is present. (3) The two horizontal fast- and slow-component waveforms are coherent. (4) The particle motion becomes linear following correction for anisotropy. And (5) successful removal of tangential energy in the case of core phases. The results at most stations are good, the error of azimuth is less than 10°, and the error of delay time is less than 0.2 s. The fast polarization directions and delay times do not depend on back azimuths, thus a single layer of anisotropic fabric is able to sufficiently explain the data without the need for additional layer. Based on 817 observations, we develop an anisotropic image of upper mantle in the northern segment of the NSTB. In the study region, the fast-waves trend in NW-SE in the northeastern margin of the Tibetan Plateau, Alxa block, and western and northern margins of the Ordos block. The fast-wave directions are in nearly E-W in the Qinling orogen. Within the Ordos block, the fast-wave directions trend in nearly N-S in the north, but switch to nearly E-W in the south. The value of delay time in the Ordos block is not only less than that in its margins, but also less than that in other tectonic units. Especially, the value of delay time in the conjunction of the northeastern margin of the Tibetan Plateau, Alxa block and Ordos block is the largest and considerably larger than other areas. This implies the value of delay time in the stable units is less than that of the active units. Analysis of the fit between the fast-wave direction of shear-wave splitting and predicted fast axis orientation calculated from the surface deformation field indicates the coherence between surface deformation and mantle deformation in the northeastern margin of the Tibetan Plateau, Alxa block, western and northern margins of the Ordos block, and the crust is coupled with the mantle. These results suggest the vertical coherent deformation of the lithosphere plays a major role in the observed seismic anisotropy. In the Qinling orogen, both the lithosphere mantle and eastward asthenospheric mantle flow contribute to the observed anisotropy. Within the Ordos block, there exist weak anisotropy and thick lithosphere, and the shallow deformation is inconsistent with the deep deformation, suggesting the anisotropy of the stable Ordos block is possibly caused by "fossil" anisotropy frozen in the ancient NCC. |
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| Keywords: | North-south tectonic belt Seismic array Shear wave splitting Lithospheric deformation Asthenospheric mantle flow |
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