QIANG Zheng-Yang,
WU Qing-Ju
.2015.Upper mantle anisotropy beneath the north of northeast China and its dynamic significance.Chinese Journal Of Geophysics,58(10): 3540-3552,doi: 10.6038/cjg20151010
中国东北地区北部上地幔各向异性及其动力学意义
强正阳, 吴庆举
中国地震局地球物理研究所, 北京 100081
Upper mantle anisotropy beneath the north of northeast China and its dynamic significance
QIANG Zheng-Yang, WU Qing-Ju
Institute of Geophysics, Chinese Earthquake Administration, Beijing 100081, China
Northeast China located in the eastern part of the Central Asian Orogenic Belt (CAOB), one of the largest Paleozoic orogens on Earth. Surrounded by the Siberian craton, North China plate and the Pacific plate, northeast China is characterized by widespread Cenozoic intraplate volcanism and lithospheric deformation caused by multiepisode extension since Late Mesozoic.As the only area developing deep earthquake in China, northeast China has suffered the subduction of the Pacific plate, which makes it become the natural laboratory of studying the lithospheric deformation, subduction and intraplate volcanos and also their interaction relationship.
Seismic anisotropy determined by the splitting of shear waves, especially the core-refracted phases like SKS, is one of the most direct and effective ways to image the structure and deformation in the interior of the Earth. We measure the shear wave splitting parameters,the fast polarization direction (φ) and the splitting time between the fast and slow waves (δt), using the SplitLab analysis software package which combine both the rotation-correlation (RC) method and the transverse minimum energy (SC) method. A quality factor (good, fair, or poor) is assigned to all the shear wave splitting measurements,according to the signal to noise ratio of the initial phase, the rectilinear polarization of the particle motion after correction, and well-defined extreme value of contour plot.Only the measurements marked as 'good’ or 'fair’ are retained in the subsequent analysis and discussion, and the delay times less than 0.4 s will be marked as null measurements based on measurement error consideration.
A total of 377 pair of SKS splitting measurements obtained at 147 temporary and permanent seismic stations in northeast China are used to infer the upper mantle structure of this area. The shear wave splitting results at most of stations in western part of our study area are characterized by homogeneously oriented fast directions trending N143°E to N199°E with an average close to N169°E, in agreement with the extensional orientation of the Late Mesozoic lithosphere. Additionally, the delay times vary from 0.4 s to 1.6 s with a mean value of 0.85±0.23 s and correspond to a 98±26 km thick layer if 4% anisotropy is assumed, suggesting the anisotropy mainly reside in the lithosphere. Several anisotropy with small delay times (~0.4 s) are observed in the Songliao basin and Jiamusi massif which are probably caused by the lithosphere delamination or the hot mantle upwelling that partially eroded the ancient deformation frozen within lithosphere.While in the eastern part, NNW-SSE trending fast directions with large delay times (larger than 1.0 s) are observed. As the missing of the deep seismicity of this area, we speculate that there exist a slab tear at the end of Pacific plate, whose roll-back may induce NW trending mantle flow. In addition, nearly W-E fast splitting directions are observed in Jiamusi massif only, which can be best explain by the LPO of metastable olivine within the Pacific slab.
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