利用剪切波分裂研究青藏高原东北缘及东南缘上地幔各向异性

Upper Mantle Anisotropy Beneath the Northeastern and Southeastern Margins of the Tibetan Plateau from Shear Wave Splitting

摘要:

利用从IRIS上下载的青藏高原东北缘238个台站以及中国地震科学探测台阵喜马拉雅一期350个台站记录到的远震波形数据，通过采用剪切波分裂方法，获取各个台站下方各向异性分裂参数——快波偏振方向（φ）和分裂时差（δt），从而得到青藏高原东北缘、东南缘上地幔的各向异性特征。研究结果表明，祁连块体、阿拉善块体和鄂尔多斯块体北部，快波方向为NNW-SSE，明显不同于GPS测量得到的近NE-SW的地表位移场方向，延迟时间平均~0.85 s；羌塘块体以及松潘-甘孜褶皱带的西部，快波方向呈现沿顺时针方向旋转的趋势，并且与GPS测量得到的地表位移场方向一致，延迟时间平均为~1.24 s；松潘-甘孜褶皱带东部、秦岭造山带与鄂尔多斯块体南部的交界处，快波方向呈现无序分布，与GPS方向表现出不一致的分布，延迟时间平均~1.08 s；川滇块体北部，快波方向近似N-S方向，与GPS测量得到的地表位移场方向相同，平均延迟时间为~0.925 s；位于北纬26°以南的川滇块体南部，快波方向近E-W方向分布，明显不同于GPS地表位移场方向，平均延迟时间~1.065 s。综合分析推测，羌塘块体、松潘-甘孜褶皱带的西部以及川滇块体北部，地表形变和深部之间的变形是相互耦合的；祁连块体、阿拉善块体和鄂尔多斯块体北部，松潘-甘孜褶皱带东部、秦岭造山带与鄂尔多斯块体南部的交界处以及川滇块体南部，壳幔之间可能存在解耦现象。

Abstract:

In this study, by collecting teleseismic data recorded by 238 and 350 stations in the northeastern and southeastern margins of the Tibetan Plateau, respectively, we used the shear wave splitting method to measure two splitting parameters at each station:polarization of fast wave and splitting time. Subsequently, the upper mantle anisotropy beneath the northeastern and southeastern margins of the Tibetan Plateau was obtained. The results can be summarized as follows:In the Qilian, Alsxan, and northern part of the Ordos Block, the fast wave direction is NNW-SSE, which is significantly different from the near NE-SW direction of surface displacement field measured by GPS, and the average delay time in this region was 0.85 s. In the Songpan-Ganzi fold belt and western part of the Qiangtang Block, the fast wave direction tends to rotate clockwise, which is consistent with the direction of surface displacement field measured by GPS, and the average delay time in this region was 1.24 s. In the junction of the Qinling orogenic belt, eastern part of the Songpan-Ganzi fold belt, and southern part of the Ordos Block, the fast wave direction shows a disorder distribution, which is inconsistent with GPS direction, and the average delay time in this region was 1.08 s. In the northern part of the Sichuan-Yunnan Block, the fast wave is in the near N-S direction, which is consistent with the direction of surface displacement field measured by GPS, and the average delay time in this region was 0.925 s. In the southern part of the Sichuan-Yunnan Block, the fast wave is generally distributed in the near E-W direction, which is distinct from the direction of surface displacement field measured by GPS, and the average delay time in this region was 1.065 s. After comprehensive analysis, we suggested two conclusions:1) in the Songpan-Ganzi fold belt, western part of the Qiangtang Block, and northern part of the Sichuan-Yunnan Block, the crustal and deep deformations are coupled, and 2) In the Qilian Block; Alsxan Block; northern part of the Ordos Block; junction of the eastern part of the Songpan-Ganzi fold belt, Qinling orogenic belt, and southern part of the Ordos Block; and the southern part of the Sichuan-Yunnan Block, the crust and mantle are likely decoupled.