阿尔金断裂西部邻区的上地幔各向异性研究

本文利用横波分裂方法对北京大学于田流动台阵记录的SKS震相进行分析,获到了阿尔金断裂西部及邻区的上地幔各向异性参数.分析结果显示,快波偏振方向在整个研究区基本呈近E-W向,与研究区内阿尔金断裂的走向几乎一致,分裂延迟时间在0.93~1.20s之间.综合研究区附近前人横波分裂研究结果,我们认为,在印度和欧亚大陆板块碰撞作用下,青藏高原北部上地幔软流圈物质向北流动,遇到塔里木盆地"克拉通"较厚岩石圈阻挡并发生了旋转,向东西两侧流动,导致在青藏高原和塔里木盆地边界地带软流圈上地幔橄榄岩中晶格沿近E-W向优势排列.这一模式显示阿尔金断裂可能是一个岩石圈尺度的大型走滑断裂:它既控制近地表的上地壳构造运动,同时也影响了上地幔软流圈物质的流动.另外,在向塔里木盆地内部延伸的台站也观测到显著的各向异性和近EW向的快波偏振方向.这些结果表明塔里木盆地"克拉通"岩石圈的中、下部分在南部边界被青藏高原北部上地幔软流圈流动"热侵蚀"而损失一部分,导致青藏高原软流圈向东西两侧的流动已经延伸到塔里木盆地内部.本文的研究结果揭示克拉通岩石圈"活化"不仅可以在垂直方向发生(如,岩石圈拆沉或软流圈上涌导致的热侵蚀),也可以在水平方向上发生,即软流圈的水平流动对克拉通岩石圈边界的热侵蚀作用.

Upper mantle anisotropy analysis around the western Altun Tagh fault

We conducted SKS wave splitting method to analyze teleseismic data from broadband temporary seismic stations deployed by Peking University near Yutian, Xinjiang province in China at the boundary of the Tarim basin and Tibetan plateau. All the fast polarization directions share a common preferred E-W orientation in the study area, showing a good correlation with the strike of the Altun Tagh fault in the area. The delay times range from 0.93 s to 1.20 s. Considering all the shear wave splitting measurements around this area, we postulate that the E-W fast polarization direction may be caused by the eastward escaping mantle flow of the northern Tibet beneath the left-lateral slipping Altun Tagh fault, which is prevented from moving northward by the thick lithosphere of the Tarim basin. Our observation implies that the Altyn Tagh fault could be a large lithospheric-scale strike-slip fault which not only controls the surface deformation observed by GPS measurements but also the upper mantle flow observed by the SKS splitting observations such as our result reported here. Another interesting observation is that this E-W fast polarization direction could be seen at the stations extending north into the interior of the Tarim basin, indicating the cold "cratonic" Tarim lithosphere could be thermally eroded away by this eastward escaping Tibetan asthenospheric flow at its southern boundary. Our inference suggests that the reactivation of the cratonic lithosphere may also take place leading by the horizontal asthenospheric flow at the boundary.