青藏高原东北缘及邻区面波方位角各向异性研究

The Northeast Tibet is the frontal zone of the Tibet Plateau northeast expansion to the Inner land of China and has been experienced strong tectonic deformation since Cenozoic. The mantle material beneath the Northeast Tibet may escape eastwards influenced by the continuous convergence of the Indian-Eurasian continents. Previous studies show the existence of the mantle flow beneath the southern and northern margin of the Ordos block, which is originated from the Northeast Tibet. However, the location and the depth of this mantle flow are still under debate. It is of great meaning for explaining the way Tibet Plateau expands to its surrounding blocks to obtain S-wave velocity and azimuthal anisotropy of the crust and uppermost mantle in the Northeast Tibet and its surrounding areas. By using the dense temporary and permanent seismic arrays deployed in the study area, this project is supposed to construct phase velocity map and azimuthal anisotropy of different periods via ambient noise tomography and two-plane-wave tomography. Furthermore, a non-linear Neighborhood Algorithm is presented to estimate the Vsv velocity model and azimuthal anisotropy in different depths beneath the Northeast Tibet and its surrounding areas. The resulting model is used to determine whether the eastwards mantle flow originated from the Northeast Tibet exists or not and the location and the depth of the mantle flow beneath the northern Ordos block if it really exists. This project will provide insight into the interactions between the hot mantle material escaped from the Northeast Tibet and the overlying lithosphere beneath the surrounding areas, which is helpful to understanding the lithospheric deformation mechanism and geodynamic pattern of the plateau growth.

青藏高原东北缘是青藏高原东北向扩展前缘，自新生代以来发生了强烈广泛的构造形变，其下方地幔物质在印度-欧亚板块持续汇聚过程中可能发生东向挤出。现有研究表明鄂尔多斯块体南北可能存在由青藏高原东北缘东向挤出的地幔流，且其所在位置和深度范围仍未有定论。明确是否存在地幔物质东向挤出及其通道位置和深度范围对解释青藏高原隆升和扩展非常关键。本项目拟利用布设在青藏高原东北缘及其周边密集流动台阵所记录的地震数据，使用背景噪音成像和地震双平面波成像获得不同周期相速度图像及方位角各向异性，并利用邻域算法反演获得该区域地壳上地幔剪切波速度结构及不同深度方位角各向异性；进而判明是否存在自青藏高原东北缘东向流经鄂尔多斯北缘的地幔物质流，并探讨潜在地幔流的通道位置和深度范围，从而加深对高原地幔物质东北向逃逸及其与周边块体相互作用的认识，为解释青藏高原如何向周缘块体过渡和扩展提供观测证据。

青藏高原东北缘是青藏高原东北向扩展前缘，自新生代以来发生了强烈广泛的构造形变，是研究高原隆升、向外扩展及其与周缘块体相互作用的关键区域。本项目利用“中国地震科学台阵探测”项目二期观测数据，使用背景噪音互相关方法提取了Rayleigh面波和Love面波经验格林函数，并利用非线性反演方法获得了研究区域高精度地壳上地幔S波速度结构及径向各向异性和方位角各向异性结果。研究结果显示，松潘-甘孜块体和祁连造山带下方均存在低速异常，但两处的低速异常范围大小及地壳径向各向异性、方位角各向异性值均存在明显差别。松潘-甘孜块体中下地壳低速异常范围较大、速度值较低且地壳内存在正径向各向异性、方位角各向异性快轴方向为NW-SE且幅值较强，表明该区域下方地壳形变以地壳流模式为主；祁连造山带中下地壳低速异常范围较小、速度值略高且地壳内存在负径向各向异性、方位角各向异性较弱，表明该区域下方地壳形变以水平缩短和垂直隆升的纯剪切模式为主。本项目认为两块体地壳径向各向异性和方位角各向异性的差别可能代表两地处于不同的高原抬升阶段。高原隆升早期，地壳形变主要以纯剪切增厚模式为主，如祁连山地区；高原隆升晚期，地壳形变以中下地壳流模式为主，如松潘-甘孜地区。地壳径向各向异性幅值在祁连山块体东北缘存在明显的正负分界，推测该分界可能为现今青藏高原地壳在东北缘的扩张边界。此外，本项目利用横波分裂的方法获得了鄂尔多斯块体北缘及其周边地壳上地幔各向异性特征。结果表明鄂尔多斯块体东北角克拉通岩石圈地幔已经被热的软流圈物质所取代，产生了一个克拉通岩石圈的凹槽。软流圈地幔流会因此岩石圈凹槽的存在而在局部改变流动方向，并进一步侵蚀克拉通岩石圈。