青藏高原东南缘上地幔顶部速度结构与各向异性特征及其构造启示

Since the late Cenozoic, structural deformation and seismic activity of the southeastern margin of Tibetan Plateau have become very strong. The southeastern margin of the Tibetan Plateau is characterized by strong seismicity and crustal deformation. It is an ideal region to investigate the lateral growth of the plateau and southeastward escape of the crustal material. The velocity structure and anisotropy of the uppermost mantle are important constraints on the thermal status and dynamic evolution of uppermost mantle. Currently, the widely-used seismic body wave and surface wave imaging methods remain problematic in revealing the reliable information for these properties in high resolution. Considering that P wave and S wave have different sensitivities to physical properties of rock, this proposal aims to improve the joint inversion method of Pn and Sn phases for the velocity structure and anisotropy of the uppermost mantle. By combining the data of China Earthquake Science Array, which includes 350 sets of broadband mobile seismic stations deployed in the southeastern margin of Tibetan Plateau, and the data of regional permanent stations, we will conduct joint inversion of the velocity structure, anisotropy and Vp/Vs velocity ratio of the uppermost mantle beneath the southeastern margin of Tibetan Plateau. The results are expected to offer reliable deep structure and dynamics information for understanding the mechanism of the southeastward expansion of the Tibetan Plateau.

青藏高原东南缘晚新生代以来地壳变形和地震活动十分强烈，是研究高原侧向生长及地壳物质向东南逃逸方式的理想地区。上地幔顶部速度及各向异性特征是研究上地幔顶部物质热状态和动力学演化的重要约束。当前广泛使用的地震体波和面波成像方法尚难以高分辨地揭示研究区上地幔顶部速度及各向异性特征。鉴于P波和S波对岩石物性有不同的敏感度，本项目拟发展联合Pn、Sn震相反演上地幔顶部速度结构和各向异性的方法。并利用中国地震科学台阵在青藏高原东南缘布设350套宽频带流动地震台站采集的地震数据，结合区域固定台站数据，联合反演青藏高原东南缘上地幔顶部速度结构、各向异性及Vp/Vs波速比，为探讨高原东南向扩展的变形机制提供可靠的深部结构和动力学信息。

围绕青藏高原东南缘壳幔结构和各向异性研究目标，利用东南缘固定台网和流动台站数据，通过Pn和Sn震相数据，开展了上地幔顶部Pn波和Sn波速度结构和各向异性反演，继而获得Vp/Vs波速比结果，结果表明研究区存在两条明显地低速异常带，暗示沿青藏高原东南缘地区主要的断裂带下方上地幔可能存在部分熔融或地幔物质的流动；利用哀牢山-红河断裂带短周期密集台阵数据，开展了接收函数和噪声成像研究，发现了剪切带下方的中上地壳存在倾斜界面，反映了新生代以来哀牢山不均匀隆升的特征，可能与上涌地幔物质在哀牢山下方不均匀底垫作用(东强西弱)、印度板块侧向碰撞引起的挤压作用及红河断裂走滑运动引起的差异性隆升等因素有关。红河断裂下方与两侧迥异的低速特征是红河断裂构造边界性质在上地壳尺度的体现，表明其在岩石和矿物组成上与两侧存在明显差异，哀牢山成矿带下方的高速异常可能为埋藏的花岗岩侵入岩体。项目研究结果为青藏高原东南缘构造-岩浆活动和动力学演化提供了深部结构约束。