面波层析成像研究喜马拉雅造山带西构造结壳幔S波速度与各向异性结构

The western Himalayan Syntaxis, as a part of the Himalaya-Tibet orogenic belt, where is characterized with variable topography, complex tectonic history, and intense seismic activity, has been the prime site to understand the processes and geodynamic of continental collision and mountain building. Previous geological and geophysical studies indicate that the western Himalayan Syntaxis has experienced intense intra-continental deformation and lithosphere shortening and thickening. Intra-continental subduction has been considered as a main mechanism responsible for lithosphere shortening and thickening in this region. However, there are still topics of debate regarding the subduction, for example, has the lower crust subducted into the lithospheric mantle? Is the subduction slab of the Himalayan orogenic belt either a single plate subduction, or two break-off opposing plates? In this project, based on the waveform data recorded by TIPAGE (deployed by GFZ Potsdam) and other permanent seismic networks in this region we propose to carry out two-plane-wave surface wave tomography (fundamental mode Rayleigh wave and Love wave) to obtain three-dimensional velocity and anisotropy (azimuthal and radial anisotropy) models of the crust and upper mantle in the western Himalayan Syntaxis, in an attempt to understand lithosphere structure and deformation in this region. With the completion of this project, we expect to answer above scientific questions and provide important constraints on the dynamic mountain building process of the Himalaya-Tibet and the mechanisms responsible for the intermediate earthquakes frequently occurred in the western Himalayan Syntaxis.

喜马拉雅造山带西构造结是青藏高原地形变化最剧烈、构造最复杂、地震活动剧烈的地区，是研究大陆碰撞及其动力学的天然实验室。前人研究表明喜马拉雅造山带西构造结的岩石圈正经历强烈的大陆内部变形和南北向缩短增厚。而大陆俯冲被认为是造成该区域岩石圈变形与缩短增厚的主要机制。然而，对该区域的大陆俯冲模式仍存不同看法。如，下地壳物质是否发生了俯冲？是单一板片的单向俯冲，还是两个板片的双向俯冲？本项目拟利用德国GFZ Potsdam的TIPAGE地震台网和其他地震台网在喜马拉雅造山带西构造结及邻区观测到的地震波形资料，通过双平面波层析成像研究，获取基阶Rayleigh波和Love波相速度分布，开展三维S波速度和各向异性（方位角和径向各向异性）结构反演，据此确定不同构造块体下方地壳和上地幔深部结构与变形特征，为探索以上关键科学问题进而理解喜马拉雅造山带的动力演化过程及西构造结中源地震的发生机制提供重要的约束。

本项目的科学目标是构建喜玛拉雅-青藏高原西构造结（帕米尔-兴都库什地区）的地壳与上地幔三维剪切波速度与各向异性模型，进而探索该区域大陆岩石圈俯冲模式、俯冲板片深部几何形态、岩石圈变形特征与中深源地震的成因。经过3年的研究，结合面波层析成像研究、多种地球物理数据与地表地质等信息，获得以下主要成果：（1）在南帕米尔的下地壳与上地幔观测到一个近水平的高速度异常，表明印度板块水平俯冲于帕米尔高原之下；而在兴都库什山脉下方，俯冲的印度板片正在发生断离，并诱发了该地区的中深源地震。（2）在帕米尔北部的上地幔顶部观测到一个低速异常区，这反映了亚洲板块的下地壳物质与岩石圈地幔耦合以倾斜角度俯冲于帕米尔高原东南部下方，其俯冲的南边界达南帕米尔中部。在向下俯冲过程中可能由于脱水脆化或者热散逸引发了中深源地震。（3）地震学证据揭露了在帕米尔高原下方印度板块与亚洲板块相向俯冲导致水平地壳流与地幔流，解释了该区域的岩石圈变形。（4）融合地震面波层析成像与背景噪音层析成像，还构建了青藏高原西缘、中部与东部的速度与各向异性结构，进一步厘清了俯冲的印度岩石圈地幔俯冲角度与模式沿造山带走向变化特征。由西向东，在兴都-库什地区，印度板块以高角度俯冲，并可能正在发生断离；在帕米尔与青藏高原西缘，印度板块以近水平角度俯冲于高原之下；在青藏高原的中部（东经89度至96度），印度岩石圈地幔是以高角度俯冲至班公-怒江缝合线附近；而在高原的东部（东经96度至100度），印度岩石圈地幔以近水平角度俯冲至金沙江缝合线北沿。（5）多种地球物理数据揭露了青藏高原内部普遍存在着软弱的中下地壳，地壳流在青藏高原内部发展，向青藏高原东南缘逃逸，在边缘变窄沿断裂带分布，地壳流会解耦上地壳与下地幔的变形。上述成果进一步的理解了帕米尔-兴都库什地区与青藏高原内部的深部结构与陆-陆碰撞动力学过程等科学问题。本项目已发表6篇国际学术论文，已培养研究生1名，另有4名在读研究生。