喜马拉雅造山带东西构造结俯冲板块顶部低速薄层结构的对比研究

Himalayan orogenic belt (HOB) generated in the collision of Indian plate and Eurasian plate is a natural experimental field for studying the collision of subducting continents. High-resolution earthquake location results show that the distributions and patterns of intermediate depth earthquakes beneath the subducting continental slabs and adjacent area beneath the eastern and western syntaxes of the Himalaya orogenic belt (HOB). And the seismic tomography for the western syntaxis shows a thin low-velocity layer atop the subducting continental slab which is generally observed atop the subducting oceanic ones. The thin low-velocity layers forms in the process of dehydration which affects greatly on the earthquakes and magma activities in subducting slabs. So, structural comparison of the thin low-velocity layers atop the subducting continental slabs beneath the HOB syntaxes is very important for understanding the continental-continental collision. In the project, observed seismograms recorded by the Chinese Digital Seismograph Network, seismic networks of adjacent countries and portable seismic arrays, especially those data with high frequency guided waves are analyzed through matching the observed and theoretical travel-times and seismograms to constrain the velocity structures of the thin low-velocity layers atop the subducting continental slabs beneath the HOB syntaxes. Combining with the focal mechanisms and high-resolution earthquake location results, the distribution difference of intermediate deep earthquakes will be analyzed, and combining with tectonic, petrological and geochemical results, the relation between the thin low-velocity layers atop the subducting continental slabs beneath HOB syntaxes, and provide fundamental parameters for studying the regional evolution processes, such as the uplift of Tibetan plateau and the formation of HOB.

印度板块与欧亚板块碰撞形成的喜马拉雅造山带是研究陆陆碰撞的天然试验场。地震精定位结果显示喜马拉雅造山带东西构造结处俯冲大陆板块中-深源地震分布形态差异较大，且西构造结地震层析成像结果还发现了通常仅见于俯冲海洋板块顶部的低速薄层。低速薄层相关的脱水及部分熔融对俯冲板块地震和岩浆活动影响很大，因而东西构造结俯冲陆块顶部低速薄层结构的对比研究对于陆陆碰撞过程的理解至关重要。本项目拟以中国数字测震台网和周边国家地震台网的固定台站及国内外流动台阵波形数据，特别是与含低速薄层的结构相关的高频导波资料，利用观测与理论的走时和波形拟合相结合的方法，确定两个构造结处低速薄层形态和速度异常值；结合定位和震源机制结果，分析两处中-深源地震分布差异性原因；结合构造地质学、岩石学及地球化学成果，分析俯冲大陆板块顶部低速薄层结构差异与板块阶段的关系，为青藏高原隆升和喜马拉雅造山带形成等区域构造演化研究提供基础参数。

俯冲板块进入地球深部，在其表面形成与脱水和部分融入相关的低速薄层。考虑到喜马拉雅造山带区域稀疏台站分布，本项目开发多(深度)事件-多震相的差异化传播路径协同成像方法，同时利用数据相对丰富的阿拉斯加数据进行了验证，发现太平洋俯冲板片顶部存在~2%的速度降；受塔里木和准噶尔盆地双向俯冲的影响，天山中部岩石圈内存在6%的低速异常，同时天山造山带一些零散区域κ值要低于1.73，应与软流圈热物质进入岩石圈并对其进行加热所产生的增生楔部分熔融体有关；受兴都库什-帕米尔双向俯冲差异性影响，其下方410-km间断面存在着不同的抬升量，分别为~30 km和~15 km；喜马拉雅东构造节的缅甸弧地区俯冲板块导致410-km间断面抬升~10 km，同时还观测到300 km深度处界面，其与陆壳深俯冲形成的超高压变质岩中的柯石英到斯石英的相关有关。从目前关于喜马拉雅造山带东西构造结的结果来看，大陆板块俯冲带顶部低速薄层较之海洋板块的低速薄层要弱，相关的研究结果对于喜马拉雅造山带形成与演化的研究提供必要的数据支撑。