华北克拉通下方多期次俯冲-拆沉的滞留体精细地震学研究

The North China Craton (NCC) with steady western part and broken eastern part is resulted from the multi-stage slab-subductions and lithosphere detachments. In the process, the stagnants of the subducted slabs and detached lithosphere produced by the thermo-chemical effects exist at different depths of the Earth's interior.Study on the detailed seismic structures of the stagnants of subducted slabs and detached lithosphere is important for understanding the formation and evolution of NCC, the mechanism of continental subduction and the activation of NCC. Based on the former results from seismic tomography and receiver function-migration, the travel-times vs distances of seismic waveform data from China Digital Seismic Network, Japanese Hinet and some transportable seismic arrays with good cross-ray traces in the crust-upper mantle beneath NCC are used to study the velocity anomalies with 3D ray-tracing method.Combined with the results from the structural geology, geochemistry, etc, the distributions and properties of subducted and detached stagnants beneath NCC are used to study the relations between the stagnants in different depths with the structural units at the Earth's surface, to analyze the formation of NCC and the processes of multi-stages of subductions and detachments and to give constrains for understanding the NCC destruction.

华北克拉通现今稳定的西部块体和被破坏的东部块体的格局源于多期次俯冲-拆沉过程。在此过程中，俯冲板块-拆沉岩石圈经过热-化学作用后成为滞留体存在于地球内部不同深度处。俯冲-拆沉滞留体的精细结构及性质的研究对于华北克拉通形成与演化过程及大陆深俯冲机理，乃至对于华北克拉通活化的认识均具有重要的意义。本项目选用在华北克拉通下方地壳-上地幔内有很好交叉射线路径的国家测震台网和日本高敏感度地震台网(Hinet)及部分中国流动观测台阵地震波形资料，基于三维射线追踪，分析特定震相时-距曲线的局部异常所对应速度异常结构，给出俯冲-拆沉滞留体在华北克拉通下方的不同深度分布及性质，并结合构造地质学和地球化学等方面的研究成果，给出存在于不同深度的滞留体与地表特定构造单元的对应关系，为深入理解华北克拉通破坏提供重要的参考。

华北克拉通相关多次俯冲拆沉残留体在地球深部的存在及异常程度可以解释华北克拉通形成的构造过程。本项目基于三重震相的走时和多频带波形拟合，探测了俯冲拆沉体在地幔中的存在，对地下结构体的速度-厚度权衡问题提供了有效约束。老的俯冲板块在上地幔中的残留相对比较弱，在既有观测数据范围内识别有相当的困难，在数据中有效信息提取需要特别仔细。通过本项目的工作，我们发现研究区域下方俯冲的海洋板块进入地球深部，在地幔转换带中存在着滞留体，同时俯冲板块进入地幔转换带中也导致了地幔转换区顶部低速层的出现。目前分析的结果看，岩石圈范围内一定深度范围内低速结构体，而地幔转换区存在着老的俯冲拆沉体，且地幔转换区顶部的低速层分布与俯冲拆沉体形成过程中的脱水有相当的关联。