华北克拉通及东邻西太平洋活动大陆边缘地区的P波速度结构:对岩石圈减薄动力学过程的探讨  

P wave velocity structure of the North China Craton and West Pacific active continental margin:exploration for dynamic processes of lithospheric thinning

作　　者：郭慧丽[1,2] 徐佩芬[1] 张福勤[1] 

机构地区：[1]中国科学院地质与地球物理研究所,北京100029 [2]中国科学院大学,北京100049

出　　处：《地球物理学报》2014年第7期2352-2361,共10页Chinese Journal of Geophysics

基　　金：国家自然科学基金面上项目(40874025)资助

摘　　要：本文通过地震层析成像研究获得了华北克拉通及其东邻地区（30°N～50°N，95°E～145°E）10×1°的P波速度扰动图像．结果显示，在西太平洋俯冲带地区，上地幔中西倾的板片状高速异常体与其上方的低速异常区构成俯冲带与上覆地幔楔的典型速度结构式样．俯冲板片高速体在约300～400km深度范围内被低速物质充填，暗示俯冲板片可能发生了断离．在华北克拉通地区的上地幔中发现三个东倾排列的高速异常带．在此基础上，本文构建了华北克拉通及其东邻西太平洋活动大陆边缘地区的上地幔速度结构模式图，并据此探讨克拉通岩石圈减薄与西太平洋活动大陆边缘的深部动力学联系．本文认为，太平洋板片的俯冲（断离），触发热地幔物质上涌并在上覆地幔楔中形成对流，使克拉通岩石圈受到改造（底侵与弱化）．随着俯冲板片后撤，地幔楔中的对流场以及对岩石圈改造的影响范围均随之东移，最终导致华北克拉通岩石圈自下而上、从西向东分三个阶段依次拆沉减薄．这一模式能很好地解释现今克拉通岩石圈自西向东呈台阶状减薄的深部现象．We determined the three-dimensional （3D） P wave velocity structure beneath the North China Craton and its east region （30°N～50°N,95°E～145°E） using 539815 arrival times from 21504 local and regional earthquakes, and 318976 data from 13062 teleseismic events, which were recorded by 2626 seismic stations in our study region. The seismic images show some new features. Low and high velocity anomalies are imaged alternately in the upper mantle, denoting a typical structure pattern of the subduction zone and the upper mantle wedge above the Western Pacific subduction zone. At depth of 300 ~ 400 km the subducting slabs with high velocityanomalies were separated by low velocity anomalies, implying that break-off of the subducting Pacific slab might have happened in the mantle. Multilayer east dipping high velocity anomalies are imaged at the depth range of upper mantle and mantle transition zone beneath the North China Craton. Based on the seismic features above, a model of seismic structure beneath the study area is given in this paper and the deep dynamics between the North China Craton and the West Pacific active continental margin is discussed. We suggest that the Pacific subduction （or/and breakoff） triggered upwelling of hot mantle material and formation of convection in the mantle wedge, which caused cratonic lithosphere modification （underplating and weakening）. As the subduction retreated, convection in the mantle wedge and the influence range of the modification for lithosphere moved eastward, resulting in lithospherie thinning of the North China Craton by three stages of delamination. This helps explain why the present lithosphere thickness of the North China Craton has step changes from west to east.

关 键 词：地震层析成像 华北克拉通 西太平洋活动大陆边缘 太平洋板块俯冲 岩石圈拆沉减薄 

分 类 号：P315[天文地球—地震学] P541[天文地球—固体地球物理学]