华北地区地幔过渡带三维电性结构及动力学状态研究

To obtain the 3-D electrical conductivity distribution in the mantle transition.zone is a leading edge research in the electromagnetic (EM) studies of the world, but we seldom see this kind of researches in China. In this study, we report the 3-D electrical conductivity distribution beneath North China in the depth range 400km-1200km, which will be imaged by inverting C-response estimates from about 30 geomagnetic observatories. First, the data will be intensively studied to obtain high quality responses; Secondly, the long period MT measurements at representative location in east and west of North China will be obtained to study the relationship between the upper and lower mantle and it also indicates the electricity structure background (<400km). As starting model in the inversion we consider a laboratory-based 3-D conductivity model of the region obtained from seismic surface wave and thermodynamic results; and then, the ocean effect will be quantitatively studied, and the NOAA ETOPO topography/bathymetry and map of sediment thicknesses will be used in the inversion. We modify the global scale 3-D EM induction codes to a regional-scale 3-D EM inversion in order to delineate more precisely the mantle transition zone conductivity structure. The inversion scheme is based on a quasi-Newton optimization method while the forward algorithm relies on a contracting integral-equation approach. The 3-D electrical conductivity distribution in the mantle transition zone beneath North China access new knowledge to the evidence for lateral variations in transition-zone temperature, water content and melt, and this will provide an adequate explanation in the mantle dynamics of North China.

通过电磁学方法研究地幔过渡带三维电性结构是地球电磁学研究的前沿课题之一，国内很少见到相关方面的研究。本研究拟通过华北地区的三十个台站的地磁响应函数C来研究华北地幔过渡带附近的三维电性结构。首先，通过多年来台站观测资料的精细处理获取高质量的响应函数；其次，选取华北地区东、西两侧代表性测点进行长周期MT观测，讨论深、浅地幔之间的联系，得到浅层（<400km）初始模型背景值，结合华北地区地震学层析成像和岩石实验的结果构建可信的初始三维模型；再次，通过数值模拟，量化和校正海洋效应对观测资料的影响，考虑引入海洋地形的相关信息进行反演；正演采用积分方程法，反演采用拟牛顿法，实现球坐标系下区域性电性结构的三维反演，并对结果的可靠性进行评价。最终，获取的华北地区过渡带三维电性结构，可以有效地限定地幔间断面的深度、形态和空间分布，得到华北地区精细结构的新认识，为推测华北地区地幔物质和动力学状态提供客观依据。

地磁测深研究可以提供地下几百千米直到核幔边界的电导率分布信息，是地球构造、演化以及动力学研究中一种非常重要的地球物理手段，是国内外近年来研究的热点。首先，本研究有效利用了华北地区多年来长期记录的珍贵地磁资料，应用不同的降噪技术和回归分析，获取了高信噪比的、更合理的地磁C响应数据；其次，采用有限元软件COMOSL和球坐标系下交错网格有限差分方法，量化研究了海洋效应对地磁观测响应的影响，利用中国地区台站平均一维电导率模型进行比值法海洋效应校正，改善了华北地区东部台站周期20天以内的响应数据；结合华北地区地震学层析成像和岩石实验的结果构建了可信的初始三维模型，三维正演模拟采用球坐标系下的交错网格有限差分法，反演采用数据空间Occam．利用检测板模型对反演方法的可靠性进行了验证，三维反演的结果显示出在地幔过渡带附近华北地区东部表现为明显的低阻异常，而在岩石圈底部存在一定的高阻体，这可能是由于岩石圈地幔受到部分熔融导致其中水含量随熔体提取而降低的缘故。华北地区东部低阻异常体的存在从侧面印证了太平洋板块的俯冲是华北地区岩石圈演化的首要动力学机制。太平洋板块的俯冲携带了大量的水到俯冲带上盘的地幔楔，而这一部分水可能是导致华北岩石圈活化、克拉通破坏的关键因素。