华北地区东部岩石圈导电性结构研究——减薄的华北岩石圈特点

华北古大陆克拉通解体、岩石圈减薄的深部过程,对于建立中国大陆中新生代演化动力学模型是亟待深入研究的重要科学问题,因而"华北克拉通破坏"也就成了当前学术界的热门话题。为了研究"华北克拉通破坏"首先需要给出较准确的华北岩石圈结构,这必须依靠包括超宽频带高精度大地电磁深探测在内的现代深部地球物理探测技术。2001和2005年在华北地区东部布置了应县—商河(HB-MT01)、文水—日照(HB-MT02)大地电磁测深剖面进行研究。研究结果表明,在华北地区东部沿地壳-上地幔探测剖面可划分为4个电性区,区内发现有下地壳高导带和上地幔高导层存在。文中依据研究区壳-幔电性结构特征,推断华北地区东部地壳和上地幔之间发生过大规模构造运动,导致壳-幔之间存在解耦现象。研究结果还发现,华北东部确实存在岩石圈减薄区,其岩石圈厚度约50~80km厚。岩石圈明显减薄的区域包括北太行山隆起、华北裂谷带北部和鲁西断隆,其范围比原先认识到的要复杂,并非以太行山重力梯度带为界划分东、西两区,简单地认为东区即是减薄区。此外,在华北地区东部的现代高精度大地电磁探测结果也进一步证明了地球物理观测对于大陆动力学研究的重要性,这使人们更加认识到在今后的研究中必须强调地球物理-地质-地球化学之间的有机结合。

Geoelectric structure of lithosphere beneath eastern North China: features of a thinned lithosphere from magnetotelluric soundings

It is widely accepted that disassembly of the North China ancient craton and the thinning process of lithosphere at depth are essential for the dynamic evolution model of the China mainland since Mesozoic and Cenozoic times. To address these questions, the deep structure of the lithosphere beneath North China should be well delineated, which relies on a new generation of geophysical technology including ultra-broadband high-precision magnetotelluric (MT) sounding. In 2001 and 2005, MT survey was carried out along the Yingxian-Shanghe (HB-MT01) and Wenshui-Rizhao (HB-MT02) profiles in eastern North China, respectively. The result shows that the crust and upper mantle along the profiles can be divided into four electric sections, and high-conductivity zones and high-conductivity layers exist in the lower crust and upper mantle, respectively. It is inferred that decoupling between crust and mantle is present beneath eastern North China which is associated with large-scale tectonic movements. The thickness of the lithosphere in eastern North China is estimated to be 50-80 km, which has been apparently thinned with respect to usual continental cratonic regions. It involves the northern Taihangshan uplift, northern North China rifted basin and Luxi (western Shandong) fault-bounded uplift, which is different from the previous claim that the lithospheric thinning is confined to east of the Taihangshan gravity gradient zone. This work also demonstrates the importance of the modern high-precision MT sounding and combination of geophysics, geology and geochemistry in research of continental dynamics.