祁连山西段及邻区电性结构及构造意义

The region north to the Eastern Kunlun Fault is now still growing and well suited to study the active crustal shortening and thickening of the Tibetan Plateau. The northern boundary of the Tibetan Plateau, the structures and the status of the deep materials of the area are not well imaged because of inadequate data, which causes inconsistent and ambiguous descriptions of the area in the geodynamic models. The focus of the proposed project is to use wideband magnetotelluric (MT) method to detect the crust-upper mantle structures of the Western Qilian Mountains and its neighboring areas. The project, which is based on the former MT work in the studying area since 2009, is planned to complement new MT survey across different places in the Western Qilian Mountains and extending into the Beishan Block at the north, across the Qaidam Basin at the south, so to form the MT network covering the eastern section of the Altyn Tagh Fault, the Northern Qaidam Basin, the Western Qilian Mountains, the Western Hexi Corridor, the Southern Alashan Block, and the Southern Beishan Block. The average site separation of the MT sites along each profile will be less than 10 km; the general bandwidth of the MT data range from 0.001 s-5000 s; some MT sites will be planned to record data with maximum period exceeding 50000 s. All the MT data will be analyzed with impedance tensor decomposition techniques and interpreted with several forward (2D and 3D) and inversion methods (NLCG, Occam, RebOcc, ABIC, RRI, WSINV3DMT) in order to obtain reliable electrical resistivity structures of these areas. The following tasks of the project will then be fulfilled: 1) the structure variation of the Altyn Tagh Fault, the Northern Qaidam Fault, and the Northern Qilian Fault along their lateral extension; 2) the structures of the Western Qilian Mountains and its neighboring areas; 3) whether the large-area weak (middle or lower) crustal layer exists from the middle section of the Eastern Kunlun Mountains to the Hexi Corridor or not? The special goals of the project are: 1) analyzing the status of the deep materials in the crust-upper mantle of the areas; 2) discussing the range of the weak crustal layer in the areas and analyzing the possible mode of material transferring; 3) analyzing the coupling mode between orogens and basins (for instances, the Hexi corridor and its neighboring orogens; the Western Qilian Mountains and its neighboring basins); 4) combining the electrical resistivity structures with the present surficial deformation, discussing the northern boundary of the Tibetan Plateau under Neotectonic environment. The implementation of the project will further accumulate geophysical data and provide new evidences for studying the evolution and growth of the Tibetan Plateau.

有关青藏高原北部边界、东昆仑以北区域的结构以及深部物质状态是认识青藏高原继续向北生长的关键。本研究在已有大地电磁剖面基础上，以祁连山西段为中心，进一步开展以密集点距宽频带大地电磁测深（平均点距<10km，频率范围在0.001－5000s,部分点最大测深周期超过50000秒）的深部地球物理探测研究，形成覆盖祁连山西段并向周边块体深入的大地电磁测深网络。采用多种阻抗张量分析和正、反演手段相联合，得到关于测区的可靠电性结构。对阿尔金走滑断裂带的横向延伸与结构变化、祁连山的整体结构样式、东昆仑中段以北区域地壳软弱层的存在性等具体问题展开研究。结合已有地质、地球物理资料，讨论青藏高原在新构造背景下的北部边界，分析青藏高原北部盆山系统与两侧地块间的接触关系及可能的耦合方式，探讨东昆仑以北地壳软弱层的规模,分析可能的物质迁移方式，提供关于青藏高原向北生长的深部证据。

本研究在祁连山西段及邻近区域采用大地电磁测深方法开展了密集点距、多剖面的结构探测，为了提高低频视电阻率和相位数据质量，我们在数据处理时引入了不同的方法，并开发相应程序进行数据联合。在此基础上利用二维、三维反演技术对电性结构进行确定。获得了如下几方面成果：.1）相位超象限数据的解释：针对在这一地区获得的相位超象限大地电磁测深数据，首先从理论上进行了分析，指出在三维各向同性介质中，与高导结构有关的电流槽效应可以从电流累积效应以及电流反转两方面引起相位超象限的发生。在此基础上，对河西走廊北缘出现的相位超象限实测数据进行三维反演，模型结构与理论模型中的“L”结构具有很好的对比性，同时，我们认为在“L”高导结构的局部异常体附近的高导体也会对相位超象限的分布范围产生影响。.2）阿尔金断裂带东端部结构研究：阿尔金断裂带切过祁连山西端后如何延伸是一个争议问题，在阿尔金断裂带东端部位布设多条平行分布密集点距的大地电磁测深剖面，通过2D和3D反演模型对比，我们提出阿尔金断裂带并没有越过河西走廊北侧的花海－金塔盆地向北延伸，而是沿河西走廊北缘向东，并终止于以逆冲为主的分枝断裂。.3）关于祁连山西段的结构样式：获取了跨过祁连山西段的密集点距大地电磁测深剖面，电性结构模型提供了祁连山西段三分的证据。同时二维电阻率模型揭露在剖面南侧存在来自南部的具高阻特征的地幔楔，它的北向推挤造就了柴达木盆地东缘以及祁连山地壳层次逆冲推覆构造的发育。也即，祁连山西段的结构受控于壳内的大型逆冲推覆构造及其次生逆断层，在中下地壳层次形成相对低阻带，在地表则发育与逆断层有关的条带状山系。.4）对青藏高原北部动力学的认识：从松潘－甘孜地块至祁连山腹地的大地电磁测深剖面显示，东昆仑造山带之下不存在显著的高导异常，也就是存在于松潘－甘孜地块之下的高导下地壳并没有连续延伸到造山带之下；柴达木盆地上地壳新生代沉积层表现为显著的低阻特性，它的厚度呈现出向北加深的趋势。在中－南祁连山之下发现有向南不断加深的低阻异常体，这一异常体支持在中、下地壳存在滑脱构造，它控制了祁连山西段发育的叠瓦状逆冲推覆构造。综合分析认为在印度－欧亚板块持续碰撞背景下，统一的东昆仑－柴达木岩石圈地幔被南侧地块推挤而向NE方向扩展，与沿河西走廊北侧向南俯冲的亚洲大陆岩石圈地幔一起控制了青藏高原北部的地壳变形。