摘要
冲断带是目前国内外研究的一个热点。近十几年来，国内外地质学家对中国中西部的冲断带做了大量的工作，并取得了许多成果。但这些工作主要集中在库车等少数几个地区，祁连山北缘冲断带的研究还刚刚起步。本论文在结合大量野外地质调查、地球物理、钻井等资料的基础上，运用最新的构造分析理论，对祁连山北缘冲断带的平面展布、剖面结构及冲断作用对河西走廊盆地群的改造和控制等方面进行了详细研究。
通过野外地质调查和地震资料的解释，将祁连山北缘冲断带在平面上划分为原地冲断系统、近距离冲断系统和远距离冲断系统，其中原地冲断系统又可划分为原地隐伏冲断系统和原地显露冲断系统。冲断带剖面结构自西向东划分为三段：酒泉盆地南缘、榆木山南缘—民乐盆地南缘西段、民乐盆地南缘东段—武威盆地南缘。酒泉盆地南缘的冲断带已被证实是个水平位移量较大的、沿着至上三个滑脱面由南而北产生收缩变形的薄皮冲断系统，冲断体系向NE方向的冲断推覆的位移距离超过50Km。榆木山南缘—民乐盆地南缘西段，冲断带自上而下只发育了两层，属于冲断带前锋的原地显露冲断系统其性质可能属于山前基底卷入式构造。民乐盆地南缘东段—武威盆地南缘发育宽度较大的原地显露冲断系统，由三叠系、石炭—二叠系、奥陶系、寒武系和岩体组成的冲断片相互叠置，形成冲断带宽广的“显露式”前锋。
通过对生长地层的识别，并结合磁性地层学资料，认为冲断带的变形时间开始于约9Ma(中新世胳塘沟组末)，冲断带表现为“前展式”变形。
建立了7条横穿祁连山北缘冲断带的构造剖面，并进行了合理的平衡恢复。恢复以后了解到，祁连山北缘冲断带由于中新世以来的冲断作用南北向空间上缩短了40～60km，构造缩短率也在40～60％之间，自西向东缩短率呈现减小趋势。
在综合地球物理、地层和地质资料的基础上，重点剖析了酒泉盆地南缘冲断带的结构。指出了冲断活动导致了酒泉盆地南缘冲断带三个不同层次的冲断变形体的产生。浅层为露头区古生界(包括志留系和奥陶系)组成的外来推覆体；中层由古生界和中生界构成，是一套完整的冲断片，目前识别出的构造窗基本上属于这一套冲断层序。深层冲断为一套双重冲断构造，这是冲断体系中规模最大，向北波及最远，对于冲断系统起着主要控制作用的冲断层序。因此，酒泉盆地南缘冲断体系是一个多层次的复杂的冲断系统。
研究了祁连山北缘冲断带对河西走廊盆地群的改造和控制作用。认为中新世以来的冲断使早白垩世相互连通的昌马盆地、酒泉盆地、民乐盆地和武威盆地最终分割开来，同时酒泉盆地内部的石大凹陷和花海凹陷、营尔凹陷和金塔凹陷也由于上新世以来的冲断而最终隔裂开来的。强烈的冲断推覆还使冲断带之下掩盖了大量的中新生界，其中酒泉盆地南缘掩盖了15km左右的中生界，民乐和武威盆地南缘冲断带之下不存在中生界。中新世以来的强烈冲断，控制了其北侧前陆盆地的形成。受冲断规模、冲断带结构的影响，河西走廊自西往东前陆盆地越来越不发育，沉积厚度减薄，变形减弱。
Thrust belts' research is a hot aspect for geologists at present. Lots of effort has been carried out in the central and western China and some achievements have been gotten for the late decades. This effort concentrated in some areas as Kuqa. However, few work concentrated in the thrust belt in north margin of Qilian mountains. The author studied the planar distributing, section structures and the reconstructing and controlling of thrusting to Hexi Corridor basins group, based on the compound of field surveys, geophysical and drilling data.
Based on the field surveys and geological interpretations of seismic data, thrust belt in north margin of Qilian mountains had been divided into three types as insitu, short- distance and long-distance thrusting systems. Insitu thrusting systems can be divided into two types as blind and exposed. Thrusting belt can be divided into three segments from west to east in the sections as south margin of Jiuquan Basin, south margin of Yumushan-west segment of south margin of Minle Basin, east segment of south margin of Minle Basin-south margin of Wuwei Basin. Thrust belt of south margin of Jiuquan Basin has been confirmed to be a thin-skinned thrusting system with large horizontal displacement and contracted deformation along three detachment levels at least from south to north. Displacement of the thrusting to NE reached more than 50km. In the south margin of Yumushan-west segment of south margin of Minle Basin, thrust belt just developed two levels from upper to lower, which belonged to exposed insitu thrusting system of thrusting front with properties of thick-skinned structures ahead the mountains. In the east segment of south margin of Minle Basin-south margin of Wuwei Basin, wide exposed insitu thrusting system developed, composed by thrusting sections of Triassic, Carboniferous- Permian, Ordovician, Cambrian and rock mass superposing each other. These formed exposed front of thrust belt.
Based on the identification of growth strata, combined with the magnetostratigraphic data, the author deemed that the deformation of the thrust belts began at about 9Ma (Getanggou Formation, Miocene) and appeared to be break-forward.
Seven sections has been built and balanced crossing the thrust belt of north margin of Qilian mountains. Through the balanced sections, the shortening reached to 40-60km and fraction shorting to 40-60% in S-N direction. Fraction shorting decreased from west to east.
Compounded with the geophysical, stratigraphical and geological data, the author stressed on the analysis of the thrust belt in south margin of Jiuquan Basin. It was risen that the thrusting caused three levels of thrust deformable bodies in the south margin of Jiuquan Basin. The upper were the exposed Paleozoic allochthonous nappes (including Silurian and Ordovician). The middle were the intact Paleozoic and Mesozoic thrust fragments. Recognised tectonic windows mostly belonged to the thrust level so far. The lower were duplexes in deep level. These structures controlled the thrust systems, with the largest scale and most displacement to the north. It could be summarized that thrust systems in south margin of Jiuquan Basin were complicated and with several thrust levels.
Controlling of thrust belt of north margin of Qilian mountains to Hexi Corridor basins group had also be studied here. It was deemed that Changma Basin, Jiuquan Basin, Minle Basin and Wuwei Basin divided each other caused by the thrusting from Miocene. These basins were connected in early Cretaceous. Shida Depression and Huahai Depression, Yinger Depression and Jinta Depression in Jiuquan Basin divided each other for the thrusting from Pliocene. Because of the thrusting, most of Mesozoic and Cenozoic strata were covered beneath the thrust belts, as about 15km Mesozoic were covered beneath south margin of Jiuquan Basin. But no Mesozoic existed beneath the south margin of Minle Basin and Wuwei Basin. Intense thrusting from Miocene controlled the formation of north foreland basins. Influenced by the scale and structures of thrust belts, from west to east, foreland basins did not develop gradually, sediments decreased and deformation weakened.
引文
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