Structural Character Study Of Thrust Belt In Northern Margin Of Qilian Mountains

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.