Epicontinental mega thrust and nappe system at north segment of the western rim of the Ordos Block
Zhang Jiasheng1, He Zixin2, Fei Anqi3, Li Tianbin4, Huang Xiongnan1
1. Institute of Geology, China Earthquake Administration, Beijing 100029; 2. Changqing Oilfield Branch Company, China National Petroleum Corp., Xi’an 710021; 3. Exploration and Production Branch Company, China National Petroleum Corp., Beijing 100011; 4. Geological Survey of Ningxia Hui Autonomous Region, Yinchuan 750021
A mega thrust-nappe system which has formed since Late Mesozoic are preserved at north segment of the western rim of the Ordos Block, in which both crystalline basement rocks and Early Paleozoic epicontinental sediments were involved. Three tectonic domains, including nine sub-tectonic elements(B1-B7, BN and BS), formed in different stage of the thrust-nappe developing were subdivided according to their stratigraphic records, deformation properties of involved rocks, geometry of the thrusts, and relationship with alternation link thrust. The system showed an overall arc-shaped structure forward eastwards,locally sheared by east-west trending link thrusts, resulting from an eastwards promoting of the consistent thrust movement: a fold-thrust zone in the front, followed by a series of autochthonous or heterochthonous nappes and thrust sheets, in which low-angle detachment zones and duplexes were developed, and finally overlapped at the back of the system by latest thrust sheet. The mega thrust-nappe system was mainly developing during Jurassic-Cretaceous and underwent at least three major thrusting events. The first thrusting event was approximately occurred at the end of Jurassic, during which a principal detachment probably formed along the boundary between two different basement of the Alxa Terrain in the west and the North China Terrain in the east, that resulted in a fold-thrust zone of Zhuozishan-Gangdeershan forming. A large scale heterochthonous nappe of Shizuishan-Jianshan with upper and lower tectonic layers was formed during the second thrusting event. These tectonic layers separated by a low-angle detachment developed in Carboniferous system with typical duplexes. Since Cenozoic(ca.65 Ma), the mega thrust-nappe system went a step further along with collision between the Indian and China plates, as a result of a northwards pushing of the Qinghai-Tibet Plateau. A third thrusting event pushed the Ordovician strata, which was previously covered by heterochthonous nappe, up to surface again at western side of the system and, at the same time, the Suhaitu back-thrusts were created in the Zhuozishan-Gangdeershan fold-thrust zone. Distance of the thrust sheets and nappes movement might be up to 60 to 80 km during the first and second thrust events, and up to 8 km during the third event. Difference of thrusting movement vectors at vicinal sheets was adjusted by series nearly east-west trending link thrusts.