Research On The Characteristics Of Structure And Tectonic Evolution Of Southern Section Of Western Margin Of Ordos Basin

The Southern Section of Western Ordos Basin which Limit in east of Qingtongxia-Guyuan fracture,west of Baiyanjing fracture,south of Lingwu fracture and north of Pingliang line is sandwiching between different tectonic units: the north China craton,alashan block,qinling-qilian orogenic belt.The particularity of geotectonic position makes it in different tectonic systems in different geological historical periods,and it is affected by different dynamic systems.The tectonic characteristics and evolution are very complex and representative.Although it has been concerned by researchers for a long time,there are still many controversies in the understanding of its basic tectonic pattern and tectonic pattern in critical tectonic period due to the limitation of data and cognitive level.At the same time,the west margin of ordos basin oil and gas exploration as an important strategic development zone has long received the attention of the oil and gas workers,also conducted lots of exploration work,but because of its basic structure characteristics and formation distribution and understanding is not clear,lead to for a long time without a breakthrough.Therefore,it is of great significance in theory and practice to further study the tectonic characteristics and evolution of the southern section of the western margin of ordos basin.This paper is based on field outcrop data,drilling data,gravity,magnetic,electrical data and seismic data,guided by the theory of plate tectonics and tectonic geology,through the processing and interpretation of aeromagnetism,gravity and CEMP data,field outcrop data analysis,drilling data calibration,complete a comprehensive interpretation of two-dimensional seismic data.Based on the detailed interpretation of various data,the basic work of the structural sequence and its structural relationship of the southern part of the western margin is completed.The main fault characteristics and their influences are analyzed.The inner sub-zones of the southern part of the western margin are redivided.On this basis,the structural characteristics are analyzed and the tectonic patterns are established,and the evolution of the southern part of the western margin is completed through the description of the palaeotectonic patterns in each key tectonic period.The research mainly achieved the following results and understanding:(1)There are three different types of unconformity have been defined in the southern section of western Ordos basin on basis of mapping,outcrop observation and seismic data.There unconformities are angular unconformity between lower Cretaceous and underlying strata,angular unconformity between lower Jurassic and underlying strata and angular unconformity between upper Paleozoic and lower Paleozoic.There unconformities represent important tectonic events in the evolution history of the southern section of western Ordos basin,respectively.(2)The comprehensive 2d seismic data interpretation method,which is calibrated by drilling,constrained by outcrop data and assisted by gravity and electrical exploration data,has achieved good results in the south section of the western margin with poor data quality and complicated structure.(3)The sub-tectonic zones in the southern part of the western margin were reclassified.Taking several important faults in the region as the boundary,the southern part of the western margin is further divided into several sub-units: Majiatan structural zone,Shigouyi-Shajingzi structural zone and Weizhou-Pengyang structural zone.The new division scheme highlights the control function of the shaft fracture,and the east-west zoning is more obvious.(4)Based on a variety of comprehensive analysis,the concept of " narrow southern section of west margin of Ordos basin " was proposed.Taking the huianbu-shajingzi fault as the boundary,the traditional southern section f the western margin is divided into majiatan structural zone and the " narrow southern section of west margin of Ordos basin ".The author thinks that majiatan structural zone is more similar to the tianhuan depression,and the " narrow southern section of west margin of Ordos basin " without majiatan structural zone is more conducive to the comprehensive discussion of its structural characteristics.(5)The structural models of majiatan structural zone and the south segment of the narrow western margin were established respectively.Majiatan structural zone is a typical low-angle thrust nappe structure with carboniferous as the slippage surface.The " narrow southern section of west margin of Ordos basin " is a high-angle thrust controlled by the deep sliding surface,and the strong horizontal extrusion pressure from west to east induced by the deep tectonic action is the main dynamic background for the structural deformation of the " narrow southern section of west margin of Ordos basin ".(6)The paleotectonic patterns of the key tectonic periods such as caledonian,indo-chinese,yanshan and Himalayan periods were defined.The " narrow southern section of west margin of Ordos basin " in the caledonian period is a transitional weak tectonic deformation belt.The response deformation of caledonian movement in this area is not as strong as that of the corridor and helan area to the west of qingtongxia-guyuan fault,and it is not as transitional as that of the ordos basin.In the late Triassic,there was also a slight folding deformation in the " narrow southern section of west margin of Ordos basin ",with the qinglongshan-pengyang fault roughly as the eastern boundary,and an ancient uplift on its western side,which extended northward and was connected with the ancient uplift under the yinchuan graben,and was collectively referred to as the yinchuan-qinglongshan-pengyang uplift.The late Triassic helan mountain basin and ordos basin are two independent basins separated by the late Triassic yinchuan-qinglongshan-pengyang uplift.Sedimentary characteristics,stratigraphic contact,stratigraphic shortening,and apatite and zircon fission tracks all reflect that the middle yanshan movement at the end of the Jurassic was the main period for the formation of the present nappe structure in the southern part of the western margin.In different regions of the southern part of the western margin,the Himalayan movement was either strike-slip or thrust nappe,which further transformed the thrust nappe tectonic belt formed by yanshan movement.(7)The tectonic evolution of the south segment of the narrow western margin can be systematically summarized as the intermittent uplift of the ancient continental beam in the western margin.During the siluran-early carboniferous,it was one of the main components of the caledonian tectonic belt in the western margin.In late Permian,Triassic extension,late Jurassic and early cretaceous,it controlled the formation and evolution of sedimentary basins to a great extent as a tectonic uplift belt.