Study On Mesozoic-Cenozoic Structural Characteristics Of The Piedmont Belt In The Northern Tuha Basin

The Tuha Basin is rich in oil and gas resources,among which the northern piedmont zone is one of the important oil and gas rich structural zones in the basin.However,the geological structure of the piedmont zone is complex and variable,and there are different viewpoints on its evolution process.In order to reveal the structural characteristics of the northern piedmont zone more clearly and clarify its evolution process,this article establishes a more reasonable structural model based on new geological outcrops,seismic and drilling data,reorganizes the structural styles of the piedmont zone,and summarizes the corresponding structural features.The structural characteristics and evolution process of the northern piedmont zone in the Tuha Basin were systematically analyzed through precise interpretation of seismic profiles and balanced profile techniques.The main understanding obtained in this thesis is:(1)The northern piedmont zone mainly develops east-west thrust faults and north-south strike slip faults.The fault styles include shovel type thrust fault,backhoe type thrust fault,blind thrust fault,thrust fault,basement involvement type thrust fault,passive roof type thrust fault,overthrust nappe type thrust fault,and vertical linear strike slip fault.The combination styles include imbricated structures,offset structures,wedge-shaped structures,serpentine structures,and nappe structures.(2)The northern thrust nappe zone consists of a series of thrust faults and anticline structures,with northward dipping Carboniferous Jurassic thrust faults and overthrust structures widely developed from west to east.The central thrust fold belt is composed of a series of nearly east-west trending thrust faults and nearly north-south trending strike slip faults.From an overall perspective,the northern thrust cover zone and the central thrust fold zone are divided into four segments on a plane,namely the western segment,the central western segment,the middle eastern segment,and the eastern segment.The southern thrust flexure zone is characterized by the distribution of Lower Jurassic strata and faults in a parallel direction.(3)According to the combination relationship of thrust faults in the compression zone structure transformation zone on the profile,the No.1 and No.5 transformation zones are divided into the same inclined transformation zone,the No.2 is the reverse inclined transformation zone,and the No.3 and No.4 transformation zones are the opposite inclined transformation zone.According to the scale and control effect of the conversion zone,it is divided into three levels,with numbers 1 to 3 being the first level conversion zone and numbers 4 and 5 being the second level conversion zone.The strike of the faults in the study area varies in a stepwise manner from west to east,from near WE direction,to EES direction,then to near ES direction,and finally from ES direction to WE direction,and then to WS direction.The regular changes in fault strike within the transition zone indicate that the transition zone has segmented control over the study area.(4)The evolution stage of the piedmont zone can be divided into four stages:(1)The Middle Permian deposited thicker strata.Bogda in the northern part of the Late Permian changed from Aulacogen to uplift,and the Permian system was subjected to intense denudation,resulting in unconformity between the Permian system and the Triassic system.During the Early Triassic,the Bogda orogenic belt slowly uplifted During the Middle Triassic to Late Triassic,the Bogda Mountains formed and uplifted,resulting in a strong tectonic inversion in the piedmont zone.In the late Triassic,the hanging wall stratum of the fault was strongly denuded,resulting in unconformity contact between the Triassic and Jurassic.(3)Jurassic Cretaceous,early Jurassic,tectonic activity is weak.In the middle and late Jurassic period,due to the strong uplift of the Bogda Mountains,the strata underwent deformation and thrust fault activity was strong From Pliocene to present,due to the continuous collision between Indian Plate and Eurasia plate,Bogda Mountain was revived and uplifted again,resulting in the northern Carboniferous system overthrusting or sliding over the Jurassic nappe in the Quaternary period,which led to the transformation of Jurassic faults into compressive overthrusting faults.