Characteristics And Genetic Mechanism Of Intraplate Tectonic Deformation In The Western Ordos Basin

Intraplate tectonics refer to the deformation zones developing on the surface of the lithosphere inside the continent and far away from the boundary of the active plates.As an important part of the theory of plate tectonics,it has been widely concerned.However,at present,most of the research focuses on the fields of petrology,geochemistry,chronology and geophysics,and the research on the deformation of intraplate tectonics is weak,which limits the understanding of the characteristics of intraplate deformation and its evolution process.From the perspective of application,intraplate deformation is mostly developed in the basin.The incomplete understanding of intraplate deformation characteristics will also bring difficulties to the comprehensive analysis of the basin.In order to clarify the characteristics of intraplate deformation and its genetic mechanism,this paper selects the western edge of Ordos Basin as the research target.On the basis of clarifying the structural deformation style of seismic profile and the distribution characteristics of faults,the intraplate tectonic deformation characteristics of the research area are described comprehensively.And then combined with fault growth process analysis,identification on growth stratum and the production of tectonic evolution profile,the tectonic evolution process was recovered.At the same time,under the constraints of tectonic background,the relationship between multi-stage deformation is clarified to determine the genetic mechanism of intraplate deformation in the study area.And then differential deformation dynamic model is established.(1)The western Ordos Basin can be divided into 3 sections and 6 sub-sections due to the difference of thrust deformation.Based on the distribution characteristics of the maximum or minimum values of regional Bouguer gravity anomalies,two first-order faults are identified: Qingtongxia-Wuzhong fault and Zhongwei-Zhongning fault.They divide the western Ordos Basin into three sections: northern section,middle section and southern section.Through the mutation characteristics of regional Bouguer gravity anomalies,Zongbieli fault is identified in the northern section which is divided into Wuhai sub-section and Hengshanbao sub-section.The Tianshuipu fault and GuyuanPengyang fault were identified in the southern section which is divided into Tianshuipu sub-section,Shajingzi sub-section and Pingliang sub-section.The middle section is called Majiatan sub-section.(2)The study area is divided into two tectonic deformation systems.The lower tectonic system mainly developed extensional deformation which includes domino type,graben type and tension-torsion flower structure.The upper tectonic system is dominated by shrinkage deformation which mainly includes thrust imbricate fan,duplex and compression-torsion flower structure.The deformation of the upper and lower tectonic systems affects each other: the normal fault of the lower tectonic system controls the reverse faults of the upper tectonic system.Their distribution was a mirror image and the combination form of them is the ‘ v ’ shape opening to the side.Besides,the reverse fault of the upper tectonic system makes the normal fault section of the lower tectonic system bend and deflect in the inclined direction.(3)The evolution stages of the study area are divided into six periods: faultdepression period(Mesoproterozoic),extension period(Cambrian-Ordovician),stable subsidence period(Carboniferous-Permian),initial compression period(Triassic),thrusting period(Jurassic)and continuous compression period(Late CretaceousCenozoic).In the Mesoproterozoic Changcheng period,intracontinental rifts were developed and subsidence were in the way of depressions in Jixian period.The subsidence in these two periods were not continuous,and there was obvious regional unconformity between them.The Cambrian-Middle Ordovician inherited the Mesoproterozoic tectonic framework and the marine sedimentary environment.In the Late Carboniferous,the study area was dominated by regional subsidence,which transformed from offshore lake basin to stable inland lake basin,developing river-delta-lake deposits.In Triassic,affected by Indosinian tectonic movement,the western Ordos Basin was subjected to uplift and denudation with the fluctuation degree of strata in different sections intensified.In the Jurassic,influenced by the Yanshan movement,the area entered the thrusting stage,taking the Carboniferous-Permian coal seam,the Yanchang Formation shale and Yan’ an Formation coal seam as main slip thrusting surface.In the late Cretaceous,the Yanshan movement was relatively weaker,and the whole region was generally uplifted,lacking the late Cretaceous deposition.Affected by Himalayan movement,the tectonic stress field of the basin changed,and the thrusting action continued to develop.(4)The comprehensive deformation characteristics of the study area and the interpretation model of the evolution stage show that the early extensional deformation developed in the whole area,and the late contraction deformation occurred early in the south and late in the north,resulting in the division and deformation difference.Before the Late Ordovician,the study area a back-arc foreland basin environment,accept widely carbonate deposits.During Silurian-Devonian,the Qinqi orogenic belt was initially formed,resulting in small-scale shrinkage deformation in the south of western Ordos basin.Subsequently,the area is dominated by stable settlement.Until Triassic,the southern part of the western margin began to form contraction deformation under extrusion,while the northern part of the western margin was extended in NW.In the early Jurassic,the tectonic activity was in an intermittent period.At the beginning of the late Jurassic,the influence of the Paleo-Tethys region caused the North Qilian area to squeeze to NE,and the southern part of the western the Ordos Basin was again differential contraction deformation.The northward extrusion of the Alxa block also exerted compression on the northern part of the western margin.There are three times of compression in the southern part while the northern part only accepts compression once,therefore,there are differences in time and shrinkage,which leads to the difference of segmentation and thrust deformation in the western Ordos Basin.(5)The evolution of faults in different directions in the western Ordos Basin affects the distribution characteristics of strata and the degree of karst transformation,thereby affecting the trap distribution pattern and the quality of reservoir space.The compressioninduced uplift matching the updip reservoir-cap assemblage can form a good structuralstratigraphic trap,which is a favorable area for oil and gas accumulation.The western Ordos basin experienced multi-stage tectonic movement,resulting in multi-directional fault deformation.The intersection point of faults in different directions is the dominant area of karst transformation,which can form good karst reservoir space and is also a favorable area for oil and gas exploration.