Study On Main Fault Activity And Deformation Characteristics Of Related Blocks In Yuncheng Basin

China is a country with frequent earthquakes.It is also a country with a wide range of active basins within the continent.A series of strong earthquakes occur in intracontinental basins.The fault depression zone is located on the southeastern margin of the Ordos block and is an important seismic tectonic unit in Northern China.Yuncheng Basin is one of the deepest basins in the fault depression of the Fen-Wei fault zone.It was formed in the Oligocene and is controlled by the North Zhongtiaoshan Faults.It is a shallow semi-graben basin in the southeast and northwest.Since the formation of the Yuncheng Basin,a number of NE-NNE-oriented faults have developed within the basin,controlling the NE or near NEE tectonic uplift and basin subsidence in the Yuncheng Basin.The North Zhongtiaoshan Faults experienced a long-term tectonic evolution,which controlled the tectonic evolution of the Yuncheng Basin,causing the Zhongtiaoshan block on the southeast side of the fault to rise continuously and the Yuncheng basin on the northwest side to continue to be fractured and accepted for sedimentation.The activity pattern and sliding rate of the North Zhongtiaoshan Faults are important for understanding the tectonic framework of the Yuncheng Basin,and are also important for the seismic risk assessment of the Yuncheng Basin and the medium-and long-term prediction of the earthquake.The South Emei Platform Fault is a controlled basin fault in the northern part of the Yuncheng Basin.It also plays an important role in basin evolution.Its activity is of great significance for constructing the movement mode of the relevant block in the Yuncheng Basin.In addition,the secondary bulge of the Mingtiaogang is developed inside the Yuncheng Basin.Through the investigation and research on the ground fissure of the uplift boundary,it is believed that the fault of the southern part of the Mingtiaogang has a certain activity.The study on the movement mode and activity of the Mingtiaogang block is of great significance to the establishment of the tectonic evolution model of the Yuncheng Basin.The paper is based on the sub-topic “Major active faults in the southeast margin of Ordos,paleo earthquakes and rupture segments”,which is part of the project “Study on seismic risk of arc extension area in southeastern margin of Ordos”.On the basis of the interpretation of high-resolution remote sensing images,detailed geological and geomorphological surveys,UAV surveys,fault profile analysis and stratigraphic dating are carried out on the Zhongtiaoshan block,the Emei Platform and the Mingtiaogang sub-uplift area.Detailed researches about the late Quaternary faults activity habits,deformation of the topography and the activity rate have been carried out.Based on the previous data on boreholes(profiles)and paleomagnetics in the area,the relevant block uplifts have been analyzed.The following conclusions have been made:(1)There is a series of secondary faults parallel to the main fault in the Haizhou section of the North Zhongtiaoshan Faults.The closer the faults is to the interior of the basin,the later the development time and the more active it is.There mainly develops three periods of alluvial fans since the late Pleistocene.The abandoned age of the first alluvial fan was 14.1-15.7 ka,and the average height of the fault steep slope on the topography was 10.2-11.7 m.The average vertical slip rate of the fault was 0.65-0.83 mm/a;The average vertical slip rate of fractures is 0.45-0.62 mm/a in the south Xiaoli Village since 15.7±2.5 ka.The average vertical slip rate is about 0.1 mm/a since 107.4±18.6 ka,and the activity of late Pleistocene faults tends to increase.(2)The cracks of the Mingtiaogang ground fissure spread from the southwest to the northeast(Village Xiawang-Village Tao-Village Banpo-Village Wucao-Village Xincao-Village Sima),with an overall trend of 50° and a total length of about 15 kilometers.The maximum activity rate of the cracks is about 8 mm/a,and the cause of the cracks in the Mingtiaogang belongs to the coupling mode of the tensile stress of the basin plus the over-exploitation and infiltration of the groundwater: the structural fault is the controlling factor of the ground fissure,which controls the location and scale of the ground fissure.Over-exploitation of groundwater and surface water infiltration is the main cause of the formation of today's ground fissures.Therefore,the cause mechanism of the cracks in the Mingtiaogang is fracture control joints,pumping and suturing the latitudinal viscous expansion joints.Through field investigation,we found that Village Wucao developed two ground fissures.According to the ground fissure shape and the ground subsidence,it is considered that the cracks in the Mingtiaogang compound "Y" crack development pattern.(3)Under the action of the regional tectonic stress field,the Zhongtiaoshan block,the Yantai block and the Mingtiaogang secondary block all undergo a clockwise skew deformation.Among them,the Emei Platform and the Mingtiaogang block are uplifted.The mode gradually rises from northeast to southwest.The Zhongtiaoshan block is about 300 m higher than that of the Emei platform since the late Pliocene.Since the Quaternary,the activity of the North Zhongtiaoshan Faults has been about 700 m.,and the tilting of the block is over 100 m.Since the Quaternary,the faulting magnitude of the South Emei Platform Fault has been about 300 m.Since the middle Pleistocene,it was about 150 m.Since the late Pleistocene,the uplift of the Mingtiaogang has been about 100-150 m,and it has risen rapidly in the middle Pleistocene,and the uplift is more than 100 m.