Study On Geometry And Kinematics Of The Nankou-Sunhe Fault And Its Relationship With Ground Fissures

The Nankou-Sunhe fault is an important concealed fault in the NW Zhangjiakou-Bohai seismic tectonic belt,which also plays an important role in controlling the distribution of ground fissures in the Beijing Sub-Plain.For a long time,due to the limitations of data and its accuracy,structural geometry and kinematics of the Nankou-Sunhe fault have not been systematically studied.Especially,it is relatively weak in the southeastern segment of the fault.The relationship between the Nankou-Sunhe fault and ground fissure has also not been studied.In this paper,the Nankou-Sunhe fault is taken as the main research object.The tectonic unit of the Daxing Sub-uplift where the southeastern section of the Nankou-Sunhe fault is taken as the key research area.The main research contents include:?1?structural geometry of the Nankou-Sunhe fault,?2?the controlling relationship between the Nankou-Sunhe fault and the northeast fault of the Huangzhuang-Gaoliying fault,Shunyi-Liangxiang fault and Nanyuan-Tongxian fault,?3?fault kinematics in the tectonic unit of the Daxing Sub-uplift,?4?the controlling effect of the Daxing Sub-uplift,?5?and restriction on the distribution of ground fissures in the Beijing Sub-Plain.The significance of this work can provide not only the basic information for judging the seismic hazards and ground fissure disasters in the Beijing Sub-Plain,serving the city for disaster prevention and mitigation,but also the geological foundation for understanding the controlling effect of the Nankou-Sunhe fault on the sedimentation of the Daxing sub-uplift,and neotectonic movement of Beijing Sub-Plain.1.Based on the combination of comprehensive regional geophysical information?include 2-D seismic,high-precision gravity,CSAMT profiles,and 3-D seismic,gravity of the key tectonic locations?and high resolution drilling data,the 2-D geometric features of the entire Nankou-Sunhe fault are summarized systematically,and its 3-D geometry model is revealed.The study shows that?1?the Nankou-Sunhe fault is a three?named the NW section,SE section and midsection,respectively?right-step faulting for left lateral shear,distributing in the NW direction 155°and 140°,?2?secondary faults are developed,and the strata suffered from deformation in the fault terrace zone,?3?comparing vertical fault throw of three right-step faulting,it is the largest at the NW section and the smallest at the SE section,?4?vertical fault throws and dips of the individual right-step faulting are characterized by large middle part and small ends.2.Previous studies have shown that the latest activity period of the NW section and the midsection in the Nankou-Sunhe fault are the Holocene and the late Pleistocene,respectively.New research indicates that the latest activity period of the SE section in the Nankou-Sunhe fault also is the early of the late Pleistocene–late of the Middle Pleistocene by using drilling and ¹⁴C ages.At the same time,Compare the latest activity period of the NE faults,The latest activity of all faults,include the Huangzhuang-Gaoliying,Shunyi-Qianmen-Liangxiang and Nanyuan-Tongxian Faults,in the northeast of the Nankou-Sunhe fault is the Holocene or Late Pleistocene,while it is older in the Northwest of the Nankou-Sunhe fault.The activity intensity of all faults in the northeast of the Nankou-Sunhe fault also is strong,while it is weaker in the Northwest of the Nankou-Sunhe fault.Therefore,the NW Nankou-Sunhe fault has a control effect on the latest active age and activity intensity of the NE Huangzhuang-Gaoliying,Shunyi-Qianmen-Liangxiang and Nanyuan-Tongxian Faults.Based on the combination of the regional geophysical evidence and the controlling effect evidence on the latest active age and activity intensity,it is not only confirmed that the NW Nankou-Sunhe fault cuts the NE Huangzhuang-Gaoliying,Shunyi-Qianmen-Liangxiang Faults,but also proposed that the NW Nankou-Sunhe fault cuts the NE Nanyuan-Tongxian fault.Furthermore,Using calculating the relative slip of the NE faults in the pre-Cenozoic depth,the left-lateral shear of the Nankou-Shanghe fault is relatively large?0.97km³.1km,mean1.61km?,which is much higher than the vertical fault throws of 0.1km⁰.5km,and it gradually decreases from the middle to the ends.Those feature may reveal that left-lateral shear of the Nankou-Sunhe fault may is the most important the kinematic characteristics,or the Nankou-Sunhe fault developed a certain amount of left-lateral shear during the pre-Cenozoic.3.With the high resolution sequence stratigraphy and the correlation analysis of seismic,cores,logs,¹⁴C,color and paleomagnetic data,different orders of sequence boundaries and facies sequence are identified,and the isochronous stratigraphic framework and the depositional evolutional model are built in two walls of the SE section of the Nankou-Sunhe fault and used to predict vertical kinematics of the active Nankou-Sunhe fault since the Quaternary.The study shows that?1?the fault activity in the SE segment of the Nankou-Sunhe fault is generally weak during the early Pleistocene,and the fault activity rate is 0mm/a⁰.202mm/a,and the average activity rate is 0.075mm/a.?2?During the Middle Pleistocene,the fault activity was generally strong,the fault activity rate was 0mm/a⁰.789mm/a,and the average activity rate was 0.74mm/a.?3?Fault activity has weakened again since the late Pleistocene,with an average activity rate of 0.069 mm/a.Therefore,the inhomogeneous time distributions of faulting are the important activity characteristic since the Quaternary.Four important periods of relatively intense activity,which occurred respectively at about 2.5Ma±,1.35 Ma±,0.79⁰.55 Ma and 0.15⁰.25 Ma,are identified,and it can be exactly the same as the NW section of the Nankou-Sunhe fault.4.The Cenozoic continuous activities of the Nankou-Sunhe fault have a controlling effect on the sedimentation of the Daxing sub-uplift.The Nankou-Sunhe fault experienced a stable period of inactivity during paleogene period,and reactivated in the Neogene.The fault activation disintegrated Pleistoch the structural pattern of the“Two sub-uplifts and Two sub-sags”in the Beijing Sub-plain.The Daxing sub-uplift in the SW wall of the Nankou-Sunhe fault still continues to rise,and provides sediment sources for surrounding areas.But,The Daxing sub-uplift in the NE wall of the Nankou-Sunhe fault began to subsidence and sedimentary during the Neogene to the early Pleistocene.During the Middle Pleistocene,the Nankou-Sunhe fault was strongly active and had an important restrictive controlling effect on the NE-trending fault activity.The thickness of the two walls in the Nankou-sunhe fault was different,and segmentation activities began to appear in the NE-trending fault,such as Huangzhuang-Gaoliying fault,Shunyi-Liangxiang fault,etc.Taking the Nankou-Sunhe fault as the boundary,its SW section is no longer active,but the NE section continues to active.Since the late Pleistocene,the Nankou-Sunhe fault activity gradually weakened,so its control on the sedimentation of the Daxing sub-uplift weakened.But its weak activity in the NW section still affects the development and evolution of the river drainage in the Beijing sub-plain.The landform appears as a linear terrain along the Nankou-Sunhe fault,becoming an important catchment channel or catchment depression.5.Widely developed ground fissures in Tongzhou area can be divided into historical ground fissures and modern ground fissures according to their formation age.Historical ground fissures are widely associated with historic earthouakes,and most of them have healed today.Modern ground fissures are also divided into two categories.One is represented by the ground fissures on the Miaoshang area.The scale of the ground fissures is small and the damage is weak.It is caused by the long-term sand and water loss of the historical ground fissures?liquefaction veins?.The other is represented by the Shuangbutou ground fissure.The scale of the ground fissure is large and the damage is strong.The Shuangbutou ground fissure is altogether caused by the two kinds of deformations under the influence of current NNW tensile stress.The long-term continuous creep slip of the Nanyuan-Tongxian fault,conceals in the lower part of the ground fissure,provides the geological conditions for the accumulation and transmission of stress for the formation of the fissures,and the long-term and excessive exploitation of groundwater resources exacerbates the horizontal and vertical deformation of the stratum.At present,the ground fissure also records the creep activity of the Nanyuan-Tongxian fault.Compared with the Huangzhuang-Gaoliying fault and Shunyi-Qianmen-Liangxiang fault,the Nanyuan-Tongxian fault has a slower inclination and a deeper buried upper faulted point depth,and it may just lead to the wider ground fissures than ground fissures along the Huangzhuang-Gaoliying fault and Shunyi fault.The Nankou-Sunhe fault has an important controlling effect on modern rivers,and also plays an important role in controlling the latest activities of concealed faults.The kinds of controlling altogether restricte the distribution of modern ground fissures in the Beijing sub-plain.The NE wall of the Nankou-Sunhe fault is relatively strong active block,and is the main distribution of modern ground fissures,especially along the buried active faults.