Quantitative Study Of The Latest Dextral Strike-slip Movement Of The Tanlu Fault Zone In Late Cenozoic And Its Tectonic Implications

The theory of active blocks divides mainland China into a series of active blocks separated and bounded by strongly active tectonic belts formed in Late Cenozoic,and still active since late Quaternary to the present.The interior of the block is usually relatively stable,and tectonic deformation and strong earthquake activity mainly occur on the boundary zone of the block.All the magnitude 8 earthquakes and more than 80%of the magnitude 7 and above earthquakes since historical records are located on the boundary of the active block.In the period of post-craton destruction,large strike-slip earthquakes occurred frequently in the North China Block.These earthquakes not only occurred along the boundary zone of the blocks,but also a considerable part of the strong earthquakes occurred inside the block.In the North China block,earthquakes above M8 occurred along the sub-block boundaries(such as the 1303 Shanxi Hongdong M8 earthquake,the 1556 Shanxi Huashan piedmont M8 earthquake,the 1739 Ningxia Pingluo M8 earthquake,etc.).However,scattered and frequent strong earthquakes in the North China Plain(such as the Sanhe Pinggu M8 earthquake in 1679,the Tangshan M7.8 and Luanxian M7.1 earthquakes on July 28,1976,the Xingtai M7.2 earthquake in 1966,the Heze M7 earthquake in 1930,the Ci County M7? earthquake in 1830,etc.)should be typical representative of intra-plate tectonic deformation inside the block.The occurrence of these strong strike-slip earthquakes indicates that the current tectonic deformation of the North China Plain is different from the extensional deformation when the basin formed.When and how did this tectonic transition occur? North China has been the cradle of Chinese civilization since ancient times,and it is still the political,economic and cultural center of China.When did the diffuse strike-slip earthquakes in the North China Plain begin? What are the mechanisms and characteristics of the latest tectonic deformation in North China? Where should we go to find the answer to this major scientific question that is closely related to Chinese civilization?The Tan-Lu Fault Zone(TLFZ)is the largest strike-slip fault system in the North China block and even in eastern China,and it is also the boundary fault between the sub-blocks in the North China block.The largest historical earthquake in North China occurred along TLFZ,that is,the Tancheng M8? earthquake in 1668,and small and medium earthquakes along the fault are still active today.The tectonic evolution of the TLFZ and its related geological processes should be a comprehensive response to regional tectonic events.A scientific understanding of the tectonic evolution of the TLFZ since Late Cenozoic,especially an in-depth study of the latest tectonic deformation of the TLFZ,is of great significance for understanding the changes in the geodynamic environment and its tectonic response in the North China block since late Cenozoic.It is also of practical significance for the understanding of the seismogenic mechanism and the risk of large earthquakes in the post-craton destruction period in North China.Due to lack of late Cenozoic geological and geomorphic offset markers along the TLFZ and serious surface artificial modification,there exists objective difficulties in the study of the late Cenozoic strike-slip movement of TLFZ.The formation and development of pull-apart basins along strike-slip fault zones is a direct response to the strike-slip movement of faults.Thus,the sedimentary record of pull-apart basins is a key to deciphering the strike-slip process of faults that control development of basin.This study takes the Banquan Basin as our study object.The Banquan Basin is the largest and most typical pull-apart basin along the surface rupture zone of the 1668 Tancheng M8? earthquake where the TLFZ shows the strongest seismic activity.Elaborate sedimentary-tectonic framework of the basin was obtained through shallow seismic reflection exploration,and complete sedimentary filling sequence of the basin was revealed through drilling.On this basis,the tectonic evolution and sedimentary response process of the Banquan Basin were discussed.The chronostratigraphic framework of the basin was established by means of magnetostratigraphy and cosmogenic nuclide chronology,giving time constraint for formation and development of the basin.This study reveals the time,displacement and the dextral strike-slip process of the TLFZ in late Cenozoic,and compares it with the eastward extrusion and tectonic orogenesis of the Tibetan Plateau in late Cenozoic and discusses the latest tectonic pattern and geodynamic background of the North China Block in the post-craton destruction period.The following conclusions are therefore obtained:(1)The Banquan Basin is a half-graben-like pull-apart basin with faulting in the east and onlapping in the west,showing typical double-layer structure of faulting and subsiding.The basin has undergone three main evolutionary stages.During the prepull-apart period,the TLFZ that controls the evolution of the basin showed very weak activity,and then fault activity intensified and led to two periods of pull-apart and extension of the basin.The earlier period of pull-apart and extension had an extensive influence on the basin,when almost all faults were active.The later period of pull-apart and extension led to expansion and subsidence of the eastern margin and center of the basin,indicating that fault activity migrated to the central basin.Then,the basin expansion reached its peak.Following that period,the basin entered the subsidence stage.New strike-slip fault formed in the center of the basin,which effectively accommodated normal faulting of the boundary faults of the basin and caused the basin to shrink and die out.(2)The sedimentary filling and depositional cycle of the basin has strong response to the episodic pull-apart and extension of the basin.Before the intense pull-apart,a thin layer of Miocene mudstone slowly accumulated in the basin due to local rifting.The loose sediments above the mudstone are growth-strata,which can be divided into three sedimentary systems.In the early and late stages of pull-apart and extension,the basin was successively filled with coarse-grained alluvial fan facies(sedimentary system I)and braided river-meandering river facies(sedimentary system II)with frequent facies changes.During the subsidence stage of post-pull-apart,the basin was filled with a set of floodplain facies deposits(sedimentary system III).(3)Using magnetostratigraphy and burial cosmogenic nuclide dating method,the chronostratigraphy framework of the loose sediment above the Miocene mudstone in the KA borehole located on the central slope of the Banquan Basin was determined.The burial age of cosmogenic nuclides shows that the bottom of the loose sediment were buried at ～4.01±1.27 Ma.Elaborate magnetostratigraphy sequence reveals 15 positive and negative polarity zones for the drilling profile.The bottom of the sediment sequence corresponds to the bottom of C3 n.1n,which is about ～4.3Ma.This indicates that the basin began to develop at ～4.3Ma,that is,the latest dextral strike-slip movement of the TLFZ in late Cenozoic began at ～4.3Ma.According to the chronostratigraphy framework and tectonic evolution of the basin,we suggest that the pull-apart and extension of the basin and the strike-slip movement of the faults show periodic cycles on a time scale of millions of years.(4)The sedimentary and tectonic evolution of the Banquan Basin was directly controlled by strike-slip movement of the TLFZ.Based on the geometric shape and internal structure feature of the basin,the dextral strike-slip displacement of the TLFZ since late Cenozoic(～4.3Ma)is about 1000～2000 meters,and the dextral strike-slip rate is about 0.2～0.5mm/a.This slip-rate is roughly comparable to geological slip-rate derived from active tectonic studies with a timescale of 100,000 years.The latest tectonic deformation of the North China Block has been governed by the eastward tectonic extrusion and orogenesis of the eastern margin of the Tibetan Plateau since late Miocene.The eastward thrusting of the Liupanshan fault zone and sinistral shearing of the Qinling fault zone led to anticlockwise rotation and pushing of secondary blocks in North China,resulting in a planar bookshelf faulting and rotation pattern.This unique deformation pattern transferred eastwards to the North China Plain at ～4.3 Ma and the process continues to the present time.Assuming that the pull-apart basin has recorded all the dextral strike-slip displacement of the TLFZ,we deduced that the North China Plain has rotated 0.16°～0.33° counterclockwise since ～4.3Ma,which is in a decreasing relationship with the rotation of the Ordos and Taihang Mountain sub-blocks and relatively consistent with the rotational angular velocity of the North China Plain revealed by contemporary GPS velocity feild.It suggests that the dextral strike-slip of the NE-trending faults in North China Block should be the result of counterclockwise rotation of sub-blocks.This counterclockwise rotation,accompanied with sinistral movement of the ～EW-trending boundary fault zones to the north and south of North China and dextral strike-slip motion of the NNE-trending boundary faults between secondary blocks,is likely to be the long-range effect of strong extrusion of the eastern margin of the Tibetan Plateau.