Timing Of Angular Unconformities In Upper Pleistocene Around The Ordos Block And Its Tectonic Significance

The basement of the North China Craton was formed in the Archean,and the destruction of it began in the early Mesozoic,reaching the peak in the Early Cretaceous.In this stage,the North China Craton experienced the late Cretaceous Yanshanian movement,Paleogene Eocene-Neogene Middle Neogene rifting and NeogeneQuaternary differential uplift movement.Therefore,types of active faults and the combination of active tectonic belts have been developed,dividing the North China Carton into the Ordos Block,the Taihang Mountains,and the North China Plain.Previous studies have shown that the destruction of the North China Craton is mainly affected by the westward subduction and retreat of the western Pacific plate since the late Mesozoic,as well as the stagnation and dehydration and back-arc expansion in the mantle transition zone.Under the influence of these effects,a series of extensional faults,extensional basins,and metamorphic core complexes were formed in the eastern part of the North China Craton,while the Ordos Block and its periphery in the western part of the North China Craton showed an overall depression or fault-depression basin pattern under the background of weak extension.As an important part of the North China Craton,the Ordos Block and its surrounding Cenozoic faulted basins have always attracted the attention of geoscientists.Based on the sedimentary stratigraphic contact relationship and the evolution history of the basins,the Cenozoic rift basin system in the periphery of Ordos has experienced three distinct extensional evolution periods: the stage of extensional rifting from the end of the Paleocene to the late Neogene,the development stage of stable lake basins in the early and middle Pleistocene,and the latest tectonic change stage since the middle and late Pleistocene.However,there are doubts about the exact time of the latest tectonic movement and its movement pattern.Therefore,the study of the start time and deformation style of the latest tectonic movement since the Late Pleistocene of the Ordos periphery has important scientific significance for perfecting the evolution history of the Ordos block and its periphery.At present,there are many discussions about the dynamic source of the tectonic framework of the Ordos Block and its surrounding areas,which can be summarized into the following three views:(1)this movement is related to the far-field extrusion effect caused by the collision of the Indo-European continent,(2)the movement is related to the upwelling of the deep lithospheric mantle caused by the Pacific subduction slab related,(3)the movement is the result of the combined effect of(1)and(2).There is a lot of evidence for all these three models,but no conclusive conclusion about the power source.Therefore,the research on the power source of the latest tectonic movement of the Ordos block and its periphery will help us to have a deeper understanding of the current movement and the seismic tectonic background of the Ordos.Regional unconformity is one of the important signs for determining tectonic movement.Through the study of location,form,and sedimentary age of unconformity,the starting time and evolution history of tectonic events in the area can be determined.Our research mainly focuses on the Hetao Basin and Shanxi Graben system,because these areas have widely developed ancient large lakes since the middle and late Pleistocene,such as Hetao ancient lake,Nihewan ancient lake,Linfen ancient lake,etc.The related lacustrine sediments can record a series of sedimentary tectonic events,which can helps us find the evidence of sedimentary discontinuities,and analyze the power source of unconformity through determine the start time of these tectonic events.Through field geological surveys in the Hetao Basin and Shanxi Graben system,we have discovered eight sections with angular unconformities.Through the study of these angle unconformities,we have the following conclusions:(1)The angular unconformities in the Upper Pleistocene strata are widely distributed in the Hetao rift basin and the Shanxi graben system,and these unconformities are distributed near the active faults in the piedmont.We speculate that the latest tectonic movement represented by these unconformities is universal in the periphery of Ordos;(2)The dating of the upper and lower strata of the unconformity shows that there are two phases of unconformity in the Hetao fault basin.The formation time of the first phase is 59-70 ka ago,and the time of the second phase is 30 ka ago,and both lasted to date;the sedimentary discontinuity of the Shanxi graben system began in the early Late Pleistocene(about 150 ka)and lasted until the Malan loess re-covered it at the end of the Late Pleistocene;(3)These Upper Pleistocene stratigraphic unconformities were mainly formed by tectonic processes,and the influence of climatic effects was limited,which means that the tectonic action led to the activity of the piedmont fault zone,then causing the traction and deformation of the strata near the fault zone;(4)The latest tectonic movement of the Ordos periphery represented by these unconformities may be caused by the eastward extrusion of the Qinghai-Tibet Plateau,pushing the Ordos block to rotate counterclockwise.