Study On Cenozoic Tectonic Deformation And Movement Of Active Fault Blocks Based On 3D Morphology Of Planation Surfaces

Located at the northern end of Shanxi graben system,the basin-ridge structural area in northwest Beijing is an important part of Shanxi graben system,which belongs to the intersection area of Shanxi seismic belt and Zhangbo seismic belt.It is a tensile fault depression area dominated by semi-graben & graben structure,with many grabens and semi-graben basins developed,and is a typical basin-ridge structural area.The margins of these basins are mostly controlled by northeast-trending active fault zones,which divide the basin-range area into a series of active fault blocks.The upwelling of molten materials in the upper mantle asthenosphere caused the whole tensile environment in the basin-range area,which was the deep dynamic cause of basin fault depression and fault block tilting.Nowadays,there are still some problems in the study of seismotectonics in this region.First of all,as far as the new tectonic movement of fault blocks is concerned,at present,the tectonic movement of all fault blocks is generally classified as tilting movement,which lacks the concrete understanding of the tilting deformation details of each fault block,and has not systematically and accurately obtained some major deformation quantities or related quantitative parameters such as tilting direction and tilting angle of fault blocks,and lacks the accurate determination of the total accumulated tectonic deformation at fault block scale.At the same time,it lacks the discrimination of the tilting movement differences of each fault block and the structural strain space of the whole fault depression area on this basis.Secondly,there is a lack of understanding of the relationship between fault blocks and the overall structure of fault blocks in this basin-range area,and a lack of grasp of the overall nature of the regional scale fault system.What is the coordination mechanism to link and combine fault blocks? In what way does the fault block connect the isolated faults together? How to evaluate the inhomogeneity of the whole structure of fault block? etc.The solution of these two problems can provide key information about the overall activity level and pattern of regional seismotectonics since Cenozoic,and provide important parameters and conditions for constraining regional seismotectonic background.Therefore,these problems deserve further study and solution.Multiple planation surfaces are generally developed in the study area,and the existence of these planation surfaces provides a certain opportunity to solve these difficult structural problems.The so-called planation surface generally refers to the gentle,undulating and nearly flat topographic surface which approaches the erosion basis due to the long-term weathering and erosion of external forces.Subsequently,in the active tectonic period,the horizontal form of the preliminary planation surfaces is often affected by fault activity and fault block movement,and the original planation surfaces are deformed in different forms and degrees.Therefore,the tectonic deformation of planation surfaces can indicate the deeper tectonic movement under the ground.In recent years,more and more scholars at home and abroad have studied the deformation,movement and evolution process of active structures based on this characteristic of planation surfaces.Predecessors have systematically combed and studied the planation surfaces in the study area.At present,it is considered that there are three mountain planation surfaces in this area: Beitai,Dianziliang and Tangxian planation surfaces.This paper focuses on the structural deformation of the highest planation surface in the region,analyzes and studies the 3-D deformation and movement of structural fault blocks in the basin-ridge area since Cenozoic,and tries to solve some structural problems that still exist in the study area mentioned above.Based on the latest high-resolution remote sensing images and topographic data,as well as the advanced spatial analysis functions and algorithms of GIS platform,combined with field geological survey and measurement,this paper makes detailed remote sensing interpretation and accurate topographic and geomorphological analysis of planation surfaces,analyzes the 3-D structural deformation characteristics of planation surfaces and extracts quantitative parameters that can reflect this deformation,and accurately obtains the structural deformation and related parameters of each fault block.This paper studies the 3-D tectonic deformation and movement of active fault blocks in the basin-range area since Cenozoic,and reveals the non-uniformity characteristics of the overall structure and tectonic strain of fault blocks in the basin-range area based on the differences of fault block movement and deformation.Through the research of this paper,some preliminary understandings are obtained as follows:(1)There is a positive correlation between the degree of cutting erosion and the lifting degree of fault block.With the increase of fault block tilting angle,the degree of undercut erosion and backward erosion in the watershed across the front of fault block tilting uplift in piedmont area increases accordingly,and the degree of retreat of the corresponding main watershed increases;(2)All the fault blocks in the middle and eastern section of Mt.Liulingshan generally show tilting movement,and some fault blocks develop secondary faults.The tilt angle of each fault block is different,and this tilt difference may be adjusted by a series of northwest-trending faults between fault blocks(3)From the spatial distribution characteristics of tilting movement degree,the tilting angle and lifting amount of fault blocks in the middle section are higher than those in the eastern section,which shows that the tilting rate of the front edge of fault blocks in the middle section is higher than that in the eastern section,with the tilting rate ranging from 1.22 mm/a to 1.55 mm/a in the middle section and from 0.45 mm/a to 0.71 mm/a in the eastern section.(4)A NE-trending structural belt with oblique section and block boundary is formed between two blocks in the middle section.The oblique tension environment may produce strain distribution,and at the same time,a normal fault and a right-lateral strike-slip fault are formed.During 1989-1999,a series of moderately strong earthquakes with Ms?5 occurred in this section,and the main seismogenic faults of this strong earthquake sequence are probably closely related to this right-lateral strike-slip fault.(5)Fault blocks in the basin-range area can be divided into three types according to the form of tectonic movement: tilting,horst and tilting.The inclined block belongs to the semi-graben structural fault block,which is widely distributed in the northwest of the basin-range area.The blocks are arranged in a domino-style combination.It is speculated that there is probably a slip surface under these inclined fault blocks,which generally controls and coordinates the movement and deformation of these inclined fault blocks.The horst-type fault blocks are mainly vertically uplifted,which are concentrated in the southeast of the basin-range area,where the topographic uplift is relatively higher.They are distributed in the exposed areas of metamorphic core complexes such as Wutaishan,Dianziliang and Xiaowutaishan area,which may reflect the strong upwelling of magma material in the deep below.Tilting-sliding fault blocks are concentrated in the southeast edge of the basin-range area,and basically develop and distribute along the structural belt of Mt.Taihang in the northeast direction.They are close to the horst fault blocks in the northwest side,and often show that many fault blocks are distributed in a nearly parallel oblique arrangement,forming a stepped pattern.(6)Taking the northeast-trending structural belt where the basin-range area is located as the center,the tilt angle of the fault block gradually decays toward the periphery of both sides of the basin-range area,from more than 6° to less than 1°.This indicates that the tilting movement degree of fault blocks gradually weakens from the center of the basin-range area to both sides,and it is speculated that the development of detachment structures in the basin-range area and the upwelling and activity degree of deep magmatic materials are obviously stronger than those in the peripheral areas.Of course,the conclusion of this paper is based on the existing data,technical methods and some new understandings,which need to be further tested and improved by other data and means in the future.