Effect Of Pre-Existing Faults On The Formation And Evolution Of Continental Rift Valleys

The continental rift is one of the main tectonic units of the lithosphere.The continental rift,which is distributed in a ’ zigzag ’ distribution on the plane and arranged alternately by extensional and shear segments,is distributed all over the world.The Shanxi graben system,which is located in the hinterland of the Chinese mainland,is a modern graben system.It is also distributed in a ’ jagged ’ shape on the plane and is formed by the alternating arrangement of extensional basin and shear basin.It has rich geological and geophysical data and is an ideal place to study the formation and evolution mechanism of continental rift.Pre-existing faults play an important role in controlling the formation and evolution of continental rifts.However,the influence of different characteristics of pre-existing faults on the formation and evolution of continental rifts is still unclear.Nowadays,physical simulation experiments have proved to be a powerful tool for studying the progressive development of continental rifts.Therefore,the thesis uses the physical simulation method to design the experimental models under different conditions based on the actual geological model,and quantitatively analyzes the experimental results to explore the influence of different pre-existing faults on the formation and evolution of the rift system.The experimental model is compared with the formation and evolution process of the Shanxi graben system.Finally,the distribution of geothermal resources in the study area is combined to explore the control effect of active faults on the distribution of geothermal resources in the Shanxi graben system.The main understanding is as follows :(1)The formation,evolution and structural characteristics of the rift system are affected by the pre-existing faults,which are mainly reflected in the angle between the extensional section and the shear section(α)and the cutting depth of the pre-existing faults.In the case of the same α angle,the greater the cutting depth of the pre-existing fault,the more the number of faults in the rift,the greater the width of the graben in the shear section,and the more obvious the strike-slip effect in the shear section on the adjacent extension section;in the case of the same cutting depth of the pre-existing fault,the overall structural style of the shear section,the relative size of the extension and strike-slip strength,the graben width,the strain rate and the dextral shear movement are all related to the size of the α angle.(2)In the formation and evolution process of Shanxi graben system,the preexisting faults play a decisive role,mainly reflected in the following three aspects : In terms of expansion sequence,the North China Craton formed ’ serrated ’ pre-existing faults under the NW-SE compression in Yanshan period,and then under the NW-SE extension stress in Cenozoic,the NE-trending basins(Datong,Taiyuan,Yuncheng Basin)were formed first,and the NNE-trending basins(Xinding,Linfen Basin)were formed later.In terms of structural geometry and kinematic characteristics,the study area is controlled by pre-existing faults,showing a ’ zigzag ’ distribution on the plane,and the faulted basins are arranged in NE and NNE directions.At the same time,the NE-trending basins are mainly extensional,and the NNE-trending basins are characterized by dextral strike-slip.On the profile,the basin-controlling boundary faults are the reactivation of the pre-existing faults,and the boundary faults of each basin are normal faults.The difference is that one side of the NE-trending basin is a high-angle normal fault,and the other side is a step-type fault group,while the faults in the NNE-trending basin are distributed in a negative flower shape on the profile.(3)There are abundant geothermal resources in the five fault basins in Shanxi graben system.The formation of boundary faults in fault basins is controlled by preexisting faults.Pre-existing faults control the distribution of geothermal resources in the whole fault basin by controlling the formation and evolution of boundary faults.The internal basin boundary faults of the Shanxi graben system can extend down to the deep crust of 20 ～ 40 km,providing a channel for the upwelling of deep materials.Combined with the division of existing geothermal fields,it is speculated that the vicinity of each basin boundary fault in the Shanxi graben system is a favorable target area for geothermal exploration and development.