Differential Structural Deformations And Controlling Factors Of Salt Tectonic In The Middle Segment Of The Kuqa Depression

The widely deposition of salt layers is the most critical factor that leads to the Kuqa depression featured by stratification compression and vertical stacking.During the Mesozoic-Cenozoic evolution process of the Kuqa depression,the compression stress changed with the pulsating uplift of the south Tianshan Mountains,and the deposition center gradually migrated from north to south,and the pre-existing base structures vary with different tectonic positions.Influenced by these factors,the main controlling factors of tectonic deformation in the N-S trending different tectonic belts,the vertical different structure layers and the E-W trending different tectonic position in the middle segment of the Kuqa depression are discrepant,while the studies on this are insufficient.Based on the latest seismic and drilling data,this paper analyses the geometrical deformation differences about the N-S trending zoning,vertical stratification and E-W trending segmentation in the middle segment of the Kuqa depression.On this basis and combining with the changes in the regional and local tectonic environment,migration of the sedimentation center and the difference in the basement paleo-structure,and supported by structural physical simulation,this paper systematically analyses the differences of the salt-related structural deformation and their controlling facts in the middle segment of the Kuqa depression.The main findings and conclusions are as follows:1.The deformation styles of different strata in the middle segment of the Kuqa depression are quite different.The deformation of subsalt layers can be divided into two types of basement thrust and cap layers detachment.The North Monoclinic structure belt is characterized by step-style thrust blocks.The Kelasu structural belt is characterized by wedge-style thrust blocks,pop-up structure,imbricate fan structure and flower-like structure,while the Qiulitage tectonic belt is characterized by ramp structure,domino thrust fault block,fault-bend fold.The deformation of salt layer can be divided into piercement and hidden piercement structure.The Kelasu structural belt is characterized by salt embankment,salt ridge and thrust salt wedge.The Qiulitage tectonic belt is characterized by hump-style salt dome and thrust salt nappe.The deformations of suprasalt layers in different tectonic belts are all mainly characterized by detachment fold and thrust fault.2.The differential defromations along the strike in the middle segment of the Kuqa depression vary largely.The vertical stratification compression deformation is unconspicuous in the eastern North Monocline tectonic belt,while the western segment of the North Monocline tectonic belt still presents a measure of stratification compression characteristic.The Dabei area,located in the western Kelasu tectonic belt,developed salt domes both in the leading edge and trailing edge of the thrust imbricate fan,while the salt dome was only developed in the trailing edge in the Keshen area.The subsalt layers in the Awate area of the western Baicheng depression mainly developed thrust faults,and the salt layer was sharply thickened and pierced the surrounding rocks.In contrast,the faulting in the middle and eastern segments of the Baicheng depression was relatively weak,the salt outflowed and thinned to form a withdrawal depression.The different scale of basement-involved faults were developed in the subsalt layers in the Xiqiu tectonic belt,and the shape and scale of the salt domes developed above these faults were different.In the western Xiqiu tectonic belt,the Quele salt nappe was developed with scale gradually decreased from west to east.In the Dongqiu tectonic belt,the subsalt layers mainly developed shovel-style decollement faults,the salt anticline developed above these faults were severely uplifted and eroded.3.The controlling factors of structure deformation varied with different tectonic belts in the middle segment of the Kuqa depression.The subsalt and basement layers in the North Monoclinic tectonic belt were southward tilted strongly,causing the salt southward flowed under gravity action and sharply thinned,while the uplift degree of the western segment of the North Monoclinic tectonic belt is smaller than that of the eastern segment,thus the tectonic deformation in the western segment of the North Monoclinic belt still presents some extent of stratification characteristic.The double salt domes developed in the Dabei area that located in the western Kelasu tectonic belt are mainly controlled by two factors:one is that the steep margin slopes of the Qiulitage palaeo-uplift restricted the southward flow of the salt rock,the second is that the Baicheng fault developed in the leading edge of the thrust imbricate fan has larger thrust displacement.While the fault displacement of the Kelasu fault is much larger than that of the other faults in the Keshen area,because of which the salt rock only formed a large salt dome above the Kelasu fault.The violent faulting in the sub salt layers and thickening of the salt layer in the Awate area that located in the western Baicheng were closely related to the narrow salt deposition range,the obstruction of the Wensu uplift on the salts flow,and the more intense compression stress.However,the deposition range of the salt is relatively wide in the middle and eastern segments of the Baicheng depression,and the barrier obstruction is absent.Influenced by these facts,the salt outflowed and seriously thined.The spectacular salt structures in the Xiqiu area of the Qiulitage tectonic belt were closely related to the pre-existing reverse faults that developed in the sub-salt layers.The resurrection of these pre-existing faults in late-stage induces the salt rocks occurring violent diapirism,and the scale of the salt dome presents a notable positive correlation with the intensity degree of the subsalt faulting.The strong fold and faulting in the supersalt layers in the Qiulitage tectonic belt were dominated by the strong compression stress,which can remotely transmit through bedding compression and gravity slide.In the western Xiqiu tectonic belt,the Quele piercement salt body was controlled by the steep slope of the XiQiu paleo-uplift,which prevented the salts from flowing southward and driven the salt to accumulate on the northern margin of the XiQiu paleo-uplift and pierce surrounding rocks.In the Dongqiu tectonic belt,the shovel-style decollement faults developed in the sub-salt layers are dominated by the weak Triassic-Jurassic coal seam.4.Regarding the characteristic that the salt layer contain competent interbed in the Kuqa depression,this paper has designed a series of physical simulation experiments to analyse the influence of different factors on the structure deformation of competent interbed,including the overlying strata thickness,compression rate,interbed thickness,basement slope and syn-sedimentary action.The results show that the deformation of interbed is mainly dominated by dense folds,and the deformation intensity is more strong than that of the salt layer,supersalt and subsalt layers.However,the deformation range of interbed is relatively smaller and only concentrated in the compression side.On the whole,the deformation of the competent interbed is controlled by the violent thrust action of the sub-salt layers,the intensive thickening of the interbed usually occurred in the footwall of the sub-salt faults with prominent thrust displacements.The main factors influencing the deformation of the competent interbed contain the interbed thickness,underlying salt layer thickness,syn-sedimentary action and basement slope.