Influence Factors Controlling The Structural Deformation Of Fold-thrust Belts In Southwestern Tianshan

The deformation of the eastern and western sections of the fold-thrust belts of the southwestern Tianshan is obviously different,the "stacking" structure and basement involved thrust structure are developed in the fold-thrust belts of the Northern in Kashgar area.It is obviously different from the thin skin thrust structure of Kalpin foldthrust belts,on the east side.In this paper,based on structural profile interpretation,using discrete element numerical simulation research and single factor variable control method,design Twenty-eight sets of numerical simulation experiments to analyze the effects of controlling factors of structural evolution in the research area,including the cohesion of decollement,the Position of decollement,the thickness of sedimentary cover,the unidirectional and bidirectional extrusion,and the pre-existing fracture.The experimental results as follows:(1)The cohesion of the decollement has an important effect on the deformation of the fold-thrust belts.the cohesion of the decollement controls the difficulty of the preexisting faults upward breakthrough.When the cohesion of the decollement is high,it is easy to form basement involved thrust structure along the pre-existing faults.When the cohesion of the decollement is low,the roof faults of thrust system developed in decollement,the fault below the decollement is prevented from spreading upward.In the area where the low cohesion decollement is developed,the deformation front propagates faster and farther forward,the deformation mainly occurs in the upper unit,mainly contains the pop-up structural and triangular belt,the structural deformation of the lower unit is relatively weak.The lower the cohesion of the decollement,the easier it is to develop thin skin thrust detachment structure.When the thickness of overlying sedimentary strata is thicker,the thrust belt is larger,the number of the thrusting belt is fewer,and the distance between thrust belt is larger.(2)In unidirectional and bidirectional experiments,the stress and strain are mainly concentrated in the weak zone such as the decollement and the fault,the stress and strain are stronger near the extrusion side.The smaller the cohesion of decollement,the more easily the stress spreads to the distance.In bidirectional experiments,while in short distance,the stress can propagate to the far side through the low cohesive decollement,while in long distance,the deformation on the other side will not be affected.(3)The "stacking" structure is developed in the fold-thrust belts of the Northern in Kashgar area,mainly controlled by the gypsum-salt layer of Paleogene and the preexisting faults of pre-Cenozoic,and affected by the south-to-north compression from the Pamir side.The gypsum-salt layer of Paleogene can be seen as an effective low cohesive decollement,In the extrusion process,the pre-existing faults that formed before Cenozoic near the extrusion side were activated successively and overlapped each other to form a series of thrust plates,the gypsum-salt decollement as the roof faults,the fault below the decollement is prevented spreading upward and southward,the "stacking" structure is developed near the extrusion end.Kashgar northern thrust belt is located in Pamir and south Tianshan intersection zone,the frontal faults of Kashgar effect by Pamir thrust-belt thrust extrusion from the south to the north,hinder faults of kashgar due to the inhibition of the southern to northern thrust faults on the Pamir side,did not continue to propagate southward and upward.Lead to that there is not formatting the thin skin thrust structure of Kalpin in the fold-thrust belts of the Northern in Kashgar area.(4)The thin skin thrust structure is developed in the fold-thrust belts of Kalpin,mainly controlled by the gypsum-salt layer of the middle Cambrian and the thick of cover sediment of Paleozoic and Cenozoic.The gypsum-salt layer of the middle Cambrian can be seen as an effective weak cohesive decollement,Paleozoic and Cenozoic strata are developed in Kalpin area formatting the thick of cover sediment,the effectiveness of the decollement is enhanced.In the extrusion process,there are a series of thrust plates are formatted.These faults were merged into the middle Cambrian decollement,forming a large scale the thin skin thrust structure.Kalpin area is far from the West Kunlun-Pamir side,the thrust deformation front was not affected by the West KunlunPamir extrusion from south to north.Inside the Kalpin thrust belt,with the Piqiang fault as the boundary,the Kalpin thrust belt is divided into two sections,the gypsum-salt layer of the middle Cambrian in the west is deeper than the east,and the overlying sedimentary strata in the west is thicker than the east.Lead to the thrust belt is larger,the number of the thrusting belt is fewer and the distance between thrust belt is larger in the west of the Kalpin thrust belt.