Research On The Structural Characteristics,Formation And Evolutions Of Guandoushan In South Sichuan Basin

The Guandoushan structure is located in the transitional part of the southeast Sichuan fold belt and the southern part of the central Sichuan uplift belt,and it is located on the west side of the Huayingshan faults zone.It is also affected by both the central Sichuan hard block and the Huayingshan faults zone.The Guandoushan structure in the south of Sichuan Basin is a typical north-east fold in the southwestern part of the south Sichuan Basin brush structural belt.The research done on the Guandoushan structure has been weak,and most of them are geological features of the surface.There is no specific study of its origin and evolution.From the viewpoint of global tectonic activity theory,based on the interpretation of 2D seismic profiles,drilling data and regional geological data,combined with basic theory and analysis methods such as structural geology and sedimentology,the tentonic characteristics,formation and evolution of the Guandoushan structure were studied by means of balanced section technology and structural physical simulation experiments.The understanding of structural geology that conforms to the characteristics of actual regional structural deformation has been obtained.The geometrical characteristics,kinematical characteristics,dynamical characteristics,and tectonical evolutional history of the study area have been analyzed.The following understandings have been obtained:(1)Geometrical characteristics: There are three detachment layers in the vertical direction of the Guandoushan structure,which is characterized by layered deformation.From the bottom to the top,it can be divided into three deformation layers.The bottom deformation layer is the basement-Sinian system,and the detachment fold formed along the slip near the interface between the basement and the overlying strata.The middle deformation layer is the Cambrian-Lower Triassic Feixianguan Formation.which is a fault formed by slipping off of the Middle-Lower Cambrian detachment layer,followed by reverse faults.The two faults are confined to the pop-up structure and the fault disappears in the detachment layer of the Jialingjiang Formation.The upper deformation layer is the reversed fault that breaks through the Jialingjiang Formation to the surface and slides off along the detachment layer of the Jialingjiang Formation.(2)Kinematical characteristics: The following main detachment layer of the Middle-Lower Cambrian strata are the boundary of the Guandoushan structure,and there is a significant gap between the shortened amount of the upper and lower strata.The existence of the detachment layer controls the overall structural deformation and displacement characteristics and strength to a certain extent,absorbs a certain amount of structural deformation and displacement,and causes significant differences between the upper and lower structural features.detachment layer from north to south controls the overlying strata gradually strengthened.Overall from north to south,the overall deformation is weakened.The amount of shortening of each line from bottom to top generally increased.It is presumed that the number and intensity of tectonic movements experienced by the upper strata are relatively large,and the deformation of the entire cap rock is less frequent,and the main detachment layer of the Middle-Lower Cambrian causes the deep deformation to be weak.(3)Dynamical characteristics: The Guandoushan structure is a pop-up structure that is confined by back thrust faults along the detachment layer of the Middle-Lower Cambrian in the Middle-Late Triassic under the action of the two-dimensional compression forces of the South East-North West.(4)History of tectonic evolution: The Guandoushan structure was formed in the Middle-Late Triassic and was squeezed along the Middle-Lower Cambrian Weak layer during the Indosinian northwest-southeast direction to form a fault,followed by reverse fault.They formed a pop-up structure.It is the most prominent feature of the Guandoushan structure.From the Late Cretaceous to the Paleogene,it was squeezed from the northwest to the southeast,forming detachment fold along the detachment surface of the basement and cap rock,and the deformation of the upper strata was strengthened.During the Neogene period,it was once again squeezed by the northwest-southeast direction.The slipping of the gypsum salt rock along the Lower Triassic Jialingjiang Formation broke through to the surface.Since then,it has continued to be affected by the Himalayan period.Cap rock generally deformed to strengthen,strongly uplift and eroded.The surface Guandoushan structural core has been eroded to the Upper Triassic Xujiahe Formation and the two wings are Jurassic strata.