TECTONIC DEFORMATION HISTORY AND PHYSICAL MODELING IN THE MIDDLE PIEDMONT ZONE OF LONGMEN MOUNTAINS

Longmenshan thrust structure belt can be divided into several belts in NE direction and into several sections in EW direction.The overthrust zone of middle section of Longmenshan structural belt is featured by occurrence of Pengxian-Guangxian Complex and development of klippe in its front edge,with well developed tight and isoclinal folds dipping towards northwest.The deformation of nappe-slumped overthrusts are very strong,developing a series of imbricate faults and related folds as well as a series klippes composed of Devonian-Lower Triassic carbonate rocks.Thick-skinned structures dominated by imbricate overthrust structures occur mainly in the hinterland,while the deformation of foreland depression is relatively weak and monoclines plunging in the southeast SE direction on surface.Structural geophysical modeling revealed that the angle between the direction of maximum principal stress and the fault lines is 70° and the deformation pattern is the structural combination of single detachment and vertical overlap controlled by double detachment.The structural deformation processes can be divided into three stages:vertical overlap of detached faults after the deposition of Xujiahe Formation in early stage,the vertical overlap of detached faults during the deposition period of Suining Formation and Penglaizhen Formation in the middle and late stage,which controlled the sedimentation.