The Structural Characteristics And Deformation Mechanisms Of Multiple-Detachments Folds In Eastern Sichuan

In this paper,we study the geometry and kinematics of Multiple-detachments folds in Eastern Sichuan to analysis the deformation mechanisms and controlling factors of study area.The results of this study concerning about the structural characteristics and evolution of multiple-detachments folds and their controlling factors might be helpful for the model building of Eastern Sichuan to find the oil and gas and the investigations of the multiple-detachments folds in other fold and thrust belts.Detachment folds are widely distributed in the Eastern Sichuan.These folds have three detachments,the Cambrian salt at the bottom,the Silurian mudstone in the middle and the Triassic salt at the top.There are 7-8 high steep detachment folds in Eastern Sichuan.The Proterozoic under the Cambrian salt did not involve in the deformation of the detachment folds.As the eastern margin of Eastern Sichuan,Fang Dou Shan Fault was a thrust fault from basement.As the western margin of Eastern Sichuan,Hua Ying Shan Fault was a thrust fault from the Cambrian salt.Detachment folds developed two groups of faults in the Cambrian salt and the Silurian mudstone.The west trend faults developed in the early time and the east trend faults developed in the late time,which lead to that the west trend faults were cut by the east trend faults.With the increase of structural shorting,the uplift rate of the fold grows higher associated with both folding and thrusting.The sequence of faults development above the Cambrian salt were disordered.According to our quantitative calculation of the structural shorting of the Cambrian salt at the bottom and the Silurian mudstone in the middle and the shorting rate of the profile,taking the example from Nanmenchang anticline,we find that the structural shorting controlled the deformation of three detachments folds:when the shorting rate<20%,two west-dipping thrust faults each developed in the Cambrian salt at the bottom and the Silurian mudstone in the middle.Detachment fold was turned into Fault-propagation fold and the salt and mudstone were gathered at the core of the anticline.When 20%<shorting rate<30%,an east-dipping thrust fault developed in the Cambrian salt and cut off the two west-dipping thrust faults.When the shorting rate>30%,thrust faults developed and cut off the core of the fold.Three detachments folds are associated with both folding and thrusting in Eastern Sichuan,composing a quite complex multiple-detachments fault related folds.