Geometrical Types,Distribution Characteristics and Genetic Mechanism of Structural Deformation in Southern Dabashan Foreland Fold-Thrust Belt,Central China

The southern Dabashan foreland fold-thrust belt lies between the Qinling orogenic belt and Sichuan cratonic basin,which is formed by the combined action of Late Triassic plate subduction and collision of the Qinling orogenic belt and subsequent intracontinental orogeny. From NE to SW,it can be divided into three sub-belts based on geometrical characteristics of its structural deformation. (1)The inner-belt adjacent to the Chengkou fault is characterized by an intense thrust system. The dominant structural assemblage is imbricate faults,accompanied by such subordinate structural assemblages as duplex,the triangle zone and pop-up structure. (2)The middle-belt between the Pingba fault and Jimingsi fault is characterized by the thrust-fold system with the fault-bend fold as its dominant structural assemblage. (3)The outer-belt between the Zhangba-Jimingsi fault and Tiexi-Wuxi concealed fault is characterized by the deformation-fold system. The dominant structural assemblage is the Jura-type-fold with different types of deformation-folds such as box anticline,kink structure,over anticline,recumbent fold,tight anticline,isoclinal anticline and detachment-disharmonic anticline. Three major regional detachment layers are found vertically: Sinian to Cambrian clay shale,Silurian clay shale and Lower Triassic halite-bearing gypsum. As the southern Qinling belt propagated southward,the fold-thrust system developed from nappe thrusting,with elevation of the three main deformation layers and detachment toward south within these layers. Actually it grows on the detachment evolvement sequence: errace slipping→secant slipping→interlayer-slipping. This model provides new insights into evolution of folding and thrusting by cap rock detachment,and is helpful to understand the deformation mechanism of the southern Dabashan foreland fold-thrust belt.