Research On Superposed Deformation Of Dabashan Foreland Structural Belt

The Dabashan Structural belt is a part of south Qinling Mountains, which has been regarded as a foreland thrusting belt of the collisional belt of the north and south China plates. According to tectonic deformation style and involved strata, the Dabashan structural belt can divided into Dabashan nappe-thrust belt, Dabashan foreland structural belt (including Basement Detachment Structural belt and Cover rock Decollement Structural belt) and Dabashan foreland depression belt.The results of the researches show that the Dabashan foreland structural belt has gone through strong superimposed deformation during the Mesozoic. The early deformation (D1) was related to the collision between the north and south China plates and is the foreland thrust belt of the Qinling collisional orogen. It occurred about the middle-late Triassic and the early Jurassic. In the western curvilinear structural belt, the deformation of Dl is of the folds with axis of nearly east to west trending. But both in central and in eastern of the belt, the mapping in lager scale shows that the fold axes of D1 are about nearly south to north. Based on the deformation sequence and the age constraints of the deformed strata, the age of the Dl recognised in the central and eastern curvilinear structural belt also is the late Triassic and the early Jurassic. The Dabashan curvilinear structural belt is composed of linear folds with curvilinear fold hinges and some curvilinear thrust faults (D2). Based on the Ar-Ar ages,161 Ma, of biotites from the ductile shearing zones in the north Dabashan thrust belt, the Dabashan structural belt was gone through a strong deformation. On the other hand, the deposition and filling process of Zigui basin show that the provenance and environment of the basin was changed between the middle and late Jurassic. Therefore, the authors believe that the age of D2 is the late Jurassic to the early Cretaceous. Because of the distinct superimposion of the two phases of deformation, it is impossible that they are progressive. So the Dabashan curvilinear structural belt records two phases of deformation which are formed during different tectonic settings. It is very important for the understanding of the evolution of Qinling orogen.