Magnetic Fabrics in Jingzhuba-Shiwo Section of Foreland Fold Belt of the South Dabashan: Constraints on Its Tectonic Evolution

A detailed investigation of the structural geometry and magnetic fabrics together with rock magnetism has been carried out along Jingzhuba-Shiwo section located in the foreland fold belt of the South Dabashan, which aims to shed light on the superimposed deformation and tectonic evolution. As a whole, the intensity of deformation decreases from the northeast to the southwest, and the axial planes dip northeastward and the hinges of large-scale folds plunge northwestward at small angle. Moreover, the maximum principal stress indicates that the compression of the South Dabashan is dominant as per palaeostress analysis. The sampling horizons before the Late Jurassic are mainly characterized by magnetic fabrics in deformed rocks, whereas the sampling horizons ranging from the Late Jurassic to the Early Cretaceous are characterized by incipient magnetic fabrics in weakly deformed rocks. The magnetic lineations are dominantly oriented NW-SE and remain almost parallel to the main tectonic structures of the section, suggesting that AMS can be ascribed to tectonic processes. Also there are two types of unusual magnetic fabrics revealed. The first one merely occurring in the northeast section is characterized by an oblique magnetic foliation with respect to bedding, which is related with parallel-bedding simple shear during fold growth. The other one caused by tectonic superimposition is characterized by an oblique magnetic lineation with respect to bedding strike. The plunges of Kmin increase with decreasing deformation intensity from northeast to southwest along the section, which can be taken as a gauge to distinguish the deformation intensity of weakly-deformed sedimentary rocks according to the relativity. Consequently it can be concluded that the superimposed structure of the section was formed by the combined compression which arose from the South Dabashan primarily and the Micangshan and the eastern Sichuan fold belt secondly during the Late Jurassic. Afterwards the compression from the South Dabashan was still dominant, the compression from the other two directions, however, became much weaker in the Early Cretaceous than in the Late Jurassic. Furthermore, the effects resulting from Qinling intracontinental orogeny during J3-K1 and Xuefeng intracontinental deformation could be reflected by the results mentioned above, which can also provide evidences for exploration of continental dynamics.