Superposition of structures in the SichuanYunnan N-S tectonic belt at the eastern margin of the Tibetan Plateau: constraints from structures and magnetic fabrics

In this study on the northern section of the Sichuan-Yunnan tectonic belt (SYNSTB), which is on the eastern margin of the Tibetan Plateau, we assess the characteristics of superposed structures in the Daliang-Shan and Hanyuan-Ganluo fault belts, as well as in the Ganluo Basin that lies between them. By integrating field observations of structures with an analysis of magnetic fabrics, we show that the northern section of the SYNSTB underwent two episodes of deformation during the Mesozoic-Cenozoic. The first episode was characterized by NE-SW shortening that was initiated in the Late Jurassic-Late Cretaceous, and it may be related to the clockwise rotation of Yangtze Block and the far field effect of the Neo-Tethys ocean basin closed; and the second by NW-SE compressional shearing that was initiated in the Miocene-Pliocene and continues to the present day, and it is the result of the eastern extrusion of the Tibetan Plateau material. The magnetic fabrics associated with the weak deformation of the Ganluo Basin are mainly the result of layer-parallel shortening before folding, which is inferred to indicate Mesozoic NE-SW compression. In addition, the following atypical magnetic fabrics are present: (1) a magnetic foliation that is oblique to the bedding and associated with layerparallel simple shearing during folding, and (2) a magnetic lineation that is oblique to the strike of bedding to varying degrees, and that is the result of two superposed structures. The integrated study of field structures and magnetic fabrics indicates, therefore, that the N-S structures of the SYNSTB were mainly developed in the Mesozoic, but were subsequently overprinted by left-lateral strike-slip shearing during the Cenozoic. The resulting composite tectonic system provides further information on the tectonic evolution of the eastern margin of the Tibetan Plateau during the Mesozoic and Cenozoic.