Mesozoic multi-directional compressional tectonics and formation-reformation of Sichuan basin

Abstract：The Sichuan basin located far away from the active plate boundaries is well known for its surrounding fold zones whose genesis and tectonic settings have long been one of the key issues in the geotectonic study of South China. Based on a compilation of regional structural geology and an analysis of superimposed folding structures, this paper presents a synthetic discussion on the Mesozoic compressional tectonics in such aspects as its deformation features and structural styles, main tectonic events and their timing. Strudies show that the Sichuan basin and its surrounding zones experienced three major tectonic events during Mesozoic, each one creating fold zones that were compounded spatially and resulted in the formation and reformation of the Sichuan basin. The Triassic collisional orogeny along the northern and western margins of the Yangtze cratonic block resulted in the formation of the proto basin, i.e., the Chuan-Yu-Dian -Qian sedimentary basin whose extending size is larger than the present-day Sichuan basin. Folds created by this tectonic event mainly trend in W-E direction. Tectonic regime changed profoundly during the Middle-Late Jurassic, dominated by multi-directional plate convergences in Asia, which caused rejuvenation of old orogenic belts and formation of new tectonic systems. In particular, the rejuvenation of the Qinling orogenic belt resulted in the development of the Micangsahn-Dabashan foreland fold and fault zone along the northern margin of the Sichuan basin; the westward subduction of the Paleo-Pacific plate activated the Xuefengshan tectonic zone and created the oroclinal folds in east and south Sichuan; the eastward extrusion of the Songpan-Ganze orogenic belt reactivated the Longmenshan-Jinpingshan thrust zone in west Sichuan. This multi-directional compressional event greatly reformed the proto Sichuan basin, and shaped the basic structural pattern surrounding the Sichuan basin. The third major event occurred in latest Cretaceous and was characterized by NW-SE compression, and it strengthened the NE-SW trending structures. It is concluded that the main fold zones in and around the Sichuan basin was characterized by pervasively distributed thin-skinned structures mainly formed in Middle-Late Jurassic, and in consequence of multi-directional intracontinental orogeny, they constituted in turn part of the convergent tectonic system in east Asia.