Early tectonic evolution of the northern margin of the Tibetan Plateau: Constraints from the sedimentary evidences in the Eocene-Oligocene of the Jiuxi basin

The Jiuxi basin, bounded by the Altyn Tagh fault and northern Qilian fault located in the northern margin of the Tibetan Plateau, is the perfect area to study the Early Cenozoic tectonic action, because the tectonic events of the two faults were recorded in the Late Eocene sediments. In this paper, an integrated research of sedimentary facies, heavy minerals, and paleocurrents on eleven sedimentary sections from the Jiuxi basin provides a detailed record of the early tectonic evolution of the northern margin of the Tibetan Plateau since the Late Eocene. Then these data indicate the sedimentary frame of the earliest sediments records, the Huoshaogou Formation, in the Jiuxi basin was constrained by the left-slip motion along the northern part of the Altyn Tagh fault. However, the Altyn Tagh fault could not extend beyond the Tibetan Plateau within the Alxa block because of the rigid nature of the Alxa block. Hence this strong crustal extrusion from the Altyn Tagh fault had to southwardly transform to the northern Qilian fault to south which resulted Early Cenozoic thrusting of the northern Qilian fault while the foreland basin stage of the Jiuxi basin had been formed.