Tectonic constraints on subduction closure processes of the BangongCo - Nujiang Tethys Ocean from the Middle Jurassic to the Late Cretaceous: A magnetic fabric study in the Linzhou Basin, Lhasa Block

Abstract: Tectonic events recorded during the Middle Jurassic to Late Cretaceous in the Linzhou Basin of the Lhasa Block are associated with the subduction closure process of the BangongCo - Nujiang Tethys Ocean. To constrain the subduction closure process of the BangongCo - Nujiang Tethys Ocean during this period, a detailed magnetic fabric study of five relatively continuous strata in the Linzhou Basin during the Middle Jurassic - Late Cretaceous was carried out. The results show that the Middle Jurassic Yeba Formation developed initial deformation magnetic fabric, pencil-like magnetic fabric and tensile lineation magnetic fabric; The Late Jurassic Duodigou and Linbuzong Formations developed initial deformation and pencil-like magnetic fabrics; The Early Cretaceous Tacna Formation developed strongly cleaved magnetic fabric and tensile lineation magnetic fabric with more intense deformation; The Late Cretaceous Shexing Formation developed initial deformation magnetic fabric and pencil-shaped magnetic fabric. Combining the movement of the fracture zone, the direction of the stress field and the analysis of the tectonic environment, it is concluded that the tectonic environment has undergone four processes of extension-extrusion-extension-extrusion from the Middle Jurassic Yeba Formation to the Late Cretaceous Shexing Formation. The fracture zone undergoes three main processes: clockwise dextral rotation - rotation stop – clockwise dextral rotation. There are also relatively frequent changes in the north-south direction of the extrusion stresses and plate drag forces to which they are subjected. Thus, under the continued northward subduction of the Neo-Tethys Ocean, the tectonic processes recorded in the Linzhou Basin are associated with altered subduction polarity of the BangongCo-Nujiang Tethys Ocean. The BangongCo-Nujiang Tethys Ocean shifted from southward subduction to northward subduction during the Middle Jurassic. It shifted again to southward subduction in the Early Cretaceous and eventually closed.