Zircon U-Pb chronology and provenance of the Paleogene sandstones in the Nima basin, Tibet: implication for coeval paleogeography

Nima basin which is located in the Bangongco suture was surrounded by several thrust faults and its paleogeographical features and tectonic evolution process can be restored by the Paleogene sediments. Based on the methods of petrology, sedimentology, and zircon chronology, the features of provenance, paleogeography and tectonic activities of the Nima basin were analyzed through studying its Paleogene section. The compositional characteristics of sandstones, paleocurrent direction, and zircon U-Pb chronology indicate that the adjacent terranes and the thrust belts in the suture zone are the provenance areas for Nima basin. Qm-F-Lt, Qt-FL triangular diagram shows that the component of sandstones mainly belongs to the recycling orogenic zone, and indicates that the southern depression experienced intensive tectonic activities during the Paleogene period. Based on the characteristics of heavy minerals and sediments, the tectonic evolution of the south depression of Nima basin can be divided into four stages: the stable stage in Early Eocene, the strong active stage in Late Eocene, the highly stable stage in Early Oligocene, and the highly active stage in Late Oligocene. The collision between the Indian plate and Eurasian plate in the Early Cenozoic period led to the crustal shortening and uplifting of the Nima basin and the reactivating of the thrust fault system. The ancient river system which developed due to the uplift of the surrounding terrain and the strong tectonic orogeny provided debris deposits to the sedimentary center of the ancient lake, and therefore the coarse debris of fan delta develops on the edge of the steeper terrains of the lakes. The southern Nima basin eventually evolved into an independent sedimentary depression with multi-directional source and its paleogeography feature depended on the regional tectonic activity.