Provenance Analysis in the Nima Basin during Paleogene and Its Implications for the Decline of the Tibetan Central Valley

Abstract

It is unclear what caused the Bangong Nujiang suture zone in the central Tibetan plateau to rise from less than 2 km in early Cenozoic to more than 4 km at present. The zircon U–Pb ages and trace elements of samples from the Niubao Formation in the Paleogene of the Nima basin were analyzed and tested. Combined with the isostasy theory, the surface uplift height of the Nima Basin during the Cenozoic period was calculated. The zircon U–Pb age results of the Niubao formation are consistent with the ages of the Lhasa terrane on the south side of the basin, the Qiangtang terrane on the north side, and the uplift in central. The zircon Eu/Eu* results show that the crust in central part of Tibetan plateau thickened by ∼20 km in Paleogene, resulting in ∼3 km surface uplift. Sediments created a total of about 1 km of surface uplift throughout the Paleogene, and the deposition rate began to slow down significantly at ∼40 Ma. Therefore, it is inferred that in the early Cenozoic, the uplift of the valley was mainly caused by sedimentation. With the continuous downward subduction of the Indian plate, at about 40 Ma, factors such as crustal shortening dominated the uplift of the central valley, and the uplift caused by deposition only accounted for a very small part. In general, the uplift of the Central Valley in the Paleogene was mainly affected by crustal shortening, but a quarter of the surface uplift was caused by the accumulation of sediments.