Research progress on the age, paleoenvironment, and paleo-elevation of Cenozoic strata in the Lunpola Basin, Central Tibetan Plateau

The most significant topographic change on Earth is the uplifted Tibetan Plateau that was caused by the India-Eurasia collision. There has been a long-standing dispute about the time of collision and the uplifted paleo-elevation history, which can help to characterize not only the uplift process, but it also serves as a link between the Earth's deep lithosphere and the upper crustal-surface spheres. In this context, it is an important scientific issue to study the paleo-elevation of the plateau in different tectonic domains and in different time-slices. Only by accurately reconstructing the paleo-elevation can we better examine the uplift mechanism of the Tibetan Plateau and its impact on regional and global climate. During the past 20 years, great progress has been made in reconstructing the paleo-elevation of the Tibetan Plateau, but we still have a long way to go to solve such a complex and difficult scientific problem. At present, there are still controversies about the specific age of many Cenozoic strata and consequently uncertainties about different paleo-elevation reconstructions. This paper aims to review the most recent progress in the chronology, paleoclimatic change, and paleo-elevation reconstructions of the Lunpola Basin in central Tibetan Plateau, where has received most comprehensive studies in recent years. First, the chronology of the Cenozoic strata in the Lunpola Basin has been well defined, particularly due to the discovery of up to ten tuff layers in the Niubao and Dingqing Formations, which have provided new absolute age constraints of their deposition. The age of the Niubao Formation has changed from Paleocene-Eocene to Eocene-Early Oligocene, and the age of the Dingqing Formation ranges from Late Oligocene to Late Miocene. Second, plant fossils from the Niubao Formation in the Lunpola Basin have revealed a tropical and subtropical humid forest in the late Early Eocene (47 Ma), and a subtropical open forest in the late Eocene (39 Ma). This ecological shift was related to global cooling after the early Eocene climatic optimum and to the reduction of water vapor transport caused by the retreat of the Neotethys Sea. The vegetation type of the Dingqing Formation is dominated by warm-temperate mixed coniferous and broad-leaved forests. The increase of coniferous forests and the existence of vertical vegetation gradient indicate that the orogenic belt surrounding the Lunpola Basin had reached a certain height at the end of the Oligocene. High-resolution paleoclimatic records indicate that the lower and middle Dingqing Formation recorded dry-wet alternations with a 400 ka quasi-cycle, implying that the eccentricity of the Earth's orbit drove the dry-wet oscillations in the Lunpola Basin between 25.5 and 20 Ma; whereas the stable isotopic records of the upper Dingqing Formation reveal that an enhanced cooling and dry climate prevailed after 14 Ma, which was a response to the East Antarctic ice-sheet expansion and to global cooling after the Mid-Miocene Climatic Optimum. Finally, the middle Niubao Formation and the Dingqing Formation are rich in animal and plant fossils, and their abundant paleontological data suggest that the paleo-elevation of the Lunpola Basin was mostly at 2000-2700 m during the late Eocene, and at 3000-3200 m in the Late Oligocene-Early Miocene (26-20 Ma). During the early Late Miocene (similar to 12 Ma), pollen evidence of aquatic floating-leaved plants suggests that the maximum paleo-elevation of the ancient lake surface in the Lunpola Basin was at similar to 3400 m, which means that the Lunpola Basin underwent a similar to 1200 m rise in elevation after the Late Miocene.