The rise and demise of the Paleogene Central Tibetan Valley

Abstract

Reconstructing the Paleogene topography and climate of central Tibet informs understanding of collisional tectonic mechanisms and their links to climate and biodiversity. Radiometric dates of volcanic/sedimentary rocks and paleotemperatures based on clumped isotopes within ancient soil carbonate nodules from the Lunpola Basin, part of an east-west trending band of basins in central Tibet and now at 4.7 km, suggest that the basin rose from <2.0 km at 50 to 38 million years (Ma) to >4.0 km by 29 Ma. The height change is quantified using the rates at which wet-bulb temperatures ( T w ) decline at land surfaces as those surface rise. In this case, T w fell from ~8°C at ~38 Ma to ~1°C at 29 Ma, suggesting at least ~2.0 km of surface uplift in ~10 Ma under warm Eocene to Oligocene conditions. These results confirm that a Paleogene Central Tibetan Valley transformed to a plateau before the Neogene.

Rapid Eocene diversification of spiny plants in subtropical woodlands of central Tibet

“…The rich assemblage of spiny plant fossils from the late Eocene (~39 Ma) along the Bangong-Nujiang Suture Zone in central Tibet traces the evolutionary history of spiny plants in Eurasia back to the Paleogene. These fossils evidence an early diversification of spiny plants in the Tibetan region contemporaneous with the emergence of open semi-wooded habitats by the late Eocene, and early in the transition of central Tibet to full plateau formation which appears to have been almost complete early in the Oligocene 36 .…”

“…Despite the initial influx of moisture, the retreat of the Neotethys during that period resulted in a reduction in the amount of water vapour entering the then shallowing valley, which, together with a growing Himalaya, led to the increasingly dry Neogene environmental conditions across the Tibet and central Asia 64 . A recent study demonstrates both drying and cooling occurred in central Tibet as the surface elevation of the Tibetan Central Valley increased from ~2.5 km to more than 4 km between 39 Ma and 29 Ma 36 .…”

“…3b ), with the valley bottom being at an elevation of ~1500 m at 47 Ma 33 and ~2600–2700 m at ~39 Ma (Supplementary Note 2 ), bounded by the Gangdese and the Tanggula (Qiangtang) highlands with crest heights > 4 km to the south and north 58 respectively. It was not until just prior to the Neogene that the modern low relief high elevation plateau formed 36 , 51 , 59 . During the middle Eocene, a warm and humid airflow evidently penetrated the valley, supporting a humid climate and subtropical biota 33 , 54 , 60 .…”

“…The Bangong-Nujiang Suture Zone hosts thick Cenozoic deposits, the lower part of which in the adjacent Lunpola and Bangor basins comprise the Paleocene-Eocene Niubao Formation and the upper part the Oligocene-Miocene Dingqinghu (Dingqing) Formation 75 – 77 . The Niubao Formation, up to 3,000 metres thick 36 , is mainly formed of fluviatile red clastic rocks interspersed with greyish green mudstones and sandstones. Plant fossils have recently been reported from mudstones in the middle part of the formation near the top of the lower member of the Niubao Formation, and include, amongst others, Ailanthus maximus 49 , Lagokarpos tibetensis 78 , Illigera eocenica 79 , and Asclepiadospermum marginatum and A. ellipticum 80 .…”

“…The Dingqinghu Formation is characterised by grey lacustrine mudstones together with sandstones and oil shales, with a total thickness of about ~1,000 metres 36 , 42 , 76 . It bears rich records of animals and plants, including two Oligocene ostracode assemblages: Austrocypis-Cyprinotus-Pelocypris and Ilyocypris-Limnocythere 81 , 82 , fishes including Plesioschizothorax 83 , and mammals such as Plesiaceratherium 42 .…”