Résumé
The Jianchuan Basin, Yunnan Province, China, is the widest continental Cenozoic sedimentary basin in the southeastern Tibetan plateau. It is located approximately 10 km east of the Red River fault zone. Climatic simulations and paleoenvironment studies suggest that SE Asia has experienced a variable intensity monsoon system for 40 Ma. Because sediments can record deformation, climate and environment changes, the Jianchuan Basin provides the opportunity to assess the relative role of climate and tectonics on the Tibetan plateau formation. Sediments consist of floodplain siltites, massive fluvial sandstone, few carbonate levels, coal and volcanosedimentary deposits. U/Pb dating of zircons from dykes, volcanodetrital deposits and lava flows respectively cutting and interbedded within the sediments was performed by in-situ LA-ICPMS. All ages range from 38 to 35 Ma. Such absolute dating is confirmed by palaeontological evidence. Dental remains of Zaisanamynodon were found in coal deposits. This giant Rhino lived in Asia during the Ergilian (late Eocene). Our data allow us to propose a revised stratigraphy for the Jianchuan Basin: contrary to what was suggested by previous studies, i.e. a continuous sedimentation from the Paleocene to the Miocene, nearly no sedimentation occurred after 34 Ma. Combined with a sedimentological analysis, the data indicate that during the late Eocene, the Jianchuan area was covered by a large (>15 km) braided river system that coexisted with local transient lakes, in a moderate-slope and semi-arid environment. This major sedimentation event was followed by a period of more humid conditions that may be related to an intensification of the monsoon. The end of the sedimentation seems to be contemporaneous with the Ailao Shan-Red River fault activation. The new stratigraphy has also implications for regional studies that need robust age constraints, for example for reconstructing paleoelevation or provenance of sediments.