[1] Detrital zircon U-Pb geochronology was applied to Cenozoic fluvial sedimentary rocks from the Jianchuan Basin, Yunnan Province, China to constrain the provenance and the nature of paleo-drainage. Local geology testifies to a large river flowing through the Jianchuan Basin during the Paleogene and this previously has been linked to a paleo drainage system that connected the Qiangtang and Lhasa blocks to the South China Sea. The detrital zircon results from this study do not fit with this model and instead show provenance consistent with a river draining a watershed within the Songpan-Garze Complex, most likely from the northeast. From the late Oligocene and thereafter zircon provenance records greater contributions from erosion of local sources that surround the basin including the South China Block and Yidun Arc rocks that suggest loss of the northern sources. The timing for these changes overlap with regional deformation related to strike-slip faulting or displacement by shear strain rather than the later uplift associated with an expanding margin of the Tibetan Plateau.Components: 5200 words, 6 figures.

[1] The Cenozoic Song Hong-Yinggehai Basin in the South China Sea contains a large volume of sediment that has been used in previous studies, together with regional geomorphology, to argue for the existence of a large palaeodrainage system that connected eastern Tibet with the South China Sea. To test this and to understand the significance of sediment volumes deposited in the Song Hong-Yinggehai Basin, this study compared erosion histories of source regions with sediment volumes deposited during the two main stages in basin evolution spanning active rifting and subsidence (30-15.5 Ma) and postrift sedimentation (15.5 Ma to present). The study of basin provenance by detrital zircon U-Pb dating revealed Hainan was an important and continuous source of sediment, and a bedrock thermochronological study quantified its overall contribution to basin sedimentation. Comparison between the accumulated mass of basin sediment and volumes of eroded bedrock, calculated from apatite thermochronometry across the modern Red River drainage in northern Vietnam as well as Hainan Island, accounted for the bulk of sediment deposited since 30 Ma. Consequently, if an expanded paleodrainage ever existed it must have predated the Oligocene.

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