Stream evolution of the southeastern margin of the Tibetan Plateau: A case study of the Jinsha-Red River system

The Red River is generally considered to constitute the downstream end of a great river system that once ran through the southeastern margin of the Tibetan Plateau. Our study tests this model by confirming numerous coarse-grained distal deposits within the Lengdun Formation (27–17 Ma), which crops out along the range front of the Ailao Shan-Red River shear zone (ARSZ). Their U–Pb zircon age and lithology and geochemical characteristics can be all compared with the Jinsha suture zone of Early Triassic age in the far northwest. We interpret this suture zone as constituting the provenance of these distal detritus. The Jinsha suture zone south of the fault is defined by the Yunling Range, characterized by a flat top surface and covered by a series of Eocene stream deposits, the largest distribution area of which is the Laojunshan Basin. It is inferred that the Paleo-Jinsha River originally flowed to the south all the way to the Yunling Range and connected with the Paleo-Red River; its diversion caused the Paleo-Red River to lost its upstream, and the Eocene deposits preserved at the top of the Yunling Range mark the abandoned course of the Paleo-Jinsha River. The left-lateral movement along the ARSZ is interpreted as the manifestation of clockwise rotation of the Yunling Range. As a result, the differential shortening of the Yangtze block on its eastern part led to the Jinsha River being migrated to the north and south, resulting in the famous First and Second bends.