Discovery of the Miocene Yuanmou Conglomerate and Its Significance for the Drainage Evolution in the Southeastern Tibetan Plateau

The active deforming southeastern margin of the Tibetan Plateau has developed series of fault basins filled with fluvial-lacustrine sediment deposited by drainage systems during every stage of its uplifting, the records of Cenozoic sedimentary sequence and detrital provenance in these basins are often obscure, making the interpretation of the drainage reorganization quite difficult. This research discovered the earliest Late Cenozoic fluvial conglomerate in the Yuanmou Basin, one of the fault basins in SE Tibet, and named the conglomerate as 'the Yuanmou Gravel Layer (YMG)'. Clast petrography, morphology and paleocurrent directions of the YMG indicate that the conglomerate is the remain of a large paleo river traversing through the Yuanmou Basin from north to south. Detrital zircon U-Pb ages can imply the YMG has similar provenance areas with Anning, Dadu and Yalong rivers at age peaks of 175-275, 600-1 000, and 2 100-2 700 Ma, but the lack of the age peak of 30-100 Ma from Qiangtang Block implies that the paleo-river was not yet connected with the Jinsha River during the YMG's accumulation. The fluvial-lacustrine strata with gravel beds containing paleosimian fossils comparable to the YMG were deposited similar to 7-11 Ma, further suggesting that the pre-formed south-flowing river was active during the Late Miocene.