Formation of the First Bend of the Yangtze River caused by the late Eocene magmatic activity

The Jinsha River (mainstream of the upper Yangtze) exhibits an unusual planform pattern reflecting its passage through a tectonically active area. The Jinsha River flows southward through a deep mountain valley, but makes an abrupt turn northward in SE Tibet forming so-called the First Bend. The cause and timing of the formation of the First Bend have been a matter of debate for one century. In this study, we utilize detrital zircon U-Pb and biotite and K-feldspar 40Ar/39Ar dating to constrain the age of sedimentary formation and sediment provenance in the Jianchuan Basin located immediately southwest of the First Bend. A combination of new detrital 40Ar/39Ar ages and published ages of volcanic tuffs suggests that the Baoxiangsi, Jiuziyan and Shuanghe formations in the Jianchuan Basin have been deposited between 38 Ma and 34 Ma. The detrital mica, K-feldspar and zircon ages from this basin detect a significant change in sediment provenance occurred between 36 Ma and 34 Ma. The detrital mineral ages vary from multimodal age patterns to unimodal age distributions, implying a switch in sediment provenance from remote source to local lithologies. We suggest that the changes in sediment provenance were caused by the reorganization of the Jinsha River. The timing of this change in sediment provenance is coincident with the emplacement age of plutons (36-34 Ma) in the Jianchuan Basin. Based on detrital mineral ages and field observation, we suggest that the reorganization of the Jinsha River was caused by the emplacement of the plutons at the First Bend. The emplacement of more than twenty plutons distributed in 250 km2 blocked the southward flowing Jinsha River and diverted it to the NE forming the modern first bend.