Pliocene to Early Pleistocene Drainage Reorganization in Eastern Tibet Inferred From Detrital Zircons

The drainage evolution in eastern Tibet during the India-Eurasia collision has attracted considerable attention because of its critical link to topographic development and plateau growth, but the discussion still persists. In the present study, new and published detrital zircon U-Pb ages are integrated from Pliocene-Early Pleistocene Xigeda Formation (XGD) and modern river sands to decipher the paleo-drainage evolution in eastern Tibet. The results confirm the eastward flow of the Jinsha River before the Pliocene, but the drainage patterns of its major tributaries (Dadu, Yalong, and Anning) in the Pliocene, differ from the modern ones. In conjunction with the previously documented fluvial incision in eastern Tibet, we interpreted that the excavation of the Paleo-Xigeda lake in Early Pleistocene time initiated and accentuated incision of the Jinsha River and its major tributaries. This led to drainage reorganization and the modern river pattern establishment in eastern Tibet. Plain Language Summary The developmental history of the Jinsha River and its major tributaries (Dadu, Yalong, and Anning) in eastern Tibet are considered to be closely associated with the topographic evolution during the Cenozoic plateau growth. However, the formation timing and mechanism of the current configuration of these large rivers remains debated. Here, we used detrital zircon U-Pb age as "provenance tracer" to identify source signatures of the Pliocene-Early Pleistocene Xigeda Formation, a sequence of lacustrine deposits sporadically distributed along the Middle Jinsha River and its large tributaries. Our provenance results demonstrate that current east-flowing Jinsha River has been established before Pliocene time, while the present-day pattern of its major tributaries, including the Yalong, Dadu, and Aning rivers, was achieved in the late Early Pleistocene. Combined with previous thermochronological and provenance studies, we infer that the spillover of dammed lake initiated the fluvial incision that led to drainage reorganization and, consequently, the formation of the modern river system. This study not only challenge existing models of Plio-Pleistocene drainage pattern of eastern Tibet that flowed southward to the South China Sea, but also illustrates how river reorganization alone can dramatically influence landscape development even in the absence of significant tectonic and climatic forces. Key Points The Jinsha River has flowed eastward before Pliocene time The present pattern of the Dadu, Yalong, and Anning Rivers achieved in the Early Pleistocene The spillover of the Paleo-Xigeda lake during the Early Pleistocene triggered incision of the Jinsha River and its major tributaries