Fast Valley Landscape Change of the Jinsha River in Southeastern Tibetan Plateau and Its Implications for Late Quaternary Tectonic and Climate Roles

The Jinsha River, the upper reaches of the Yangtze River, winds and incises through the eastern margin of the Tibetan Plateau, where active tectonics and Asian monsoon climate prevail. Quaternary valley landscapes are a combined record of both tectonics and climate over distinctive time scales. Along the Jinsha River, extensive and thick Quaternary valley fillings with hundreds of meters' thick and multiple river terraces have been widely developed in deep-incised bedrock valleys. However, their specific relations to climate change and tectonics remain elusive. In this study, we conducted field investigation, chronological studies and numerical modeling in its lower reaches. Our study indicates that the extensive valley fillings in the studied segment were formed in cold-arid climate with Optically Stimulated Luminescence (OSL) ages of ~23-19 ka B.P., coinciding with the timing of the Last Glacial Maximum (LGM). Multiple subsequent fill-cut terraces, postdating the LGM, were formed and nested in the valley fillings. OSL data indicate that sediments of three terraces (T1-T3) were formed at ~10 ka B.P., ~11.5-12.5 ka B.P. and ~13.5 ka B.P., respectively (Fig. 1). Taking climate records into account, we suggest that the post-LGM warming and humidifying, by restrained sediment supply while enhanced discharge, switched the valley from prevailing aggradation into incision and allow subsequent terrace formation. We propose that river terraces are sensitive records of millennial and sub-millennial cold-arid climate perturbations superimposed over the post-LGM deglaciation. Numerical modeling study provides evidence for this high sensitivity of valley aggradation and incision to cold-arid and warm-wet climate change over short time scale under high sediment transport influx. Our study supports that even in the tectonically active southeastern Tibet, climate occurs as a dominant role in landscape evolution outweighs tectonics, which tend to be expressed over long-term scales.

Fig. 1. Terraces ages obtained in the lower reaches of the Jinsha River plotted against the multiple climate records.