Multiple climatic cycles imprinted on regional uplift-controlled fluvial terraces in the lower Yalong River and Anning River, SE Tibetan Plateau

Abstract

The development of fluvial systems on the southeastern margin of the Tibetan Plateau is linked to significant and rapid late Cenozoic uplift. The relatively complete fluvial terrace sequence preserved along the Yalong River valley and that of its tributary, the Anning River, provides an excellent archive for studying the development of terraces in rapidly uplifting mountainous areas. This study reveals that terrace development is predominantly controlled by multiscale climate cycles and long-term uplift, as shown by terrace dating, sedimentary characteristics, and incision rates. At least six alluvial terrace units were identified in 20 transverse sections through the terraces along about a 600km length of river and were dated using Electron Spin Resonance (ESR) and Optically Stimulated Luminescence (OSL). The climatostratigraphic positions of the terrace deposits and their respective age constraints suggest that fluvial aggradation was concentrated during Marine Isotope Stages (MIS) 32, 22, 18, 4, 2, and the Younger Dryas (YD) and that incision occurred during the succeeding cold-to-warm transitions. The changes in fluvial style marked by terraces 6, 5, and 4 predominantly occurred in synchrony with the 100-ka Milankovitch climate cycles, while terraces 3 and 2 were controlled by the obliquity-driven 41-ka climate cycles. Finally, the aggradation of terrace T1 occurred in response to the YD stadial. During the intervening time between 0.72 and 0.063Ma, terraces either did not form or were not preserved, which may suggest that uplift rates varied through time and influenced terrace formation/preservation. The progressive valley incision recorded by these fluvial terraces cannot be entirely explained by climate cycling alone. Temporal and spatial variations in incision rates indicate that the continuing long-term incision has been driven by uplift. The temporal distribution of the incision rates reveals two rapidly uplifting stages in the southeastern Tibetan Plateau, including an accelerated uplift that has been taking place since 0.06Ma. The spatial distributions of differing incision rates reflect the geomorphological response to crustal shortening and differential uplift in this region.