Late Cenozoic Landform Evolution Of Jinsha River Valley And Its Driving Mechanism

The evolution of fluvial systems in the south-eastern Tibetan Plateau is closely related to the plateau rapid uplift since Cenozoic Era.The Jinsha river is an important river in the south-eastern Tibetan Plateau where has great relative relief.Its river valley landforms and fluvial deposits in the valley are ideal archives for the study of geomorphic evolution,tectonic uplift and climatic change in this area.However,the geomorphic evolution of its valley is a current focus of attention and disputes in the field of geology.Although several complete terrace series exist in the Jinsha river valley,few researches have resolved the problem of these terrace sequences and their forming ages.Besides,the driving mechanism of these terraces remains to be seen.The Taoyuan paleolake is one of the faulted lake basins in the Jinshajiang river valley whose formation age is unknown.The relationship between its formation age and the plateau uplift needs further investigation.Despite the evidence for the uplift of the Tibetan plateau in the tectonic landform,the quantitative constraints are still lack.To solve these problems,detailed investigation is performed in the Jinsha river from Quagong to Qiaojia and its tributary Yalongjiang river downstream.Optically stimulated luminescence(OSL)and electron spin resonance(ESR)methods are used to determine the terrace order and the formation age for the Taoyuan paleolake.Several results are achieved as follows:(1)The developments of the river terraces in the Jinshajiang river basin have close corresponding relationships with Milankovitch cycles.They have at least recorded 17 climate variations during glacial-interglacial cycles.The accumulation should take place in the glacial period and the erosion should perform at the end of the glacial period when the climate gets warm.Different climate stages are controlled by different climatic periods.(2)The river terraces in the Jinshajiang river basin from Longkaikou to Jinjiangping are sensitive to the great climate changes in this area.The T13 terrace records the climate shift in 1.55 Ma B.P.and the T9 terrace keeps the information of climate variation in 0.99 Ma B.P..In about 0.30 Ma B.P.,when this area is drought,the T4 terrace is formed.And in the subsequent drying period,sand-gravel deposits of the T3 terrace begin accumulating.(3)In the last glacial period,5 terraces are formed responding to the climate changes,especially in the last glacial maximum period.The extreme climatic events and regional crustal uplifts during the Last Glacial Maximum period are the driving force for the river erosion.In addition to this,the accumulation and erosion in the Jinsha river basin respond to the 5 extreme drought events in Holocene.(4)The terraces are the joint products of both climate change and tectonic uplift.The climate shifts are the trigger for the switch between accumulation and erosion and the tectonic movements provide continuous driving force for the erosion creating sufficient space for next terrace.The terraces formed in a century scale in the sinking river valley in the Jinshajiang river basin can be independent products of climate shifts.(5)According to the incision rate in the Yalong river downstream area,quickly tectonic uplift has happened in 1.10?0.02 Ma B.P.and since 0.06 Ma B.P.and the relative intense uplift took place in the Jinpingshan area.The incision rate of the terraces in the Jinsha river basin from Longkaikou to Jinjiangping reveals 4significant tectonic uplift events of the Dadong-Dingxiling Block subordinated to the Dianzhong active-tectonic Block happened in 1540-1510 ka B.P.,1200-990 ka B.P.,320 ka B.P.and 80 ka B.P.respectively.Before 80 ka B.P.,this area uplifts as a whole and after 80 ka B.P.the south part uplifts more quickly than other parts.(6)The widely distributing lacustrine stratas in the area from Longkaikou to Taoyuan formed in about 2.6-1.55 Ma B.P.,which can be compared with Sheshan fm in the Heqing Basin.Faulted lake basins have widely existed in the Jinsha river basin such as Xigeda,Taoyuan and Shigu since Pliocene epoch.Their formations are the results of the local subsidence caused by fault activity at the boundaries between blocks which can be interpreted as the uplift of the south-eastern Tibetan Plateau as whole.Their extinctions are attributed to the river headward erosion caused by the intense overall uplift.