The First Bend along the Changjiang River formed in the southeast margin of the Tibet Plateau and the middle of Three Parallel Rivers area,located within the fold belt that formed between the northward subducting Indian plate and the stable Yangtze Platform.The rivers evolution in this region is critically related to the tectonic deformation.Large-scale strike-slip faults,linear geomorphic features,block mountains and graben basins widely formed,near the First Bend along Changjiang River,where the river landscape is one ideal indicators for studying the tectonic,climate and other geological processes.There are four levels of terraces(T1～ T4)formed at the Judian-Daju segment of Jinsha River.In this paper,we mainly studied the T1 and T2 terraces of this segment from two aspects,including the analysis of the morphological characteristics and the chronology study of terraces.We collected the sand samples of the first-order terrace and the second-order terrace to constraint the ages by the optical luminescence dating.And we also analyzed the longitudinal profile,cross profile and plane distribution characteristics and other morphological characteristics of terraces.We employed data from digital elevation model(DEM) to study the plane features of the terraces and compared with the valley cross section.We suggest that the preservation of the terraces is closely related to the river bend and the width of the valley.The terraces were well preserved in a wider and more crooked valley.The height of fluvial terrace staircases above the local river level is used for inferring relative tectonic motions between the segments.The correlated longitudinal terrace profiles show no distinct relative tectonic movements within the catchment area.However,the elevation of terrace above the river water is increased along the Jinsha River to the lower reaches of the Tiger Leaping Gorge.During our field investigation,we found that the downstream incision rate of our study area is higher than upstream.Those results demonstrate that the downstream segment behaved as different tectonic block under differential tectonic movement and the Haba-Yulong micro-block possibly experienced a faster uplift.The OSL ages of T1 and T2 are 9～ 12 ka and 15～20 ka respectively.We complemented the chronology of the terraces around The First Bend.Terraces (T2 ～ T4) near the First Bend are generally covered by the paleosol-like deposits,and height difference between each terrace is relatively light.The second terrace was made up by several binary-structured deposits cycle that indicate Bouma sequence.The sedimentary features and the chronology suggest that the deposition of T2 is formed during the Last Glacial Maximum(LGM:21～ 18 ka),and the subsequent erosion process may occurred after 15 ka or even later.T1 deposited in the warm period during the Holocene.It's clear that the formation and incision processes of the two terraces occurred under different climatic conditions.This may indicate that the stronger the tectonic movement was,the better the climatic imprint as expressed in the form of the development of the terrace.In addition,the ages and incision rates of the terrace are much more lager in the downstream,so we speculate that the inferred tectonic motions played a key role in the terrace incision process.%通过DEM及野外调查,提取和分析长江第一弯河流阶地的空间位置及分布特征,认为在河流弯曲及宽阔河段阶地的保存范围大于其他河段;根据阶地序列年代学研究提出,长江第一弯河段T1和T2堆积阶地分别形成于9～12 ka和15～20 ka,这两级最新阶地分别形成于不同的气候背景.T2开始堆积于末次冰盛期(LGM),下切于15 ka前后;而T1堆积于全新世初期的大暖期,下切于9 ka前后.而且,两级阶地均存在向断裂方向的上翘,河谷摆向断裂上升盘的相反一侧;河流下切速率也向着断裂方向增大.因此,河流阶地的形成应该受到构造因素的控制,而气候作用对河流下切有明显的影响.
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