Study On The River Terrace In The Lower Reaches Of The Yalong River And Its Significances On The Neotectonic Movement

The lower reaches of Yalong River are located in the south-east edge of theTibetan plateau and the southern tip of the south-north seismic belt, with a strongtectonic activity and frequent earthquake, resulting in the typical tectonic landform. Inthe meantime, this area is greatly affected by East Asian monsoon, India monsoon andplateau monsoon, leading to the frequent climatic fluctuations during the QuaternaryPeriod. The river terrace is a better carrier of the neotectonic movement and climatechanges, through the study of which the processes of the regional climatic changesand the formation uplift can be explored. The lower reaches of Yalong River and itstributary the Anning River preserve the relatively complete river terraces, forming theinformation base of the climatic changes, the tectonic uplift and the evolution oflandforms on the south-east tip of the Tibetan pla teau. Over the long periods in thepast, there has been no systematic study on the river terrace of the lower reaches ofthe Yalong River, especially no reliable data of the river terrace and the formation age,which resulted in the limitation in acquiring the neotectonic movement data. As far asthe above problems are concerned, this paper takes the terraces of the Yalong Riverand its tributary Anning River as the subjects, through a lot of geological surveys inthe open air, and defines the terrace sequence of the lower reaches of the YalongRiver. Moreover, geochronological approaches such as OSL, ESR and14C have beenemployed to establish the age framework of this river terrace, resulting in a furtherinvestigation of the significance of its neotectonic movement. The findings show:1. Systematically define the terrace sequence in the lower reaches of YalongRiver and its tributary Anning River. The lower reaches of the Yalong River and theAnning River developed6cut terraces or erosion terraces, which are in goodcomparison in general. But there exist some differences in some local areas. The Jinpingshan section of the Yalong River developed8terraces while the middlereaches of the Anning river just deceloped3accumulation terraces.2. Geochronological approaches such as OSL, ESR and14C have been employedto establish the framework of the6terraces in the lower reaches of the Yalong Riverand the Anning River. The formation age of T6、T5、T4、T3、T2、T1are respectively1.05Ma、0.92Ma、0.72Ma、0.063Ma、0.021Ma、0.01Ma.3. Structural geomorphology has been employed to analyze the nature of theneotectonic movement, the formation times sequence and the uplift range recorded inthe terrace sequence of the Yalong River and to establish the uplift limit in the Yalongriver since the Late Cenozoic in the east edge of the Tibetan Plateau. The lowerreaches of the Yalong River and the Anning River showed sensitive response to theuplift and reflected clearly the two rapid tectonic uplift stages1.05～0.72Ma and0.063～0.01Ma since the mid Pleistocene. The uplift ratios during the two stages are0.63m/ka and1.84m/ka respectively.