River Landform And Geolo Environment Evolution In The Yarlune Zangbo River Valle

The Yarlung Zangbo River is a great river on the Tibetan Plateau, the landform of river valley and geological evolution history of development for the study of the Tibetan Plateau geology has an important significance. As a result of the Yarlung Zangbo River basin with a harsh geographical environment, predecessor did less research work for the river valley landform and geological environment evolution of the area, even if some work, but also the lack of exact age determination, it makes the development history of the Yarlung Zangbo River is still not very clear. This paper, through a large number of field work, is not limited to the former more research work at the Grand Bend of the Yarlung Zangbo River, but also all over the middle reaches of Yarlung Zangbo River. The anatomy of profiles for the Grand Bend of Yarlung Zangbo River, Linzhi, Gyaca, Jiedexiu and Dazhuka et al., thus the development history of Yarlung Zangbo River is to know. Discussion of the glacier debris flow and distribution of Aeolian sand in the Yarlung Zangbo River valley is important for the geological disaster prevention. Discussion of the glacial deposits, outwash and ancient soil and other topography has significance for the paleoclimate research on the area.Field investigation found the Yarlung Zangbo River valley across strait widely distributed fluvial-lacustrine sediments, moraine, ancient soils and river sediments with acquisition of a large number of samples. Using ESR,14C, OSL and other dating methods analysis of the collected samples we determine many ancient dammed lakes for many period. A combination of a sample of pollen analysis obtained the ancient climate evolution of the Yarlung Zangbo River basin. In the study area we gather a lot of granite samples and known the valley landform of Yarlung Zangbo River basin with the Zircon and Apatite fission track analysis and planation surface evidence. Measurement of the cross-strait terrace of Yarlung Zangbo River valley, combined with the sample to determine the time, we known the development history of Yarlung Zangbo River in the Quaternary period.We collected22granite samples in different locations on the basin of the Yarlung Zangbo River at Lazhi County, Renbu County, Qushui County, Sangri County and Linzhi County for zircon and apatite fission track dating. To analyze the basin uplift denudation of the Neogene tectonic geomorphology and evolution it provides a thermal time data constraint. The obtained samples of apatite fission track age of (1.6+0.2)～(21.8+1.2) MaBP, track length is (12.1+2.6)～(14.2+1.4) μm, single particle age and track length showed a single peak distribution. Zircon fission track age center in33～23MaBP,22～9MaBP. The match of1.6～21.8MaBP fission track age and9～5MaBP after collision events, which represents the main uplift and exhumation period, is the high rate deposition event record in the zone. Furthermore, combination of the Yarlung Zangbo River basin with the planation surface morphology characteristics and distribution regularity and forming age of the Yarlung Zangbo River valley during the early development.Field investigations of the geomorphology of river valleys and tectonics in Gyaca sector of the Yarlung Zangbo River in Tibet show that the sector is characterized by a parallel type drainage pattern and narrow and wide valleys. The sector has undergone a tectonic evolution from collision through compression to extension, thus forming such tectonic deformation styles as folds-thrusts, strike-slip shear, rheological shear, normal faults and graben. The geomorphology of river valleys in Gyaca sector has been controlled by tectonic movement and climate. The drainage system in Gyaca sector of the Yarlung Zangbo River has been formed by headward erosion along faults of different characteristics generated by tectonic movement and climate variation since Pliocene at least.Field investigation of the geomorphology of river valleys found that the Yarlung Zangbo River middle reaches of the valley developed more than11level river terraces. Application dating methods of ESR (Electron Spin Resonance),14C, Aeolian sand and red clay-paleosol sequence initially identified the formation age of these terraces were T11>2000kaBP, T10-1783kaBP, T9-1238kaBP, T8-684kaBP, T7-382kaBP, T6-150kaBP, T5-82kaBP, T4-67kaBP, T3-43kaBP, T2-28kaBP and T1-10kaBP respectively, therefore we have a clear understanding of the development history of the Yarlung Zangbo River. According to terraces of sedimentary features and age of data analysis, found base level change, tectonic movement, climate change and meander migration joint controlled the terrace formation of the Yarlung Zangbo River. These terrace ages of the middle reaches of Yarlung Zangbo River close to the ages of river terraces near the margin rivers of the Tibetan Plateau, shows that the uplift of Tibetan Plateau and geomorphic of both the overall response have consistency and also differences within the region. Analyzing the linear relation between terrace ages and heights above the river, we calculated the average incision rates of the middle reaches of Yarlung Zangbo River, which is not coincident with the incision rates of otner rivers near the Tibetan Plateau, and reflects the Tibetan Plateau tectonic movement in the time and space differences since Pleistocene. We found Oligocene to early Miocene Dazhuka Formation gravel on the highest order terrace of elevation of3700m (560m above the river level at Xiari in Gyaca County and630m above the river level at Beige in Lang County), which illustrates the Yariung Zangbo River the highest order terrace formed after Oligocene to early Miocene, as it close to the altitude of Main Planation Surface of Tibetan Plateau, so at least7MaBP (Late Miocene) the Yarlung Zangbo River has already existed. These old and thick fluvial gravel layers near the lower location of the Yarlung Zangbo River valley were found, show that the Yarlung Zangbo River has already been cut down2000m or more from the Summit Surface of Tibetan Plateau, has been close its current location on the bottom, advance the uplift era of controversy of the Tibetan Plateau to the previous Quatsrnary. This study of the relationship between the ancient valley development of the Yarlung Zangbo River and the uplift of the Tibetan Plateau has an important significance.The Yarlung Zangbo River has a history of being blocked many times to form some large lakes. It has been reported with Saga, Jiangdang, Dazhuka, Renbu, Jiedexiu and Gega ancient lakes, but they lack a comprehensive understanding of ancient lakes in the Yarlung Zangbo River valley. This paper based on the observation of lake sediments of Quaternary typical profiles in the Yarlung Zangbo River downstream, and put forward to develop four stage dammed lakes during Quaternary, and determine lake depositional age, and discuss the relationship between lakes and glaciers. The lacustrine sediments14C (10537±268aBP at Linzhi Brick and Tile Factory,22510±580aBP and139251204aBP at Bengga,21096±1466aBP at Yusong) and a series of ESR ages at318road in Linzhi town with previous data by other experts, paleolakes persisted for691-505kaBP middle Pleistocene ice age,75-40kaBP the early stage of last glacier,27-8kaBP Last Glacier Maximum (LGM),1.8-1.2kaBP New Glacier Period, existence time of lakes gradually shorten represents glacial scale and dam moraine supply potential gradually cut down, paleolakes and dam scale also gradually diminished. This article analyzed formation and decay cause about the Zelunglung glacier on the west flank of Mount Namjagbarwa got into the Yarlung Zangbo River valley and blocked it for tectonic and climatic factors. There is a site of blocking the valley from Gega to Chibai about12km. This article according to moraines and lacustrine sediments yielded paleolakes scale:the lowest lake base altitude2850m, the highest lake surface altitude3585m,3240m,3180m and2940m, area2885km2,820km2,810km2and100km2, lake maximum depth of735m,390m,330m and90m.The vegetation and physiognomy in the Yarlung Zangbo River valley are closely related. The vegetation distribution in the Yarlung Zangbo River valley has close relationship with the unique landforms-vertical natural zone of bigger difference distribution in the Yarlung Zangbo River valley. The quaternary palynology in the Yarlung Zangbo River valley is mainly in woody plants pollen or herb pollen or ferns pollen with its content varies. The plant landscape was mainly in alpine shrub meadow, grassland and meadow, grassland and forest change alternately. Analysis of Palynological Assemblages of typical deposition profiles in the Yarlung Zangbo River valley, the paper reveals the paleovegetation evolution history and ancient climate changes in the southern Tibetan Plateau during late Quaternary.