Studies On Quaternary Fluvial Incision Process And Landscape Evolution In Typical Canyons Of The Mid-upper Reaches Of The Yellow River

landscape.The river,acting as a link between the source area and the sedimentary area,is sensitive to tectonic deformation and climate fluctuations.The Yellow River originates from the northern foot of the Bayan Kala Mountains on the northeastern Tibetan Plateau and cuts through the whole NE Tibet in the northeast direction,with rich geological-geomorphological information.Determining geological-geomorphological processes of the mid-upper Yellow River is essential for the geodynamics of the growth and expansion of the northeastern Tibetan Plateau.Although an amount of studies have been conducted on the Yellow River for more than one hundred years,there remains a lot of debates and uncertainties.They include the development of paleo-channel of the Yellow River,the formation time of the modern Yellow River,the earliest appearance of the Yellow River in various reaches,and the geological and geomorphological processes in the Yellow River Basin.In this thesis,we selected three typical canyons?i.e.the Jishi Gorge,the Mijia Gorge and the Qingtong Gorge?that are located on the mid-upper reaches of the Yellow River.The fluvial incision history and local erosion rates of the three canyons were investigated.Firstly,we determined the catchment-averaged denudation rates on both sides of the Jishi Shan.It is concluded that the differential fault activities on both sides of the range mainly controls higher erosion on the eastern Jishi Shan.Secondly,we quantified the incision history of the Yellow River in both the Mijia Shan and the Laolongwan Basin in the northeastern Tibetan Plateau.The results show that the Haiyuan fault plays an important role in accelerating the incision rate in the Mijia Shan,given that the same climatic condition in both Mijia Gorge and Laolongwan Basin.Thirdly,we conducted research on eight fluvial terraces along the southeastern bank of the Yellow River in the Qingtong Gorge.It is suggested that the average incision rate of the Yellow River over the past 350 ka was?200 m/Ma and may have increased in the past?60 ka.In addition,the first appearance of the Yellow River in the Qingtong Gorge was?0.6Ma.At that time,the Yellow River can be divided into two sections with completely different evolution histories.The details of the three major studies are introduced below.The north-trending Jishi Shan is located within the central part of the northeastern Tibetan Plateau,as the geomorphologic boundary between the main body of the Tibetan Plateau and the Longzhong Basin.Modern precipitation across the Jishi Shan shows a strong orographic effect,since mean annual precipitation on the windward east flank are?716.1 mm/a but?265.8 mm/a or less in the west of the range.The two range-front faults on both sides respectively thrust upon the Xunhua and Linxia Basins.The two faults juxtapose mafic magmatic rocks and low-grade metamorphic rocks of late Paleozoic and early Mesozoic ages that are rich in quartz over abyssal Quaternary sedimentary deposits.Therefore,the Jishi Shan and its neighboring Linxia and Xunhua basins provide a perfect setting to differentiate the relationships among climatic factor,tectonics and topography.Our goal was to explore the main controlling factor?tectonic/fault activity or orographic rain?on erosion process across the Jishi Shan in the northeastern Tibetan Plateau.Firstly,a data set of 14 new catchment averaged denudation rates on both sides of the Jishi Shan were quantified over millennial timescales by cosmogenic 10Be nuclides.Secondly,climatic conditions across the range were determined by regional vegetation cover and decade-scale rainfall data.Thirdly,stream profile analysis was conducted to derive the basic geomorphological characteristics of the Jishi Shan.The results show that catchments draining from the eastern Jishi Shan are almost two times higher denudation rates than those from the western side?with mean values of 0.85 mm/a and 0.42 mm/a on eastern and western sides,respectively?.In addition,channel steepness index,Enhanced Vegetation Index?EVI?and rainfall are also much higher on the eastern side of the Jishi Shan,while the concavity values of drainages on both sides are similar.Based on the comprehensive analysis of regional geomorphological and climatic characteristics,it is concluded that the differential fault activities on both sides of the range mainly controls higher erosion on the eastern Jishi Shan,the northeastern Tibetan Plateau.The Mijia Shan is a prominent geometrical "knot" on the left-lateral Haiyuan Fault.The fault curves with multi-strand splay bound the Mijia Shan in plane and behave as a positive flower shaped structure in the upper crust.Oblique shear supports rapid uplift of the Mijia Shan positive flower structure.The Laolongwan Basin is a pull-apart basin,and mainly controlled by the Haiyuan Fault.The basin has experienced rapid subsidence and accumulated extremely thick sediments.In this study,we employed cosmogenic nuclide 10Be analysis to determine the formation ages of the Yellow River terraces in the Mijia Gorge and the Laolongwan Basin at the northeastern periphery of the Tibetan Plateau.The aim of this study was to determine local fluvial incision rates and the effect from tectonic activities.The two sets of terraces were surveyed in field and dated by cosmogenic 10Be exposure dating method.The results show that the best-fitting simulation ages in the Mijia Shan are 68.0+-9.4 +11.5 ka for T11 and 174.7+-38.4 +52 ka for T14.In addition,in the Laolongwan basin,sample S3 constrains an age of 123.9 ± 2.7 ka and S2 yields an age of 45.0 ± 1.8 ka.Combining the strath heights of the sampled terraces,the fluvial rates were determined.The results show that the fluvial incision rate of the Yellow River is?1.94 mm/a in the Mijia Shan over the last 175 ka,while a much lower incision rate of?0.73 mm/a is yielded in the Laolongwan Basin since the last?120 ka.The incision rate at the Mijia Shan is almost twice more than that in the Laolongwan Basin.Considering the same climatic condition in both the Mijia Gorge and the Laolongwan Basin,the difference between the two incision rates?-1.21 mm/a?might be due to local tectonic deformation derived from the Haiyuan active strike-slip fault.Consequently,our results show that the Haiyuan fault plays an important role in accelerating the incision rate in the Mijia Shan.Hence,it is suggested that the role of the main geological structures across the main river should also be considered when studying on the evolution of terraces of big rivers.The Niushou Shan sits on the outermost side of the northeastern Tibetan Plateau,and is bound by the Sanguankou-Niushoushan Fault that is an active fault that separates the North China Block to the northeast from the Tibetan Plateau to the southwest.The Niushou Shan represents a link between the upper reaches of the Yellow River where it has incised deep canyons into the Tibetan Plateau,and the relatively low-gradient reaches of the river farther north.In this study,we conducted research on 8 fluvial terraces along the southeastern bank of the Yellow River in the Qingtong Gorge,focusing on detailed interpretation of the terrace distribution and chronological determinations of four of the primary terraces.These data allow us to establish the pattern and history of fluvial incision and to evaluate the role of climatic and tectonic drivers of fluvial terrace formation.The results can be used to refine the regional history of incision and drainage integration along the middle reaches of the Yellow River.A flight of 8 relict fluvial terraces preserved along the canyon.Four of these?T2,T3,T6 and T8?were dated using OSL and cosmogenic-nuclide exposure dating.The results show that the four terraces were formed at 51.6 ± 2.9 ka,60.7_-8.3^+14.4 ka,346.1_-27.3^+30.0 ka,and 580-640 ka,respectively.Precise measurement of terrace heights suggests that the average incision rate of the Yellow River over the past 350 ka is?200 m/Ma and may have increased in the past?60 ka.According to the comparison of incision histories with regional climatic records and local tectonics,it is suggested that climatic variations since the late Quaternary drove cutting and abandonment of the terraces during progressive growth of the Niushou Shan.The results provide constraints on the timing of incision and linkage of various segments of the modern Yellow River fluvial network.