Geomorphic Characteristics Of The Eastern Pamir Syntaxis And Its Controlling Factors

The Pamir,as the northwestern continuation of the Himalayan-Tibetan orogen,has been impacted by the far-field effects of the Cenozoic Indian-Eurasia collision.Because of substantial north-south crustal shortening,the width of the Pamir is markedly different with that of the continuous Tibetan terranes.Its tectonic process in late Cenozoic has been systematically studied by geologists,including the different sliding rates of the strike slip faults on sides,as well as the modern geomorphology,which surface deformation can respond to the crustal shortening and deformation characteristics of the Pamir.Therefore,the formation and geomorphological evolution of the Pamir,as well as its relationship with the uplift of the Tibetan Plateau,are urgent issues that need to be addressed.The Eastern Pamir Syntaxis(EPS)links the Pamir with the western Tibetan Plateau.Due to the barrier of the Karakoram and the Hindu-Kush Mountains,the water vapor brought by the Indian summer monsoon and the Westerlies is difficult to reach the EPS,leading to the tendency of the region to drought.The development of fault in basins of the EPS were influenced by the the Karakoram Fault,Karakax Fault and Kashgar-Yecheng Transfer System from southwest to northeast and magmatic,metamorphic and sedimentary rocks are widely exposed in the EPS.It is an ideal area for studying the tectonic evolution,terrane division,geomorphological response of the Pamir,and the comparative analysis with the Tibetan Plateau.On the basis of determining the appropriate resolution Digital Elevation Model(DEM)and stability threshold,the Hydrological Analysis Tool in Arc GIS is used to obtain the Hypsometric Integral(HI)and the drainage system of the EPS,and quantitatively studies the geomorphic development characteristics and control factors of the EPS.Then,the χ value and channel steep coefficient of the EPS are extracted by using the Topo Toolbox tool on Matlab based the appropriate resolution DEM to analyze the channel steep coefficient,watershed migration between different basins and its control factors in the EPS.Finally,combining geomorphological characteristics and field investigations,the modern-link rivers of the EPS are systematically studied by using detrital zircon U-Pb ages to analyze their provenance change,tectonic deformation and geomorphological response process.The main conclusions are as follows:(1)DEM resolution has little effect on extracting parameters of hydrology and HI of the EPS.36000 is the stable threshold for hydrology and HI parameters of EPS.HI values of catchments in the EPS range from 0.0129 to 0.6453,with mean HI of 0.3981,identified geomorphological development trend in Mature Stage.And order of mean HI is larger in Fault Zone(0.4101)> Whole Zone(0.3981)> Non-Fault Zone(0.3880),which suggest that tectonics is a crucial factor affecting geomorphic feature in the EPS.And then,sequence of mean HI in different catchments covered by different lithology in the Non-Fault Zone is Igneous rock(0.4866)> Metamorphic rock(0.4386)>Continental sedimentary rock(0.4119)> Marine sedimentary rock(0.4093)>Quaternary sedimentary rock(0.3363),which indicated different erosion resistance of rocks is also an important factor for geomorphic feature in the EPS.And due to the strong erosion ability of glaciers on the surface,its mean HI(0.5167)is significantly higher than other rocks,which has a significant impact on geomorphic development characteristics.In addition,the climate mainly affects the erosion and transportation of rivers by controlling the river runoff through precipitation and temperature,thus affecting the geomorphic development in the basin.However,mean annual precipitation has less influence on geomorphic development than mean annual temperature.This suggests that mean annual temperature is the main climatic factor affecting the erosion in the EPS.The above analysis shows that tectonics,lithology and climatic feature has jointly influenced geomorphic development in the EPS.(2)Climatic conditions and lithologic erodibility have little impact on the channel steep coefficient of the EPS,and tectonic activity is the main control factor of channel steep coefficient.Lithologic erodibility is the main influencing factor for moving northward in the central section of watershed between the Yarkant River basin and the Gaz River basin and the west section of watershed between the Yarkant River basin and the Yigeziyeer River basin.Tectonic activity is an important factor for moving northward in the central section of watershed between the Yarkant River basin and the Kalatashen River basin.Lithologic erodibility is an important factor for moving westward in the north and south section of watershed between the Yigeziyeer River basin and the Kalatashen River basin.Tectonic activity and lithologic erodibility jointly control moving eastward in the north section of watershed between the Kalatashen River basin and the Gaz River basin.The Kalate River basin obviously captures river system of the Gaz River basin.Lithologic erodibility is the main control factor for moving southward in the south section,tectonic activity is the main control factor for moving eastward in the central section,tectonic activity and lithologic erodibility property are the main factors for moving eastward in the north section of watershed between the Kalate River basin and the Gaz River basin.(3)The main age peaks of detrital zircon U-Pb in the EPS are consistent with the ages of the rock masses in the basin,indicating that different periods rocks exposed in the region are the main source of detrital zircon.The distribution of detrital zircon UPb ages from sediments of the modern rivers in the Pamir are quite different indicating that the provenance is evidently different.The Permian-Triassic magmatic rocks widely exposed in the North Pamir provide a large amount of detrital materials for the Kalate River and Gaz River,and the Cenozoic magmatic rocks in the Central Pamir provide a large amount of detrital materials for are the Vanch River,the Murghab River,the Bartang River and the Yazgulom River,and the Cretaceous magmatic rocks in the South Pamir provide a large amount of detrital materials for the Gunt River and the Yarkant River.The modern fluvial detrital zircon U-Pb age of the Pamir support that the South Pamir,Central Pamir and North Pamir are related to the Lhasa terrane,Qiangtang terrane and Songpan-Ganzi Complex respectively.In the early Cretaceous,the Cretaceous igneous rock developed when the Meso-Tethys began to close provided Cretaceous materials for the Pamir and the Lesser,Greater,and Tethyan Himalayan Strata.In the late Cretaceous,the 100-80 Ma igneous rock developed when the NeoTethys was closed may be the material source of the ～80 Ma age peak of the Yarkant River.Since the Eocene,the continuously uplifted South Pamir has undergone intense exposure and denudation,providing a large amount of Cretaceous materials for the Gunt River and Yarkant River.In the Oligocene,the Paleogene magmatic rocks developed in the Central Pamir provided a large amount of detrital materials for the rivers in the Central Pamir.In Miocene,Neogene magmatic rocks developed in the Central-North Pamir provided a large amount of detrital materials for the Yarkant River.Due to the Cenozoic Indian-Eurasia collision,dragging the South Pamir,Cnetral Pamir and North Pamir to bend,relative to the Lhasa terrane,Qiangtang terrane and SongpanGanzi Complex in the Tibetan Plateau,arced northward for about 280 km,forming the arc-shaped shape of the Pamir.The establishment of the modern-like river pattern in the EPS contemporaneous with activity on the Kongur Extensional System which influenced the material sources of modern rivers in the EPS.In the EPS,detrital zircon U-Pb age change significantly from south to north,and the influence of Cretaceous proto-sources gradually diminishes northward,and the influence of Permian-Triassic magmatic rocks gradually increased.The provenance changes of the EPS indicate that its tectonic activity and deformation may have begun in the Cretaceous,and its terrain evolution is related to the growth of the Tibetan Plateau.