Quantitative Study On Surface Erosion And Topographic Evolution In The East Tibet

Earth's surface is the critical interface between Earth's internal and external systems.In this circulation,surface erosion is a key factor governing the landscape evolution and provides a source of multiple feedback mechanisms linking climates and tectonics.Quantifying erosion rates is necessary to clarify potential feedbacks between climate and tectonics,and is fundamental to understand the uplift history,topographic evolution,and geodynamic processes in active mountain ranges.Over recent decades,rapid improvement in the digital representation of topography and erosion quantification techniques has promoted the surface erosion research.Based on those new techniques and knowledge,numerous papers have been published,and pave the way for further understanding of the erosion processes and the interactions between climate and tectonics.However,some topic questions still unanswered.Eastern Tibet is the most dramatic transient landscape in the margin of the Tibetan Plateau.It is affected by active tectonic related to the collision between India Plate and Euro-Asia Plate since early Cenozoic as well as intense and highly variable climate.It provides a great opportunity to study the coupling between climate,erosion,and tectonics.It is fundamental for understanding the uplift and geodynamic processes of the Tibet.The different geomorphology,high-gradient rainfall,and high frequency episodic erosion events in eastern Tibet also provide a unique laboratory to study different surface erosion processes.Longmen Shan,located in the eastern Tibetan plateau,is characterized by steep topography.However,no significant shortening across the entire Longmen Shan was found prior to the 2008 Wenchuan earthquake.To explain this peculiar configuration,it needs to understand the role of upper crustal faults in the evolution of the Longmen Shan and the interactions between tectonic and erosion processes in shaping the topography along this margin.Here,we provide a new data set of millennial erosion rate derived from beryllium-10 concentration in river sediments from 15medium-sized catchments in the southwestern Longmen Shan.Combined with previous researches,we found that subsurface tectonic is the first-order control the erosion pattern in the Longmen Shan while the rainfall has a minor effect.The ¹⁰Be-and low-temperature thermochronology-derived erosion rates as well as river terrace,Paleoseismology studies both indicate the strong activity of the Dachuan-Shuangshi and Yanjing-Wulong faults in the southern section and the Yinxiu-Beichuan fault in the central of the Longmen Shan.We also suppose that slip rates on thrust fault system as well as their structural geometry together control the vertical deformation and the recent landscape along the Longmen Shan.The 2008 M_w 7.9 Wenchuan earthquake,China,ruptured the parallel Beichuan and Pengguan faults along the Longmen Shan,which induced a large amount of landslides,volume comparable to the coseismic uplift.Quantifying the removal of co-seismic landslide material after the earthquake is central to our understanding of geomorphic recovery from seismic events and the topographic evolution of tectonically active mountain ranges.In order to get more insight into the fluvial erosion response to co-seismic landslides,over the post-seismic period of 2008-2013,we annually collected river sediment samples?0.25-1 mm?at 19 locations and measured the concentration of cosmogenic ¹⁰Be in quartz.When compared with published pre-earthquake data,the ¹⁰Be concentrations declined dramatically after the earthquake at all sampling sites,but with significant spatial differences in the amplitude of this decrease,and were starting to increase toward pre-earthquake level in several basins over the 5-year survey.Our analysis shows that the amplitude of¹⁰Be decrease is controlled by the amount of landslides directly connected to the river network.Calculations based on ¹⁰Be mixing budgets indicate that the sediment flux of the 0.25-1mm size fraction increased up to 6 fold following the Wenchuan earthquake.Our results also suggest that fluvial erosion became supply-limited shortly after the earthquake,and predict that it could take a few years to several decades for fluvial sediment fluxes to go back to pre-earthquake characteristics,depending on catchment properties.We also estimate that it will take at least decades and possibly up to thousands of years to remove the co-seismic landslide materials from the catchments in the Longmen Shan.Considering the effect of isostasy,we think that the Wenchuan earthquake will lead to a net material surplus in the Longmen Shan.As other thrust mountainous ranges,the vertical motion and denudation at surface during the inter-seismic period and cosesimic rupture have different spatial pattern,which together control the topographic evolution of eastern Tibet.Following the change in boundary condition,fluvial adjustment will propagate from the outlet to the upstream,and thus sets the lower boundary condition for adjacent hillslopes.Meanwhile,migration of drainage-divide also alters the catchment condition and increase or decrease the disequilibrium of the channel profile.Assessing the rates of fluvial propagation and divide mobility is central to understand catchment-wide responses to perturbation as well as practices of topographic analysis and interpretation.The Liqiu River in the east Tibet,as a transient topography,provides an opportunity to study this question.By comparing knickpoint and divide migration rates,we found that the divide mobility is an order of magnitude slower than the fluvial propagation rate.In such situation,the quickly adjustment of interior river profile will prevent the formation of low-relief surface by divide migrate alone.However,suddenly stream capture might slow the interior river propagation rate and thus amplify the influence of exterior rivers in shaping the drainage topography.Based on these results,we have a better knowledge of the surface erosion in the east Tibet,in terms of tectonic-controlled erosion,episodic events,and transient topography evolution.This thesis aims to give more information about surface erosion which can be used to interpret the topography evolution and dynamic mechanism of the east Tibet as well as the interaction between climate and tectonic.