Stream-power River Incision Model And Its Applications To Tectonic Geomorphology

The landscape of active orogens can be regarded as the product of the interaction between tectonics and climate.Among orogens,fluvial system is the most sensitive to changes in tectonics,lithology,and climate and can also record the related signals.Extracting and retrieving these details from fluvial system can help study the feedbacks between tectonics,climate,and topography and understand the landscape evolution of active orogens.The Tibetan Plateau,resulting from the continuous convergence of Indian and Eurasian plates since the early Cenozoic,has grown to be the highest plateau on Earth.In the inner land of Tibet,the climate is dry,reducing the transport capacity of streams.Detrital materials that are caused by glacial erosion and fault activity deposit in the intermountain basins,which helps form the flat terrain.On the contrary,the margin of the Tibetan Plateau is constrained by a lot of active orogens,where steep topography forms owing to the intense rock uplift and rapid river incision.Thus,the tectonic and climatic settings vary a lot within the vast plateau,making Tibet a natural laboratory to understand the feedbacks between tectonics,climate,and topography.So far,studies on the tectonic geomorphology of the Tibetan Plateau have been mainly focused on two hot issues:1)How does the plateau grow and expand;2)What contributes to the sustainment of the steady margin and the high elevation of Tibet?Most studies were focused on traditional geologic methods,e.g.,chronology,stratigraphy,and fault activity,but limited work on quantifying the tectonic geomorphology and fluvial system of the Tibetan Plateau.In fact,a lot of rivers that originate from or flow through the margin of the plateau have been affected or reshaped by the intense tectonic deformation and rapid rock uplift.Simultaneously,these tectonic signals have been recorded and stored in the bedrock channels.Via the well known stream-power river incision model,we can extract and retrieve such details from these river channels to quantify the effects of tectonic activity and climatic change on topographic evolution.These findings can help study the expansion style of-and the sustainment of the plateau.Therefore,in the thesis,we first show an introduction to the fundamentals,solutions,and applications of the stream-power river incision model.Then,we illustrate the advantages and disadvantages of two general solutions,slope-area analysis and the integral approach,to the steady-state form of the model in details.We propose a coupled process that combines the merits of the two solutions and statistic tests.We utilize the coupled river profile analysis process and low-temperature thermochronology to study the expansion style(taking the Yumu Shan as an example)and the sustainment of the high topography(taking the Yarlung Tsangpo Gorge for example)of the Tibetan Plateau.1.Stream-power incision model and its implications:discussion on the urgency of studying bedrock channel across the Tibetan PlateauOwing to its sensitiveness to tectonics and climate,fluvial channel can record the tectonic and climatic signals.How to retrieve the related information from river profiles has long been a hot issue in the earth science.Fortunately,the stream-power river incision model,based on physical experiments and rigid mathematic theory,combines bedrock uplift and river incision inherently,making it possible to extract rock uplift history from river profiles.Under the assumption of steady-state river profiles(the rates of rock uplift are balanced by that of river incision),the derived slope-area analysis yields steepness index to quantify the spatial pattern of rock uplift.For its heavy rely on binning and smoothing of elevation data,this method may lack the temporal and spatial resolution of tectonic activities.The integral approach,or called chi-plot,makes up for such drawbacks.Integrating the contributing drainage area with upstream distance as a x value,the river profile can be transformed into a straight line.In addition,we can recognize transient signals,i.e.,knickpoints,from the chi-plots more easily.With the development of the numerical and analytical solutions to the transient equation,we can extract the temporary and spatial information of rock uplift rates.Based on the assumption of a linear relationship between local channel gradient and river incision rate,the vertical velocities of knickpoints migrating upstream were demonstrated to be controlled only by tectonic uplift rates.Then all the knickpoints can be preserved in the river profile,which results a full rock uplift history without loss.However,under non-linear conditions,velocities of knickpoints migration vary a lot,which may result consuming knickpoints and incomplete uplift histories.When a knickpoint passed the whole river profile,the previous tectonic information might be erased.In other words,whether we can get a full rock uplift history also depends on the scale of the fluvial system.Drainage network evolves,accompanied not only by vertical changes in stream profiles but by water divides migration and fluvial network reorganization.Knowing the migrating orientation of water divides is critical to understand the geological and earth surface process.For catchments with the similar elevation difference,lower ? values mean larger steepness indices,or higher erosion rates.Field work and numerical model both have showed that water divides migrate toward areas with lower erosion rates.Hence,this feature can be a good indicator to network reorganization.Although the research and application of stream-power river incision model has got a great progress,problems still remain.We have got the temporary pattern of rock uplift rates under linear assumption.However,solutions to the non-linear transient equation are still confusing.What's more,we treat these problems based on an assumption of uniform lithology and climate.How to take these variant parameters into consideration is difficult.Throughout the paper,we give a brief introduction to the stream-power river incision model,including the model theory,solutions to both steady and linear transient equations,applications,and problems unsolved.We think that many of the problems will be solved in the near future.By presenting the available studies on the Tibetan Plateau,we also highlight that systematic analysis of bedrock channels across the Tibetan Plateau is urgent to understand the deformation history and landscape evolution.2.Coupling slope-area analysis,integral approach and statistic tests to steady state bedrock river profile analysisSlope-area analysis and the integral approach have both been widely used in stream profile analysis.The former is better at identifying changes in concavity indices but produces stream power parameters with high uncertainties relative to the integral approach.The latter is much better for calculating channel steepness.Limited work has been done to couple the advantages of the two methods and to remedy such drawbacks.Here we show the merit of the log-transformed slope-area plot to determine changes in concavities and then to identify colluvial,bedrock and alluvial channels along river profiles.Via the integral approach,we obtain bedrock channel concavity and steepness with high precision.In addition,we run bi-variant linear regression statistic tests for the two methods to examine and eliminate serially correlated residuals because they may bias both the estimated value and the precision of stream power parameters.We finally suggest that the coupled process,integrating the advantages of both slope-area analysis and the integral approach,can be a more robust and capable method for bedrock river profile analysis.3.Using slope-area and apatite fission track analysis to decipher the rock uplift pattern of the Yumu Shan,a thrust-fault-bounded mountain range at the northeastern margin of the Tibetan PlateauAvailable studies have shown that the growth of the Qilian Shan,NE margin of Tibet,started?10 Ma ago.However,when and how it expanded northwards is still under debate.Here we focus on the rock uplift pattern of the Yumu Shan,an active fault-related fold in the Hexi Corridor north to the Qilian Shan.Channel steepness here shows a spatially variant rock uplift pattern,with higher rates in the middle part and lower rates towards the west and east tips.In addition,apatite fission track thermochronology reveals that the growth of the Yumu Shan started-4 Ma ago,similar to the work on active tectonics.Combining the onset ages of the growth of the Qilian Shan(10 Ma),Laojunmiao anticline(3-4 Ma),Baiyanghe anticline(3-4 Ma),Wenshu Shan(4.5 Ma)and Heli Shan(2 Ma),we draw an conclusion that the northeastern margin of Tibet initiated growth at mid-Miocene and expanded to the Hexi Corridor and to the south of Alxa block in the early Pleistocene.4.How a stationary knickpoint is sustained:New insights into the formation of the deep Yarlung Tsangpo GorgeIn order to test the hypothesis that the stationary nature of the Yarlung Tsangpo Gorge is tectonically controlled,the rock uplift pattern in the southeast Tibetan Plateau and the critical condition to sustain a stable knickpoint must be derived.Via slope-area analysis and the integral approach,we first quantify the pattern of channel steepness in southeast Tibet and find that the steepness index shows higher values around the gorge but lower values toward the inner land andthe mountain front.Such a pattern of channel steepness indicates that the active rock uplift is restricted in the zone just around the Yarlung Tsangpo Gorge.Then,we derive a general knickpoint migration model that accounts for spatially variant rock uplift rates.From the model,a critical condition for maintaining a stable knickpoint is concluded that the difference of incision rates in the downstream and upstream reaches of the knickpoint should match that of rock uplift.Employing a stream-power river incision model,we calculate the incision rate in the gorge and find a higher correspondence with differential rock uplift rates in the downstream and upstream reaches of the knickpoint.Therefore,we favor tectonic control as the primary mechanism to explain the stability of the knickpoint within the Yarlung Tsangpo Gorge.Based on our studies on the method of river profile analysis and its applications,we first propose a coupled process to steady-state river profile analysis that combines slope-area analysis(identify channel types based on variable concavities),the integral approach(obtain channel steepness with high precision)and statistic tests(eliminate serially correlated residuals).We combine the coupled process and low temperature thermochronology to study the tectonic geomorphology of Tibet,of which the goal is to figure out the expansion style of and the sustainment of the Tibetan Plateau.We take the Yumu Shan as a case to study the expansion style of NE Tibet.The channel steepness of the Yumu Shan is higher in the central part but decrease toward both sides,which is similar to the Qilian Shan.It means that the parallel ranges in NE Tibet may share the similar rock uplift pattern.AFT thermochronology reveals that the growth of the Yumu Shan started?4 Ma.We thus derive a northward decreasing trend in the onset ages of the growth of the ranges and can conclude a protracted deformation pattern in NE Tibet.We study the formation of the Yarlung Tsangpo Gorge to reveal what contribute to the sustainment of the steady margin of Tibet.We derive the channel steepness pattern in SE Tibet with higher values around the gorge but decreasing toward the inner land and the mountain front,indicating that the active rock uplift is restricted in the zone just around the gorge.Then,we derive a critical condition for maintaining a stable knickpoint,i.e.,the difference of incision rates in the downstream and upstream reaches of the knickpoint should match that of rock uplift,from a general knickpoint migration model.We calculate the incision rate in the gorge and find a higher correspondence with the difference in rock uplift rates between the downstream and upstream reaches of the knickpoint.We thus favor tectonic control as the primary mechanism to keep the stability of the knickpoint within the Yarlung Tsangpo Gorge and to sustain the steady margin of the Tibetan Plateau.