Spatial Patterns Of River Incision Rates In The Northern Chinese Tian Shan And Its Foreland

The rate of erosion is critical for characterizing the landscape evolution and understanding the interactions between tectonics,climate,and surface processes in an active orogenic belt.The study on the change of Cenozoic global erosion rate has always been a hot topic and the mountain is an important source of global sediments.To some extent,the mountain erosion is a key factor controlling the evolution of mountain range.As one of the most important external forces causing mountain erosion,accurate river incision rate helps to quantify the interaction of topographic and landscape processes.At the mountain front of the northern Chinese Tian Shan,well-developed fluvial geomorphologic units?river terraces and alluvial fans?are preserved along the courses of the rivers.Among them,fluvial terraces are residual features that developed at the top of the sedimentary infilling,thus recording the final riverbed before river incision.At present,the formation/abandonment ages of fluvial terraces and alluvial fans in this region have been well constrained.These progresses in geomorphological ages provide us with an opportunity to estimate the rate of erosion?caused by river incision?from fluvial geomorphic investigations.In the past decade,the analysis method based on geographic information systems?GIS?and digital elevation model?DEM?has successfully been used to estimate the erosion rates in the Tian Shan.In this paper,we selected four rivers that are the Kuitun River,the Jingou River,the Manas River,and the Urumqi River as the research objects to discuss the river incision rates in the late Quaternary in the Tian Shan based on the reconstruction of paleogeomorphology.These selected four rivers all have developed obvious terraces in front of the mountain.Based on the several field investigations and the Google Earth satellite images,we first classified the sequences of river terraces developed by the above rivers.Furthermore,according to the characteristics of terraces?for example,no loess cover in flat surface?and the age of terraces,one of the identified terraces is used as a reference surface to reconstruct the paleogeomorphology before the previous floodplain was abandoned as the terrace.The volume of eroded materials caused by river incision is estimated by comparing the DEM of the present-day topography and the reconstructed paleogeomorphology.The depth of erosion is averaged over the erosion area constrained by the used reference terrace,and thus this is actually the depth of river incision.When combined with the terrace formation age,the average rates of river incision in the mountain and the piedmont is calculated,respectively.Finally,we analyze the correlation between the spatial pattern of incision rates and its controlling factors?lithology,climate and tectonics?in the northern Chinese Tian Shan.This work has obtained the following main understandings:?1?The remote and field-based observations define the terrace sequences of the analyzed four rivers preserved at the mountain front,which is the T₁-T₅ in the Kuitun River,T₁-T₅ in the Jingou River,T₁-T₅ in the Manas River and T₁-T₉ in the Urumqi River,respectively.For each analyzed river,one of the defined terraces is used as the reference surface for the reconstruction of the paleogeomorphology before the previous floodplain was abandoned as the terrace due to river incision.These referec terraces are T₅ of the Kuitun River,T₅ of the Jingou River,T₅ of the Manas River,and T₄ of the Urumqi River,and their ages are 13.4±1.6 ka,12.6±1.3 ka,12.4±0.8 ka and 12±1.2 ka,respectively.?2?The volume of eroded materials caused by river incision is estimated by comparing the DEM of the present-day topography and the reconstructed paleogeomorphology.The amount of mountain erosion of the four rivers in the mountain and in the piedmont are 0.80±0.30 km3and 2.22±0.3 km³?the Kuitun River?,0.22±0.12 km3and 1.46±0.19 km³?the Jingou River?,0.66±0.40 km³ and2.08±0.33 km³?the Manas River?,and 0.07±0.05 km3and 0.41±0.10 km³?the Urumqi River?,respectively.?3?When combined with the volume of eroded materials and terrace formation age,we have calculated the average incision rates since the formation of the terraces that are,in the mountain,4.04±1.47 mm/y?the Kuitun River?,3.53±1.38 mm/y?the Jingou River?,3.38±1.63 mm/y?the Manas River?and 1.87±1.33 mm/y?the Urumqi River?and,in the piedmont,4.27±0.84 mm/y?the Kuitun River?,2.53±0.41 mm/y?the Jingou River?,2.43±0.58 mm/y?the Manas River?,1.35±0.77mm/y?the Urumqi River?.?4?Based on the reconstruction of the palegeomorphology,the river incision rates and the amount of erosion appeared to decrease from west to east.In the piedmont,the incision depth also decreased towards the downstream basin.Furthermore,we find lithology and climate?precipitation?may not be the main factors to control the spatial distribution of the river incision rates in the northern Tian Shan.However,tectonics setting?the shortening of the north-south crust,the mean slope and relief controlled by the shortening amout?may play an important role.?5?The spatial features of the topography are significantly controlled by the tectonics setting in the Tian Shan,and the spatial difference of the topography is further intensified by the river evolution,especially the river incision.