Bedrock Channel Form And Its Response To Tectonic Uplift In The Northern Qilian Mountains

In rapid erosion and tectonic uplift area,river incision on bedrock dictates the relationship between climate,lithology,tectonic and topography.Bedrock river faithfully records the tectonic deformation of the mountain massif,and its channel morphology systematically adjusts in response to the rates of the tectonic deformation thus,has become the important carrier to explore spatial distribution information of rock uplift rates.Research on bedrock channel form in active orogene has a profound significance which not only provides a new method for exploring the pattern of tectonic deformation,but also deepens the understanding of the dynamic mechanism of the interaction between tectonic uplift and surface processes.The Qilian Mountains,located along the northeast margin of the Tibetan Plateau,where develops a lot of typical bedrock channels with the same erosion base level(Hexi Corridor),have been regarded an ideal region for bedrock channel form studies.For a long time,the researches of tectonic deformation mainly focus either on active structures(e.g.,fault,anticline,fold etc.)or on spatial geomorphologic surfaces(e.g.,terraces and alluvial fan).Although a certain amount of rates of data have been accumulated,it is difficult to effective parallel comparison due to the constraints of sections preservation,field survey conditions,dating methods and objects and so on.The shear stress incision model or stream power incision model base on the physical mechanism of river incision processes,not only lays a solid theory foundation for quantitative research on bedrock channel form,but also builds the functional relationship between channel form and rock uplift rate.The steepness index and concavity index in incision models provide accurate quantitative standards for channel longitudinal profile,and the key parameters,erosion coefficient and slope index provide conditions for the quantitative isolation of various influence factors including lithology,discharge,sediment flux and so on.Therefore,the shear stress incision model or stream power model has been regarded a powerful tool for assessing bedrock channel form,by which we can obtain the spatial distribution information about rock uplift rate and the deformation pattern of differential tectonic uplift in orogene.In conclusion,this paper combines the extraction of steepness index and concavity index with the calibration of the key parameters base on shear stress incision model in order to analyze the response mechanism of bedrock channel form to tectonic uplift rate in the northern Qilian Mountains.Moreover,we try to explore the tectonic deformation pattern in the northeastern margin of the Tibetan Plateau base on the analysis of the spatial distribution regularities of rock uplift rates.In the northern Qilian Mountains,the type of bedrock channel geomorphology is extremely complex that all of the cascade,step-pool,plane and riffle-pool beds are developed,and the combination is diverse among different topographic zones.The bedrock channel form shows an obvious geographical distribution law in east west and north south directions in this area,and the channel morphological index also varies regularly.The average value of steepness index in the middle section of the massif is 1.6 times higher than the eastern section,and the high mountain zone is about 2 times than the low mountain zone in the northern Qilian Mountains.The values of concavity index for the channels intersecting with main faults show extremes(?<0 or ?>0.6),otherwise are in the normal range(??0?0.6).The downstream adjustments of channel width with drainage area in the eastern section of massif are more rapid than in the middle section,and the index b values in width-area scaling are 0.40 and 0.27,respectively;the low mountain zone are more rapid than the high mountain zone,and b' values are 0.43 and 0.35,respectively.According to our analysis and discussion on major geologic factors-tectonics and lithology,affecting bedrock channel forms,tectonic uplift controls the overall trend of channel form changes,while lithologic resistance dominates the change characteristics of channel width in local reach.Our quantitative calibration for the key parameter erosion coefficient in shear stress incision model base on the main influence factors of channel width and discharge show that the erosion ability of bedrock channel in the middle section is more intense than in the eastern section,and the erosion coefficient K value is 1.1?1.2 times between the two sections;the high mountain zone is more intense than the low mountain zone,and the K value is 1.1?1.3 times between the two zones.According to our inversion calculation on the slope index n with terrace incision rates,the range of n values is 0.83?1.76 in the northern Qilian Mountains,which is completely consistent with empirical research(2/3<n<5/3)and illustrate that rock uplift rate shows a power function positive correlation with steepness index in the area.Our estimation for shear stress erosion coefficient k,shows that the erosion ability in the middle section is higher than in the eastern section,the high mountain zone is higher than in the low mountain zone on account of the differences in lithologic resistance and sediment flux,and the ke values are 2.3 times and 1.6 times between the two groups,respectively.In the northern Qilian Mountains,bedrock channel form is mainly controlled by the differential rock uplift,thus the channel longitudinal steepness indicates the spatial distribution of uplift rates in a certain degree and the steepness index ksn predicts the intensity of tectonic uplift.On the whole,the tectonic uplift activities in the middle section of the massif are more intense than in the eastern section;the high mountain zone is more intense than in the low mountain zone.Combining the calculation on steepness index with the calibration on erosion coefficient K,we infer that the rock uplift rate in the middle section is 2?3 times higher than in the eastern section,the high mountain zone is 2?4 times higher than in the low mountain zone.Obviously,the difference in rock uplift in the north south direction is more remarkable than in the east west direction in this orogene.The spatial distribution characteristics of rock uplift rates inferred from the bedrock channel form are generally consistent with the results of river terraces in our study area.Therefore,according to the results of strath terraces we preliminary assess that the average rock uplift rate in the middle section between the high and low mountain zones are 1.6?4.8 mm/a and 0.8?1.2 mm/a,respectively,and in the eastern section are 0.8?1.6 mm/a and 0.4 mm/a in the two zones,respectively.Changes in bedrock channel concavity in our study area are primarily controlled by the activities of major thrust faults.The extreme values of concavity index(?<0 or ?>0.6)indicate the locations of the faults,and the extent of variation in the steepness index when channel transects the fault reflects the intensity of the vertical fault activity.According to our study in key drainages including Beida,Fengle,Liyuan,Xiying,Jinta,Zamu and Huangyang basins,we infer that the differential rock uplift in the northern Qilian Mountains is largely controlled by the edge faults(e.g.,Hanxia-Dahuanggou Fault,Fodongmiao-Hongyazi Fault and Huangcheng-Shuangta Fault)instead of the internal faults(e.g.,Sunan-Qilian Fault and Lenglongling Fault),and the active nature and intensity of the main boundary faults(e.g.,Fodongmiao-Hongyazi Fault and Huangcheng-Shuangta Fault)show obvious segmentation characteristics.Generally,our inferences about the distribution,active nature and intensity of main thrust faults base on bedrock channel form is generally consistent with the studies of active structures in the northern Qilian Mountains.Our research on bedrock channel form in the northern Qilian Mountains suggests that the prevailing tectonic deformation in this region is thrust-napping controlled by the pushing to the north of the Tibetan Plateau.The Beida,Hongshuiba and Maying drainages in the middle section are located just at the action center of pushing force,thus the thrust-nappe activity is most intense,the steepness index value is highest and the rock uplift is most rapid.To the east,due to far from the action center gradually,the thrust-nappe activity is weakened and the tectonic deformation transformed to horizontal strike-slip movement,so the steepness index values and rock uplift rates obviously decrease.Obviously,the differential rock uplift in the north south direction is more intense than in the east west direction,which suggests that the tectonic deformation is mainly driven by compressional thrust of active faults.Additionally,the active intensity of the edge faults is greater than the internal faults in the northern Qilian Mountains,which indicate that the tectonic deformation is transmitted from south to the north.