Tectonic Movement And Erosion Of The Western Qilian Shan Duirng Late Cenozoic

The Qilian Shan, located in the northeastern margin of the Tibetan plateau, is oneof the youngest parts of the growing plateau. With the uplift of the Tibetan plateau,the Qilian Mountains has experienced intense deformation since Cenozoic, which haschanged the landscape and drainage network as well as the atmospheric circulationthere. It is viewed as the most ideal place for studying the interaction betweentectonics, climate, and geomorphology. This thesis analyzes the relations betweenerosion, tectonics, climate and topography based on the published decadal erosionrates. Meanwhile, it studies the relations between geomorphology, tectonics, erosionand climate of Hongshuiba drainage basin in the middle Qilian Mountains.1. Controls on decadal erosion rates in Qilian Shan: re-evaluation and newinsights into landscape evolution in the northeastern Tibetan plateauPrevious work suggested that decadal-scale erosion rates are closely correlated withlocal topographic gradients rather than climatic factors. However, the arid trend in aclimatic change was proposed to explain the discrepancy between short-term andlong-term erosion rates. In order to reevaluate the influences of tectonics and climateon erosion, this work adopted2previously used parameters (slope and mean localrelief) and reintroduced3new parameters (historical cumulative seismic moment,runoff coefficient of variation and fault density) to study11drainage basins in theQilian Shan area. Different from previous conclusions, this work found that theerosion rates are also closely related to both surface rupture density and rainstormintensity in addition to terrain steepness. Limited60-year cumulative seismicmoments of historical earthquakes show weaker correlations. The slopes greater than20o correlate well with erosion rates, indicating the spatial difference of erosion withinthe basins. Thus either averaging erosion rates of large drainage basins or comparingwith site-specific thermochronologic exhumation rates directly will be misleading ifthe deposition effects within the basins are not considered. The present study alsosupports the ‘bath-tub’ model to interpret the low relief of intermountain basins. 2. Geomorphic features of the Hongshuiba drainage Basin in the QilianShan Mountains and their tectonic implicationsOwing to the intensive uplift of the Qilian Shan Mountains since Late Cenozoic,the drainage basins have experienced strong tectonic deformation as evidence by theirgeomorphic features. This research took the Hongshuiba drainage basin as the object,which originates from the interior of the Qilian Shan. Stream networks of this basinwere extracted from the DEM data, which were graded into five levels with theStrahler method. Five sub-catchments, the Heishui Drainage basin, Yanglu Drainagebasin, Zhangrui Drainage basin, Sanchahe Drainage basin and upstream region werefurther chosen for detailed analysis. The bifurcation ratio, basin relief, mean elevation,basin area, stream length, drainage density, and average slope of the HongshuibaDrainage basin and the sub-catchments were determined in terms of GIS tools. Inaddition, this thesis analyzed the slope spectrum of the Hongshuiba drainage basin.Both the drainage density and basin area are small, which indicates the drainagenetwork here is still on an initial stage. High bifurcation shows that the drainage basinhas experienced intensive tectonic disturbance. From south to north, the bifurcationratio varies greatly. This work eliminated the impact of lithology, and regards thedifference of bifurcation ratio as an indicator of spatial diversity of tectonic movement.In the south part with a high average altitude of4000m, the bifurcation ratio is4.08,which reflects less tectonic disturbance. The gentle slope occupies a so large areaproportion that the slope spectrum shows a bimodal pattern and a peak value is in thelow-slope region. The area with low relief can be regarded as a residual planationsurface or the result of fluvial deposition. The middle part may be affected by glacierowing to its high altitude, so the network here developed not perfectly and thebifurcation ratio can not reveal the tectonic movement. However, the increasing basinrelief and the activity of the Changma fault can reflect the increasing impact oftectonic movement. In the north part, the basin relief is over2000m and thebifurcation ratio is above4.50, both of which indicate that this area has experiencedintensive tectonic movement. As a result of the thrust uplift of the east part ofFodongmiao-Hongyazi fault, the Hongshuiba River incises terrace deeply near thepiedmont outlet. Although the basin relief varies in accord with the tectonicmovement, average altitude and slope do not show the same tendency. The increasingaverage altitude may be resulted from the tectonic background and the slopeindicating the local topography steepness may be affected by climate or lithology, both of which require further research.3. Using fission track thermochronology to study the erosional elevationdistribution of the source regionErosion plays an important role in the interaction between tectonics and climate,as well as landscape evolution. However, current methods are limited in reflecting theerosional areas in source region. This work studied the erosional distribution in sourcethe region via fission track (FT) thermochronology. First, the age probability densityfunction (PDF) was determined by an age-elevation plot. Then, the age of PDF ofdetrital grains were calculated based on the assumed normal distribution. At last, thiswork compared the two PDFs and determined the erosional elevation and relativeerosion velocity. This thesis applied this approach to study the elevation distributionof gravel deposition in the Hongshuiba River diluvial fun. It found that the finegravels show a similar and younger PDF despite the different deposition chronology,which may indicate the fine gravels were from the low part of the basin. The coarsegravels show the different age PDF values, which are older than fine ones. Such aphenomenon cannot be surely attributed to tectonics. Considering the averageelevation, mean temperature and the mountain glaciers here, this work suggested thatthe coarse gravels might be the product of freezing and thawing. Although it focusedon the Holocene conglomerate deposition, this work might provide a new approachand viewpoint for research of the genesis of gravel sedimentation in earlier times.Based on the analysis of the whole Qilian Mountains and local drainage basin, thiswork found that terrain steepness control erosion in decadal or millennial scale, whilein the long run, the variation of erosion and deposition may affect local relief.Bifurcation ratio and basin relief of sub-catchments indicated a growing tectonicmovement from south to the north in Hongshuiba drainage basin. Spatial difference oferosion may result from climatic variation, but with limited relation of tectonics. Thisthesis provided plenty of fundamental data and new insights into the landscapeevolution of the Qilian Shan in the northeastern Tibetan plateau.