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The Research on Temporal and Spatial Distributions of Erosion Rates and Tectonic Deformation in the Northern Qilian Shan

Author: HuXiaoFei
Tutor: PanBaoTian;Eric Kirby
School: Lanzhou University
Course: Quaternary Geology
Keywords: Qilian Shan thermochronology river terrace erosion rate tectonic deformation
CLC: P542
Type: PhD thesis
Year: 2010
Downloads: 151
Quote: 1
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Abstract


The studies on the time and rate of uplift and deformation in the Tibetan Plateau have gained significant improvement along of numerous researches on the plateau and its surrounding mountains, although one of the basic theories that the dynamic mechanism of uplift and deformation of the plateau, is still in a heated controversy. In the northeast part of the Tibetan Plateau, the Qilian Shan, as the newest part of the plateau, would be one of the key areas in studying the mechanism of uplift and extension of the plateau. Most previous studies focused on slip rates on the Alyn-Tagh Fault in west end of the Qilian Shan, the Haiyuan Fault in the east and the faults on the mountain front of the Qilian Shan. Among these studies, data are accumulated in the Holocene, and the study on the deformation rate within the mountain range is lack. Limited by few study areas and relatively short time period, the temporal and spatial distributions of the deformation rates in the Qilian Shan are poorly displayed. Due to these problems, we conducted series studies on low-temperature thermochronology, river incision rates and stream long profiles in the northern Qilian Shan. Based on these studies, the temporal and spatial distributions of erosion rates and deformation in the Qilian Shan are analyzed in order to better recognize the deformation pattern of the Qilian Shan, and which will strongly help us to understand the dynamic mechanism of the uplift and extension in the northeastern Tibetan Plateau.In the eastern Qilian Shan, low-temperature thermochronology of apatite from Baishugou gives Fission Track ages of 56-80 Ma B.P. and (U-Th)/He ages of 22-45 Ma B.P. Modeling of thermo history by Fission Track length data indicates that rock has experienced a low cooling rate of 0.41-0.74℃/Ma from 80 Ma B.P. to 10 Ma B.P., and experienced a relatively fast cooling rate of 7.5±1.8℃/Ma since 10-8 Ma B.P. Spatial distributions of fission track ages and modeling cooling histories suggest that fast cooling/erosion rate is located in the upstream area of Huajian, where has a high topography and high relief, and that relatively lower cooling/erosion rate is located in downstream area of Huajia, where has a lower topography and a lower relief.Studies on the terrace ages in eastern Qilian Shan with OSL, ESR, and 14C dating methods document that rivers incision mainly happened on five periods:67-71 ka B.P.,-50 ka B.P.,30-37 ka B.P.,20-25 ka B.P. and recent 10 ka. During Holocene, rivers incised 1-3 times, and correspondingly 1-3 terraces were formed. With precisely GPS survey on terrace heights, rates of river incision are determined within 0.3 mm/a and 2.5 mm/a. From 70 ka B.P. to 10 ka B.P, incision rates did not change along with time in individual tectonic area, but vary among different tectonic area. These distribution characters of incision rates showed a steady state of tectonic uplift since 70 ka B.P. in each tectonic area and a different uplift rate in different area. In the Xiying River and the Nanying River, terrace surfaces were apparently deformed by tectonic deformation. The average vertical slip rates on Kangning thrust fault are calculated as 0.20±0.05 mm/a,0.20±0.11 mm/a,0.22±0.10mm/a and 0.68±0.30 mm/a, since 70 ka B.P.,50 ka B.P.,30 ka B.P. and 10 ka B.P., respectively. The average vertical thrust rate of Huangcheng-Taerzhuang fault in Huajian is determined as 0.22±0.07 mm/a since 50 ka B.P., and the average vertical slip rate of Nanying fault near Qinzuiwan is 0.21-0.37 mm/a since 70 ka B.P.Analysis of the longitudinal profiles of bedrock channels along the northern Qilian Shan reveals systematic differences in the channel steepness index along the trend of the frontal ranges. Local comparisons of channel steepness reveal that lithology and precipitation have limited influence on channel steepness. Similarly, there is little evidence suggesting that channel steepness is influenced by differences in.the sediment loads. We argue that the distribution of channel steepness in the Qilian Mountain is mostly the result of differential rates of rock uplift. Thus, channel steepness indices reveal a lower rock uplift rate in the eastern portion of the Qilian Mountain and a higher rate in the middle and west. The highest rates appear to occur in the middle-west portions of the range, just to the west of the Yumu Shan.Different geological evidences present that during 70 Ma B.P. and 10 Ma B.P., the northern Qilian Shan, with a low relief, had experienced a low erosion rate with low depositing rates in closure basins, and since 10-8 Ma, the mountain area began to uplift in a relatively fast rate. Comparing of thermochronology data distributed along the northern Qilian Shan indicates that the initiation of fast uplift of the mountain range was simultaneously along the northern Qilian Shan, although the uplift rate might be different, which led to different topographies. Fast river incision rate in late Pleistocene and depositing evidence showed an acceleration trend with the uplift rate since Miocene. The temporal and spatial distribution of uplift rates along the Qilian Shan reveal that the faults on the mountain front would be merging into a big fault in deep crust, and the magnitude of the fault indicates it may extend below the crust. Ohterwise, vary uplift rates bounded by thrust fault in eastern Qilian Shan also show a rigid character of the Qaidam-Qilian block. Based on these evidences, we propose that the movement of the Qaidam-Qilian block to northeast is probably driven by the movement of the Tibet Plateau in corresponding to the collision of Indian and Euro-Asian plates, and the slip of the Alyn-Tagh fault is acting as a boundary condition of the Qaidam-Qilian block. Since 10-8 Ma, the collision of Qaidam-Qilian block with Ala-Shan block was happened in whole lithosphere, and which induced the deformation and uplift of the northern Qilian Shan.

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