【作者】 胡小飞；

【导师】 潘保田； Eric Kirby；

【作者基本信息】 兰州大学 ， 第四纪地质学， 2010， 博士

【摘要】 随着对青藏高原研究的逐步深入,隆升时代及其变形速率方面取得了重要进展,但最基本的理论问题之一：隆升变形模式与机制问题目前有较大分歧。位于高原最北缘的祁连山,隆升时代较晚,是高原的最新组成部分,其构造变形研究对解决高原隆升扩展机制具有重要作用。以前的研究,主要集中在祁连山西端阿尔金断裂和东部海原断裂的走滑速率和祁连山山前洪积区域的断裂变形速率方面,对于山体内部的变形及剥蚀研究较少,而且变形研究多集中在全新世。另外,由于研究点与时段的限制,对祁连山构造抬升变形的时空分布特征较少涉及,而这些研究方面的深入对理解祁连山及高原北部扩展变形模式有重要的意义。本研究针对这些问题,在祁连山北部开展了低温热年代学、河流阶地及变形、河道分析等方面的研究,在这些研究的基础上对祁连山侵蚀和抬升的时空分布特征进行分析,以探讨高原东北部抬升变形的动力学机制问题。祁连山东段低温热年代学研究得到西营河柏树沟磷灰石裂变径迹年龄为56-80Ma,磷灰石(U-Th)/He年龄为22-45 Ma。热历史模拟结果显示在-70 Ma至10-8 Ma期间,样品经历了较慢的冷却速率(0.41-0.74℃/Ma)；10-8 Ma至今样品快速冷却,冷却速率为7.5±1.8℃/Ma。柏树沟地区10-8 Ma以来的平均剥蚀速率约为0.23mm/a。较快冷却剥蚀出现在山体较高、地形起伏较大的铧尖上游区域,下游山体较低、地形起伏较小的区域冷却剥蚀较慢。利用光释光、ESR、14C测年手段对祁连山东段的河流阶地形成时代进行了定年,发现晚更新世祁连山东段阶地主要形成于20-25 ka、30-37 ka、51-56 ka左右、67-71 ka。全新世以来,祁连山东段河流普遍发生1-3次下切,而形成1-3级河流阶地。结合阶地形成年代,在高精度GPS测量的基础上,计算分析得到祁连山东段河流下切速率主要分布在0.3-2.5 mm/a之间。从70 ka到10 ka之间,在各个构造区下切速率随时间变化较小,显示了区域抬升速率的稳定性。河流下切速率的空间分布特征揭示了在垂直于祁连山走向的方向上,祁连山东段的构造抬升速率存在空间差异。在皇城-塔尔庄断裂以南的区域构造抬升速率较高,祁连山北缘断裂与皇城-塔尔庄断裂之间的区域抬升速率较低。抬升速率的差异而导致皇城-塔尔庄断裂南北出现不同的地形起伏。由于构造变形的作用,在西营河与南营河的河谷中,阶地面发生了明显的变形,计算得到康宁桥断裂70 ka、50 ka、30 ka、10 ka以来的平均错断速率分别为0.20±0.05mm/a、0.20±0.11 mm/a、0.22±0.10mm/a、0.68±0.30 mm/a; 50ka以来,皇城-塔儿庄断裂在铧尖附近的垂直错断速率为0.22±0.07mm/a；70 ka以来清咀湾附近断裂垂直滑动速率为0.21-0.37 mm/a。50 ka以来,祁连山北部的水平缩短速率为0.33-0.54 mm/a,吸收了青藏高原高原北部约1/10的缩短量。分布于祁连山北翼的河流河道陡峭指数沿祁连山走向有系统的变化,在东段较小,中西段较大。通过对比分析显示岩性和降水条件对河道的陡峭指数影响有限,也没有明显的证据显示河流负载对河道的陡缓有明显的作用,河道陡峭指数的变化主要受区域构造抬升速率的影响。河道陡峭指数的变化反映了祁连山北翼构造抬升速率的分布特征：祁连山东段抬升速率较低,中、西段较高,抬升速率最大处位于中段榆木山以西区域。综合分析以上研究结果,得到东西绵延约1000 km的北祁连山在10 Ma之前,侵蚀速率很小,周缘沉积速率较低,多为湖湘沉积,地形较平缓。近10-8 Ma以来,整个北祁连山开始同步快速抬升,抬升速率在空间分布上的差异导致山体海拔与地形起伏的差异,在晚中新世以来祁连山的抬升有后期加速的趋势。祁连山北部抬升速率的时空分布特征显示祁连山北缘断裂在深部是一条连在一起的深大断裂,其深度应该可达到地壳以下,祁连山东段的差异性抬升也揭示了祁连山块体的刚性特征。柴达木-祁连块体向北东运动的驱动力应主要来源于高原主体逐渐向北东方向推进的动力,阿尔金断裂的走滑是柴达木-祁连块体向北运动的边界表现。10-8 Ma以来,柴达木-祁连块体与北部较稳定的阿拉善块体的挤压可能发生在整个岩石圈厚度上,同时造成祁连山的逐步抬升。更多还原

【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.更多还原