【作者】 李清洋；

【导师】 潘保田；

【作者基本信息】 兰州大学 ， 第四纪地质学， 2010， 硕士

【摘要】 关于青藏高原隆升—扩展的过程、时间及其机制,一直是国际地球科学界普遍关心的科学问题和学者们争论的热点。祁连山位于青藏高原的东北缘,新生代构造研究表明,逆冲断裂、活动褶皱、构造隆升在该区普遍发生,整个地区经历了强烈的构造变形。祁连山的抬升历史在一定程度上记录了青藏高原隆升的过程,近年来受到众多学者的越来越多的关注。相比在祁连山西段已经取得的科学进展,东段的研究工作略显不足,这在一定程度上妨碍了从整体上把握和认识青藏高原隆升及扩展对周缘地区的影响,因此有必要加强在祁连山东段的研究工作。磷灰石裂变径迹测年方法是近年来兴起一种低温热年代学方法,由于其封闭温度较低(100±20℃),对温度变化反应敏感,因此能够对构造活动和地表过程之间相互关系进行有效研究。本文以祁连山东段为研究区域,利用磷灰石裂变径迹测年方法对一组采自该区奥陶系花岗岩岩体的样品进行了测年分析,对其热演化历史进行了模拟,计算出该区长尺度地表剥蚀速率,并初步讨论了该区的构造活动和地形对裂变径迹年龄及长尺度地表剥蚀速率的影响。主要结论如下：1.祁连山东段13个的磷灰石裂变径迹视年龄结果为晚白垩纪到古新世(～56-86Ma),且大部分集中在70Ma到75Ma,均远远小于地层年龄。裂变径迹长度分布特征表明,样品在通过磷灰石裂变径迹部分退火带时发生了严重的退火。2.热历史模拟研究揭示出研究区从白垩纪晚期以来经历了三个主要演化阶段：①～75Ma到18Ma,祁连山东段处于构造稳定期,地表侵蚀速率为～0.01mmm/a,②～18Ma到～8Ma,祁连山东段进入快速抬升—侵蚀期,侵蚀速率为～0.07mm/a,③～8Ma至今,祁连山东段发生强烈抬升—侵蚀,侵蚀速率为～0.24mm／a,并且此次构造活动具有多阶段性。3.祁连山东段热演化历史说明,青藏高原东北缘的持续向外扩展促使祁连山从中新世中晚期开始,沿着祁连山活动断裂系由北西向南东逐渐抬升,最近8 Ma以来为祁连山抬升的主要阶段。4.地形对于磷灰石裂变径迹年龄可能有很大影响,尤其是对待类似本文中密集采样的测年结果,年龄普遍倒置现象很可能是由于地形和岩体抬升—地表剥蚀相互反馈使得样品发生差异性冷却退火所致。长尺度地表剥蚀速率在根本上受控于构造作用,并且能够反映构造影响下的地貌发育过程。更多还原

【Abstract】 The process, time rate, and mechanism are key issues in understanding uplift and extension of the Tibetan Plateau, and attract consistent general concern in the geo-science field in recent decades. Margining the northeast Tibetan Plateau, the Qilian Mountain suffers dramatic tectonic deformation since Neogene, deduced from researches of ubiquitous thrust faults, active folds, and tectonic uplift in situ. Moreover, owing to its sensitive reflection upon the uplift, it provides an outstanding field laboratory for studies of plateau uplift and therefore receives much more concern recently. Nevertheless, the investigations on eastern Qilian Mountain is still insufficient comparing to the western Qilian Mountain, and in some extent, lack of data in this area impedes the full understanding of the effects of uplift of the Tibetan Plateau on surround regions.Thus, an enhancement of investigation on eastern Qilian Mountain is urgently required. Apatite fission track (AFT) dating is one of the most important thermochronology method springing up recently, and because of its sensitivity to temperature variation and lower closure temperature (100±20℃), AFT serves as a powerful approach in studies of tectonic activities,surface processes and mountain evolution.Here, we analyzed a suite of apatite fission track data from local ordovician granite samples in eastern Qilian Mountain, modelled their thermal history evolution, calculated the long-term surface denudation rates, and then discussed the regional tectonics and the effects of topography on fission track ages and long-term surface denudation rates.Our investigation draws some conclusions as follows:1.Apparent ages of 13 apatite fission track samples from eastern Qilian Mountain range from late Cretaceous to Paleocene(-56-86Ma) and concentrate upon 70Ma to 75Ma, far younger than the age of strata. The horizontal confined tracks are relatively short, in the order of～12.5μm, the distributions of track length are broad and display single skewed peak, which suggest a strong annealing with the samples while going through the upper part of partial annealing zone.2.The modeling result of thermal evolution history with fission track data indicates three main cooling stages in eastern Qilian Mountain since late Cretaceous:①～75 Ma to 18 Ma, a relative stable state, with an low erosion rate of～0.01mm/a.②～18 Ma to～8 Ma, a stage of relatively rapid exhumation, and the erosion rates is-0.07 mm/a.③～8Ma to present, a dramatic high exhumation stage, which characterized a multiple sub-stage, and the average erosion rates is-0.24 mm/a.3.The thermal evolution history of eastern Qilian Mountain illustrates the outward growth of the Tibetan Plateau prompting the gradual uplift of Qilian Mountain from northwest to southeast along the Qilian active faults system since middle-late Miocene, and the latest 8 Ma is the major stage of uplift of Qilian Mountain.4. Topography may have significant influence on apatite fission track ages, especially to the intense sampling dating as this paper involves. The age inversion may come from the discrepant annealing induced by feedback between topography and rock uplift-surface denudation process. Long-term surface denudation rates are in first order controlled by tectonic uplift and could reflect the geomorphical evolution under the influence of tectonics.更多还原