Fission Track Geochronology of Xiaonanchuan Pluton and the Morphotectonic Evolution of Eastern Kunlun since Late Miocene
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摘要: 对昆仑垭口地区小南川岩体7件样品进行磷灰石裂变径迹年代学测试, 分析了岩体的冷却过程及岩体的剥露与构造地貌演化的关系.结果表明东昆仑山区中新世晚期视剥蚀速率极为缓慢, 为0.020~0.035mm/a, 反映的是构造隆升作用微弱、地貌缓和的地质环境, 因而构造隆升速率与低的视剥蚀速率相当.上新世以来小南川岩体突发性快速隆升冷却, 造成超过3km的物质揭顶, 这不是由单纯的剥蚀过程导致, 而是反映了昆仑山上新世以来的强烈构造隆升驱动下的成山作用过程.岩体上新世的裂变径迹年龄与近东西向的昆仑河-野牛沟谷地断裂断陷、昆仑垭口盆地断陷以及后期西大滩谷地断陷的综合构造地貌演化有密切的成因联系.此外裂变径迹年龄的空间分布格局反映了区域性的差异隆升作用, 由南向北、由西向东, 隆升和剥蚀作用逐渐衰减, 这与东昆仑山南北向以及东、西昆仑山之间地貌发育的差异性以及新生代火山作用分布是吻合的.Abstract: Apatite fission track (AFT) thermochronology of seven samples from the Xiaonanchuan pluton in the Kunlun pass area was carried out to determine the timing of cooling and the relation between the exhumation and the morphotectonic processes. The AFT ages yield low denudation rates of 0.020-0.035 mm/a during the Late Miocene, which correspond to a stable geomorphic and weak tectonic uplifting environment. The low denudation rates can be considered as the approximate tectonic uplifting rates. The AFT geochronology shows paroxysmally rapid cooling since the Pliocene and an apparent material unroofing of more than 3 km in the Xiaonanchuan area. This was not the result of simple denudation. The rapid cooling was coupled with the intensive mountain building processes since the Pliocene, which were driven by tectonic uplifting. The accelerated relief building was accompanied by a series of faulting, which caused the basin and the valley formation and sinking. The space pattern of the AFT ages also shows differential uplifting, which decreases northwardly. This trend is supported by the regional AFT data, which indicate that the exhumation decreases northwardly in eastern Kunlun. This trend also exists in east-west orientation from the western Kunlun range to the eastern. The uplifting trend is also supported by geomorphic characteristics including the elevation and the relief differences as well as the distribution of the Late Cenozoic volcanism.
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Key words:
- eastern Kunlun /
- Tibet /
- Xiaonanchuan /
- apatite fission track /
- morphotectonic /
- Pliocene
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图 1 研究区构造位置及小南川地区地质、地貌简图(据1∶25万不冻泉幅, 1∶20万纳赤台幅地质图修改)
1.中元古界万保沟群; 2.下古生界纳赤台群; 3.志留系赛什腾组; 4.三叠系; 5.花岗岩; 6.上白垩统海相碎屑岩; 7.第四系; 8.活动断裂; 9.一般断裂; 10.样品点
Fig. 1. Geological and geomorphology sketch map of Xiaonanchuan and tectonic position of study area
图 2 小南川岩体遥感影像及样品点分布
星号代表样品点位; 箭头指示性线影像走向及位置
Fig. 2. Sample positions in Xiaonanchuan pluton in the remote image
图 3 小南川岩体年龄高程分布及封闭径迹长度分布
Fig. 3. AFT age-elevation plot and fission track length distribution of Xiaonanchuan pluton
表 1 小南川岩体磷灰石裂变径迹年龄测试结果
Table 1. Apatite fission track dating results of Xiaonanchuan pluton
表 2 小南川岩体中新世晚期以来阶段性视剥蚀速率
Table 2. Apparent denudation rates of the Xiaonanchuan pluton at different stages since Late Miocene
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