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天山是印度与欧亚板块碰撞过程中,欧亚大陆内重新复活的一条重要的造山带,其拓展过程一直备受关注。南天山西部山前新生代晚期巨厚的砾岩层为构造变形和磁性地层研究带来了困难,限制了该地区新生代晚期造山带拓展过程的研究。本文选取南天山西部山前乌鲁克恰提剖面,对新生代晚期的阿图什组和西域组地层进行详细的描述、测量,并对砾石出露点进行砾石统计。结果显示南天山西部山前乌鲁克恰提地区新生代晚期阿图什组存在3期沉积旋回,第一旋回由冲积扇远端、河流相变为该旋回上部的冲积扇扇中沉积环境。砾石统计结果显示由该旋回中部至上部砾石磨圆度变差、大粒径砾石增多。第二旋回沉积环境以较为平静的湖相及少许河流相开始,变为该旋回上部的冲积扇环境。砾石统计结果也显示出与第一旋回相似的变化特征,即由该旋回中部至上部砾石磨圆度变差、大粒径砾石增多。第三旋回下部为冲积扇远端、冲积扇扇中环境,上部变为扇根环境。砾石统计显示该旋回下部至上部,火成岩和变质岩等不稳定成分增多,磨圆度则呈现棱角状砾石显著增多的趋势,砾石粒径也明显增大。西域组则延续第三旋回的趋势,表现为山前快速堆积砾石沉积,砾石磨圆、分选呈显著变差趋势,粒径持续变大。从阿图什组第一旋回至西域组,砾石的磨圆度呈现变差的趋势,而粒径则表现为明显的增大趋势,这表明晚新生代(5.6Ma以来),褶皱冲断带不断向盆地拓展,物源区距离沉积区越来越近。这些结果表明南天山西部山前新生代晚期(5.6Ma以来)存在3期构造活动,为南天山山前晚新生代的多期拓展提供了证据。
The Tianshan Mountain is an important orogenic belt revived in Eurasia during the collision between India and the Eurasian Plate. Its expansion process has drawn much attention. Late Cenozoic massive conglomerate strata in the western part of southern South Tianshan have brought difficulties in tectonic deformation and magnetic stratigraphy research, which limits the study of the extension of the Neozoic orogenic belt in the area. In this paper, we select the Urukkatti section of the Piedmont Piedmont in the western part of the southern Tianshan Mountain to describe and measure the Atushi Formation and the Western Group formations in the late Cenozoic. Gravel statistics are also made for the gravel outcrops. The results show that there are three sedimentary cycles in the Late Cenozoic Atushi Formation in the Urucachati area, western Piedmont of the southern Tianshan Mountains. The first cycle is from the alluvial fan distal to the sedimentary environment of the alluvial fan in the upper part of the cycle. Gravel statistics show that from the middle of the cycle to the upper gravel roundness worsened, large gravel increased. The second cyclonic sedimentary environment begins with a relatively calm lacustrine facies and a little river facies and becomes an alluvial fan environment in the upper part of the cycle. Gravel statistics also show similar characteristics to the first cycle, ie, the roundness of the gravel from the central part of the cycle to the upper part has deteriorated, and the increase of large-size gravel. The lower part of the third cycle alluvial fan distal alluvial fans in the environment, the upper fan-shaped environment. The gravel statistics show that the unstable components such as igneous rocks and metamorphic rocks increase from the lower part to the upper part of the cycle, while the roundness shows a significant increase of angular gravel, and the gravel size also increases obviously. The Western Regions continued the trend of the third cycle, showing the rapid accumulation of gravel deposits in front of the piedmont. The gravel rounding and sorting showed a significant trend of deterioration, and the particle size continued to increase. The degree of roundness of gravel tends to deteriorate from the first cycle of the Atushi Formation to the Western Formation, while the grain size shows a clear trend of increase. This indicates that the Late Cenozoic (5.6Ma), fold-thrust belt Continuously expanding to the basin, the source area is getting closer and closer to the sedimentary area. These results indicate that there are three tectonic activities in the late Paleozoic of the southern South Tianshan Mountains (since 5.6 Ma), which provide evidence for the multi-stage expansion of the southern Tianshan Mountains in the late-Cenozoic.