Newly discovered backthrust fault and related active fold scarp in the first piedmont anticline belt of the foreland thrust belt of the North Tianshan
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摘要:
构造楔是活动挤压区重要的发震构造。北天山前陆冲断带发育复杂的挤压构造楔, 并在地表形成3~4排褶皱带, 前人普遍认为最南部的第一排山麓背斜带自第四纪以来已不再活动。野外调查发现, 第一排山麓背斜带——南安集海背斜北翼单斜区发育2条近平行排列的、向南逆冲的晚第四纪活动反冲断层(喀沙反冲断层, KBT), 断错安集海河两岸的晚第四纪地貌面, 并形成一系列坡向南的反向断层陡坎。安集海河组灰绿色泥岩沿KBT顺层向南逆冲至沙湾组砖红色砂岩之上, 造成地层的重复出露。在安集海河西岸, 南侧的KBT断层面顺层呈铲型构成断弯, 断层的持续活动使上盘阶地和基座面均发生同步弯曲, 形成向斜断弯褶皱陡坎。褶皱陡坎南侧的阶地堆积较薄, 向北明显增厚, 具有生长地层的特征, 表明KBT在阶地堆积过程中有持续活动。喀沙反冲断层陡坎及活动褶皱陡坎的发现表明, 南安集海背斜晚第四纪以来仍在活动。这些地表反冲断层及相关褶皱构造为研究准噶尔南缘构造楔的活动性提供了窗口, 为进一步限定活动构造楔的变形机制、速率和生长演化历史提供了可靠便捷的途径。
Abstract:The tectonic wedge is an important seismogenic structure in the active compression area.The foreland thrust belts of the North Tianshan developed a complex extruded tectonic wedge and formed 3~4 groups of fold belts on the surface.It was generally believed that the first group of piedmont anticline belt in the southern part had ceased to be active since the Quaternary.In the field investigation, we found that there were two nearly parallel Late Quaternary active back thrusts (Kasha backthrust fault, KBT) arranged southward in the first piedmont anticline belt, the monocline area on the north wing of South Anjihai anticline.It had faulted the Late Quaternary landform on both sides of the Anjihai river and formed a series of southward thrust fault scarps.The gray-green mudstones of the Anjihaihe Formation (E2-3a) thrust southward along the KBT bedding onto the brick-red sandstones of the Shawan Formation (E3-N1s), it causing repeated exposure of the strata.On the west of the Anjihai river, the KBT plane on the south side is shovel-shaped along the bedding to form a fault bend.The continuous activity of the fault caused the simultaneous bending of the upper terrace and the base surface, forming a slanting fault-bending fold scarp.The terraces on the south side of the fold scarp are relatively thin, and they have obvious thickening to the north.They have the characteristics of growing strata, indicating that KBT has continuous activities during the terrace accumulation process.The discovery of the Kasha backthrust fault scarp and active fold scarp indicate that the South Anjihai anticline has been active since the Late Quaternary.These backthrust faults and related fold structures provide a window for us to study the activity of tectonic wedges in the southern margin of Junggar, and provide a reliable and convenient way to further define the deformation mechanism, rate and growth history of the active tectonic wedge.
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Key words:
- active fold /
- backthrust fault /
- fold scarp /
- South Anjihai anticline /
- Northern Tianshan
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表 1 T3阶地面沉积物光释光年龄测定值
Table 1. OSL age of sediments on terrace T3
阶地 埋深/m 238U /(Bg·kg-1) 226Ra /(Bg·kg-1) 232Th /(Bg·kg-1) 40K /(Bg·kg-1) 含水量/% 内剂量/(Gy·ka-1) 环境剂量率/(Gy·ka-1) 等效剂量/Gy 年龄/ka T3 1.7 33.9±6.7 35.4±0.6 33.0±0.4 567.4±10.1 5 0.545±0.026 3.5±0.2 382.6±45.4 108.2±14.0 
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