Late Quaternary Tectonic Deformation of the Latgan Fold in the Southern Margin of the Bole Basin, North Tianshan
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摘要: 研究天山地区活动逆冲断裂、褶皱对于认识整个天山再生造山带的隆升和地震危险性评估具有重要意义。以天山北麓博乐盆地南缘库松木楔克断裂东段勒塔干褶皱为研究对象,通过无人机航拍提取高精度DEM和野外实地调查结果,将勒塔干背斜东部迪里克河附近的洪积扇分为5期,从新到老分别为T1、T2、T3、T4、T5。其中,T4洪积扇完整记录了褶皱的变形历史,其后翼褶皱陡坎高度为(8.1±0.6)m。自T4洪积扇废弃以来,勒塔干断层的滑移量为(33.0±2.6)m。T3洪积扇仅发育在迪里克河出水口处,即勒塔干背斜北侧,(16.9±0.2)m的断层陡坎高度揭示了自T3洪积扇废弃以来,控制背斜形成的逆断层发生了21.4~21.7 m的滑动。通过与相邻地区洪积扇期次进行对比,认为T4洪积扇的废弃年龄为(74.01±6.14)ka,勒塔干背斜下断坡晚第四纪滑动速率为(0.45±0.05)mm/a,勒塔干褶皱晚第四纪地壳缩短速率为(0.37±0.04)mm/a。Abstract: The study of active thrust faults and thrust folds in the Tianshan ranges is of great significance for understanding the uplift characteristic and earthquake hazard for the Tianshan. This paper focuses on the Latgan fold in the eastern segment of the Kusongmuxieke fault on the southern margin of the Bole Basin in the North Tianshan. Based on the high-precision DEM extracted by drone aerial photography and field surveys, we divided the alluvial fans which developed along the Dilik River in the eastern part of the Latgan fold into five generations, T1-T5 from young to old. The alluvial fan T4 completely records the deformation history of the fold. The height of the fold scarp of T4 is 8.1±0.6 m. Since the abandonment of the T4 alluvial fan, the dip-slip offset of the Latgan fault is 33.0±2.6 m. The T3 alluvial fan is only developed at the outlet of the Dilik River, that is, on the north side of the Latgan anticline. The 21.4-21.7 m dip-slip offset can be derived from the 16.9±0.2 m fault scarp since the T3 alluvial fan was abandoned. By comparing with the alluvial fan in adjacent areas, we can determine the abandoned ages of the T4 alluvial fan is 74.01±6.14 ka. Based on the lower dip-slip offset and T4 alluvial fan abandonment ages, the late Quaternary slip rate of the Latgan anticline and crustal shorting are 0.45±0.05 mm/a and 0.37±0.04 mm/a, respectively.
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Key words:
- Activity fold /
- Tianshan northern piedmont /
- Bole basin /
- Latgan anticline /
- Tectonic deformation
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引言
印度板块和欧亚板块碰撞的远程效应使晚新生代以来中亚地区发生了剧烈的变形,并导致天山的再度隆升和活化(Molnar等,1975;Tapponnier等,1979;Hendrix 等,1994;Yin等,1998)。天山南侧为塔里木地块,北侧为哈萨克地块和准噶尔地块,各地块之间现今海拔高度差别极大,不同地貌单元之间的转换部位地震频发,这些资料表明天山地区具有强烈的构造活动性(图1)。地球科学工作者针对天山地区断层和褶皱活动性、天山深部构造和现今地壳运动进行了研究(Molnar等,1984;Avouac等,1993;Abdrakhmatov等,1996;Burchfiel等,1999;Thompson等,2002;Hubert-Ferrari等,2007;沈军等2008;Yang等,2008;Makarov等,2010;杨晓平等,2010;Campbell等,2013;Stockmeyer等,2014,2017;Saint-Carlier等,2016;Grützner等,2017;Rizza等,2019;Wang等,2020a),天山地区主要活动断裂由山前东西走向逆冲断层褶皱带、北西-南东右旋走滑断层和北东-南西左旋走滑断层组成(Hubert-Ferrari等,2007;Charreau等,2008,2020;Campbell等,2013;Rizza等,2019;Wu等,2020),如图1所示。GPS速度场显示西天山南北向缩短速率约为20 mm/a,占据了印度板块与欧亚板块之间缩短速率的20%。天山地区南北向缩短速率不均一,呈向东降低的趋势。在东天山,即吐鲁番盆地的位置,南北向缩短速率降为5 mm/a (Burchfiel等,1999;Zubovich等,2010;Wang等,2020a)。在天山两侧和天山内部,大部分的逆冲断层为东西走向,但在博乐盆地-博罗可努山-吐鲁番盆地造山带内,发育许多北西西-南东东走向的逆冲断层或逆走滑断层,如库松木楔克断层、包尔图断层、库米什断层(Wang等,2020b;Ren等,2021)。部分学者(Avouac等,1993;Yin等,1998;Yang等,2008)认为天山应变主要集中在天山南、北侧山前逆冲断层褶皱带上,也有部分学者(Thompson等,2002;Huang等,2015;Charreau等,2017)认为天山内部逆冲断层和走滑断层在吸收天山应变中起了相当大的作用。虽然大部分7级以上地震(1902年M81/4阿图什地震、1906年M7.7玛纳斯地震、1911年MW8.0克敏地震、1885年M6.9Belovodskoe地震)发生在天山南、北侧的逆冲断层褶皱带或相邻的山脉中,但更高频率的6.0级左右中强地震(2017年MW6.3精河地震)和大地震(1812年M8.0尼勒克地震)发生在天山内部,这表明天山内部也吸收了许多的地壳应变。
本文主要研究对象为天山北麓博乐盆地南缘库松木楔克断裂东段的断层相关褶皱——勒塔干褶皱带,如图2(a)所示。库松木楔克断裂西起赛里木湖,沿着库松木楔克山呈弧状向东延伸。东段库松木楔克断层可分为3个近平行的逆冲断层和3个近平行的背斜,分别为精河南断层、查汗图断层、永集断层、精河南背斜、勒塔干背斜和阿拉沙克背斜(图2(b))。研究库松木楔克断层的活动性对地震灾害评估具有重要意义,尤其是该断层紧邻著名的风景区赛里木湖,且距博乐市和精河县仅50 km。
为评估勒塔干断层缩短速率,在勒塔干东段沿迪里克河使用无人机获得了高精度的数字高程模型(DEM)影像。利用获得的DEM数据和野外实地调查资料,得到勒塔干褶皱的地壳缩短量。结合相邻地区洪积扇的废弃年龄可推测迪里克河不同期次洪积扇的废弃年龄。综合以上数据,可确定勒塔干断层晚第四纪滑动速率。
1. 构造背景
博乐盆地位于中国新疆境内北天山西北部。天山北侧是稳定的哈萨克地块和准噶尔地块,南侧是相对稳定的塔里木地块,西南侧是快速变形的帕米尔高原。几个古生代的构造板块拼接碰撞组成天山山脉,且在晚新生代由于印度板块和欧亚板块的碰撞再次活化(Molnar等,1975;Tapponnier等,1979;Avouac等,1993;Hendrix等,1994;Allen等,1999)。裂变径迹结果显示天山再次活化的年龄大概在渐新世到中新世之间。之后,变形逐渐向天山南、北侧传递,5~7 Ma时天山构造变形传递到了拜城盆地和准噶尔盆地南缘(Sun等,2004,2009;Zhang等,2018)。天山地貌的典型特征是发育一系列东西走向的山脉和盆地,如伊犁盆地,吐鲁番盆地,博乐盆地,盆山边界往往是中高角度、错断基底卷入的逆冲断层(Ghose等,1998;Thompson等,2002;Makarov等,2010)。除天山内部的逆冲走滑断层外,天山南、北侧还发育大量的薄皮逆冲推覆构造(Avouac等,1993;邓起东等,2000)。
勒塔干背斜位于天山北麓博乐盆地南缘。三角形的博乐盆地北侧为阿拉套南缘断裂,东侧为博罗可努-阿齐克库都克断裂,南侧为库松木楔克断裂(图2(a))。文献和仪器记录显示,博乐盆地内未发生过大于7级的地震,仅有4个6.0~6.9级中强地震(1765年M6.5地震、1958 年M6.5地震、1962年M6.2地震和2017年MW6.3精河地震)和一些小震记录。2017年精河MW6.3地震发震构造为长度20 km的精河南断层和精河南褶皱,该次地震仅破裂了精河南断层的下断坡,最大滑移量为0.25 m(Gong等,2019;Hu等,2021)。博罗可努-阿齐克库都克断裂晚第四纪走滑速率为2~4 mm/a(Campbell等,2013;Hu等,2021)。阿拉沙克背斜长约10 km,核部宽约3 km。永集断层陡坎长约10 km,其晚更新世以来永集断层的垂直速率为0.23~0.25 mm/a(陈建波等,2007)。
2. 洪积扇褶皱变形测量与断层滑动量计算
为获得勒塔干背斜的变形信息,对南、北向流经勒塔干褶皱东部的迪里克河不同期次的洪积扇进行了划分和测量。研究工作包括识别和测绘不同期次的洪积扇、使用DJI Phantom 4 RTK无人机获取洪积扇地形数据、利用PIX 4D程序和无人机数据生成高分辨率的数字高程模型影像,精度为5.52 cm/pix。
(1)洪积扇期次的划分
洪积扇期次划分需利用洪积扇拔河高度和洪积扇面上的河道密度、深度。废弃年代越久的洪积扇,其拔河高度越大,上面发育的河流越多,且河流的下切深度越大。勒塔干背斜东段迪里克河两岸的洪积扇分布如图3所示,图中蓝色实线表示褶皱剖面线,黑色虚线表示横跨多期洪积扇地形剖面线,白色实线表示断层陡坎剖面线。
(2)洪积扇面褶皱变形剖面的绘制
在Arcgis中获取褶皱变形量,利用获得的DEM数据提取。每个数据点包括该位置的经、纬度和高程信息,两点之间的距离<6 m。将提取的数据点投影到垂直于褶皱轴的方向上得到活动褶皱的变形曲线,从而计算褶皱变形量,并利用洪积扇的海拔高度减去河床海拔高度消除初始洪积扇坡度的影响(Lavé等,2000)。在本研究中,褶皱构造变形量主要通过洪积扇砾石顶部的变形量估算。使用洪积扇表面表示变形量和速率的原因如下:①假设洪积扇表面在废弃前与现代河床平行,可将洪积扇表面与现代河床进行比较,得出其变形;②对比天山内部洪积扇研究结果,确定迪里克河附近洪积扇废弃年龄。
(3)断层滑动量计算
本文利用地层产状和洪积扇褶皱特征计算断层滑动量,即使用Thompson等(2002)提出的断弯褶皱、铲状褶皱的计算公式计算断层滑动量,按照Thompson等(2002)的方法,从高精度DEM中提取垂直断层陡坎的地形剖面,并计算断层陡坎高度。
3. 勒塔干活动断层与褶皱的变形
3.1 背斜区洪积扇分布特征
勒塔干背斜是长20 km、宽2.5 km的近东西走向的断层相关褶皱,背斜南翼出露黑色的西域砾岩。在该褶皱北翼,西域砾岩被剥蚀,新近系上新统的黄色泥岩出露。迪里克河由南向北流经勒塔干背斜东段,其发生褶皱变形的洪积扇记录了褶皱变形。洪积扇从新到老可被划分为T1~T5期次,其中T2、T3和T4洪积扇的基座为上新统泥岩(图4)。T1洪积扇仅保存勒塔干背斜北翼,且大部分分布在迪里克河左岸,右岸的T1宽约30 m、长约500 m。T2洪积扇沿迪里克河两侧分布,特别是在勒塔干背斜以南的迪里克河上游。T3洪积扇仅保存于勒塔干背斜北翼,高出T2洪积扇3~5 m。T4洪积扇由砾石组成,砾石覆盖于泥岩之上,T4洪积扇并不单单保存在褶皱区,在勒塔干褶皱南侧保留了1个宽20 m、长800 m的条带状T4洪积扇。T5洪积扇拔河高度最大,约为35 m,迪里克河左、右岸均有零星分布,其洪积扇上的河流下切深度较其他洪积扇深。需注意的是,勒塔干背斜地形为西侧高、东侧低,地形坡向东。
3.2 勒塔干断层下断坡滑移量
如图3、图5所示,残存的T1、T3、T5洪积扇分布连续性差,且分布面积有限,并不能通过这三期洪积扇确定褶皱活动的变形历史。T2洪积扇分布在背斜的核部和南、北侧,但T2洪积扇面褶皱变形轻微,褶皱变形不能较好地识别。T4洪积扇在背斜南翼、核部均有分布,较好地记录了勒塔干背斜的褶皱变形。该洪积扇在背斜核部残存约1 500 m(沿南北向),相对河床的地表坡度为0.03°,这表明背斜核部的T4洪积扇与河床近平行。背斜区的T4洪积扇可分为南、北段,拔河高度分别为25.2、26.0 m,由此得到背斜顶部T4洪积扇面的拔河高度为(25.6±0.4)m。后翼(南翼)的T4洪积扇水平长度约为1 000 m,相对河床的地表坡度为 0.56°。T4洪积扇面褶皱的后翼(南翼)平均拔河高度为18.0 m,最低位置拔河高度约为17.1 m,由此得到褶皱后翼(南翼)拔河高度为(17.5±0.5)m。褶皱后翼与褶皱顶部高度差为(8.1±0.6)m。
平缓且与河床近水平的褶皱顶部、距离较长的褶皱后翼是铲状褶皱的典型特征(Hu等,2015;Zhong等,2020)。勒塔干断层下断坡的倾角可由上断坡的断层倾角和有关拔河高度确定。野外断层露头揭露了勒塔干断层的上断坡倾角为55°,通过公式(h1/sinα1=h2/sinα2)计算得到下断坡的倾角为35°,进而通过铲状断层后翼抬升动力学模型计算得到自T4洪积扇废弃以来勒塔干断层的滑动量为(33.0±2.6)m(图5(e))。
3.3 勒塔干断层近地表的滑移量
勒塔干断层错断了迪里克河出水口处T1、T3、T5洪积扇。在T1洪积扇上可识别出(2.3±0.3)m高的断层陡坎。结合断层55°的倾角,2.3 m垂直断层位错揭示了2.5~3.4 m的断层滑动量,这个滑动量很可能揭示了最后一次地震位错量。T3洪积扇上断层陡坎的高度为(16.9±0.2)m,较大的断层陡坎高度表明T3洪积扇自被废弃以来经历了多次地表破裂事件,利用55°的断层倾角可得到(21.4~21.7)m的断层滑动量。与T1、T3洪积扇上的断层不同,T5洪积扇上断层分为南、北支。北支断层位于南支以北600 m处。南支断层陡坎高度为(16.4±0.2)m,与T3上断层陡坎高度近似。T5洪积扇南支的断层滑动量为(21.1~22.5)m,T5洪积扇北支断层陡坎高度仅为南支的1/2,为(8.2±0.3)m。由于T5洪积扇北支断层附近的基岩被第四纪的砾石遮盖,北支的断层面未出露,因此假设断层倾角为30°~60°(Grützner等,2017)。T5洪积扇北支断层滑动量为(9.5~18.9)m。自T5洪积扇废弃以来,断层垂向上的位错和断层滑动量分别为(24.6±0.6)m和(30.6~41.4)m(图6)。
4. 结论与讨论
通过无人机航拍和野外勘探,将勒塔干背斜东段迪里克河附近的洪积扇分为5期,从新到老分别为T1、T2、T3、T4、T5。其中T4洪积扇的断层滑动量与T5洪积扇一致,T4、T5洪积扇很可能是同时形成的次级洪积扇。自T4洪积扇废弃以来,断层的滑动量为(33.0±2.6)m。晚第四纪以来勒塔干褶皱的地壳缩短速率为0.38 mm/a。通过比较天山南、北侧褶皱缩短速率和天山内部断层缩短速率,可知天山内部断层缩短速率较小。
洪积扇、河流阶地的形成与废弃、气候旋回息息相关(张培震等,1996;Huang等,2019)。Huang等(2019)通过宇宙成因核素的测年手段计算了焉耆盆地内的洪积扇废弃年龄,开展了天山地区首个系统性总结洪积扇废弃年龄的工作。但考虑到Huang等(2019)研究区与本研究区距离达上百公里,因此,本文未直接采用其研究成果。Hu等(2021)测定了距勒塔干背斜仅30 km处洪积扇的废弃年龄,其中Fan1洪积扇侵蚀程度较T5洪积扇严重,海拔高度远高于T5洪积扇。因此,认为Hu等(2021)研究中的Fan2、Fan6洪积扇分别对应本研究的T5、T4洪积扇。因此,T4洪积扇废弃年龄为(74.01±6.14)ka。结合前文研究可知自T4洪积扇废弃以来,勒塔干断层滑动量为(33.0±2.6)m,勒塔干断层滑动速率为(0.45±0.05)mm/a,勒塔干背斜地壳缩短速率为(0.37±0.04)mm/a。GPS速度场显示,沿博乐盆地天山的南北缩短速率为10 mm/a(Hu等,2021),天山南北缩短量的3.7 %是由库松木楔克断层所吸收。相较于天山南、北侧较快的缩短速率2~6 mm/a(Avouac等,1993;Saint-Carlier等,2016;),天山内部的逆冲断层滑动速率均相对较小,如巴音布鲁克盆地北侧的逆冲断层缩短速率为0.04~0.25 mm/a,焉耆盆地北缘的哈尔莫墩褶皱地壳缩短速率为0.3 mm/a。库米什盆地南、北侧的库米什断层和包尔图断层的晚第四纪滑动速率分别为0.65、0.31 mm/a(Wang等,2020b;Ren等,2021)。
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