2020, Vol.40
2 云南大学地球系统科学研究中心, 云南省地球系统科学重点实验室, 云南 昆明 650500;
3 中山大学地球科学与工程学院, 广东省地球动力学作业与地质灾害重点实验室, 广东 广州 510275;
4 中国地震局第二监测中心, 陕西 西安 710054;
5 新疆维吾尔自治区地震局, 新疆 乌鲁木齐 830011)
帕米尔位于喜马拉雅-青藏造山带的西北部，是印度、欧亚大陆会聚带的西构造结(图 1a)。新生代以来，伴随着印度板块向北的强烈推挤，帕米尔发生了显著的地壳缩短增厚，并向欧亚大陆楔入达约300 km[1~4]。根据活动构造调查、GPS速度场和震源机制解等资料，现今的帕米尔在其北缘和西缘强烈挤压缩短的同时[4~9]，中-西帕米尔正以近南北走向的公格尔拉张系为界与东帕米尔分离[8~16](图 1a和2b)。
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图 1
帕米尔及邻区的主要构造单元(a)和帕米尔东北部的主要新生代构造图(b)
据Burtman和Molnar[1]、Robinson[19]以及Li等[15]修改 图 1a中粗红色箭头代表GPS速度矢量分析指示的中-西帕米尔与东帕米尔的分离，白色边框指示图 1b的位置； 图 1b中黄色区域为公格尔拉张系，蓝框指示图 2a位置 Fig. 1 Major structural elements of the Pamir and adjacent areas (a) and major Cenozoic structure map in the northeastern Pamir (b) (modified from Burtman and Molnar[1], Robinson[19] and Li et al.[15]). The red thick arrow in Fig. 1a represents the GPS velocity vector which indicates the separation of mid-west and east Pamir. Fig. 1b is the enlarged view of the region in the white frame; Kongur Extensional System is shown in yellow zone, and the blue area indicate the location of Fig. 2a |
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图 2 (a) 木吉断层的空间展布图(位置见图 1b，据Li等[15]修改)和(b~e)典型地点的位错特征(位置见图 2a) Fig. 2 (a)Spatial distribution map of the Muji Fault(location shown in Fig. 1b, modified from Li et al[15]) and (b~e) offsets of geomorphic markers observed on Google Earth images(location shown in Fig. 2a) |
木吉断层位于公格尔拉张系的最北段，整体呈NWW向延伸，断层向东与公格尔拉张系剩余段呈大角度相交，向西可能与近东西走向的帕米尔逆断层带相接(图 1b和2a)。木吉断层作为中-西帕米尔与东帕米尔最北部的边界转换断层，其运动性质和滑动速率的准确限定对于理解帕米尔现今应力状态和运动学特征、造山带内部大型正断层端部与走滑断层的运动学转换过程等至关重要。Chevalier等[17~18]通过详细的地貌填图认为木吉断层西段以右旋走滑为主，东段以正断为主；通过对位于断层中部的阿克萨依处断错河流阶地的研究，估算断层右旋滑动速率为4.5~11.0 mm/a。Li等[15]通过进一步的填图则认为断层西段以右旋走滑为主，而东段虽有显著的正断分量，但仍以右旋走滑为主；结合无人机航拍提取的高精度地形和宇宙成因核素10 Be深度剖面测年法进一步限定阿克萨依处右旋滑动速率介于5.9±1.2 mm/a和9.4±1.0 mm/a之间；综合区域内GPS观测数据，估算中-西帕米尔和东帕米尔的相对拉张以1.4°~2.0°/Ma的角速度进行，旋转中心大致位于公格尔拉张系的最南端。那么木吉断层的运动性质如何？沿走向是如何变化的？随着断层运动性质的变化，其滑动速率是否发生了改变？
为了回答上述问题，本文选取阿克萨依向东约20 km的布拉克村为研究对象(图 2a)：布拉克村以北约10 km的山前高冰碛台地(39.2020°N，74.3910°E)的水平和垂直位错特征显著。在高分辨率卫星影像解译基础上，结合详细的野外踏勘与差分GPS精准测量限定冰碛物的水平位错量和垂直位错量；利用宇宙成因核素10 Be暴露测年法对位错冰碛物上的冰川漂砾进行测年，限定其形成时代，获得了断层在该处的晚第四纪右旋滑动、垂直抬升和水平拉张量及速率。
1 区域构造背景帕米尔造山带整体呈向北突出的弧形，其周缘为不同性质的块体环绕(图 1a)：西侧为以三叠系或侏罗系为基底的相对软弱的塔吉克盆地[1, 8, 20]，东侧为以寒武系为基底的相对刚硬的塔里木盆地[8]，北侧为阿莱谷地-南天山山脉。受不同边界条件的影响，帕米尔西缘发育左旋压扭性的达瓦孜断层，东缘发育右旋走滑为主的喀什-叶城转换带和喀喇昆仑断层，北缘发育向北逆冲的帕米尔逆断层带；此外，帕米尔内部的现今构造变形以公格尔拉张系和卡拉库尔地堑的显著拉张为主(图 1a和2b)。
达瓦孜断层走向NNE，是帕米尔与塔吉克盆地的边界断层(图 1a)。由于达瓦孜断层的左旋压扭运动，帕米尔西部发生了显著的逆时针旋转[1, 13, 21]。断层的总滑动量、滑动速率等运动学参数至今未得到很好的限定。
NWW走向的喀什-叶城转换带由多条近平行、陡倾的右旋走滑断层组成[2]，延伸长度约350 km(图 1a和1b)。断层自晚始新世[22]或晚渐新世[23~24]开始活动，调节了帕米尔相对于塔里木盆地的向北运动，累积右旋滑移量约280 km，平均滑动速率7~15 mm/a[2]。由于断层沿线未发现明显的断错地貌[25~26]，断层两侧GPS速率基本一致[9]，表明喀什-叶城转换带晚第四纪以来活动已基本停止，东帕米尔和塔里木盆地现今已拼接为一整体向北移动。
喀喇昆仑断层位于喀什-叶城转换带的SW侧，是帕米尔与青藏高原的边界断层(图 1a和1b)。断层活动可能始于晚渐新世[19, 27]，其北段的累积右旋走滑量为149~167 km，平均滑动速率7~11 mm/ a[28]；由于沿断层地貌面未发生明显断错，晚第四纪以来断层北段活动已基本停止[12, 14, 28]。
帕米尔逆断层带由数条近东西走向、向北逆冲的断层组成(图 1a和1b)，是新生代以来帕米尔与欧亚大陆缝合拼接的位置[1]。断层带活动逆断层和褶皱变形显著，GPS观测数据表明其缩短速率高达10~15 mm/a[9]。帕米尔逆断层带内地震活动频繁，是1974年玛尔坎苏Mw7.1地震、1985年乌恰Mw6.9地震和2008年Nura Mw6.6地震的发震断裂[15, 29]。帕米尔内部现今的构造变形以沿公格尔拉张系[10~11, 30~31]和卡拉库尔地堑近EW向拉张作用为主[32~34]。由于卡拉库尔地堑拉张量(＜3 km)和速率(＜1.0 mm/a)非常有限[32, 34]，帕米尔内部的EW向拉张主要集中在公格尔拉张系上。公格尔拉张系整体呈NNW走向，倾角20°~45°，总长约250 km[31]。公格尔拉张系开始活动的时间起始于6~8 Ma[10~11]；总拉张量由北部的30~35 km向南逐渐减小至 < 3 km[11, 16]。由GPS限定的现今东西向拉张速率也表现为由北向南递减的趋势[15]：由北部的约7 mm/a逐渐减小到南部的0 mm/a。沿公格尔拉张系地震活动强烈，如1895年塔什库尔干7级地震[35]、2016年阿克陶Mw6.6地震[15]和2017年Mw5.4地震[15]等。根据走向和构造变形特征公格尔拉张系由北向南划分为多段，本文研究的木吉断层位于公格尔拉张系的最北段(图 1b)。
2 布拉克村北冰碛物位错特征和年龄木吉断层发育在昆盖山山前，整体沿NWW走向呈线性展布(图 2a)。断层向东与公格尔拉张系近乎正交，向西切过喀拉阿特河谷最终可追索至帕米尔逆断层带附近，延伸长度近80 km。断层沿线发育一系列位错地貌，包括河流阶地、洪积扇、水系、冰碛垄和山脊等的同步位错。大致以阿克萨依(见图 2a的地点3)为界，断层东段在右旋走滑的同时具有显著的正断分量(图 2d和图 3)。以地点2为例(图 2a和2c)，断层活动不仅使河流阶地右旋位错了约26 m，还形成了一宽度约200 m的地堑：坡向南的主断坎高约21 m，而坡向北的断坎包括5条，高度0.3 m至7.5 m不等。与东段不同，断层西段运动性质几乎为纯右旋走滑，垂直位错分量很小(地点4，见图 2a和2e)：如在地点4，断层活动使一冲沟右旋位错约160 m，但冲沟附近断层陡坎高度不足约20 m。断层运动性质的变化也体现在木吉盆地的宽度上：在阿克萨依以东，木吉盆地宽度超过约15 km，与木吉断层具有显著的正断分量相符；自阿克萨依向西，盆地宽度迅速变窄并在喀拉阿特河谷附近消失，表明断层正断分量已不明显。为了对木吉断层东段的运动性质和滑动速率进行定量限定，本文选择布拉克村北断错冰碛物进行研究：在布拉克村北，覆盖于昆盖山上(海拔> 5000 m)的山麓冰川强烈侵蚀古生代基岩，并沿冰川槽谷将碎屑沉积搬运出昆盖山，在槽谷出口的斜坡上堆积了连续分布、保存完好的冰碛物(图 3a)；木吉断层切过这些冰碛物，使冰碛物外缘(图 3b和3c)和发育在其表面的冲沟(图 3b和图 4a)发生了明显的位错，根据位错量、冰碛物的年龄，可准确限定断层的运动性质和滑动速率。
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图 3
布拉克北Google Earth卫星影像图(a)、地貌解译图(b)和冰碛物M2西缘右旋走滑位错量(c)(位置见图 2a)
图 3a中p1、p2蓝色线条为野外实测剖面位置；图 3b中M1冰碛物内黑色虚线指示冰碛垄；M2冰碛物西缘的右旋走滑位错量为190 m，由东向西的水系位错量分别为105 m、70 m和50 m，断坎的垂直断距为105±12 m，NE倾次级陡坎的垂直断距为约4.5 m； 图 3b中黄色圆圈代表采样位置 Fig. 3 Google Earth satellite images of north Bulake (a) and the corresponding geomorphic interpretation (b), and dextral-slip offset of the western edge of the moraine deposits (c)(location shown in Fig. 2a). The blue line of p1 and p2 in Fig. 3a indicate the location of measured sections; the black dashed lines of M1 marine in Fig. 3b indicate the distribution of moraine deposit; The displacement of right lateral strike slip of the west margin of M2 marine is 190 m, the displacement of the drainage from east to west are 105 m, 70 m and 50 m respectively, the vertical displacement of the main scarp is 105±12 m, and 4.5 m for the secondary scarp; The yellow circles in Fig. 3b indicate the sample location |
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图 4 木吉断层布拉克北水平断错地貌(a)和垂直断错地貌(b)(位置见图 3a) Fig. 4 Horizontal faulted geomorphology (a) and vertical faulted geomorphology (b) of Bulake north segment from Muji Fault(location shown in Fig. 3a) |
据相对位置、高度和表面风化程度，可将冰碛物划分为两期(图 3b)。较年轻的冰碛物(M1)靠近山前，海拔介于4300 m至4700 m。冰碛物表面发育一系列近平行、向南凸出、形态完整的弧形冰碛垄，无植被和黄土覆盖(图 3a)。较老一期冰碛物(M2)堆积在较年轻冰碛物(M1)的外围，海拔介于4000 m至4300 m，其外缘高出周围冲洪积扇达100 m。冰碛物表面被黄土覆盖，整体呈灰黄色；表面还零星镶嵌有花岗岩、石英岩漂砾，漂砾直径通常为0.4~2.0 m，最大者超过3 m。
沿木吉断层，冰碛物M2西缘右旋位错量约190 m(图 3c)，该位错量可用以代表冰碛物M2形成以来的断层的右旋滑动量。在冰碛物M2表面还可识别出3条可准确测量位错量的冲沟，由西向东位错量分别为50 m、70 m和105 m(图 3b)。由于这些冲沟形成于冰碛物沉积之后，其位错量应不超过冰碛物沉积以来断层的实际滑动量。综上，冰碛物M2形成以来断层的右旋滑动量为190 m。
在右旋位错的同时，冰碛物M2也发生了显著的垂直位错(图 3b和4b)。根据差分GPS实测地形数据(图 5a)，断层下(北)、上(南)盘地形坡度分别为约6.4°和11.5°，断坎高度105±12 m。另外，在该陡坎南侧的洪积扇上还识别出两条走向NW、坡向N的次级断层形成的陡坎，其高度分别为约4.5 m(图 5b)和 < 1 m。这些坡向北的次级断坎和坡向南的主断坎构成局部地堑。
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图 5 冰碛台地断坎(a)和冲积扇断坎实测剖面(b)(位置见图 3a) Fig. 5 Vertical offsets of moraine (a) and vertical offsets of alluvial fan (b)(location shown in Fig. 3a) |
冰川和河流等外动力地质作用将未暴露的岩石带到地表接受宇宙射线的照射，从而在矿物晶格中生成宇宙成因核素(如10 Be、21 Ne、26 Al等)。核素浓度随着暴露时间的增长而不断积累[36]。根据地表岩石中累积的核素浓度可测定岩石的暴露年龄[37]。
在木吉断层以北的冰碛物M2表面选择7个直径较大、无明显覆盖、翻滚和后期搬运痕迹的花岗岩漂砾进行样品采集(图 3b，图 6a~6 g和表 1)。在每个花岗岩漂砾顶面厚度小于5 cm的表层进行取样，对各采样点拍照，并详细记录砾石粒径、风化程度和位置信息。对周围有高山遮挡的砾石，测量采样面至山顶的仰角(从0°至360°每隔20°读取一个值)，后期进行地形屏蔽系数校正。
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图 6 布拉克北冰川漂砾采样点照片(a~g)和10 Be年龄结果图(h) (h)年龄平均值由灰色框内5个样品的暴露年龄算术平均计算得出；(a~g)具体位置见图 3b Fig. 6 Photos of the sampled glacial boulder from Bulake north site(a~g)and the distribution of 10 Be age (h). The light brown band in Fig. 6h indicate the arithmetic mean of the ages of five samples in the grey frame; The location of Fig. 6(a~g) are shown in Fig. 3b |
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表 1 木吉断层地貌面样品10 Be测试分析数据与年龄结果 Table 1 The 10 Be concentration and exposure ages of samples from Muji Fault |
样品中石英矿物的提纯在中国地震局地质研究所新构造年代学实验室进行：首先将砾石样品粉碎并筛选出粒径250~500 μm的颗粒；用盐酸浸泡去除碳酸盐矿物；对样品进行磁选去除磁性矿物；最后用5 %的HF/HNO3的混合酸刻蚀样品以充分去除石英矿物中大气成因的10 Be和长石矿物，结合多钨酸钠重液分离将石英进一步提纯。
提纯后的石英矿物在中国地震局地壳应力研究所宇宙成因核素前处理实验室进行10 Be核素的化学分离：称取约30 g石英样品加入9 Be载体，加入HF并加热使其完全溶解，通过阳离子交换树脂柱分离提取Be，过柱后的溶液加入氨水并调节pH=9沉淀Be(OH)2，于马弗炉中高温煅烧制成BeO。最后的靶样制作和加速器质谱测试在美国普渡大学PRIME实验室完成。样品具体信息和测试结果见表 1。
利用华盛顿大学网站提供的宇宙成因核素测年CRONUS-Earth在线程序(Version 3)(http://hess.ess.washington.edu/)计算样品的10 Be暴露年龄(表 1)。考虑到花岗岩漂砾相对较易受到风化侵蚀的影响，Owen等[38]得出的塔尔库什干谷地的冰川漂砾的侵蚀速率为2.3 m/Ma，与Seong等[39]通过慕士塔格和公格尔山地区最古老的冰碛物巨砾的10 Be浓度计算得出的1~3 m/Ma的侵蚀速率结果相一致。假定布拉克北冰川漂砾的侵蚀速率同为2.3 m/Ma，计算得出不同产率模型对应的暴露年龄(表 1)。本文选择LSDn产率校正模型[40]计算冰碛物年龄和滑动速率。
根据测试结果，样品2017MJ-1至2017MJ-5的年龄介于约12.6 ka至22.7 ka之间(图 6h)，而样品2017MJ-6和2018MJ-6的年龄明显大于上述5个样品的年龄，可能说明其在最后一次暴露地表之前经历了多次搬运和暴露，所测得的10 Be浓度含有显著的继承性组分。我们用相对一致的5个样品的年龄的平均值代表冰碛物的年龄，为16.8±3.5 ka(图 6h)。该年龄与Seong等[39]在公格尔-慕士塔格地区识别出的Olimde冰阶的起始年龄(17.1±0.3 ka)和Owen等[38]在塔什库尔干谷地识别出的库孜滚冰阶的年龄(19±4 ka)大体一致，也与全球末次冰盛期的年龄相当[41]。
3 讨论与结论基于高精度卫星影像解译、野外实地勘察和差分GPS测量，可得到冰碛物M2形成以来木吉断层的右旋位移量和垂直位移量分别为190 m和105±12 m。在利用断层垂直位移量计算水平拉张量时需已知断层面的倾角，但在野外勘察中并未找到可靠的断层露头。在2016年阿克陶Mw6.6级地震中，研究点附近发生了3次强余震[15, 43]。震源机制解给出的与断层平行的节面的倾角分别为65°、68°和83°。本文取3个倾角的平均值72±8°作为断层倾角，可得断层南北向拉张量为34±12 m，南北向拉张量与右旋走滑位移量的比值约为1：6。宇宙成因核素10 Be暴露测年法得到的冰碛物M2的年龄为16.8±3.5 ka。由于冰碛物西缘累积的右旋位错量达190 m，表明冰碛物形成以来经历了数十次地震循环，因此该年龄可大致代表冰碛物的位错年龄。综上，断层的右旋走滑、垂直抬升和南北向拉张速率分别为11.3±2.4 mm/a、6.3±1.5 mm/a和2.0±0.8 mm/a；断层在水平方向的总滑动速率(右旋走滑速率和南北向拉张速率的矢量和)为11.5±2.3 mm/a。
与大致位于断层中部的近乎纯走滑的阿克萨依处相比[15, 17]，在布拉克北断层虽仍以右旋走滑为主，其正断分量也明显增加。这可能是木吉断层与区域内其他主要断层相互作用的结果(图 2a)：在布拉克北(东段)，木吉断层靠近公格尔拉张系近南北走向段，以东西向拉张为主的公格尔拉张系可能在NWW走向的木吉断层上产生一定的南北向拉张分量；随着断层的向西延伸，断层逐渐靠近其北侧的帕米尔逆断层带，逆断层带上的南北向挤压应力逐渐抵消拉张应力，从而使木吉断层的运动性质逐渐变为纯走滑。木吉断层整体以右旋走滑为主的运动性质与公格尔拉张系近南北走向、以东西向拉张为主的剩余段均指示了中-西帕米尔与东帕米尔正在发生分离。尽管木吉断层运动性质发生了变化，但断层在水平方向的滑动速率(11.5±2.3 mm/a)与阿克萨依处(中段)右旋走滑速率的最大值(9.4±0.9 mm/a)[15]大致相当，表明断层的滑动速率沿断层走向向西大致保持恒定。上述特征可能代表了造山带内部大型正断层端部走滑转换断层的运动学特征：靠近正断层具有显著的正断分量，远离正断层逐渐变为以走滑为主；沿断层的滑动速率大致保持恒定；走滑断层和正断层指示的两侧块体的相对运动方向一致。
木吉断层作为中-西帕米尔与东帕米尔的最北部边界断层，其滑动速率的准确限定对于估算中-西帕米尔和东帕米尔的相对拉张速率至关重要。由于断层的滑动速率沿断层走向大致保持恒定，Li等[15]利用阿克萨依处右旋走滑速率计算得到的中-西帕米尔和东帕米尔的相对拉张速率1.4°~2.0°/Ma是有代表性的，帕米尔构造结内部沿公格尔拉张系正发生快速拉张。
致谢: 感谢匿名审稿专家以及编辑部杨美芳和赵淑君老师建设性的修改意见。
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2 Research Center for Earth System Science, Yunnan Key Laboratory of Earth System Science, Yunnan University, Kunming 650500, Yunnan;
3 Guangdong Provincial Key Laboratory of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong;
4 The Second Monitoring and Application Center, China Earthquake Administration, Xi'an 710054, Shaanxi;
5 Seismological Bureau of the Xinjiang Uygur Autonomous Region, Vrümqi 830011, Xinjiang)
Abstract
The Pamir orogen lies at the northwestern syntaxis of the India-Asia convergence zone. Active tectonic investigations, geodetic GPS measurements and focal mechanisms of major earthquakes all indicate that modern deformation of the Pamir is characterized by strong crustal shortening along its northern and western margins as well as significant crustal extension along the Kongur Extensional System, which divides the central-western Pamir from the eastern Pamir. The SEE-trending Muji Fault is the northernmost segment of the Kongur Extensional System and acts as the northern boundary fault between the central-western and the eastern Pamir. Well determinations of the slip sense and slip rate of the fault are critical for our understanding the current stress status and kinematic process of the Pamir. This study focuses on the displaced moraine deposits at north Bulake (39.2020°N, 74.3910°E), eastern segment of the Muji Fault. Based on high-resolution satellite image interpretation, detailed field observations and differential GPS topographic measurement of the moraine deposits, the dextral displacement, vertical displacement and north-south extension of the fault are well determined to be ca.190 m, 105±12 m and 34±12 m, respectively; the age of the moraine deposits is determined to be 16.8±3.5 ka by cosmogenic nuclide 10Be exposure dating of seven granite boulders on the surface. These yield a dextral-slip rate, vertical-slip rate and north-south extension rate of 11.3±2.4 mm/a, 6.3±1.5 mm/a and 2.0±0.8 mm/a, respectively. The ratio of the dextral-slip rate, vertical slip rate and north-south extension rate is around 6:3:1 and the horizontal slip rate is 11.5±2.3 mm/a. Compared to the central segment of the fault at Aksayi, where the fault is characterized by mostly dextral slip, the fault at north Bulake (eastern segment) is characterized by dextral slip with significant normal-faulting component. Because the horizontal slip rate of 11.5±2.3 mm/a at north Bulake is broadly consistent with the maximum dextral slip rate of 9.4±0.9 mm/a at Akesayi, the slip rate maintains roughly constant along the fault in spite of significant change of its slip sense. Our result can make two conclusions:(1) Although the Muji Fault has significant different strike (SEE-trending) and slip sense (dextral slip) from the rest segment of the Kongur Extensional System (SSE-trending and normal-faulting, respectively), both of the fault segments indicate that the central-western Pamir is separating from the eastern Pamir; (2) The extension rate of 1.4°~2.0°/Ma between the central-western and eastern Pamir, which is estimated from slip rate at Aksayi, can be used to represent the modern extension rate of the Pamir interior.