【作者】 黄凯；

【导师】 肖安成； 吴磊；

【作者基本信息】 浙江大学 ， 构造地质学， 2023， 博士

【摘要】 受阻双弯曲（restraining double bends）由走滑断层发生两次互为镜像的走向偏转形成,多发育在走滑断裂中部,是研究走滑断裂运动学特征和演化的重要窗口。长达1600 km的阿尔金左旋断裂沿线发育五个非常显著的受阻双弯曲,其形成过程与机制对研究阿尔金断裂的演化乃至青藏高原向北的扩展有着重要的启示意义。本文以阿尔金断裂中段的茫崖受阻双弯曲为研究对象,利用遥感解译、野外构造填图、沉积学分析、宇宙成因核素埋藏测年等方法,对茫崖受阻双弯曲的变形特征与形成时间进行了系统研究,结合地震反射数据所揭示的邻近柴达木盆地西南缘的新生代变形特征,分析了茫崖受阻双弯曲的形成过程与机制。主要获得以下认识:（1）茫崖受阻双弯曲具有复杂的几何学形态和走滑压扭的运动学特征。该双弯曲构造东西长70 km,南北最宽约30 km,其西侧的弯曲角约26°,东侧的弯曲角约15-17°,在弯曲内侧分别形成了海拔相对较高的阿卡腾能山和平顶山,向东通过一个宽约3-4 km拉张阶区与其东侧的阿尔金断裂平直段相接。这些弯曲角和拉张阶区将茫崖受阻双弯曲划分成了具有不同几何学特征的4段,自西向东分别为乌尊肖尔段、白石头沟段、野马滩段和戈壁岭段。茫崖受阻双弯曲内发育大量与阿尔金主断裂近平行或斜交的、具有走滑压扭特征的次级断裂和褶皱。所有这些特征表明,茫崖受阻双弯曲在几何学特征上是非常不成熟的,这与其东、西两侧具有相对成熟的几何学结构且规模较大的阿克塞和阿卡图受阻双弯曲明显不同。（2）茫崖受阻双弯曲与其南侧的柴达木盆地西南缘具有多期复杂的变形历史,整个研究区存在三期主要的构造变形。第一期发生在侏罗纪,发育两组正交的正断层,受区域伸展背景控制。第二期发生在始新世到中中新世,上述正断层发生反转,且越靠近阿尔金断裂变形越强烈,形成了沿着阿尔金断裂呈线性展布的中中新世角度不整合面,其上被晚中新世及以后地层所覆盖,受控于阿尔金断裂系中中新世的构造调整。第三期发生在中新世晚期-第四纪,来自南侧祁曼塔格逆冲系统的变形向北东传递到阿尔金断裂南侧,使得北西向断层再次活化,并在第四纪时强烈活动,与茫崖受阻双弯曲的变形之间发生复杂的相互叠加与改造。（3）茫崖受阻双弯曲形成于约3 Ma。对茫崖受阻双弯曲东侧狼沟剖面、西侧依吞布拉克剖面开展了包括岩性观测、砾石成分统计、古水流测量等在内精细的物源分析工作,发现这两个剖面均存在一个明显的物源转换事件,物源区从北侧阿尔金山或南侧祁曼塔格-东昆仑山转变为现今邻近的平顶山和阿卡腾能山。该物源转换点代表了平顶山和阿卡腾能山的初始隆升,可用于指示茫崖受阻双弯曲的形成时间。利用宇宙成因核素¹⁰Be-²⁶Al埋藏定年方法得出该物源转换点的时代分别为2.92±0.48 Ma（狼沟剖面）和3.15±0.39 Ma（依吞布拉克剖面）。据此本文认为茫崖受阻双弯曲形成于约3 Ma,远远晚于阿尔金断裂中段的形成时间,而与祁曼塔格逆冲体系传递至阿尔金断裂附近的时代较为一致。（4）茫崖受阻双弯曲是北西走向逆冲系统对阿尔金断面改造所形成。基于上述研究,结合前人已有成果,本文提出现今茫崖受阻双弯曲的形成机制模型。南侧祁曼塔格冲断系统在斜向汇聚背景下沿着中地壳滑脱面向北东方向扩展,在约3 Ma左右传递至阿尔金断裂附近,持续的斜向挤压导致平直的阿尔金断裂发生弯曲,形成了茫崖受阻双弯曲,双弯曲北侧的平顶山和南侧的阿卡腾能山发生强烈隆升变形。南部的阿卡腾能山由于同时受到来自祁曼塔格逆冲系统的干涉改造,形成了一个相对更大更复杂的变形区。茫崖受阻双弯曲的形成使得其东侧阿尔金主断裂从走滑压扭转变为走滑张扭,形成了狭长的索尔库里走廊箕状断陷盆地。青藏高原北部可能存在多处北西走向挤压构造对阿尔金主断裂的几何形态进行改造的现象,改变了阿尔金主断裂上的局部应力状态和应变分配,形成局部复杂变形区的同时也影响高原北部的隆升和扩展。更多还原

【Abstract】 Restraining double bends（RDB）usually form by two mutually-mirrored strike deflections along strike-slip faults,and are commonly observed in the middle,but not at the ends,of the strike-slip faults.They provide an important window to reveal the kinematic characteristics and evolution of the strike-slip faults.Along the～1600-km long left-lateral strike-slip Altyn Tagh fault（ATF）,five prominent RDBs develop.Their formation processes and underlying mechanisms have significant implications for the evolution of the ATF and northward expansion of the Tibetan plateau.By using remote-sensing interpretation,field mapping,sedimentary provenance analysis and burial dating of cosmogenic nuclides,we herein systematically investigated the structural deformation and formation time of the Mangya RDB along the ATF.Together with the Cenozoic deformation features in the nearby southwestern Qaidam Basin revealed by seismic reflection data,we further analyzed the formation process and mechanism of the Mangya RDB and discussed the associated implications for the Cenozoic deformation of the northern Tibetan Plateau.We drew draw the conclusions as follows:（1）The Mangya RDB is characterized by complex geometry and transpressional kinematics.The Mangya RDB is～70 km long from east to west,and～30 km wide from north to south.The western and eastern bending angles are～26°and～15-17°,respectively.Two relatively high mountains,the Akatengneng Shan and Pingding Shan,was formed in the two inner-coners of the bend.To the east,the Mangya RDB connects with the straight segment of the ATF via a 3-4 km wide extensional stepover.The two bends and the stepover divide the Mangya RDB into four segments with different geometries.From west to east they are Wuzunxiaoer,Baishitougou,Yematan and Gebiling segments,respectively.Field mapping indicated that many transpressional faults and folds develop within the Mangya RDB,and are parallel or oblique to the main trace of the ATF.Taken together,these observations indicate that the Mangya RDB is geometrically immature,and thus contrasts to the Akato and Aksay RDBs to the west and east,respectively,which are much larger and geometrically more mature.（2）The Mangya RDB and the Southwestern Qaidam Basin to the south have experienced three-stage complex deformation history.The first occurred in the Jurassic Period,and is characterized by the development of two orthogonal normal faults under regional extensional background.The second happened in the Eocene to the Middle Miocene when the two-forementioned sets of faults were inverted.The associated intensity of deformation was much stronger close to ATF,forming a Middle Miocene angular unconformity that was linearly restricted along the ATF and covered by younger strata.This stage of deformation is likely controlled by the mid-Miocene tectonic reorganisation of the ATF system.The third stage took place since the Middle Miocene when the Qiman Tagh thrust system successively propagated toward northeast and reached the southern side of the ATF,reactivating the pre-existing NW-striking faults.Much intensive deformation occurred in the Quaternary and interacted with the deformation associated with the Mangya RDB.（3）The Mangya RDB formed at～3 Ma.Detailed provenance analyses,including lithology observation,gravel composition statistics and palaeocurrent measurement,have been carried out in two sedimentary sections in the Langgou area（wolf pass）on the north side of Pingding Mountain in the east and Yitunbulak area on the northwest side of Akatenneng Mountain in the west.A striking provenance change was revealed from the Altyn Shan and the Qiman Tagh to the more proximal Pingding Shan and Akatengneng Shan.This provenance change is interpreted to result from the initial uplift of Pingding Shan and Akatengneng Shan,and thus was used to quantify the formation time of the Mangya RDB.Cosmogenic nuclides ¹⁰Be-²⁶Al burial dating of gravels at the change points indicate that the provenance change occurred at 2.92±0.48 Ma and 3.15±0.39 Ma in the Langgou and Yitunbulak section,respectively.Therefore,the Mangya RDB likely formed at～3 Ma,much later than the formation time of the middle part of the ATF,but consistent with the time when Qiman Tagh thrust system met the ATF system.（4）The Mangya RDB likely formed by the NW-striking thrusts altering the fault surface of the ATF.Combining the above results and published studies,we proposed a mechanism model for the formation of Mangya RDB.The Qiman Tagh thrust system propagated northeastward along the middle crust detachment,touching and further bending the fault surface of the ATF to form the Mangya RDB at～3 Ma.Strong deformation took place in the two inner-coners of the bend to raise the Pingding Shan and the Akatengneng Shan.The latter grew in size to become a larger deformation area due to the interplay of deformation of both ATF system and Qiman tagh thrust system.The stress of Suerkuli segment of ATF was shifted from transpression to transtension in response to the formation of the Mangya RDB,creating the narrow half-graben basin named as the Suerkuli corridor basin.We further recognized some other NW-trending compressive structures deflecting the geometries of the ATF in the northern of the Tibetan Plateau,which likely changed the stress state and strain partitioning along the ATF,and thus affecting the uplift and expansion of the northern plateau.更多还原