【作者】 任俊杰；

【导师】 徐锡伟；

【作者基本信息】 中国地震局地质研究所 ， 构造地质学， 2013， 博士

【摘要】 欧亚板块与印度板块的碰撞，导致了青藏高原的大幅度隆升，并造就和改变了整个欧亚大陆的构造格局，并对亚洲地区的气候与环境产生了重大影响。同时，还造成了青藏高原向东运动。青藏高原东缘形成了整个高原最为陡变的地形梯度，是目前研究青藏高原向东运动及青藏高原东缘隆升机制的重要场所。2008年汶川Ms8.0级地震造成了龙门山断裂带的破裂，这更激发了对青藏高原东缘隆升机制研究的兴趣。关于青藏高原东缘的变形机制主要有两个端元模式：一种是大陆逃逸模式，认为青藏高原东缘的变形主要集中在重要的活动边界断裂上；另一种是管道流模式，认为变形在青藏高原东缘地区广泛分布，活动断裂所承担的变形量很小。已建立的用来解释龙门山隆升和汶川地震的发震机制的模式均认为青藏高原东缘的变形主要集中在龙门山断裂带上，但这些模式未对川西高原在青藏高原向东运动中变形吸收的作用进行讨论。龙日坝断裂带位于龙门山以西约200km，与龙门山断裂带大致平行。初步研究认为该断裂带中段晚第四纪具有较强的活动。从构造上来看，该断裂带是一个重要的次级边界构造带，它把巴颜喀拉块体分为阿坝和龙门山两个次级块体。断裂带以西地貌上为相对平坦的高原面，断裂以走滑运动为主；以东为地形起伏的龙门山隆起，断裂以逆冲作用为主。那么，龙日坝断裂带详细的几何展布特征怎样？其晚第四纪活动强度如何？最新一次大震的离逝时间是多少？断裂上的大震破裂模式是什么？2008年汶川地震造成了相距～15km的两条近平行的映秀－北川断裂和灌县－江油断裂的级联破裂，而龙日坝断裂带中段上的两条近平行的断裂仅相距～20-30km，是否也会发生级联破裂事件呢？但在龙日坝断裂带上缺少历史大震的记录。另外，作为青藏高原向东运动中遇到的第一个阻隔的龙日坝断裂带在青藏高原东缘变形分配中承担什么样的作用？龙日坝断裂带的北段似乎与昆仑断裂带相连，那么其与昆仑断裂带具有什么样的运动学关系？这些问题的解决不仅能为龙日坝断裂带上的地震危险性评价提供定量资料，而且有助于对青藏高原向东运动及青藏高原东缘隆升机制的理解。本论文在分析整理前人资料的基础上，通过高分辨率卫星影像解译，进行了详细的野外地质调查，确定龙日坝断裂带的几何展布特征；并对断错微地貌进行详细测量和测年（碳14和光释光），计算其晚第四纪滑动速率；同时，开挖古地震探槽，恢复断裂晚第四纪以来的破裂历史，分析断裂的破裂行为。此外，还对昆仑断裂最东段的塔藏断裂的晚第四纪活动特征进行了详细调查。最后，通过地质学滑动速率和GPS速度资料综合分析龙日坝断裂带在青藏高原东缘变形分配中的作用。通过上述研究，主要得到如下认识：1)龙日坝断裂带中段由两条近平行的断裂（毛尔盖断裂和龙日曲断裂）组成，且断错地貌清晰。其中毛尔盖断裂主要沿着毛尔盖河和羊拱沟的北侧基岩山坡展布，发育断错山脊和断错水系，其晚第四纪活动表现为右旋走滑；龙日曲断裂主要沿龙日曲河和雀儿登河北侧山坡通过，发育断错山脊和断错山前洪积扇，形成显著的断层陡坎，表明该断裂以右旋走滑为主，兼有向南东方向的逆冲作用。断错地貌表明龙日曲断裂自18ka以来右旋走滑速率为2.5±0.4mm/yr，而自约11ka以来的右旋走滑速率为1.4+0.4/-0.3mm/yr，其上的垂直滑动速率均非常小（0.1～0.2mm/yr）。毛尔盖断裂自约21ka以来的右旋走滑速率为2.3+0.4/-0.3mm/yr，但自约9.5ka以来右旋走滑速率为0.7±0.1mm/yr。这些结果表明整个龙日坝断裂主要表现右旋走滑，兼有非常小的向南东逆冲的分量。而整个龙日坝断裂带中段的晚更新世晚期以来总的平均右旋走滑速率为4.8mm/yr，但晚更新世晚期至全新世滑动速率约为7.5mm/yr，而全新世以来的滑动速率约为2.1mm/yr，滑动速率的降低可能与青藏高原向东运动的减慢有关。2)龙日坝断裂带南段断续展布，在观音桥一带出露最好，其全新世水平滑动速率为0.3～0.5mm/yr，垂直滑动速率为0.2～0.3mm/yr。龙日坝断裂带北段走向转为近南北向，变形分散在至少两个分支上。西支活动性较弱，断错晚第四纪地貌不清晰，而东支出露较好，具有向西逆冲分量，全新世以来的右旋平均走滑速率约为0.8mm/yr，平均垂直滑动速率约为0.3mm/yr。3)在昆仑断裂的东端，存在着多条晚第四纪活动的断裂。塔藏断裂西段以走滑运动为主，全新世以来左旋走滑速率为2.9±0.7mm/yr。而东段表现为向南西方向的逆冲，全新世平均滑动速率向东逐渐降低，从约1～1.5mm/yr到～0.3mm/yr，到最东端的塔藏镇仅表现为全新世河流阶地中地层的褶皱变形。塔藏断裂应为昆仑断裂的最东段，与昆仑断裂的主干断裂（玛曲段）之间以一个拉分盆地相隔。而岷江断裂以向东的逆冲作用为主，全新世以来的垂直滑动速率为0.37～0.53mm/yr。4)在龙日坝断裂带中段上开展的古地震探槽研究表明，毛尔盖断裂晚第四纪以来共有了三次古地震事件，分别发生在距今5170±80、7100±70和8510±420cal yr BP，基于断裂长度和估算得到的同震水平位错量可计算出其震级为Mw7.2～7.4。而龙日曲断裂上晚第四纪以来共有四次古地震事件，分别发生在距今5080±90、11100±380、13000±260和17830±530cal yr BP，而基于断裂长度可估算得到震级为Mw～7.4。从两条断裂上破裂图像来看，最新一次事件可能造成了两支断裂的共同破裂，其震级为Mw～7.6。而在最新事件之前，毛尔盖断裂和龙日曲断裂表现为交替的活动。如果把龙日坝断裂带中段作为一个整体来看，除了最新一次事件外，整个断裂带似乎符合约2000年复发间隔的准周期复发特征。超过5000年的离逝时间和较高的滑动速率，预示着龙日坝断裂带中段未来具有较高的大震危险性。5)青藏高原从中部以正东的方向向东运动，而在青藏高原东缘受到走向约NE60o龙日坝断裂带和龙门山断裂带的阻挡。来自青藏高原内部的运动一部分转化为沿龙日坝断裂带上的右旋走滑，另一部分继续向东南运动与刚性的四川盆地碰撞，相对较软的龙门山次级块体承担了来自青藏高原内部和四川盆地块体的挤压，导致了山体抬升和龙门山断裂带上的逆冲兼右旋走滑作用。与龙门山断裂带一样，龙日坝断裂带的两个分支断裂交汇于深约20km的滑脱面上。GPS速率和地质学速率均表明龙门山断裂带在吸收青藏高原东缘变形中承担重要的作用，但龙日坝断裂带也这个应变分配过程中也承担着同等重要的作用。龙门山次级块体在来自青藏高原内部的向东的推挤和来自四川盆地北西向的挤压，龙日坝断裂上较陡的断层倾角使得其主要承担走滑运动，而龙门山断裂带较缓的断层倾角使得其主要承担逆冲作用。6)横穿岷山隆起近东西向的GPS剖面和昆仑断裂东端的活动断裂的地质学滑动速率结果表明，昆仑断裂主断裂玛曲段上的左旋走滑位移除了一小部分转化为塔藏断裂东段的逆冲作用外，大部分转化为沿龙日坝断裂带北段、岷江断裂、虎牙断裂等南北向构造上近东西向的地壳缩短，进而导致了岷山的隆升。若尔盖盆地可能是一个与岷山西部隆升有关的小型前陆盆地。更多还原

【Abstract】 The ongoing collision of Eurasian and Indian plates caused the widespread uplift of theTibetan Plateau and has built the tectonic framework of the whole Eurasian area. This collisionalso has influenced significantly the climate and environment in the Asian area. The uplift of theTibetan Plateau is accompanied by the eastward motion of the plateau, leading to the greatestrelief in eastern Tibet than anywhere else on the plateau. Eastern Tibet has become an importantsite for studying the mechanism of eastward motion of the Tibetan Plateau and the uplift of easternTibet. The2008Wenchuan earthquake (Ms8.0) ruptured the Longmen Shan fault zone andstimulated numerous studies on eastern Tibet.Two end-member hypotheses are proposed to illustrate the deformation mechanism of easternTibet. A continental extrusion model predicts that the deformation is concentrated on majorboundary faults, whereas a lower crustal model suggests that the deformation is distributedthroughout the whole region of eastern Tibet, and the role of major faults played in the eastwardmotion of the Tibetan Plateau was not considered.The Longriba fault zone,~200km west of the Longmen Shan, is subparallel to the LongmenShan fault zone. Preliminary investigation showed a strong activity along the central Longribafault zone. Tectonically, the Longriba fault zone separates the Bayan Har block into the Aba andLongmen Shan subblocks. West of the fault zone, the landform is characterized by the flat surfaceof the Tibetan Plateau and strike-slip faults. East of the fault zone, it is marked by high relief ofthe Longmen Shan and thrusts. So, what about the fault geometry of the Longriba fault zone? Howabout the magnitude of its fault activity in the late Quaternary? How long is the elapsed time sincethe last event on this fault zone? Which model is suitable for ruptured behavior of largeearthquakes on this fault zone? In addition, the2008Wenchuan earthquake ruptured the twosubparallel faults with~15km apart: Yingxiu-Beichuan and Guanxian-Jiangyou faults. The twosubparallel strands in the central Longriba fault zone are separated by~20-30km, that raising thepossibility that a large earthquake could rupture both strands. However, no large historicalearthquake is known on this fault zone. Finally, as the first barrier on the way of eastwardmovement of the Tibetan Plateau, what role does the Longriba fault zone take in strain partitioningin eastern Tibet? The northern Longriba fault zone is linked with the Kunlun fault, how about thekinematic relation between them? The answers to these questions not only provide the quantitativeparameters for seismic hazard assessment on the Longriba fault zone, but help the understandingof the mechanism of eastward movement of the plateau and uplift of eastern Tibet.Based on the collection of previous data, we determined the geometry of the Longriba faultzone combined with high-resolution satellite images and field geological investigations. Using topographic survey of displaced landforms and dating of geomorphic surfaces (radiocarbon andOptically Stimulated Luminescence), Late Quaternary slip rates on this fault zone are calculated.We excavated trenches to reconstruct the paleoseismic history and then studied the rupturebehavior on this fault zone. In addition, we determined the slip rates along the Tazang fault.Finally, we discussed the role of the Longriba fault zone in strain partitioning in eastern Tibetusing geological and GPS slip rates. Based on the above studies, we draw the followingconclusions.1) The central Longriba fault zone consists of two strands (Maoergai and Longriqu faults)and has a clear faulted landforms. The Maoergai fault extends the northern bedrock slope ofMaoergai and Yanggong rivers and is characterized by shutter ridges and displaced channels,showing right-lateral strike-slip motion in late Quaternary. The Longriqu fault runs primarilyalong the slopes north of Longriqu and Queerdeng rivers and is marked by shutter ridges and faultscarps on the mountain-front alluvial fan. Displaced terraces indicate that the dextral rate along theLongriqu fault since~18ka is2.5±0.4mm/yr, but is1.4+0.4/-0.3mm/yr since~11ka. Its verticalslip rate is very small (0.1-0.2mm/yr), demonstrating that the Longriqu fault is predominantlystrike-slip. On the Maoergai fault, the dextral rate since~21ka is2.3+0.4/-0.3mm/yr, but is0.7±0.1mm/yr since~9.5ka. These results shows that the whole Longriba fault zone isdominantly right-lateral motion accompanying a very small south-verging thrust component onthe Longriqu fault. The dextral rate since the latest Quaternary is~4.8mm/yr, whereas the dextralrate in the Holocene is~2.1mm/yr. The slip rates on the central Longriba fault zone decreasesfrom~7.5mm/yr in the latest Pleistocene to~2.1mm/yr in the Holocene. The drop is probablyrelated to the slowdown of the eastward motion of the Tibetan Plateau.2) The southern Longriba fault zone is intermittently distributed and is only exposed nearGuanyinqiao Town. Its horizontal and vertical slip rates are0.3-0.5mm/yr and0.2-0.3mm/yr,respectively. The strike of the northern Longriba fault zone is nearly N-S trending and thedeformation spreads in at least two fault strands. The west strand probably undergoes a weaktectonic activity, where no faulted landform in late Quaternary is found. While the east strand iswell exposed and has a west-verging reverse component. It has a dextral slip rate of about0.8mm/yr and vertical slip rate of about0.3mm/yr.3) At the eastern end of the Kunlun fault, there are numerous faults active in late Quaternary.The western Tazang fault is dominated by left-lateral strike-slip motion with a Holocene rate of2.9±0.7mm/yr. Whereas, he eastern Tazang fault is characterized by south-west-verging reversemotion and has an eastward decrease along the fault from about1-1.5mm/yr to0.3mm/yr. At theeasternmost termination of the Tazang fault, it is marked by folded layers in the Holocene riverterrace at Tazang Town. The Tazang fault is probably the easternmost of the Kunlun fault zone andis linked with the master Kunlun fault (Maqu fault) via a pull-apart basin. The Minjiang fault ischaracterized by east-verging reverse motion with a vertical Holocene slip rate of0.37-0.53mm/yr.The Huya fault may have a relatively strong left-lateral motion and an east-verging reversecomponent with a vertical slip rate of0.3mm/yr in the Holocene. The Longmen Shan fault zone is dominated by thrusting with right-lateral component and has a late-Quaternary crustal shorteningrate of about3mm/yr. The Bailongjiang fault undergoes a minor slip rate in late Quaternary (＜1mm/yr).4) The trenches on the central Lognriba fault zone show that three events ruptured theMaoergai fault in late Quaternary and occurred at5170±80,7100±70, and8510±420cal yr BP.Based on fault length and the estimated coseismic horizontal offsets, the magnitude of theseevents are approximately Mw7.2-7.4. Four surface-rupturing events occurred on the Longriqufault at5080±90,11100±380,13000±260, and17830±530cal yr BP. The last event probablyruptured the Longriqu and Maoergai faults and has a magnitude of Mw~7.6. Prior to the last event,the two fault strands of the Longriba fault zone appear to experience an alternating activity. If wetake the Longriqu and Maoergai faults as a whole, the Longriba fault zone appears to undergo aregular recurrence with an interval of~2000years before the last event. Considering the elapsedtime of over5000years and relatively high slip rate, the central Longriba fault zone has a highpotential of a large earthquake in the future years.5) GPS rates and the long-term geological slip rates derived from displaced landformsdemonstrate that the Longriba fault zone plays an important role in strain partitioning in easternTibet. The Tibetan Plateau is moving due eastward and is resisted by the Longriba and LongmenShan fault zones in eastern Tibet and then collides with the rigid Sichuan block. However, theLongriba and Longmen Shan fault zones fault zone strike N~60oE. So, one portion of the motionwas transformed into dextral slip along the Longriba fault zone, and other portion continues tomove southeastward and collides with the rigid Sichuan block. The relatively soft Longmen Shansubblock accommodates the collision and finally leads to the uplift of the Longmen Shan andthrusting with dextral motion along the Longmen Shan fault zone. The two strands of the Longribafault zone merge at the decollement~20km deep similar to the Longriba fault zone. The deep dipof the Longriba fault zone helps them undertake dominant dextral motion, and the gentle dip ofthe Longmen Shan fault zone favors thrusting.6) The E-W-trending GPS section across the Min Shan platform and geological slip rates ofactive faults at the eastern end of the Kunlun fault reveals that a small portion of the sinistraldeformation on the master Kunlun fault is transformed into the reverse motion at the easternTazang fault, and a big portion is transferred to the N-S-striking Minjing, Huya, and northernLongriba fault zone characterized by E-S crustal shortening that causes the uplift of the Min Shan.This pattern at the eastern end of the Kunlun fault is somewhat similar to that at the easterntermination of the Altyn Tagh fault zone. The Roergai basin is probably a small-scale forelandbasin associated with the uplift of the western Min Shan.更多还原