Crustal structure of northeastern Tibet inferred from receiver function analysis

Sponsored by National Science & Technology Committee, the cooperation between China Academy of Geoscience and Institute of Geophysics and Tectonics, University Joseph, France conducted a lithospherical experiment using 40 Minititan 3-component and 13 CEIS 1-component seismometers along the road from Gonghe to Yushu in Qinghai Province during 5 months after June, 1998. The interested area is on the north of Bangong- Nujiang fault, the east of Qaidam basin, the south of Center Qilian fault and the west of Longmenshan fault. And the profile across most tectonic parts of Eastern Tibet such as Southern Qilian, Eastern Kunlun fault, Bayan Har terrane, Jinshajiang suture (Figure 1), which is the first seismological profile across Eastern Tibet (Qinghai-Tibet) and will be beneficial on the comparison with the results of its center parts, especially on the understanding of the effect of the thousands-kilometer-faraway collision between Eurasia Plate and Indian Plate on the uplifting of south and north part of Eastern Kunlun fault, and on the thickening of crust and the feature of deep structure of Qilian mountain on the north of Tibet Plateau.     

青藏高原重力场与壳幔结构分析

本文基于重力资料, 分析了青藏高原壳幔结构模型、 高原陆内形变动力学条件、 高原深部物质运动特征及动力学机制。 研究表明, 重力布格异常和自由空间异常除了分别反映大地水准面之下的“剩余”密度信息和大地水准面之上的“附加”密度信息之外, 还可以组合在一起反映壳幔结构的流变学信息。 在整体处于Airy 重力均衡状态下, 如果局部布格异常与空间异常同向减小, 则是弱地壳强地幔的反映; 如果布格异常减小空间异常增大, 则是强地壳弱地幔的反映。 笔者认为, 青藏高原南部多为强地壳弱地幔地段, 东部既有强地壳弱地幔地段, 也有下地壳柔性-上地幔脆性地段, 北部多为弱地壳强地幔地段。 高原南北两侧板块边界的挤压力对高原做功, 重力位能使高原物质向低位势转移, 产生流变变形, 导致南区和北区主要为挤压变形区, 东区主要为构造伸展-侧向挤出区。 由于壳幔结构的差异, 不同地区驱动变形所需位能大小不同。 相同位能条件下, 南部更易于隆升, 东部更易于流变伸展。     

青藏高原东缘龙门山逆冲构造深部电性结构特征

通过对汶川地震前观测的碌曲—若尔盖—北川—中江大地电磁剖面的数据处理和反演解释,揭示了沿剖面的松潘—甘孜地块、川西前陆盆地、龙门山构造带及秦岭构造带50 km深度的电性结构特征及相互关系,表明青藏高原东缘向东挤压,迫使向东流动的地壳物质沿高原东缘堆积,并向扬子陆块逆冲推覆.龙门山恰好位于松潘—甘孜地块与扬子陆块对挤部位,主要受松潘—甘孜地块壳内高导层滑脱和四川盆地基底高阻体阻挡的约束,地壳深部存在着西倾且连续展布的壳内低阻层,表明龙门山深部确实存在着逆冲推覆构造,其逆冲断裂系中的三条断裂不仅以不同的倾角向西北倾斜,并且向深部逐渐汇集,但茂县—汶川断裂可能在深部与北川—映秀断裂是分离的.龙门山两翼的四川盆地和松潘甘孜褶皱带的电性结构既具有明显差异性,又具有一定的相关性.四川盆地显示巨厚的低阻沉积盖层和连续稳定的高阻基底的二元电性结构,而松潘—甘孜地块则表现为反向二元结构,即上部大套高阻褶皱带,下部整体为低阻的变化带,龙门山逆冲构造带本身又表现为松潘地块逆冲上覆在四川盆地之上,构成上部高阻褶皱带、中部低阻逆冲断裂带和底部盆地高阻基底的三层电性结构.对比龙门山逆冲构造断裂带的西倾延伸上下盘两侧的两个反对称的二元电性结构,松潘区块深部推断的结晶基底与龙门山断裂带下盘推断的下伏盆地结晶基底又存在某种内在对应关系,推断可能存在一个西延至若尔盖地块的泛扬子陆块.因此,龙门山构造带地壳电性结构研究对于揭示青藏高原东缘陆内造山动力过程,探索汶川大地震的深部生成机理都具有重要意义.     

青藏高原松潘—西秦岭—临夏盆地深地震反射剖面——采集、处理与初步解释

由于印度洋板块向亚欧板块俯冲使青藏高原不断隆起,其形成不仅导致了亚洲大陆内部强烈的晚新生代构造变形,还对其边缘地区的地貌格局产生重大影响.青藏高原东北缘是青藏高原向北东方向扩展的前缘部位,是印度与欧亚两大板块碰撞作用由近南北方向向北东、东方向转换的重要场所.本文利用2004年和2008年完成的深地震反射剖面资料,采用关键处理技术和参数开展唐克-合作剖面与合作-临夏剖面联线处理,获得总长约400 km的深地震反射剖面,完整揭示了西秦岭造山带及其两侧盆地的地壳结构和构造变形样式.结果显示西秦岭造山带下地壳向若尔盖逆冲推覆的深部构造特征;西秦岭下地壳北倾的强反射及其北侧南倾的强反射特征揭示出扬子与华北两个大陆板块在西秦岭造山带下的汇聚行为.Moho的埋深和起伏形态表明青藏高原东北缘地壳经历了高原隆升后强烈的伸展减薄作用.     

青藏高原岩石圈三维电性结构

本文报道通过综合大地电磁调查数据研究青藏高原岩石圈三维电阻率模型的初步成果.大地电磁法调查区域已经覆盖了高原大部分面积,为全区三维电阻率成像研究打下了可靠的基础.对多个测区大地电磁数据进行精细的同化处理和反演成像,取得了青藏高原可靠的岩石圈三维电阻率结构图像.成像的区域为28°N—35°N,80°E—104°E.三维反演计算时采用的网格尺寸为20km×20km,垂直方向不等间距剖分为26层.结果表明,青藏高原现今岩石圈电阻率扰动主要反映印度克拉通对亚欧大陆板块俯冲引起的热流体运动和大陆碰撞和拆离产生的构造.在岩石圈地幔,察隅地块、喜马拉雅地块和拉萨地块东部联成统一的高电阻率地块,它们反映了向北东俯冲的印度克拉通.雅鲁藏布江、班公—怒江和金沙江缝合带都有明显的低电阻率异常,表明岩石圈深处有热流体活动.雅鲁藏布江、班公—怒江和金沙江缝合带都有明显的低电阻率异常,也表明它们的岩石圈还有流体活动.青藏高原东部的低阻区沿100°E向地幔下方扩大,反映了金沙江断裂带有切穿岩石圈的趋势.地幔电阻率平面扰动的模式显示,青藏高原东西部的地体碰撞拼合形式和方向是不同的.在青藏高原西部,羌塘、拉萨和喜马拉雅等地体从北到南碰撞拼合.在青藏高原东部,羌塘—拉萨、察隅、印支、雅安和扬子等地体多方向拆离拼合,在地壳造成不正交的拆离带和压扭构造系.从高阻-低阻区的分布看,东部的地体拼合有地幔的根源,今后还会进一步发展.察隅地块岩石圈对青藏高原东部的楔入,使其北部和东部地块的岩石圈发生拆离撕裂,也造成热流体上涌的低电阻率异常.     

Comparison of River Terraces in the Middle Reach Valleys of the Yellow River and Analysis on the Multi-Gradational Features of Tectonism in the Formation of Terrace Series

Where the Yellow River flows through the Haiyuan-Tongxin arc-form tectonic region on the northeastern side of the Qinghai-Xizang (Tibet) Plateau, as many as 10～21 basis and erosion terraces have been produced, among which the biggest altitude above river level is 401m and the formation age of the highest terrace is 1.57 Ma B.P. Based on comparative analysis of the Yellow River terraces located separately in the Mijiashan mountain, the Chemuxia gorge, the Heishanxia gorge and the other river terraces in the vast extent of the northern part of China, it has been found that the tectonic processes resulting in the formation of the terrace series is one of multi-gradational features, i.e., a terrace series can include the various terraces produced by tectonic uplifts of different scopes or scales and different ranks. The Yellow River terrace series in the study region can be divided into three grades. Among them, in the first grade there are 6 terraces which were formed separately at the same time in the vast extent of the northern part of China and represent the number and magnitude of uplift of the Qinghai-Xizang Plateau since 1.6 Ma B. P. ; in the second grade there are 5 terraces which were separately and simultaneously developed within the Haiyuan-Tianjingshan tectonic region and represent the number and magnitude of uplift of this tectonic region itself since 1.6Ma B. P.; in the third grade there are 10 terraces which developed on the eastern slope of the Mijiashan mountain and represent the number and amplitude of uplift of the Haiyuan tectonic belt itself since 1.6Ma B.P. Comparison of the terrace ages with loess-paleosoil sequence has also showed that the first grade terraces reflecting the vast scope uplifts of the Qinghai-Xizang Plateau are very comparable with climatic changes and their formation ages all correspond to the interglacial epochs during which paleosoils were formed. This implies that the vast extent tectonic uplifts resulting in river down-cutting are closely related to the warm-humid climatic periods which can also resnit in river downward erosion after strong dry and cold climatic periods, and they have jointly formed the tectonic-climatic cycles. There exists no unanimous and specific relationship between the formation ages of the second and third grade terraces and climatic changes and it is shown that the formation of those terraces was most mainly controlled by tectonic uplifts of the Tianjingshan block and the Haiyuan belt. The river terraces in the study region, therefore, may belong to 2 kinds of formation cause. One is a tectonic-climatic cyclical terrace produced jointly by vast extent tectonic uplifts and climatic changes, and the terraces of this kind are extensively distributed and can be well compared with each other among regions. Another is a pulse-tectonic cyclical terrace produced by local tectonic uplifts as dominant elements, and their distribution is restricted within an active belt and can not be compared with among regions.     

昆仑构造带强震活动的特点

分析了昆仑构造带的强震活动后认为，昆仑地震带强震活动在时空分布上具有轮回性、等间距性及由西向东的迁移特点；昆仑地震带强震震级西弱东强，而强震发震频次西高东低；西昆仑构造带帕米尔结点的强震活动对大陆地震活跃幕有前兆意义。     

红河断裂带白垩纪古地磁及青藏高原地质构造演化

红河断裂带两侧古地磁结果表明,羌塘地体与扬子地台至少从早白垩世以来已连接为一个整体。青藏高原是由四个发育历史不同的地体组成的大地构造复合体,在晚古生代它们分别是劳亚古陆、华夏复合古陆和冈瓦纳古陆的组成部分。拉萨地体与羌塘地体碰撞拼合形成欧亚板块构造格局。喜玛拉雅地体、印度板块与欧亚板块碰撞拼合、推挤,使青藏高原隆起,并使欧亚板块的块体沿已存在的断层产生左行走滑,这种作用至今仍在继续     

2001年11月14日新疆青海交界东昆仑山8.1级地震构造背景初探

2 0 0 1年 11月 14日中国新疆青海交界昆仑山中的 8.1级地震发生在东昆仑断裂的西段。这是一条大型活动块体边界断裂。青藏高原东北部向东逃逸而产生左旋剪切运动使该断裂成为一条青藏高原北部强震密集带。这次地震震中在北西西向东昆仑断裂与北东东向次级断裂的交汇部位 ,破裂表现为自西向东单向扩展的特点     

Tomographic study of the deep tectonics in the Yecheng-Shiquanhe area of the Tibetan Plateau

The profile crossing the west Kunlun region cov-ers the area between the Tarim Basin and the Kara-korum Fault, which is the highest (~5000 m a.s.l.) andthe most narrow part (400—500 km from north tosouth) of the Tibetan Plateau, thus attracting great at-tention of the geological community because of itsunique tectonic features. The Geological Bureau of Xinjiang Uygur Auton-omous Region and Chinese Academy of Sciences(CAS) carried out mineral exploration and geologicalmapping in th…     

利用GPS和水准数据分析东昆仑断裂带东部及其邻区构造变形特征

基于1999—2016年GPS数据和1980—2010年区域精密水准数据,获取了东昆仑断裂带东部及其邻区主要断裂的滑动速率和区域构造变形特征。结果表示:东昆仑断裂带自西向东的走滑速率衰减非常明显,走滑速率从西大滩—东大滩和阿拉克湖段的约10 mm/a向东到塔藏段衰减至约2 mm/a,速率自西向东每100 km下降梯度约1 mm/a;东昆仑断裂带阿拉克湖段、托索湖段、下大武段和塔藏段均表现出一定的弱挤压特征。跨岷江断裂剖面显示区域挤压变形自西向东由龙日坝断裂至龙门山断裂带有逐渐减弱的特征。区域最大主应变方向为E-NEE向,最大剪切应变高值区位于阿拉克湖段和托索湖段交汇区域以及巴颜喀拉块体的龙日坝断裂中段区域。分析东昆仑断裂带东部及其邻区主要断裂间的构造转换关系认为,岷山地区的隆起变形主要是因为巴颜喀拉块体自西向东的运动受到了华南块体的阻挡,而非东昆仑断裂带向东延展引起的构造转换。     

青藏高原北部NNW向构造活动方式及形成年代

对青藏高原北部ＮＮＷ向构造的地质地貌组合特征、活动方式及形成年代等问题进行的研究表明,ＮＮＷ向构造在地貌上由西向东表现为隆、拗相间排列的构造格局。其中ＮＮＷ向活动断裂依次发育在隆起带的东缘,形成伴有褶皱构造的逆冲－挤压构造带。拗陷带则由一系列位于隆起带之间的压陷性盆地带组成。ＮＮＷ向构造活动强烈,特别是在第四纪中、晚期以来,它们不仅制约着青藏高原北部区地质地貌的发生和发展,而且还在地震活动及强震孕育和发生过程中扮演着重要的角色,ＮＮＷ向隆起从第三纪末期至第四纪初期开始发育,但大规模隆起发生在第四纪中更新世以来     

Tectonic-climatic events in eastern Qilian Mountains over the past 0.83 Ma

The eastern part of Qilian Mountains experienced strong tectonic uplift during the late Quaternary, and climate record there was influenced by Tibetan Plateau to some extent. Based on studies on the fluvial terrace series and eolian loess deposition, we find that the tectonic uplifts of the Tibetan Plateau had coupled with climatic changes in our studied region and others since the mid-Pleistocene. The uplift that occurred at 0.83 Ma corresponded to significant desert expansion in L6 and periodic variation since MIS16, while the 0.14Ma one to the further drying in northwest China. Those coupled events may indicate that tectonic uplift drove climatic changes, and the Tibetan Plateau has important impacts on East Asian Monsoon system.     

Numerical Simulation of Annual Change Patterns of Contemporary Tectonic Stress-Strain Field of the Chinese Mainland

INTRODUCTIONInthe late 1980’s ,aninternational cooperation project in earth sciences ,the World Stress MapProject ,wasinitiated underthe World Lithosphere Program. Measurements andresearch achievementsof the present-daytectonic stress field worldwide were analyzed and sorted out .The project achievedgood results and based on them, a world stress database was set up,the world stress map wascompiled,andthe general andregional crustal stress patterns were discussed (Zoback,et al .,1989 ;…     

中国海陆莫霍面及深部地壳结构特征——以阿尔泰—巴士海峡剖面为例

中国海陆莫霍面及深部地壳结构特征研究是东亚地区宏观构造格架研究中的重点内容之一.本文以地震测深等数据为约束信息,以重力数据为基础,通过分区计算,反演了中国海陆莫霍面深度.依据地壳性质与莫霍面深度分布特征,划分了莫霍面深度梯级带与分区,并对各分区的莫霍面分布特点进行了归纳、总结.并选取阿尔泰—巴士海峡典型剖面进行了重、震反演,建立了密度结构.剖面上莫霍面深度和深部结构能够清晰地反映中国大陆"三横、两竖、两三角"构造格架中的两横和两竖,在昆仑—秦岭—大别以北的准噶尔地块和中朝地台莫霍面深度45~50 km,而其以南至贺兰山—龙门山之间的祁连、柴达木至松潘—甘孜的莫霍面呈"W"型起伏,莫霍面深度由祁连地块北部的50 km,加深至68 km,在柴达木盆地抬升至58 km,在阿尼玛卿山莫霍面降至68 km,向南逐渐抬升至四川盆地的44 km,经大兴安岭—太行山—武陵山这一竖的台阶式抬升至华南褶皱带的35 km,在江绍—南岭以南缓慢抬升至南海北部陆架区的20~25 km.在巴士海峡处南海沿马尼拉海沟向东俯冲,莫霍面形态较复杂.同时剖面上祁连—柴达木地块的中下地壳存在一个低速、低密度体,推测其可能是由于部分熔融引起的,是青藏高原东北缘壳内物质流动的通道.     

基于Monte Carlo方法数值反演区域初始构造应力场——以巴颜喀拉块体为例

构造应力场往往对地震活动性具有控制作用,应力快速集中的地方常常是地震频繁发生的地方.本文以巴颜喀拉块体及其边界断裂带近20年来的7次中强震为例,结合区域历史地震震源信息、地质背景及GPS等观测数据,利用Monte Carlo方法和库仑-摩尔破裂准则为计算依据,反演该块体的震前初始构造应力场.通过将初始应力场反演中不确定部分限定在一个合理的上下限范围内进行独立的重复性随机试验,并运用统计学方法得到了巴颜喀拉块体1997年玛尼M_W7.5地震震前区域初始应力场.计算结果显示：（1）巴颜喀拉块体10 km深度处最大水平主应力方向自西向东呈顺时针旋转趋势,由NS向转变为近EW向,与浅部实测地应力数据、历史地震类型和板块运动方向吻合较好.（2）最大/最小水平主应力和二者差值自西向东均逐渐增加,最大水平主压应力值~400 MPa,最小水平主压应力值~250 MPa.差应力在昆仑山断裂带与阿尔金断裂带交汇处及甘孜—玉树断裂带西段较低（~150 MPa）;在昆仑山断裂带东端和甘孜—玉树断裂带的东南段局部地区较高（~220 MPa）.     

上新世以来构造隆升对亚洲夏季风气候变化的影响

大量地质证据表明,上新世以来(最近5 MaB.P.)青藏高原北部及非洲东部和南部地区出现过显著的构造隆升,而与此同时亚洲季风也经历了显著变化,这两者之间是否存在着因果联系一直是地学界所关心和争论的一个重要科学问题.本文利用美国国家大气研究中心(NCAR)的公用大气模式(CAM 3.1)就上新世以来青藏高原北部及东-南非高原的构造隆升对亚洲夏季风气候变化的影响进行了数值试验研究.结果表明,上新世以来亚洲夏季风的增强与两地构造隆升密切相关,但两者隆升对于亚洲季风子系统的作用是有区别的.青藏高原北部隆升主要造成东亚北部夏季风的增强及季风降水的增多,但对南亚夏季风的作用较小;东-南非高原的隆升明显增强南亚夏季风,但对东亚北部夏季风的影响有限.     

The crustal structures of the central Longmenshan along and its margins as related to the seismotectonics of the 2008 Wenchuan Earthquake

In 2010, a 500-km-long wide-angle reflection/refraction seismic profile was completed, running northwest from the central Sichuan Basin. This profile orthogonally crosses the meizoseismal area of great Wenchuan earthquake of 12 May 2008, which occurred in the central part of the Longmenshan. The profile also passes through the northwestern Sichuan Plateau, along which a new deep seismic sounding observation system was set up that was much improved over previous datasets and enabled abundant observations to be recorded. Seismic wave phase records that reflect the structural characteristics of different tectonic blocks, especially the complicated phase features associated with the Wenchuan earthquake, were calculated and analyzed in detail. A 2D crustal P-wave velocity model for the orogenic belt in the central Longmenshan and its margins was determined, and crustal structure differences between the stable Sichuan Basin and the thickened northwestern Sichuan Plateau were characterized. Lithological variations within the upper and lower crust in the interior of the plateau, especially a great velocity decrease and plastic rheological properties associated with strong lithologic weakening in lower crust, were detected. From west to east in the lower crust beneath the orogenic belt lying between the Sichuan Basin and the northwestern Sichuan Plateau, a giant shovel-like upwelling is observed that dips gently in the lower part and at higher angles in the upper part; this is inferred to be related to the fault systems in the central Longmenshan. An upwelling in the upper-middle crust along the eastern margin of the orogenic belt is associated with steeply dipping thrusts that strongly uplift the upper crust and crystalline basement beneath a central fault system in the Longmenshan. The data, combined with an understanding of the regional tectonic stress field and previous geological results, enable a discussion of basin-and-range coupling, orogenic tectonics, the crustal fault system, and the seismogenic tectonic environment of the central Longmenshan along the eastern margin of the Qinghai-Tibet Plateau.     

Discovery of the Longriba fault zone in eastern Bayan Har block,China and its tectonic implication

Re-measured GPS data have recently revealed that a broad NE trending dextral shear zone exists in the eastern Bayan Har block about 200 km northwest of the Longmenshan thrust on the eastern margin of the Qinghai-Tibet Plateau. The strain rate along this shear zone may reach up to 4-6 mm/a. Our interpretation of satellite images and field observations indicate that this dextral shear zone corresponds to a newly generated NE trending Longriba fault zone that has been ignored before. The northeast segment of the Longriba fault zone consists of two subparallel N54°±5°E trending branch faults about 30 km apart, and late Quaternary offset landforms are well developed along the strands of these two branch faults. The northern branch fault, the Longriqu fault, has relatively large reverse component, while the southern branch fault, the Maoergai fault, is a pure right-lateral strike slip fault. According to vector synthesizing principle, the average right-lateral strike slip rate along the Longriba fault zone in the late Quaternary is calculated to be 5.4±2.0 mm/a, the vertical slip rate to be 0.7 mm/a, and the rate of crustal shortening to be 0.55 mm/a. The discovery of the Longriba fault zone may provide a new insight into the tectonics and dynamics of the eastern margin of the Qinghai-Tibet Plateau. Taken the Longriba fault zone as a boundary, the Bayan Har block is divided into two sub-blocks: the Ahba sub-block in the west and the Longmenshan sub-block in the east. The shortening and uplifting of the Longmenshan sub-block as a whole reflects that both the Longmenshan thrust and Longriba fault zone are subordinated to a back propagated nappe tectonic system that was formed during the southeastward motion of the Bayan Har block owing to intense resistance of the South China block. This nappe tectonic system has become a boundary tectonic type of an active block supporting crustal deformation along the eastern margin of the Qinghai-Tibet Plateau from late Cenozoic till now. The Longriba fault zone is just an active fault zone newly-generated in late Quaternary along this tectonic system.     

Numerical Simulation on Coseismic Effect of the November 14,2001 Great Kunlun Earthquake,Northern Tibet,China

The November 14, 2001 Ms8.1 Kunlun Mountains earthquake in northern Tibet is the largest earthquake occurring on the Chinese mainland since 1950. We apply a three-dimensional （3-D） finite element numerical procedure to model the coseismic displacement and stress fields of the earthquake based on field investigations. We then further investigate the stress interaction between the Ms8.1 earthquake and the intensive aftershocks. Our primary calculation shows that the coseismic displacement field is centralized around the east Kunlun fault zone. And the attenuation of coseismic displacements on the south side of Kunlun fault zone is larger than that on the north side. The calculated coseismic stress field also indicates that the calculated maximal shear stress field is centralized around the east Kunlun fault zone; the directions of the coseismic major principal stress are opposite to that of the background crustal stress field of the Qinghai-Xizang （Tibet） Plateau. It indicates that the earthquake relaxes the crustal stress state in the Qinghai-Xizang （Tibet） Plateau. Finally, we study the stress interaction between Ms8.1 earthquake and its intensive aftershocks. The calculated Coulomb stress changes of the Ms8.1 great earthquake are in favor of triggering 4 aftershocks.