用Rayleigh面波方位各向异性研究中国大陆岩石圈形变特征

中国大陆地质构造历史非常复杂,岩石圈长期积累的形变较大,而利用地震面波传播的各向异性是研究岩石圈形变特征的强有力手段. 本文利用双台窄带通滤波-互相关方法与基于图像分析的相速度频散曲线提取技术,提取Rayleigh面波相速度频散资料,进而反演中国大陆及邻区20～120 s周期Rayleigh面波相速度方位各向异性空间分布图像. 检测板测试结果显示：中国大陆大部分区域的方位各向异性横向分辨率在5°左右. 各向异性研究结果表明：中国大陆地壳上地幔方位各向异性特征存在显著的空间差异,反映出形变特征的空间差异;104°E以东地区地壳上地幔各向异性弱于西部地区,表明其构造变形总体弱于西部地区. 青藏地块及其东缘地区地壳与上地幔顶部变形最为强烈. 但东部的局部地区如华南地块与珠江口地区、鄂尔多斯盆地西南缘以及秦岭-大别造山带,较强的各向异性显示这些区域在不同时期也经历了强变形. 青藏地块内中短周期快波方向自西向东顺时针旋转变化可能指示板块碰撞与挤压过程中软弱物质的流变方向. 青藏地块西部中下地壳和上地幔形变模式相似,可能处于壳幔耦合状态;而中东部及东缘地区地壳上地幔形变模式存在明显差异,壳幔似乎不具备垂直连贯的形变特征. 位于青藏地块北部的塔里木盆地、柴达木盆地以及祁连褶皱带同样经历了强变形. 包括四川盆地在内的上扬子地块快波方向的变化显示中地壳与下地壳上地幔形变模式不同,而形变特征一致的下地壳与上地幔应为强耦合. 大约以103°E为界,龙门山断裂带可分为南西段和北东段,南西段处于低速区,而北东段位于高速区,且方位各向异性强度明显大于南西段;2008年5月12日汶川M_S8.0级地震沿断裂带的单侧破裂模式除与北东段的高应力积累有关外,还可能与北东段地下介质物性存在密切关系,高速坚硬岩体的发育有利于应变能的积累与集中释放.     

用转换函数研究青藏高原地壳S波速度结构——"Hi-CLIMB"剖面

本文对青藏高原中部Hi-CLIMB(Himalayan-Tibetan Continental Lithosphere During Mountain Building)宽频带数字台站探测剖面资料进行处理,用转换函数模拟退火算法得到了83个台站下方S波速度结构,转化为二维速度结构剖面,并与接收函数偏移成像结果进行了对比.反演结果可清晰显示出工作区地壳内S波速度界面和不连续的莫霍面.研究表明,从N29.4°至 N31.5°地壳平均厚度为75 km;在N 32°以北地壳平均厚度为65 km.壳内界面在N31.5°以南向北倾斜,而在其北部向南倾斜.在雅鲁藏布江缝合带和班公-怒江缝合带附近地壳具有明显低速区.这一复杂结构反应了该区域内部强烈挤压变形与物质交换,为进一步研究岩石圈变形与构造演化提供了新的地球物理学证据.     

中国大陆及邻区岩石圈三维流变结构

依据地震波速得到的上地幔温度和气象台站记录的地表温度为约束,结合地表热流和热导率观测数据,利用有限元方法计算了中国大陆及邻区岩石圈三维热结构.基于此温度结果和GPS观测得到的应变率数据,以滑动摩擦、脆性破裂和蠕变三种强度机制为约束,计算得到了中国大陆及邻区岩石圈三维流变结构.结果显示:弱强度和低等效黏滞性系数的下地壳在中国大陆及邻区普遍存在,并且下地壳的流变强度和等效黏滞性系数比上地壳和岩石圈地幔一般要低1~2个数量级;中国大陆范围内青藏高原存在着厚度最大、强度最低的下地壳;青藏高原的岩石圈强度和等效黏滞性系数比华北、华南和印度板块的都要低;岩石圈流变结构的横向分布特征与重力梯度带和地形过渡带比较一致.     

用转换函数方法研究腾冲—临沧地区地壳结构

根据流动数字地震台网提供的三分量地震波形记录资料,应用转换函数及快速模拟退火算法对腾冲－临沧地区30个地震台站下的地壳横波速度结构进行了反演.反演结果说明,研究区壳幔边界清晰、莫霍界面附近速度跳跃明显,由此得出该区地壳厚度在40 km左右、并具有从南向北增厚趋势.一个普遍的现象是,在腾冲-宝山地块下地壳存在明显的低速带,低速带的厚度在10～20 km间.研究结果进一步表明各台站下方上地幔速度结构复杂.这些结果为探讨青藏高原东南缘下地壳的侧向黏性流动、碰撞板块边界处壳幔物质交换等均提供了重要的地球物理证据,为探讨印-藏汇聚过程中青藏高原东构造结岩石圈变形、高原隆升及其深部动力学有一定理论意义.     

Rayleigh wave phase velocities of South China block and its adjacent areas

Using records of continuous seismic waveforms from 609 broadband seismic stations in the South China Block and its adjacent areas in 2010–2012, empirical Green's functions of surface waves were obtained from cross-correlation functions of ambient noise data between these stations. High quality phase velocity dispersion curves of Rayleigh waves were obtained using time-frequency analysis. These interstation dispersion curves were then inverted to build Rayleigh wave phase velocity maps at periods of 6–50 s. The results of phase velocity maps indicate that phase velocities at 6–10 s periods are correlated with the geological features in the upper crust. Major basins and small-scale grabens and basins display slow velocity anomalies; while most of the orogenic belts and the fold belts display high velocity anomalies. With the gravity gradient zone along Taihang Mountain to Wuling Mountain as the boundary for the phase velocity maps at period of 20–30 s, the western area mainly displays low velocity anomalies, while the eastern side shows high velocity anomalies. Phase velocities in the eastern South China Block south to the Qinling-Dabie orogenic belt is higher than that in the eastern North China Block to the north, which is possibly due to the differences of tectonic mechanisms between the North China Craton and the South China Block. The phase velocities at periods of40–50 s are possibly related to the lateral variations of the velocity structure in the lower crust and upper mantle: The low-velocity anomalies in the eastern part of the Tibetan Plateau are caused by the thick crust; while the Sichuan Basin and the southern part of the Ordos Basin display distinct high-velocity anomalies, reflecting the stable features of the lithosphere in these blocks. The lateral variation pattern of phase velocities in the southern part of the South China Block is not consistent with the surface trace of the block boundary in the eastern Yunnan Province and its vicinities. The phase velocities in the Sichuan Basin are overall slow at short periods and gradually increase with period from the central part to the edge of the basin, indicating the features of shallower basement in the center and overall stable lithospheric mantle of the basin. The middle and upper crust of the southern Ordos Basin in the North China Block is heterogeneous, while in lower crust and the uppermost mantle the phase velocities mainly exhibit high anomalies. High-velocity anomalies are widespread at the middle of the Qinling-Dabie orogenic belt, as well as the areas in southeastern Guangxi with Caledonian granite explosion, but its detailed mechanism is still unclear.     

中国东部海域岩石圈结构面波层析成像

本文通过面波层析成像得到了中国东部海域及邻近地区的地壳上地幔S波速度图像,给出了主要构造单元的区划及其结构特征,并讨论了速度结构与现今构造活动及构造演化历史的关系.研究区内中下地壳的平均速度与地震活动存在比较显著的关系,强震基本都发生在低速区内或高低速过渡区.太行山以东地壳内存在几条北西向低速带,其中张家口—渤海地震带下方的低速带最为显著.东部海域划分成北黄海、南黄海、东海、和冲绳海槽等4个构造块体.北黄海具有较薄较高速的岩石圈,与南华北盆地类似,推测是中生代特提斯洋向北俯冲造成岩石圈减薄的遗迹.北华北地区具有低速的地壳和较厚的岩石圈,岩石圈地幔速度偏低且上下比较均匀,可能反映中生代沿北方缝合带持续碰撞作用的特点.南黄海具有相对较厚的岩石圈,较多地保存了下扬子克拉通的特征.在下扬子与华北地块的拼合过程中,洋壳俯冲可能是北黄海和苏皖地区上地幔低速特征的成因.在125°E以东的朝鲜半岛地区未发现这一拼合过程的遗迹.有可能整个朝鲜半岛都是华北地块的一部分;但也有可能是太平洋俯冲和日本海张开的作用完全改造了朝鲜半岛的岩石圈上地幔,抹去了以往构造运动的痕迹.东海地区的地壳厚度,特别是岩石圈厚度向冲绳海槽方向减小,反映出菲律宾海板块俯冲在弧后广大地区都有影响.冲绳海槽地区可见俯冲的菲律宾海板片以及板片上方显著低速的地壳和上地幔,为冲绳海槽的弧后扩张机制提供了证据.     

华北克拉通中西部地壳S波速度结构及其地质意义

华北克拉通是世界上最古老的克拉通之一.我们利用布设于华北中部的ChinArray计划461个宽频带地震台阵的连续波形资料, 基于背景噪声成像技术, 获得了克拉通中西部5~45 s的Rayleigh波群速度频散曲线, 并利用线性反演方法获得了研究区地壳上地幔顶部的S波速度结构.密集流动地震台阵使我们能够揭示研究区精细的地壳上地幔顶部速度变化, 以深入探讨华北克拉通中西部深部结构及其对岩浆和地震的控制作用.8 km深度的S波速度切片显示低速与高速异常分别与地表的盆地和山脉对应良好.不同经度和纬度方向的S波速度剖面均表明, 西部克拉通地壳大致可以分为上、中、下地壳三层.克拉通西部鄂尔多斯块体的下地壳S波速度介于3.7~3.8 km·s^-1, 暗示其下地壳以长英质岩石为主.大同火山区下方的S波低速异常从中地壳延伸至上地幔顶部, 推测源自软流圈的地幔热流提供了近垂直的主干上涌通道, 并控制了该区新生代岩浆活动.强震集中分布在上地壳高速体内部或高低速相间区, 其下地壳乃至上地幔顶部都呈现明显的低速异常, 推测源自上地幔/下地壳的深部热流沿地壳尺度的陡深断裂上侵, 诱发上覆高应力刚性块体发生蠕动破裂与应力释放, 进而诱发大震.

     

基于Love波相速度反演南北地震带地壳上地幔结构

收集了南北地震带区域地震台网中292个地震台站2008年1月至2011年3月期间的地震波形数据,由频时分析方法提取了Love波相速度频散曲线,经过反演得到了研究区内的Love波相速度分布.根据Love波纯路径频散,采用线性反演方法对0.25°×0.25°的网格点进行了一维S波速度结构反演,利用线性插值获取了南北地震带地区的三维S波速度结构.结果显示了松潘-甘孜地体和川滇菱形块体地区的下地壳具有明显的S波低速层分布,该异常分布特征支持解释青藏高原隆升及其地壳物质运移的下地壳流模型.在100至120 km深度上,川滇菱形块体西北部呈现较强的S波高速异常,这可能是印度岩石圈板块沿喜马拉雅东构造结下插至该区域所致,该区域下地壳的低速软弱物质与上地幔的高速强硬物质形成了鲜明对比,暗示了地壳和上地幔可能具有不同的构造运动和变形方式,这为该区域的壳幔动力学解耦提供了条件.     

天山及其邻区地壳上地幔S波速度结构的接收函数与面波频散联合反演

根据天山及其邻近区域88个宽频带地震台站的接收函数和欧亚大陆的基阶瑞利波群速度图像, 联合反演了这些台站下的地壳上地幔一维S波速度结构. 在这些速度模型的基础上, 利用线性各向同性变差克里金空间插值技术得到了该区域的地壳上地幔三维S波速度模型. 通过在垂向不同深度上的切片和横向上剖面投影的方式, 显示和分析了天山及其邻区地壳上地幔的剪切波速度特性与构造特性之间的关系. 结果表明, 天山及其邻区的地壳结构垂向上分为上、 中、 下3层, 每层的界面大致位于20, 40和50 km, 并且这些界面的起伏随不同块体的构造差异而变化. 天山地区和塔里木盆地隆起区的上地壳表现为高速特征, 而沉积盆地大部分地区和山前坳陷区的上地壳则表现为低速特征. 东、 西天山之间下地壳存在的近南北向低速带以及上地幔高速盖层在深度上的差异均说明东、 西天山在构造活动和形变上有明显的差别, 这种差别可能是由于印度板块与欧亚大陆的碰撞对它们产生的不同影响而造成的.     

利用接收函数、背景噪声频散和Rayleigh波ZH振幅比联合反演青藏高原东北缘及周边地区地壳结构

本文对在青藏高原东北缘及邻近地区架设的571套宽频带流动地震台站记录到的连续波形数据进行处理,通过联合反演P波接收函数、背景噪声频散和Rayleigh波ZH振幅比,获得了研究区高分辨率三维地壳S波速度（V_S）结构.研究结果显示,松潘—甘孜块体和西秦岭造山带以及北祁连造山带下方15~40 km的深度范围内存在很显著的S波低速异常体.其中,松潘—甘孜块体和西秦岭造山带下方的低速异常体很可能与部分熔融有关,且造成部分熔融的原因除了剪切加热外,还有可能是软流圈物质上涌和地壳内部的放射生热.而北祁连造山带的低速异常体则可能由地表隆升和地壳增厚所造成.阿拉善块体内部分布有很多不太显著的低速异常体,这可能与阿拉善块体经历了复杂的构造变形有关.在银川—河套地堑下方20~35 km的深度范围内同样观测到了相对不太显著的低速异常体,这更可能与基性岩浆的底侵作用有关.

     

华北克拉通岩石圈有效弹性厚度及其各向异性

华北克拉通是典型的克拉通破坏的区域,研究该区域的岩石圈有效弹性厚度(Te)及其各向异性特征有助于了解华北克拉通的强度构造及破坏机制.我们根据空间分辨率为30″×30″的地形数据和由自由空气异常解算得到的完全布格重力异常数据,利用Fan小波分析方法计算了两者之间的相关性,并基于Forsyth理论和正交各向异性薄板模型计算了华北克拉通地区的岩石圈Te和其各向异性分布情况.结果表明:(1) 从各向同性Te分布来看,华北克拉通岩石圈在东部、中部和西部存在着明显的差异.鄂尔多斯地块、河淮盆地的Te值均较高;中华北克拉通、南北重力梯度带及鲁西隆起Te值较低,约10~25 km;郯庐断裂带两侧Te有非常大的差异,西侧的Te明显小于东侧,推测郯庐断裂带在华北克拉通破坏过程中起着非常重要的作用.(2) 从Te的各向异性来看,不同块体Te各向异性的大小或方向存在差异,并且研究区内地震大多分布在Te各向异性大小或方向转变的区域.(3) 从地震波SKS各向异性和Te各向异性的比较来看,在华北克拉通西部阿拉善块体岩石圈变形趋于垂直连贯变形模式;鄂尔多斯地区各向异性源自历史构造事件的"化石"各向异性;山西裂谷带地区Te的弱轴方向和SKS的快波方向平行,而在山西裂谷带南部的秦岭—大别区域,SKS快波方向和Te弱轴方向相垂直,这可能与地幔热物质上涌等作用有关.此外,Te各向异性与现今构造应力场间的相关性不明显,体现出华北克拉通复杂的构造应力特征.     

ESTIMATING THE LOWER CRUSTAL VISCOSITY OF THE WESTERN QINLING-SONGPAN TECTONIC NODE AND ITS ADJACENT AREAS BY USING LANDFORM MORPHOLOGY

The western Qinling-Songpan tectonic node is located at the intersection of three major tectonic units of Tibetan plateau, the South China Block and the Ordos Block, and is at the forefront of the northeastern margin of Tibetan plateau. It has unique geological and dynamic characteristics from the surface to the deep underground. Based on the model for ductile flow in the lower crust, the geomorphological form is used to estimate the viscosity of the lower crust, and how the rheological process of the deep lithosphere acts on the upper crust deformation and structural geomorphology. And combined with GPS velocity field data, the current crustal deformation is analyzed to further study the regional dispersive deformation process. The results show that the viscosity of the north and northeast of the Zoige-Hongyuan Basin is smaller than that of the east and southeast. Therefore, the lower crust flow has a tendency of flowing to the northeastern low viscosity zone. We believe that when the lower crust flows from the central plain of the Qinghai-Tibet Plateau to the rigid Sichuan Basin with a higher viscosity of the lower crust, it cannot flow into the basin, and part of the lower crust flow accumulate here, causing the upper crust to rise, and the uplifting led to the formation of the Longmen Mountains and a series of NNE-striking faults as well. When the lower crust flows to the northeast direction with a low viscosity, the brittle upper crust is driven together. Because of the remote effects from the Ordos Basin and the Longxi Basin, the mountains in this region are built slowly and the stepped arc-shaped topography of the current 3 000-meter contour line and the 2 000-meter contour line are developed. At the same time, a series of NWW-trending left-lateral strike-slip faults are developed. This explains the seismogenic tectonic model of the western Qinling-Songpan tectonic node as from NWW-trending left-lateral strike-slip faulting to the NNE-trending right-lateral strike-slip faulting and both having a thrust component. The current crustal movement direction revealed by the GPS velocity field is consistent with the direction of historical crust evolution of the lower crust revealed by the viscosity, implying that there is a good coupling relationship between the lower crust and upper crust. The results provide a basis for studying the development of fault systems with different strikes and properties, the formation of orogenic belts, the macroscopic geomorphological evolution characteristics, and the rheological and uplift dynamics of the lithosphere in the northeastern margin of the Tibetan plateau. In addition, our research differs from the previous studies in the spatial and temporal scale. Previous studies included either the entire Qinghai-Tibet Plateau or only the eastern margin of the Qinghai-Tibet Plateau. However, our analysis on the contours and topographical differences in the topography of the western Qinling-Songpan tectonic knot reveals that the study area is controlled by the lower crust flow. Our results are confirmed by various observations such as seismology, magnetotellurics and geophysical exploration. Moreover, the previous studies did not point out enough that the elevation contours are elliptical, and the elliptical geomorphology further illustrates that the formation and evolution of the Qinghai-Tibet Plateau has rheological characteristics and also conforms to the continuous deformation mode. Meanwhile, in terms of time scale, the evolution time of the study area is divided into three types of simulation time according to geochronology. And the GPS velocity field is introduced to observe the present-day crustal deformation.     

哈山冲断带构造变形数值模拟

岩石圈流变结构是控制大陆碰撞造山的重要因素.哈山冲断带的构造变形、地表地形与青藏高原周缘冲断带差异较大,指示需要开展地壳流变学研究.本研究采用二维数值模拟,设计了盆山上地壳强度横向差异的单因素实验,模拟结果分析表明：若上地壳强度"山弱盆强",构造变形集中于造山带,断块垂向叠置造成造山带隆升,使地表地形的构造高点位于造山带;若上地壳强度"山强盆弱",构造变形集中于冲断带和盆地,盆山相互作用造成地表地形的构造高点位于冲断带,而非后陆.本文基于此单因素实验,模拟了哈山冲断带构造演化,发现哈山冲断带晚二叠世发育推覆构造后,构造变形逐渐减弱,扩展方式由前展式变为后展式,基底倾向由向后陆变为向前陆.结合哈山及龙门山的多学科观测资料,本文认为哈山和龙门山冲断带的构造变形、地表地形分别符合地壳强度"山强盆弱"模式和"山弱盆强"模式.研究成果可以为中西部冲断带的地球动力学模型和实验模型的搭建提供一定启示,同时对研究区内构造控藏分析和油气勘探具有指导意义.     

华北地区地壳P波和S波速度结构的双差层析成像

基于华北地区(37°N—42°N,113.5°E—118.5°E)133个固定地震台站收集到的P波和S波震相数据,利用双差层析成像法反演了该地区地壳三维速度结构并对所用地震进行了重定位.结果显示:地震走时残差均方根的平均值由重定位前的0.265 s下降至0.008 s;重定位后的震源主要分布于6—16 km深度范围内;...     

华北不同构造块体地壳结构及其对比研究

华北古大陆块体经多期构造运动的改造使地壳构造具有明显的分块特征. 利用华北地区近30条、共约两万公里的深地震测深资料及成果,进一步研究华北各次级块体内部地壳细结构,对比分析各块体的结构差异. 根据不同的地壳结构特征,华北地壳可分为三大类：西部鄂尔多斯盆地地壳结构简单,基底结构完整,为稳定古大陆地壳;华北中部隆起区太行山及北部阴山、燕山隆起区地壳结构相对简单,中部地壳和下地壳局部区域轻微速度逆转,可能与该区域地壳增厚隆升的壳内介质解耦形变有关;华北东部裂陷盆地地壳结构复杂, 基底下陷、破碎,壳内介质松散、速度低,Moho上隆、地壳减薄,横向结构差异明显,显示了新生地壳构造特征. 在此基础上,综合研究、探讨了华北地壳分块构造以及与之相关的动力学演化.     

松原前郭地震区孕震构造的地震层析成像研究

本文选取2013年10月到2018年12月期间松原市周边17个地震台记录到的515个地震事件的2926条P波和2665条S波到时数据,采用双差成像方法联合反演松原地震区震源参数和中上地壳(0~25 km)高分辨率三维体波速度和波速比结构,进而利用O'Connell-Budiansky理论估计了该区岩石介质的裂缝密度和饱和度参数.重定位后的震源参数精度有了显著提高.松原震区P波和S波的速度、波速比结构均表现出明显的不均匀特性,主要表现为低P波、低S波、低波速比结构,推测松原地震区发生更大地震的可能性较低.松原震群区浅地表区域(< 5 km)整体表现为低P波、低S波和高波速比结构,而岩石介质的裂缝密度和饱和度表现为高值.浅层高波速比、高裂缝密度和高饱和度结构可能与流体有关;从区域构造出发表明松原地震发震构造与东北地区的区域构造应力场密切相关.松原震区的三维精细速度和高精度震源位置信息为深入了解该地区的地壳结构、孕震构造和地震活动性提供重要参考.

     

青藏高原腹地中新世从造山阶段向造高原阶段的转变及其动力学机制

通过对前人研究的综述,发现青藏高原新生代地质演化与高原东南缘构造演化密切相关.俯冲下插的印度地壳在藏南发生部分熔融并注入青藏高原中部地壳,这些塑性流变的地壳物质在高原东南缘先后沿两个通道流出高原内部：早期为印支通道,开放时间为35 Ma以前并持续到12 Ma;后期为川滇通道,开放时间为12 Ma至今.由于喜马拉雅东构造结与四川盆地之间强烈的挤压,印支通道不断变窄,并在12 Ma被关闭.两个通道的差异,通道的打开和关闭,造成高原中地壳物质流出速率在中新世发生明显变化,在23 Ma以来流出速率小于注入速率,在12 Ma流出速率最小,部分熔融的印度地壳物质不断滞留于高原地壳内部,使得地势相对平坦、面积巨大的青藏高原逐渐形成并分别向南和向北扩展.通过简单的力学分析,本文将高原腹地变形划分为两个阶段：大于35~23 Ma的造山阶段,受控于造山机制;23 Ma至今的造高原阶段,受控于造高原机制.     

中国大陆及周边地区现代岩石圈演化动力学模拟

采用有限元方法模拟了近20万年来青藏高原岩石圈形变演化过程,探讨了印度-欧亚大陆的碰撞对中国大陆岩石层形变和应力场的影响以及它们与强地震活动性的关系.结合现代GPS、地震和地质学观测的结果,对比分析了中国大陆在百万年、十万年和十年尺度上的形变和构造应力场的基本格局.研究表明：（1）印度-欧亚大陆的碰撞以及印度大陆的持续向北推进、挤压所产生的应力环境,一直主导了以青藏高原为核心的我国西部地域岩石圈构造、运动和演化,但其影响随着远离青藏高原地区而逐渐变小.（2）断层滑移和重力势作用对于青藏高原东西部以及塔里木盆地的影响相当大,它们导致青藏高原岩石层东西向形变速率增大,对青藏高原的中南部地区产生拉张效应,同时导致塔里木盆地出现整体的右旋趋势.（3）青藏高原区域水平方向形变速率和GPS观测结果吻合较好.但在垂直方向上,一些地区计算结果与观测数据相差较大,这说明单纯的挤压作用不是现代青藏高原隆升的惟一机制.现代青藏高原的隆升可能与其他驱动机制,如地幔对流、重力均衡以及剥蚀作用等有关.（4）印度板块的挤压作用基本上决定了中国大陆西部的主压应力场分布.（5）印度板块的碰撞对中国大陆的强地震活动性有重要影响,但华北地区是个例外,该地区的地震活动性很强而印度板块的挤压在该区域产生的影响却很小,说明其他的驱动力在一定程度上活化了华北地块.     

JOINT INVERSION OF AMBIENT NOISE AND SURFACE WAVE FOR S-WAVE VELOCITY OF THE CRUST AND UPPERMOST MANTLE BENEATH WEIHE BASIN AND ITS ADJACENT AREA

The Weihe Basin is the main component of the extrusion and escape shear zone between the ancient North China craton block in Ordos and the ancient Yangtze platform in Sichuan Basin, and carries the dynamic transmission from the main power source of the Qinghai-Tibet Block in the west to the North China and South China regions in the east. The basin itself plays multi roles in the east-west and north-south tectonic movement, and is an excellent site for studying the structural interlacing, dynamic transformation and transmission. At the same time, Weihe Basin is also a famous strong earthquake zone in China. Historically, there was a strong earthquake of magnitude 8 1/4 occurring in Huaxian County in 1556, causing huge casualties and property losses. In view of the special geological structures and the characteristics of modern seismicity activities in the Weihe fault-depression zone, it is necessary to carry out fine three-dimensional velocity structure detection in the deep part of Weihe Basin and its adjacent areas, so as to study the relationship between velocity structure and geological structural units and their evolution process, as well as the deep medium environment where earth￣quakes develop and occur. We investigate the S-wave velocity structure beneath Weihe Basin and its adjacent regions based on continuous background noise data and teleseismic data recorded by 257 broadband stations in Shaanxi Province and its adjacent regions and China Seismological Science Array Exploration Project, and by adopting seismic surface wave inter-station method and background noise cross-correlation method, a total of 10 049 fundamental-mode Rayleigh surface wave phase velocity dispersion curves in the periods of 5~70s are obtained. Firstly, using the average dispersion curve in this study area, we obtain the one-dimensional average S-wave velocity structure model of the study area, and then we apply the ray-tracing surface-wave-dispersion direct inversion method to obtain the S-wave velocity structure of the crust and uppermost mantle (3~80km) beneath Weihe Basin and its adjacent regions. The test results of a 1°×1° grid checker board show that the recovery is good, except for the areas east of 111° and south of 32° of the study area, where there is almost no resolution. The imaging results show that the velocity structure beneath each tectonic unit in the study area has a certain distribution rule, and there is a good correlation between surface geological structure and deep velocity structure. Based on the analysis of velocity slices at different depths and S-wave velocity structures of three profiles, and combined with existing geological structures, geophysics and other deep exploration research results, we obtain the following knowledge and conclusions:1)The thick sedimentary layer covering the top of Weihe Basin is the cause of low velocity anomaly in its shallow crust, the middle and upper crust of the basin are of low velocity structure, and the low-velocity zone extends about 25km, the Moho interface uplifts abruptly relative to both the Ordos Block and the Qinling orogenic belt on opposite sides, and high-speed materials from the upper mantle intrude into the lower crust, which may be related to the underplating of mafic-ultramafic materials from the upper mantle in Mesozoic-Cenozoic period; 2)The south Ordos Block is not a homogeneous whole, the low-velocity structure of the shallow crust in southern Ordos Block is thin in east and thick in west, which may be related to the overall tilting of the Ordos Basin since the Phanerozoic, as well as the differential uplift and strong and uneven denudation of the Ordos Block since the Late Cretaceous. The crustal structure of the south Ordos Block is relatively simple and homogeneous. There is no significant low-velocity structure in the curst of the block, which shows that the low-velocity structure in the crust does not penetrate the whole Ordos block. We speculate that the southern Ordos Block still maintains the stable craton property, and has not been reformed significantly so far; 3)The variation characteristics of deep structure of the Qinling orogenic belt reflect the deep crustal structure and tectonic deformation characteristics of the orogenic belt which are strongly reformed by land-land collision and suture between North China plate and Yangtze plate, intracontinental orogeny, uplift of Qinghai-Tibet Plateau and its northeastern expansion since the Late Hercynian-Indosinian period. The deep structure beneath the eastern and western Qinling orogenic belt is different and has the characteristics of segmentation. The low-velocity anomaly at the bottom of the lower crust of the orogenic belt may be affected by tectonic activities such as uplift and outward extension of the NE Tibetan plateau, and the analysis considers that there is little possibility of the existence of lower crustal circulation channel for the eastward flowing of Tibetan plateau materials in the Qinling orogenic belt. However, since the maximum depth from the inversion of this paper is 80km, which is located at the top of the upper mantle, our results cannot prove that there exists a mantle flow channel for the eastward flow of Tibetan plateau material beneath the Qinling orogenic belt.     

基于背景噪声和地震面波联合反演华北克拉通中部岩石圈结构

基于ChinArray三期项目布设于华北克拉通中部的流动台阵观测数据,利用背景噪声互相关和地震面波层析成像获取了研究区内6—140 s周期的瑞雷面波频散,使用蒙特卡罗非线性反演方法获得了华北克拉通中部岩石圈的高分辨率三维S波速度结构。结果显示华北克拉通不同地块的岩石圈速度结构存在显著的横向差异：其中鄂尔多斯盆地腹地整体表现为高速特征,延伸至200 km以下,但其东南缘存在小范围的低速异常;东部的华北盆地整体表现为低速特征,具有较薄的地壳和岩石圈厚度;中部造山带南北两端以及南北重力梯度线下方存在相连接的低速区域,在深处延伸至华北盆地下方;在下地壳和上地幔顶部,大同火山群区域的低速体逐渐向西偏移至鄂尔多斯盆地东北角下方;而在上地幔中,该区域的低速异常随深度增加而逐渐减弱,低速体延伸至东南方向的华北盆地下方。基于本研究获得的S波速度模型,我们认为：鄂尔多斯盆地腹地保持了克拉通特性,但其东南缘存在局部的岩石圈改造作用;华北盆地发生了强烈的岩石圈破坏减薄和地壳伸展变形;中部造山带南北端以及南北重力梯度线下方的岩石圈发生了局部的改造减薄,其机制可能都来源于华北盆地下方地幔热物质的上涌;大同火山群下...