Seismic anisotropy of upper mantle in eastern Tibetan Plateau and related crust-mantle coupling pattern

By using the polarization analysis of teleseismic SKS waveform data recorded at 116 seismic stations which respectively involved in China National Digital Seismograph Network, and Yunnan, Sichuan, Gansu and Qinghai regional digital networks, and portable broadband seismic networks deployed in Sichuan, Yunnan and Tibet, we obtained the SKS fast-wave direction and the delay time between fast and slow waves of each station by use of the stacking analysis method, and finally acquired the fine image of upper mantle anisotropy in the eastern Tibetan Plateau and its adjacent regions. We analyzed the crust-mantle coupling deformation on the basis of combining the GPS observation results and the upper mantle anisotropy distribution in the study area. The Yunnan region out of the plateau has dif-ferent features of crust-mantle deformation from the inside plateau. There exists a lateral transitional zone of crust-mantle coupling in the eastern edge of the Tibetan Plateau, which is located in the region between 26° and 27°N in the west of Sichuan and Yunnan. To the south of transitional zone, the fast-wave direction is gradually turned from S60°―70°E in southwestern Yunnan to near EW in south-eastern Yunnan. To the north of transitional zone in northwestern Yunnan and the south of western Sichuan, the fast-wave direction is nearly NS. From crust to upper mantle, the geophysical parameters (e.g. the crustal thickness, the Bouguer gravity anomaly, and tectonic stress direction) show the feature of lateral variation in the transitional zone, although the fault trend on the ground surface is inconsis-tent with the fast-wave direction. This transitional zone is close by the eastern Himalayan syntaxis, and it may play an important role in the plate boundary dynamics.     

Seismic anisotropy of upper mantle in Sichuan and adjacent regions

Based on the polarization analysis of teleseismic SKS waveform data recorded at 94 broadband seis-mic stations in Sichuan and adjacent regions, the SKS fast-wave direction and the delay time between the fast and slow shear waves were determined at each station using the grid searching method of minimum transverse energy and the stacking analysis method, and the image of upper mantle anisot-ropy was acquired. The fast-wave polarization directions are mainly NW-SE in the study area, NWW-SEE to its northeast and NS to its west. The delay time falls into the interval [0.47 s, 1.68 s]. The spatial variation of the fast-wave directions is similar to the variation of GPS velocity directions. The anisotropic image indicates that the regional tectonic stress field has resulted in deformation and flow of upper mantle material, and made the alignment of upper mantle peridotite lattice parallel to the di-rection of material deformation. The crust-upper mantle deformation in Sichuan and adjacent regions accords with the mode of vertically coherent deformation. In the eastern Tibetan Plateau, the crustal material was extruded to east or southeast due to SE traction force of the upper mantle material. The extrusion might be obstructed by a rigid block under the Sichuan Basin and the crust has been de-formed. After a long-term accumulation of tectonic strain energy, the accumulative energy suddenly released in Yingxiu town of the Longmenshan region, and Wenchuan MS8.0 earthquake occurred.     

山东地区上地幔各向异性研究

通过分析山东地区数字地震台网37个宽频带地震台站的远震SKS波形资料,使用最小能量法和旋转相关法求得每一个台站的SKS快波偏振方向和快、慢波时间延迟,获得了山东地区上地幔各向异性图像.该研究区的各向异性快波方向基本呈WNW-ESE方向,快、慢波时间延迟为0.73-1.71 s.研究表明,山东地区上地幔存在明显的各向异性...     

华北上地幔各向异性研究

对华北地震科学台阵的200个宽频带和甚宽带地震台站所记录的远震SKS(SKKS)波形资料作偏振分析,采用最小切向能量的网格搜索法和叠加分析方法求得每一个台站的SKS(SKKS)快波偏振方向和快、慢波的时间延迟,并结合已发表的固定台站的结果,获得了华北上地幔各向异性图像.从得到结果看,华北东部各向异性快波方向基本为NWW-SEE方向,而西部的快波方向转到NW-SE或NNW-SSE.快、慢波时间延迟范围是0.50~1.47 s,华北西部的平均快、慢波时间延迟小于华北东部.在华北东部,快波方向与绝对板块运动(APM)方向基本一致,预示了NWW向的软流圈地幔流是引起该区域上地幔各向异性的主要原因,它使得上地幔橄榄岩等晶体的晶格优势取向沿地幔物质流动方向,从而导致了NWW趋向的快波方向.然而,在稳定的西部,快波方向既不与绝对板块运动方向一致,也不与构造走向一致,这种弱各向异性很可能是遗留在古老克拉通的厚的岩石圈内的"化石"各向异性.     

青藏高原东北缘上地幔各向异性研究

通过分析位于青藏高原东北缘的区域数字地震台网30个台站的远震SKS波形资料,采用最小切向能量的网格搜索法和叠加分析方法求得每一个台站的SKS快波偏振方向和快、慢波的时间延迟,获得了青藏高原东北缘上地幔各向异性图像.从得到结果看,青藏高原东北缘的各向异性快波方向基本上呈NW-SE方向,并有一顺时针旋转趋势,快、慢波时间延迟是0.70～1.51 s.青藏高原东北缘的SKS快波偏振方向与区域内主要构造断裂走向基本一致;各向异性快波偏振方向变化与区域内最小平均主压应力方向变化相似,也与由GPS测量得到的速度场方向变化相似.研究表明青藏高原东北缘上地幔物质在区域构造应力场的作用下,发生了顺时针旋转的形变以至流动,使得上地幔中橄榄岩的晶格排列方向平行于物质形变或流动方向,上地幔变形和上覆地壳变形可能存在垂直连贯变形特征.     

华北地区地壳上地幔速度各向异性研究

本文介绍面波反演得到的华北地区地壳上地幔速度各向异性分布图像,并与S波分裂的结果作初步的定性比较.不同周期瑞利波群速度的方位各向异性图像呈现显著的横向变化,与华北地区地壳上地幔的构造分块和垂直分层结构有比较密切的联系.在鄂尔多斯和阿拉善等稳定地块中,岩石圈地幔到160 km深度都保持比较一致的显著各向异性;而在发生过岩石圈减薄的华北克拉通东部,大约80~150 km深度范围内基本没有探测到方位各向异性,可能说明岩石圈减薄过程抹去了原有的各向异性印迹而且没有显著的水平构造运动造成新的方位各向异性.地球介质的各向异性具有明显的分层特征,面波的反演结果是如此,而S波分裂测量所表现的离散性,也可能是由各向异性的分层差异和倾斜的各向异性对称轴等因素引起.采用多层的各向异性模型,在多数情况下可以定性地解释面波反演和S波分裂结果之间的差异.进一步的工作要求增大面波的探测深度和改善分辨能力,获取更多的S波分裂测量资料,从而建立定量或半定量的三维各向异性模型.     

The upper mantle anisotropy in Yunnan area, China

Introduction The study of the upper mantle anisotropy in Yunnan area benefits the research of deep structure of Sichuan-Yunnan active block and the characteristics of deformation field, the analysis of the coupling relations among different layers of the earth and the promotion of understanding the relation between anisotropy and stress-strain field and geological construction processes. The research results would be of important significance for the interpretation of movement of plates, the …     

中国东北地区上地幔P波方位各向异性研究

地震体波走时层析成像是以地震体波到时作为观测数据,反演地下介质的地震波传播速度分布的一种反演技术方法.基于弱各向异性介质的假定,通过在走时反演方程中引入各向异性参数,可以同时进行介质的速度扰动和各向异性属性的联合反演.对三维介质的各向异性分析,有利于对区域结构构造演化、深部物质变形、动力学模式等问题的进一步解释.本文基...     

SKS splitting beneath Capital area of China

Based on the polarization analysis of teleseismic SKS waveform data recorded at 49 seismic stations in Capital Area Seismograph Network,the SKS fast-wave direction and the delay time between the fast and slow shear waves at each station were determined by using the grid searching method of minimum transverse energy and the stacking analysis method,and then we acquired the image of upper mantle anisotropy in Capital area.In the study area,the fast-wave polarization direction is basically WNW-ESE,and the delay time falls into the interval from 0.56 s to 1.56 s.The results imply that the upper mantle anisotropy in Capital area is mainly caused by the subduc-tion of the Pacific plate to Eurasian plate.The subduction has resulted in the asthenospheric material deformation in Capital area,and made the alignment of upper mantle peridotite lattice parallel to the deformation direction.And the collision between the Indian and Eurasian plates made the crust of western China thickening and uplifting and material eastwards extruding,and then caused the upper mantle flow eastwards,and made the upper mantle de-formation direction parallel to the fast-wave direction.The deformation model of the crust and upper mantle is possibly vertically coherent deformation by comparing the fast-wave polarization direction with the direction of lithospheric extension and the GPS velocity direction.     

首都圈地区SKS波分裂研究

通过分析首都圈数字地震台网的49个宽频带和甚宽带台站的远震SKS波形资料,采用最小切向能量的网格搜索法和叠加分析方法,求得每一个台站的SKS快波偏振方向和快、慢波的时间延迟,获得了首都圈地区上地幔各向异性图象.首都圈地区的各向异性快波方向基本上呈WNW-ESE方向,快、慢波时间延迟为0.56-1.56 s.研究表明,首都圈地区上地幔存在明显的各向异性,引起各向异性的主要原因是研究区受太平洋板块俯冲作用下软流圈物质变形,使得上地幔橄榄岩等晶体的晶格优势取向沿物质流动方向.另外,中国大陆受印度板块与欧亚板块的强烈碰撞,大陆西部地壳增厚隆起,同时造成物质东向挤出,使得首都圈地区上地幔物质沿快波方向变形.通过研究区各向异性快波方向和伸展运动方向与GPS测量得到的速度场对比分析,首都圈地区壳幔变形可能具有垂直连贯变形特征.     

远震剪切波分裂研究华南西南部上地幔各向异性特征

华南板块由扬子克拉通和华夏地块在新元古代碰撞拼合形成, 其西南部的缝合带位置存在强烈争议.本文基于一条南北向宽频带流动台阵天然地震数据, 利用SKS波分裂研究华南西南部上地幔各向异性特征的南北向变化.分裂测量结果显示, 26°N以南的右江盆地快波偏振方向为E-W或NEE-SWW方向, 分裂时差为0.5~2.5 s, 变化范围较大; 川东褶皱带存在较多空解, 快波偏振方向以近东西向为主, 分裂时差在0.5~1.5 s范围内变化; 在右江盆地北缘和川东褶皱带的过渡区域(约26°N附近)各向异性明显较弱.结合前人的上地幔速度成像结果, 我们认为川东褶皱带与右江盆地过渡带是扬子克拉通的南边界, 在其下方北侧的川东褶皱带巨厚的岩石圈向南转变成较薄的岩石圈, 陡变的岩石圈边界引起了软流圈上涌, 软流圈的垂向运动导致其水平方位各向异性较弱.

     

Upper mantle anisotropy and crust-mantle deformation pattern beneath the Chinese mainland

Over the past 10 years,the number of broadband seismic stations in China has increased significantly.The broadband seismic records contain information about shear-wave splitting which plays an important role in revealing the upper mantle anisotropy in the Chinese mainland.Based on teleseismic SKS and SKKS phases recorded in the seismic stations,we used the analytical method of minimum transverse energy to determine the fast wave polarization direction and delay time of shear-wave splitting.We also collected results of shear-wave splitting in China and the surrounding regions from previously published papers.From the combined dataset we formed a shear-wave splitting dataset containing 1020 parameter pairs.These splitting parameters reveal the complexity of the upper mantle anisotropy image.Our statistical analysis indicates stronger upper mantle anisotropy in the Chinese mainland,with an average shear-wave time delay of 0.95 s;the anisotropy in the western region is slightly larger(1.01 s)than in the eastern region(0.92 s).On a larger scale,the SKS splitting and surface deformation data in the Tibetan Plateau and the Tianshan region jointly support the lithospheric deformation mode,i.e.the crust-lithospheric mantle coherent deformation.In eastern China,the average fast-wave direction is approximately parallel to the direction of the absolute plate motion;thus,the upper mantle anisotropy can be attributed to the asthenospheric flow.The area from the Ordos block to the Sichuan Basin in central China is the transition zone of deformation modes between the east and the west regions,where the anisotropy images are more complicated,exhibiting"fossil"anisotropy and/or two-layer anisotropy.The collision between the Indian Plate and the Eurasian Plate is the main factor of upper mantle anisotropy in the western region of the Chinese mainland,while the upper mantle anisotropy in the eastern region is related to the subduction of the Pacific Plate and the Philippine Sea Plate beneath the Eurasian Plate.     

中国东北地区北部上地幔各向异性及其动力学意义

中国东北地区广泛发育新生代板内火山,晚中生代以来岩石圈遭受过多期拉张作用.作为中国唯一的深震孕育区,中国东北地区受到太平洋板块的西向俯冲,使得其成为研究岩石圈变形、板块俯冲和板内火山成因及其相互作用关系的天然实验室.通过分析架设在中国东北地区北部的147个流动和固定台站的SKS波形数据,共计得到了377对各向异性参数和251个无效分裂结果.结果表明,中国东北地区东西两侧具有不同的各向异性分布:西部地区各向异性方向变化范围为N143-199°E,平均N169°E,与晚中生代岩石圈伸展方向一致;其各向异性延迟时间平均值约为0.8s,说明来自地幔的各向异性比较微弱,主要由残留在岩石圈中的古老变形所引起.同时,在松辽盆地和佳木斯地块部分区域,观测到延迟时间较小的各向异性(~0.4s),可能是由于岩石圈的拆沉和热地幔物质的上涌侵蚀了保留在岩石圈的古老形变所致.在研究区东部,NNW-SSE朝向的各向异性被观测到,并伴随较大的延迟时间(大于1.0s),可能与太平洋板块撕裂回撤而产生的地幔流动有关.此外,近W-E方向的各向异性只在佳木斯地块被观测到,而太平洋板块在地幔过渡带中的俯冲可能是其产生的主要成因.     

Azimuthal anisotropy in lithosphere on the Chinese mainland from observations of SKS at CDSN

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青藏高原东北缘岩石圈厚度与上地幔各向异性

利用青海地震台网和甘肃地震台网2007-2009年记录的远震波形资料,提取S波接收函数和SKS分裂参数,得到了青藏高原东北缘的三维岩石圈厚度分布和上地幔各向异性特征.S波接收函数结果表明:昆仑-阿尼玛卿缝合带以南的松潘-甘孜地块东北缘和西秦岭造山带下方岩石圈较薄,厚度为125~135 km;昆仑-阿尼玛卿缝合带以北具有较厚的岩石圈,在昆仑和祁连地块下方岩石圈厚达145~175 km,并向柴达木盆地(175~190 km)和克拉通(鄂尔多斯南部约为170 km、阿拉善南缘约为200 km)下方增厚.上地幔各向异性结果显示:东北缘地区的SKS快波偏振方向为NW-SE向,与前人得到的昆仑断裂带南侧的快波方向存在较大差异,南侧自高原内部呈顺时针旋转,表明昆仑断裂带可能为上地幔变形的转换带.SKS快、慢波延迟时间为0.8~1.9 s,且在昆仑-阿尼玛卿缝合带以北,延迟时间与岩石圈厚度呈正相关关系,推断该区各向异性主要来源于地幔盖层的初期伸展变形.     

大别—苏鲁及邻区上地幔的各向异性

大别—苏鲁是扬子与华北的碰撞造山带,对该地区上地幔各向异性的研究有助于了解该区的地幔动力学机制.本文选用了中国数字化地震台网和区域数字地震台网(山东、安徽、江苏、河南、湖北)三分量宽频带的远震地震波形数据,分别采用最小能量法和旋转相关法,对大别—苏鲁及邻区进行剪切波偏振分析,计算了研究区台站下方介质的各向异性分裂参数:快波偏振方向(Φ)和快慢波延迟时间(δt).本文研究结果发现,研究区内快、慢波延迟时间0.5~1.63 s,推测各向异性层深度为57.5~187.6 km,由软流圈和岩石圈地幔的各向异性共同作用引起.快波偏振方向在4个不同构造区表现出不同的特点:华北板块快波偏振方向为近E-W向,根据地质资料,我们分析认为华北板块的各向异性受地幔软流圈流动的影响明显;大别造山带各向异性平行于大别主构造,反映造山过程中岩石圈物质沿大别造山轴部NW-SE向迁移的特点; 在大别南侧和东侧的扬子板块快波偏振方向分别表现为近垂直于造山带走向和NEE-SWW,苏鲁造山带各向异性结果为NEE-SWW,与地表构造有一定的夹角,同时与板块运动方向相差较大,分析认为扬子板块和苏鲁造山带各向异性是由地幔软流圈流动和印支—燕山期构造运动残留在岩石圈地幔的"化石各向异性"共同作用的结果.     

大别—苏鲁及邻区上地幔的各向异性

大别—苏鲁是扬子与华北的碰撞造山带,对该地区上地幔各向异性的研究有助于了解该区的地幔动力学机制.本文选用了中国数字化地震台网和区域数字地震台网(山东、安徽、江苏、河南、湖北)三分量宽频带的远震地震波形数据,分别采用最小能量法和旋转相关法,对大别—苏鲁及邻区进行剪切波偏振分析,计算了研究区台站下方介质的各向异性分裂参数:快波偏振方向(Φ)和快慢波延迟时间(δt).本文研究结果发现,研究区内快、慢波延迟时间0.5~1.63 s,推测各向异性层深度为57.5~187.6 km,由软流圈和岩石圈地幔的各向异性共同作用引起.快波偏振方向在4个不同构造区表现出不同的特点:华北板块快波偏振方向为近E-W向,根据地质资料,我们分析认为华北板块的各向异性受地幔软流圈流动的影响明显;大别造山带各向异性平行于大别主构造,反映造山过程中岩石圈物质沿大别造山轴部NW-SE向迁移的特点; 在大别南侧和东侧的扬子板块快波偏振方向分别表现为近垂直于造山带走向和NEE-SWW,苏鲁造山带各向异性结果为NEE-SWW,与地表构造有一定的夹角,同时与板块运动方向相差较大,分析认为扬子板块和苏鲁造山带各向异性是由地幔软流圈流动和印支—燕山期构造运动残留在岩石圈地幔的"化石各向异性"共同作用的结果.     

湖北地区上地幔各向异性及其动力学意义

SKS波分裂测量是研究大陆地幔的形变特征、探索大陆动力学和演化过程的重要工具.本文选用湖北数字地震台网(HB台网)和中国数字化宽频带地震台网(CB台网)提供的三分量宽频带数字化地震资料,使用旋转相关法和切向能量最小法,计算得到了湖北地区23个地震台站下方上地幔各向异性参数.结果表明,快波偏振方向较为复杂:大别造山带内,呈平行于大别造山带构造走向的NWW向,且与绝对板块运动方向基本一致,反映印支—燕山期扬子板块与华北板块南北向挤压碰撞造山的过程,而地幔流动则是造成这种南北挤压的力源.扬子板块内NWW向虽与绝对板块运动趋势一致,但比较离散,受地幔对流影响不明显,而NE向各向异性主要与燕山后期之后应力场的转变有关.分析认为,该地区的上地幔各向异性为地幔流动和固结在岩石圈中的"化石各向异性"共同作用的结果.根据上地幔各向异性方向与主要构造带的走向平行,可以作为地壳变形与上地幔连贯变形的证据.整个研究区域得到的分裂延迟时间在0.5~1.35 s之间,厚度在58~155 km之间.     

Upper mantle low anisotropy channels below the Pacific Plate

A new 3D anisotropic model has been obtained at a global scale by using a massive dataset of seismic surface waves. Though seismic heterogeneities are usually interpreted in terms of heterogeneous temperature field, a large part of lateral variations are also induced by seismic anisotropy of upper mantle minerals. New insight into convection processes can be gained by taking seismic anisotropy into account in the inversion procedure. The model is best resolved in the Pacific Plate, the largest and the most active tectonic plate. Superimposed on the large-scale radial (ξ parameter) and azimuthal anisotropy (of V_SV velocity) within and below the lithosphere, correlated with present or past Pacific Plate motions, are smaller-scale (<1000 km) lateral variations of anisotropy not predicted by plate tectonics. Channels of low anisotropy down to a depth of 200 km (hereafter referred to as LAC) are observed and are the best resolved anomalies: one east-west channel between Easter Island and the Tonga-Kermadec subduction zones (observed on both radial and azimuthal anisotropies) and a second one (only observed on azimuthal anisotropy) extending from the south-west Pacific up to south-east Hawaii, and passing through the Polynesia hotspot group for plate older than about 40 Ma. These features provide strong constraints on the decoupling between the plate and asthenosphere. They are presumably related to cracking within the Pacific Plate and/or to secondary convection below the rigid lithosphere, predicted by numerical and analog experiments. The existence and location of these LACs might be related to the current active volcanoes and hotspots (possibly plumes) in the Central Pacific. LACs, which are dividing the Pacific Plate into smaller units, might indicate a future reorganization of plates with ridge migrations in the Pacific Ocean.     

The thermal state and composition of the lithospheric mantle beneath the Leizhou Peninsula, South China

Two localities on the Leizhou Peninsula, southern China (Yingfengling and Tianyang basaltic volcanoes) yield a wide variety of mantle-derived xenoliths including Cr-diopside series mantle wall rocks and two distinct types of Al-augite series pyroxenites. Metapyroxenites have re-equilibrated granoblastic microstructures whereas pyroxenites with igneous microstructures have not thermally equilibrated to the mantle conditions. An abundant suite of megacrysts and megacrystic aggregates (including garnet, plagioclase, clinopyroxene, ilmenite and apatite) is interpreted as the pegmatitic equivalents of the igneous pyroxenite suite. Layered spinel lherzolite/spinel websterite xenoliths were formed by metamorphic differentiation caused by mantle deformation, inferred to be related to lithospheric thinning. Some metapyroxenites have garnet websterite assemblages that allow calculation of their mantle equilibration temperatures and pressures and the construction of the first xenolith geotherm for the southernmost China lithosphere. Heat flow data measured at the surface in this region yield model conductive geotherms (using average crustal conductivity values) that are consistent with the xenolith geotherm for the mantle. The calculated mean surface heat flux is 110 mW/m². This high heat flux and the high geotherm are consistent with young lithospheric thinning in southern China, and with recent tomography results showing shallow low-velocity zones in this region. The xenolith geotherm allows the construction of a lithospheric rock type section for the Leizhou region; it shows that the crust–mantle boundary lies at about 30 km, consistent with seismic data, and that the lithosphere–asthenosphere boundary lies at about 100 km.