Crustal and upper mantle S-wave velocity structure beneath the Bransfield Strait (West Antarctica) from regional surface wave tomography

Regional surface wave tomography in the sub-Antarctic Scotia Sea is helpful in revealing the nature of the crust and the S-wave seismic velocity profile beneath the Bransfield Strait. The joint use of our regional network, global seismographic network stations and local temporary arrays provide better lateral resolution than that obtained in our previous studies concerning the Scotia Sea region.Tomographic analysis of data obtained using 10 broad band seismic stations and more than 300 regional events, shows that the Bransfield Basin is characterised by a strong group velocity reduction of 8% with respect to the surrounding areas, in the period range from 15 s to 50 s.The crustal and upper mantle models of the eastern, central and western Bransfield Basin are obtained by joint inversion of Rayleigh and Love local dispersion curves from 15 s to 50 s. In addition our data set is expanded to a broader period interval (1–80 s), in central Bransfield Strait in order to better constrain the upper mantle and shallow crust.The main results can be summarized as follows: (a) the crust thins distinctly from W toward E; the variation is consistent with the type of volcanism, earthquake distribution and bathymetric observations, (b) low upper mantle velocities (soft lid) extend down to depths exceeding 70 km as a consequence of elevated temperatures, (c) the crust beneath the central Bransfield Basin displays continental characteristics with a gradually increasing S-wave velocity distribution versus depth analogous to the East African Rift structure of Kenya, (d) negative velocity gradients are present in the lower crust beneath the eastern Bransfield Basin; these could be interpreted as magmatic bodies originating from decompression melting of the mantle.     

中国大陆深层区域构造格架——系列解释之上地幔剪切波速度模型及其地质意义

上地幔剪切波速度(V_S)差异表明了上地幔的横向不均匀性,通过在线下载V_S模型,并结合断裂构造图,分析发现:中国大陆主要成矿域的V_S结构存在明显差异,如特提斯-喜马拉雅成矿域的V_S为高速,而滨西太平洋成矿域的V_S为低速。编制上地幔剪切波速度结构图可以增进对中国大陆深层区域构造格架的认识,为中国大陆主要成矿域的划分和解释提供深部地球物理背景资料。     

Upper mantle structure of the Northern Eurasia from peaceful nuclear explosion data

Several long-range seismic profiles were carried out in Russia with Peaceful Nuclear Explosions (PNE). The data from 25 PNEs recorded along these profiles were used to compile a 3-D upper mantle velocity model for the central part of the Northern Eurasia. 2-D crust and upper mantle models were also constructed for all profiles using a common methodology for wavefield interpretation. Five basic boundaries were traced over the study area: N1 boundary (velocity level, V = 8.35 km/s; depth interval, D = 60–130 km), N2 (V = 8.4 km/s; D = 100–140 km), L (V = 8.5 km/s; D = 180–240 km) and H (V = 8.6 km/s; D = 300–330 km) and structural maps were compiled for each boundary. Together these boundaries describe a 3-D upper mantle model for northern Eurasia. A map characterised the velocity distribution in the uppermost mantle down to a depth of 60 km is also presented. Mostly horizontal inhomogeneity is observed in the uppermost mantle, and the velocities range from the average 8.0–8.1 km/s to 8.3–8.4 km/s in some blocks of the Siberian Craton. At a depth of 100–200 km, the local high velocity blocks disappear and only three large anomalies are observed: lower velocities in West Siberia and higher velocities in the East-European platform and in the central part of the Siberian Craton. In contrast, the depths to the H boundary are greater beneath the craton and lower beneath in the West Siberian Platform. A correlation between tectonics, geophysical fields and crustal structure is observed. In general, the old and cold cratons have higher velocities in the mantle than the young platforms with higher heat flows.Structural peculiarities of the upper mantle are difficult to describe in form of classical lithosphere–asthenosphere system. The asthenosphere cannot be traced from the seismic data; in contrary the lithosphere is suggested to be rheologically stratified. All the lithospheric boundaries are not simple discontinuities, they are heterogeneous (thin layering) zones which generate multiphase reflections. Many of them may be a result of fluids concentrated at some critical P–T conditions which produce rheologically weak zones. The most visible rheological variations are observed at depths of around 100 and 250 km.     

中国大陆上地幔顶部地震波平均速度分布特征及中新生代陆相盆地成因模式探讨

由中国大陆地震级三维速度图象求取中国大陆上地幔顶部(50—200km深度内)平均速度图,通过它的分布特征分析,结合中国大陆1°×1°平均布格重力异常图,把中国大陆地幔顶部分为5个不同特征的构造域,并探讨了各域内中新生代陆相盆地的成因模式及其动力学原因。     

华北克拉通上地幔变形及其动力学意义

华北克拉通从稳定到破坏的演化过程对有关地球动力学的经典理论提出了挑战,研究其独特的演化历史是固体地球科学研究的一项重要内容。上地幔矿物晶体的各向异性记录了上地幔发生构造变形的信息,研究上地幔地震波各向异性能够揭示现今和构造历史时期所发生的构造运动。本文总结了近年来作者在华北克拉通地区所进行的高密度、覆盖广泛的地震波横波分裂观测研究结果。横波分裂的快轴方向与绝对板块运动方向的不一致,以及横波分裂参数快速的空间变化特征表明了华北克拉通的SKS横波分裂主要反映上地幔的变形。观测结果表明：鄂尔多斯块体保留了克拉通较弱的各向异性特征,其西端体现了元古代克拉通拼合的变形特征; 中新生代华北克拉通破坏事件以不同的机制主导了华北克拉通中部和东部的上地幔变形,在东部地区北西—南东向的拉张应力作用使得快轴方向平行于拉张方向,而在中部则因受到较厚岩石圈的阻挡使得地幔流动改变了方向,因此造成了北东和北北东向的岩石圈拉张。     

华南地壳及上地幔三维速度结构成像

利用国家地震科学数据共享中心的地震目录及临时台网资料,挑选出11 113个区域地震的77 093条P波走时和93 541条S波走时,采用1°×1°的经纬度网格划分,反演获得了深至60km的华南南部地区的地壳及上地幔三维P波和S波的速度结构。研究结果表明,纵波速度结构与横波速度结构从整体来看具有较好的一致性,说明该研究获得的深部速度结果具有较高的可信性,但是在50km的深度纵、横波速度结构的一致性较差,可能是由于该深度的纵横波走时数据存在着较大的差异所导致的。本研究显示了研究区域内的速度结构存在着明显的横向不均匀性,东南沿海地区的地壳中出现了大规模的低速异常,可能与该区地幔物质的上涌有关;而在珠江三角洲、雷州半岛、北部湾及海南岛等地区莫霍面下方出现的低速异常,则与该区的热运动有关。经分析认为,华南南部地壳及上地幔的速度不均匀性和华南板块与扬子地块的相互作用有关,因此开展进一步研究能为探索和分析华南再造以及中国南海北部的构造演化提供重要信息。     

上地幔三维S波速度结构与大别苏鲁超高压变质带俯冲折返机制探讨

本文分析了中国东部的上地幔剪切波速度结构及其与超高压变质岩带之间关系的构造意义。结果表明,在华北块体下面150km深处的速度高于扬子块体的速度值。大别-苏鲁造山构造带下面存在着一条地震波速度变化带。苏鲁、山东半岛下面的速度分布与大别造山带下面的速度分布处于同一个速度等值区域上。横跨大别造山带的南北走向速度结构剖面上,在100km以上的地壳和上地幔区域,华北块体下与扬子块体下面的速度均略低平均值。100km以下,大别造山带南北两侧的扬子与华北块体下面的速度结构分布形态大相径庭。华北下面的波速高于扬子块体下面的波速。大别造山带下呈现速度异常,界线的南侧,有一个略低于零速度的负波速异常区,呈现由南向华北块体的下方斜冲形态,下冲角度大约为30°,其先端部位下冲深达300多公里,其外围零速度等值线的分布区,斜向下延伸超过400km。在速度结构变化分界线的北侧,一个零速度值的分布区带,呈现出从由100多公里深处从北向南朝地表面斜上冲形态。这些速度结构成像的几何形态可能意味着200Ma前大别超高压变质岩带的形成与演化的俯冲、折返的构造运动在上地幔和岩石圈中留下的“痕迹”。     

南海热幔柱构造与油气分布

南海处于欧亚、印度—澳大利亚和太平洋—菲律宾海三大板块的夹持地带,区内以NE向深海区-海盆为中心,周围有众多的含油气盆地。南海区具有"北断(裂)、南褶(皱)、东(俯)冲、西(碰)撞"的构造特征。南海及其周缘新生代玄武岩和花岗岩广为分布,故有潜在的大火成岩省之称。其中,火山岩以碱性玄武岩为主,多为OIB型成因,其成岩年龄自南海中心至外围呈由新逐渐变老的趋势。深部地幔流动呈现出涡旋式上涌的特点,上地幔明显具环带状结构,中心部位为上升流,外围为下降流,表现出热幔柱和冷幔柱活动"双模式"对流。从区域S波速度扰动异常来看,在670km间断面,对热流体上涌确有阻挡作用。通过层析成像研究,证实本区存在巨型复蘑菇云状地幔低速体,演化过程和相邻板块活动构成相辅相成关系。由于地幔热流体上涌,促使地壳-岩石圈上隆、熔融、减薄和断陷,形成南海从边缘向中心(海盆)热流温度逐步升高的轨迹,基本控制油气田"外油内气"环形有序分布的格局。     

南海复蘑菇状地幔低速柱结构及其地幔动力学

运用高分辨率天然地震面波层析成像和体波层析成像技术,研究东亚西太平洋地区及全球地幔三维速度结构时发现南海地区地幔存在深达2000km以上的巨型复蘑菇状地幔低速柱,结合地质、地球化学和地球物理相关标志,将复蘑菇状地幔低速柱称为南海复蘑菇状地幔柱。本文在论述南海复蘑菇状地幔柱的地质地球物理特征基础上,将地幔柱划分为柱头、柱体、柱尾、幔枝和热点等部分,建立起地幔柱三维几何结构模型,探讨了复蘑菇状地幔柱在南海海盆扩张过程中的主导作用以及欧亚板块、菲律宾海板块和印度洋板块相互作用对南海演化过程的影响。     

华南显生宙时期岩石圈地幔的性质与形成时代

火山岩携带的橄榄岩捕虏体是研究深部岩石圈地幔成分特征与热结构状态的最直接样品。湖北大洪山早奥陶世钾镁煌斑岩携带的石榴二辉橄榄岩具有富集的地球化学特征,指示当时的岩石圈厚度可达110 km。华南内陆地区宁远和道县早侏罗世玄武岩所携带的地幔包体具有饱满的成分特征,代表遭受了较低程度部分熔融的地幔残留。宁远地幔包体的全岩Re-Os同位素特征显示该地区中生代大陆岩石圈地幔为从软流圈新增生而形成的新生地幔。这表明内陆地区古生代存在的富集地幔被完全拆沉,并被新生地幔所取代;中生代内陆地区的岩石圈拆沉作用可能与该地区自225 Ma以来大规模的岩石圈伸展作用有关。华南新生代地幔包体主要分布在沿海地区。通过地幔包体矿物成分估算获得的温度与压力资料揭示新生代沿海地区岩石圈厚度约为80～90 km,并具有热的地温梯度。无论是全岩还是硫化物的Re-Os同位素特征都表明沿海地区在新生代仍残留有古元古代岩石圈地幔,表明新生代沿海地区的拉张作用仅导致了岩石圈地幔的部分拆沉和减薄。     

Crustal and upper mantle S-wave velocity structures across the Taiwan Strait from ambient seismic noise and teleseismic Rayleigh wave analyses

Although orogeny tapers off in western Taiwan large and small earthquakes do occur in the Taiwan Strait, a region largely untouched in previous studies owing mostly to logistical reasons. But the overall crustal structure of this region is of particular interest as it may provide a hint of the proto-Taiwan before the orogeny.By combining time domain empirical Green’s function (TDEGF) from ambient seismic noise using station-pairs and traditional surface wave two-station method (TS) we are able to construct Rayleigh wave phase velocity dispersion curves between 5 and 120 s. Using Broadband Array in Taiwan for Seismology (BATS) stations in Taiwan and in and across the Strait we are able to derive average 1-D Vs structures in different parts of this region. The results show significant shear velocity differences in the upper 15 km crust as expected. In general, the highest Vs in the upper crust observed in the coastal area of Mainland China and the lowest Vs appears along the southwest offshore of the Taiwan Island; they differ by about 0.6–1.1 km/s. For different parts of the Strait, the upper crust Vs structures are lower in the middle by about 0.1–0.2 km/s relative to those in the northern and southern parts. The upper mantle Vs structure (Moho – 150 km) beneath the Taiwan Strait is about 0.1–0.3 km/s lower than the AK135 model. The overall crustal thickness is approximately 30 km, much thinner and less variable than under the Taiwan Island. The inversion of seismic velocity structures using shorter period band dispersion data in the sea areas with water depth deeper than 1000 m should take water layer into consideration except for the continental shelves.     

Shear wave splitting as a diagnostic of variable anisotropic structure of the upper mantle beneath central Fennoscandia

We analyze splitting of shear waves recorded during the SVEKALAPKO passive seismic experiment in south-central Finland to study fabrics of the mantle lithosphere of the Precambrian region and thus to bring information into a debate on existence of plate tectonics or its forms in the early stage of continent formation. Geographical variations of the splitting parameters and their distinct dependence on direction of wave propagation through the upper mantle allow us to identify six domains of the central Fennoscandian mantle lithosphere, including the Proterozoic–Archean transition, and to model their fabrics by joint inversion of body wave anisotropic parameters. Fabrics of the Archean mantle lithosphere can be approximated by a peridotite aggregate with lineation a dipping to the NE. On the other hand, anisotropy of the Proterozoic mantle lithosphere is weaker and we model its fabric by the (a, c) foliations dipping to the SE. We present a 3D self-consistent anisotropic model of the Proterozoic and Archean upper mantle along the SW-NE profile in the south-central Finland. Boundaries of inter-growing wedges of the Proterozoic and Archean mantle lithospheres explain the longitudinal and shear wave propagation and polarization, mantle xenolith ages, surface wave tomography and location of the upper mantle reflectors. We interpret the six anisotropic domains as fragments of mantle lithosphere retaining an old fossil olivine fabric which was created before these micro-continents assembled.     

Asymmetry in oceanic crustal structure of the South China Sea basin and its implications on mantle geodynamics

ABSTRACT

We investigated the oceanic crustal structure and lithospheric dynamics of the South China Sea (SCS) basin through a comprehensive analysis of residual gravity anomaly and bathymetry combined with seismic constraints and interpretation from geodynamic modelling. We first calculated the residual mantle Bouguer anomaly (RMBA) of the oceanic crustal regions of the SCS by removing from free-air gravity anomaly the predicted gravitational attractions of water-sediment, sediment-crust, and crust-mantle interfaces, as well as the effects of lithospheric plate cooling, using the latest crustal age constraints including IODP Expedition 349 and recent deep-tow magnetic surveys. We then calculated models of the gravity-derived crustal thickness and calibrated them using the available seismic refraction profiles of the SCS. The gravity-derived crustal thickness models correlate positively with seismically determined crustal thickness values. Our analysis revealed that the isochron-averaged RMBA are consistently more negative over the northern flank of the SCS basin than the southern conjugate for magnetic anomaly chrons C8n (~25.18 Ma) to C5Dn (~17.38 Ma), implying warmer mantle and/or thicker crust over much of the northern flank. Computational geodynamic modelling yielded the following interpretations: (1) Models of asymmetric and variable spreading rates based on the relatively high-resolution deep-tow magnetic analysis would predict alternating thicker and thinner crust at the northern flank than the southern conjugate, which is inconsistent with the observed systematically thicker crust on the northern flank. (2) Models of episodic southward ridge jumps could reproduce the observed N-S asymmetry, but only for crustal age of 23.6–20 Ma. (3) Southward migration of the SCS ridge axis would predict slightly thinner crust at the northern flank, which is inconsistent with the observations. (4) Models of higher mantle temperatures of up to 25–50°C or >2% less depleted mantle sources on the northern flank could produce large enough anomalies to explain the observed N-S asymmetries.     

Constraints on crustal and mantle structure of the oceanic plate south of Iceland from ocean bottom recorded Rayleigh waves

From April to July 2002 we carried out a deployment of 6 ocean bottom seismometers and 4 ocean bottom hydrophones in the North Atlantic south of Iceland. During the deployment period we recorded clear Rayleigh waves from 2 regional and 14 teleseismic earthquakes. This corresponds to a Rayleigh wave detection rate of nearly 92% for events with M_W ≥ 6.06.0 and epicentral distance less than 110°, close to detection rate estimates based on noise level variability. We measured Rayleigh wave event-station group dispersion and inter-station phase dispersion for one Mid-Atlantic Ridge event. The group dispersion curve is sensitive to the structure of the North-East Atlantic with an average age of  39 Myr. The phase dispersion curve is sensitive to the structure just south of Iceland (average plate age 33 Myr). Both dispersion curves indicate faster velocities than previously postulated for oceanic plate generated at the Reykjanes Ridge. A grid search approach was used to constrain the range of models fitting the data. The high velocity seismic lid just south of Iceland in the model for the phase dispersion path is slower or thinner than in the group dispersion model, which averages over a larger area and a somewhat older plate age, but the velocities in the low velocity half space are similar. We further consider the residual bathymetry in the experimental area. The residual anomaly decreases by 300–400 m from the Reykjanes Ridge to the  30 Myr old plate south of Iceland. This decrease can be explained by the disappearance of a mantle thermal anomaly associated with the Iceland plume. Both the residual bathymetry and the surface wave data are thus consistent with the notion that the southward spreading of the Icelandic plume is channelised underneath the Reykjanes Ridge and does not spread far outside this channel.Based on the experience from the pilot experiment, we estimate that a minimum recording time of 13–15 months in favourable weather conditions (April–September) is required to record enough data to map the spreading plume with surface waves, and to produce a tomographic image to a depth of 1000 km using body waves. This can be achieved by a continuous deployment of at least  20 months, or by two or three deployments during the spring and summer of consecutive years.     

Xenolith evidence for polybaric melting and stratification of the upper mantle beneath South China

Mantle xenoliths from Hainan and Qilin, South China have been studied to constrain the nature of the upper mantle and mantle processes beneath a continental margin. The extremely low Ti (160–245 ppm) contents in clinopyroxenes from some spinel lherzolites, indicative of high degrees of partial melting are inconsistent with the relatively high clinopyroxene modes (7.4–13%) in these samples. This inconsistency could be due to polybaric melting that started in the garnet stability field, then, after the breakdown of garnet to pyroxene and spinel, continued in the spinel stability field. Polybaric melting, due to adiabatic decompression of upwelling mantle, would leave a residual mantle in which the degree of depletion decreases with depth. The predicted stratified lithospheric mantle is evidenced by the negative correlation between the forsterite content in olivine and the equilibration temperature, proportional to the depth in the lithosphere from which the xenolith was derived. The lower part of the lithospheric mantle beneath South China consists predominantly of fertile and moderately depleted peridotites, which are either devoid of LREE enrichment, or show the trace element signature of incipient metasomatism, and plot within the Phanerozoic mantle domain. In contrast, the upper part of the mantle contains harzburgite and cpx-poor lherzolite, which are strongly affected by metasomatism of melt/fluid of highly variable composition. The anomalously high orthopyroxene mode (up to 47%) makes some of these refractory samples compositionally similar to the Proterozoic/Archean mantle. Their low equilibrium temperature (800–900 °C) points to the presence of old lithospheric relicts in the uppermost mantle beneath South China. Such lithosphere architecture may have resulted from partial replacement of Archean–Proterozoic lithosphere by asthenosphere that rose adiabatically subsequent to lithospheric thinning during the Cenozoic.     

Rayleigh wave tomography in the North Atlantic: high resolution images of the Iceland, Azores and Eifel mantle plumes

Presented in this paper is a high resolution S_v-wave velocity and azimuthal anisotropy model for the upper mantle beneath the North Atlantic and surrounding region derived from the analysis of about 9000 fundamental and higher-mode Rayleigh waveforms. Much of the dataset comes from global and national digital seismic networks, but to improve the path coverage a number of instruments at coastal sites in northwest Europe, Iceland and eastern Greenland was deployed by us and a number of collaborators. The dense path coverage, the wide azimuthal distribution and the substantial higher-mode content of the dataset, as well as the relatively short path-lengths in the dataset have enabled us to build an upper mantle model with a horizontal resolution of a few hundred kilometers extending to 400 km depth. Low upper mantle velocities exist beneath three major hotspots: Iceland, the Azores and Eifel. The best depth resolution in the model occurs in NW Europe and in this area low S_v-velocities in the vicinity of the Eifel hotspot extend to about 400 km depth. Major negative velocity anomalies exist in the North Atlantic upper mantle beneath both Iceland and the Azores hotspots. Both anomalies are, above 200 km depth, 4–7% slow with respect to PREM and elongated along the mid-Atlantic Ridge. Low velocities extend to the south of Iceland beneath the Reykjanes Ridge where other geophysical and geochemical observations indicate the presence of hot plume material. The low velocities also extend beneath the Kolbeinsey Ridge north of Iceland, where there is also supporting geochemical evidence for the presence of hot plume material. The low-velocity upper mantle beneath the Kolbeinsey Ridge may also be associated with a plume beneath Jan Mayen. The anomaly associated with the Azores extends from about 25°N to 45°N along the ridge axis, which is in agreement with the area influenced by the Azores Plume, predicted from geophysical and geochemical observations. Compared to the anomaly associated with Iceland, the Azores anomaly is elongated further along the ridge, is shallower and decays more rapidly with depth. The fast propagation direction of horizontally propagating S_v-waves in the Atlantic south of Iceland correlates well with the east–west ridge-spreading direction at all depths and changes to a direction close to NS in the vicinity of Iceland.     

华南东南部上地幔远震P波速度结构及意义

本研究利用114个固定台站记录的121个远震事件,以钦杭结合带为中心,采用天然地震层析成像构建了华南东南部上地幔P波速度结构模型。研究结果表明：（1）钦杭结合带、武夷成矿带以及南岭成矿带的深部结构存在着差异,说明3个成矿带经历了不同的构造演化过程;（2）江绍断裂的上地幔中存在着低速异常,推测该低速异常为从地幔过渡带或者下地幔上涌的热物质,与钦杭结合带和武夷成矿带的成矿作用有着密切的关系;（3）下扬子地区上地幔底部的高速异常可能为拆沉的岩石圈,而华夏板块上地幔顶部的高速异常则有待进一步研究。本研究的结果为认识华南东南部的深部结构提供了新的证据。     

A multi-scale approach to study the lithospheric structure of the southern Upper Rhine Graben: from seismic tomography through reflection seismics to surface geology

This paper integrates the results of different techniques—local and regional travel time tomography, reflection seismics, and surface geology. With this integration of different techniques, working on different scales, it is possible to derive a comprehensive picture of the present-day structures in the lithosphere of the Upper Rhine Graben. It is shown that the stucture of the lithosphere is dominated by structures related to the Variscan orogeny. Late stage strike-slip reactivation of the internal faults of the Rhine Graben is observed in the field. This reactivation is of dominant influence on the geomorphology in the southern end of the Upper Rhine Graben.