The concealed Caledonide basement of Eastern England and the southern North Sea — A review

Summary Field mapping, analysis of borehole core and studies of geophysical potential field and seismic data can be used to demonstrate the existence of a number of distinct crustal blocks within Eastern Avalonia beneath eastern England and the southern North Sea. At the core of these blocks is the Midlands Microcraton which is flanked by Ordovician volcanic arc complexes exposed in Wales and the Lake District. A possible volcanic arc complex of comparable age in eastern England is concealed by late Palaeozoic and Mesozoic cover. These volcanic arc complexes resulted from subduction of oceanic lithosphere beneath Avalonia prior to collision with Baltica and Laurentia in late Ordovician and Silurian time, respectively. The nature of the crust north and east of the concealed Caledonides of Eastern England and south of the lapetus Suture/Tornquist Sea Suture, which forms the basement to the southern North Sea, is unclear. Late Ordovician metamorphic ages from cores penetrating deformed metasedimentary rocks on the Mid-North Sea High suggest these rocks were involved in Avalonia-Baltica collision before final closure of the lapetus Ocean between Laurentia and Avalonia.     

Crust and upper mantle structure of the Ailao Shan-Red River fault zone and adjacent regions

Using arrival data of the body waves recorded by seismic stations, we reconstructed the velocity structure of the crust and upper mantle beneath the southeastern edge of the Tibetan Plateau and the northwestern continental margin of the South China Sea through a travel time tomography technique. The result revealed the apparent tectonic variation along the Ailao Shan-Red River fault zone and its adjacent regions. High velocities are observed in the upper and middle crust beneath the Ailao Shan-Red River fault zone and they reflect the character of the fast uplifting and cooling of the metamorphic belt after the ductile shearing of the fault zone, while low velocities in the lower crust and near the Moho imply a relatively active crust-mantle boundary beneath the fault zone. On the west of the fault zone, the large-scale low velocities in the uppermost mantle beneath western Yunnan prove the influence of the mantle heat flow on volcano, hot spring and magma activities, however, the upper mantle on the eas     

Subcrustal structure of the black sea basin from seismological data

The P-wave travel time data from the earthquakes offshore and onshore around the Black Sea are used for the tomographic reconstruction of the three-dimensional (3D) velocity distribution in the lithosphere of the region. The preliminary refinement of the foci parameters (the coordinates and origin time) has reduced the random errors in the travel-time data. The earthquake data were supplemented by the previous deep seismic sounding (DSS) data on the profiles in Crimea and offshore off the Black Sea. The dataset included more than 4000 travel times overall. In order to eliminate the crustal effect, the travel times were reduced to a surface at a depth of 35 km corresponding to the mean Moho depth in the region. The improved crustal model was used for removing the contribution of the crust from the initial data. The new tomography method, which was recently developed by one of the authors and which relies on the assumption of smoothness of the lateral velocity variations, was applied for reconstructing the velocity structure of the upper mantle beneath the Black Sea up to a depth of 95 km. The lateral velocity variation maps at different depths and the vertical velocity distributions along the meridional and sublatitudinal cross sections across the Black Sea were constructed. High velocities were revealed in the subcrustal lithosphere, and the structural difference below two subbasins—the West Black Sea (WBS) and the East Black Sea (EBS) ones—was established. It shows that the high-velocity body below the WBS is located deeper than below the EBS and is distinguished by higher velocities. Based on these results, it is concluded that the lithosphere beneath the Black Sea has a continental origin.     

Development of basins in the Inner Moray Firth and the North Sea by crustal extension and dextral displacement of the Great Glen Fault

Reflection seismic data provide evidence that Mesozoic dextral movements along the Great Glen Fault line have had an important influence on the development of the Inner Moray Firth Basin. Geophysical evidence further indicates that deep structure beneath the inner basin is dissimilar to that beneath the outer part and Viking and Central Grabens in the North Sea. Tectonic development of the inner basin can nevertheless be fitted into a pattern of North Sea extensional movements which led to the formation of the graben system with which the major North Sea hydrocarbon resources are associated.     

环渤海地区Pn波速度结构与各向异性

环渤海地区位于华北克拉通的中东部,是岩石圈破坏和减薄的主要地区,同时也是我国大陆东部强震的多发区和油气田产区,一直是国内外学者研究的重点区域.本研究利用环渤海地区1980—2015年期间中国地震台网高质量Pn波到时数据,反演得到环渤海地区Pn波速度结构及各向异性.结果显示,环渤海地区上地幔顶部的Pn波速度结构存在明显的横向不均匀性,且与区域地质构造有一定相关性.在地形隆起区,如太行山隆起、燕山隆起、鲁西隆起、胶辽隆起及苏鲁褶皱带地区,呈现为低波速异常,说明这些隆起区下方的上地幔存在热物质上涌,而凹陷地区,如华北盆地、南黄海北部盆地和南黄海南部盆地,则表现为高波速异常,说明这些凹陷地区上地幔顶部岩石圈强度较大.地壳内强震主要发生于低波速异常区和高低波速异常过渡带上,说明华北地区地壳强震的发生有可能受到上地幔深部构造的影响.太行山造山带地区Pn波各向异性快波方向为近NNE向,苏鲁褶皱带区域的Pn波各向异性快波方向为近NE向,与断裂带的走向基本一致,表明在地壳形变剧烈的地区,可能受上地幔顶部的深部动力学影响较大.华北盆地的北部和南部各向异性方向存在差异,可能与岩石圈的厚度及热状态的不均匀性有关.     

Summary of electromagnetic investigations in the western part of the Black Sea

Two geoelectric models of the transition zone between the continent and the Black Sea basin were constructed from a variety of independent exploratory methods using natural electromagnetic fields. The models presented characterize some important features of the deep structure of the sea-shore transition zone. In particular, geoelectric properties of the lithosphere beneath the Black Sea basin are found to be approximately oceanic.     

黄海及其邻近地区的Pn波速度与各向异性

利用中国东部地震台网和ISC 报告1980~2004年的地震走时数据,反演了黄海及其邻近地区的Pn波速度和各向异性,根据岩石层地幔的横向非均匀性分析了区域地质构造的深部特点.Pn波速度的变化与区域地质构造有一定的对应关系,黄海地区上地幔顶部的P波平均速度较高,没有发现明显的低速异常,表明上地幔顶部不存在大范围的地幔扰动.速度异常的分布表明,南黄海东部和西部有可能分属于不同的构造块体,其间的分界大致对应于南北走向的黄海东部断裂带,具有相对较低的Pn波速度.边界东、西两侧的Pn波各向异性存在明显的差异：南黄海西部Pn波的快波方向以北东—北北东方向为主,反映了海区内部扬子块体向北运动产生的构造变形;南黄海东部Pn波的快波方向为南北方向,与黄海东部断裂带的走向基本一致,说明黄海东部和西部之间存在一个深达岩石层地幔的南北向转换边界.结合相关资料估计黄海东部断裂带在中生代时期发生了右旋走滑运动,以响应中国东部郯庐断裂带的大规模左旋剪切以及南黄海扬子块体的向北嵌入.     

西藏高原上地幔的精细结构与构造--地震层析成像给出的启示

用布置在亚东—格尔木的164个流动地震台站记录的926个远震事件的24241条射线,进行远震P波层析成像处理,高分辨率的西藏高原上地幔的速度结构图,显示了印度巨厚地幔岩石圈在向高原之下推进的过程中,在高喜马拉雅之下拆分成上、下两层,这是发生的第一次拆沉. 下层从高喜马拉雅以下约以22°的角度向高原北部插入到350km 深;而其上层则向北伸展直到雁石坪,并构成了高原薄的地幔岩石圈. 在雁石坪北（33.7°N）,当其与亚洲大陆岩石圈地幔相遇后发生断离并下沉. 再次证实了五道梁（35.27°N）深部低速体的存在,本区内地壳内低速物质可能与上述运动有联系,反映了深层热物质的上涌.     

Seismic wave attenuation in the upper Mantle beneath the Aegean region

A detailed study of P and S waves from earthquakes located along the Hellenic arc and recorded by the Greek seismological network, shows an abnormal distribution of the seismic waves in the central Aegean Sea. The data indicate a zone of anomalously high seismic wave attenuation in the Upper Mantle beneath the Cyclades plateau and the inner part of the volcanic arc. Several independent observations suggest the presence of magma beneath the Cyclades massif.In addition, geophysical data indicate the presence of low-density and rigidity material. Theoretical consideration of the propagation of elastic waves corroborate the observed absence of shear waves.     

The distribution of split shear-wave parameters in the backarc region of the South Kuril subduction zone

This paper reports a study in the distribution of parameters of split shear waves excited by deep earthquakes in the Sakhalin and Hokkaido area, with the orientation of the axes of symmetry in the earth being estimated assuming a viscoelastic anisotropic model for the mantle. A mantle flow along NW 310° ± 20° has been identified beneath the Japan Sea. The flow is inclined at an angle of 20–30° relative to the horizontal plane and is consistent with the motion of the Pacific plate. Beneath the southern Sea of Okhotsk the fast a-axis [100] of olivine is oriented NE 30 ± 15°, nearly parallel to the trend of the Kuril arc, while the c-axis [001] is inclined at ～35° relative to the horizontal plane. Bearing in mind the increased heat flow in the region, we assume the development of a mantle flow along the olivine c-axis at NW 300° ± 20° and with the shear plane [010] in the conditions of partial mantle melting (the B-type LPO). The lowest anisotropy (1–2.5%) was identified beneath Sakhalin and the greatest (3–5%) beneath the Japan Sea. An increasing degree of anisotropy is also noted to occur with an increasing depth of focus (down to 350 km).     

The lithosphere structure and geodynamics of the West and East Black Sea Basins

The velocity structure of the Black Sea lithosphere has been studied using the local seismic tomography method based on the Backus-Gilbert approach and applied for a quite large amount of seismological data. As seismic sources, we used the earthquakes that occurred within the Black Sea and adjoining regions and generated seismic waves recorded by seismic stations around the Black Sea. This yielded information on the 3D distribution of P-waves within the most heterogeneous and poorly investigated uppermost layer of the Black Sea region (down to a depth of 60 km). The interpretation of these results, together with new data on the velocity structure of the crust of the Black Sea led to the conclusion about the different lithosphere beneath the West and the East Black Sea Basins. This can be explained by origination of the depressions in the western and eastern parts of the Black Sea at different microplates and by specific features in their development at rifting and post-rifting stages.     

Origin of the Willaumez-Manus Rise,Papua New Guinea

The aseismic Willaumez-Manus Rise on the Bismarck Sea floor separates the Manus Basin from the New Guinea Basin. The rise does not appear to be an extinct spreading axis, or a remnant arc, but may be the result of excess magmatism possibly related to an inferred mantle hot spot beneath St. Andrew Strait. A preferred interpretation is that the rise is the raised edge of the New Guinea Basin, formed in response to a thermal anomaly beneath the extensional Manus Basin which formed later than the New Guinea Basin.     

环渤海地区的地震层析成像与地壳上地幔结构

利用环渤海地区的天然地震P波到时资料,采用纬度和经度方向分别为05°×06°的网格划分,反演了该地区地壳上地幔的三维P波速度结构.初步结果表明,环渤海地区地壳上地幔的速度结构具有明显的横向不均匀性：京津唐地区地壳中上部的速度异常反映了浅表层的地质构造特征,造山带和隆起区对应于高速异常,坳陷区和沉积盆地对应于低速异常;地壳下部出现大规模的低速异常与华北地区广泛存在的高导层相对应,估计与壳内的滑脱层和局部熔融、岩浆活动有关;莫霍面附近的速度异常反映了地壳厚度的变化及壳幔边界附近热状态的差异;上地幔顶部大范围的低速异常可能是上地幔软流层热物质大规模上涌所致.     

利用远震接收函数研究黄海周边地区的地壳上地幔结构

利用宽频带流动台站(YSBSN)记录的远震波形数据和远震接收函数方法,反演了黄海东、西两侧地壳上地幔的S波速度结构.结果表明,莫霍面深度在30~38 km之间变化,位于中方一侧的JNN台下方地壳厚度最大,可以归因于华北板块和扬子板块的碰撞;韩方一侧的地壳厚度自北向南逐渐变厚,但仍然难以厘定朝鲜半岛南部潜在碰撞带的位置,这些问题的解决需要更大范围的流动台站观测.由于部分台站位于巨厚的沉积层和多孔的火山岩之上,与浅部构造的相关性使得接收函数表现出较大振幅的混响,从而影响了来自深部结构的转换震相.     

新生代渤海中部强烈沉降的物理条件和深部过程

新生代时期渤海中部的强烈沉降,是多种物理条件的共同作用结果,这些条件是由裂谷期和后裂谷期的深部过程产生成的.裂谷期在异常热地幔背景下的渤海地壳隆起、减薄、张裂,地幔热物质上升侵入地壳,莫霍界面位置升高,积累了重力势能;后裂谷期岩石圈（层）的冷却、收缩及下地壳的相变导致密度增大,加上巨厚沉积物的持续增生,使地壳处于重力不平衡状态,向下的垂直力远大于向上的浮托力,同时还有东部后退位移性板块边界和下地壳侧向流动的支持,使渤海中部成为下沉速率最快、沉降幅度最大的凹陷盆地.后裂谷期的早期的盆地下沉具有分散、局部性特征,表现为多个凹陷和凸起交替组合格局;晚期转变为大范围的整体沉降,显示重力均衡和补偿过程是从浅往深发展的.先存的郯庐断裂带对沉降的空间范围有局部边界控制性作用,其本身可能受到盆地发展的强烈改造影响.盆地的基本变形机制是上地壳的水平向脆性张破裂和垂直向或近垂直向的正断层-剪切破裂,地震震源机制解和大地震时的地表破裂表现的水平错动反映中、下地壳的走滑-平移型应力状态及相应的瞬间水平剪切破裂,它与上地壳残留的伸展、下沉相容并存于三维地壳体内.     

Seismicity in the vicinity of Ross Island, Antarctica

Earthquakes in the Ross Sea, Antarctica, are of two types: volcanic, and those which appear to be of tectonic origin. Volcanic events in the vicinity of Ross Island are associated exclusively with Mount Erebus, Ross Island; this volcano erupts regularly, generating several earthquakes per day whose characteristics are quite distinct from non-volcanic events. These nonvolcanic earthquakes are recognizable by their distinct P- and S-wave arrivals, and a lack of the high frequency, often monochromatic character typical of Erebus events.One hundred fifty-seven tectonic microearthquakes (M < 2.0) were recorded in 1983 and 1984 by the ten station network on Ross Island; these events were located using the least-squares routine, HYPOELLIPSE. Of these events, 106 have RMS residual traveltime errors of less than or equal to 0.6 seconds; they are clustered in the vicinity of Ross Island, but are not restricted to it. There is a linear trend of epicenters cutting across the island and continuing northward. Most activity seems to center beneath Mount Terra Nova, between Mount Erebus and Mount Terror. Mean depth for events is 8.2 km; however, depths are rather evenly distributed over a range of 0 to 25 km.Modelling based on Bouger gravity anomalies and seismic refraction studies indicates a depth to the Moho of about 40 km beneath the continent, shallowing to 27 km beneath the Ross Sea. This 27 km depth is approximately equal to the lower limit of the tectonic seismicity detected by the Erebus network; hence, events are of crustal origin. These data suggest, with the rift-type geochemistry of Erebus' magma, that the Ross Sea is a site of active crustal extension and rifting.     

声学探测技术在天津隐伏断层探测中的应用

利用单道地震等水上声学探测技术,对天津海河断裂的浅部构造和地层错断信息进行了探测。探测结果表明,在水深较深,相对宽阔的河道内该方法可以获得较好的结果。在天津塘沽地区,海河断裂的上断点深度位于河底面以下30m左右,对应的最新活动时代为Q_p~3—Q_h~1,这与前期的钻孔勘探结果基本吻合。在渤海近海海域,海河断裂呈一组NWW-NEE向分布的断裂带,其上断点距离海底的深度小于30m,断层断错和影响的地层年代为全新世早中期,断层活动特征与塘沽地区的基本一致。     

阿留申—阿拉斯加俯冲带及周边地区地幔过渡带结构研究

利用国家测震台网固定台站和"中国地震科学台阵探测"项目在南北地震带北段布设的宽频带流动台阵记录到的极远震事件,通过SS前驱波震相研究,获得了阿留申—阿拉斯加俯冲带东段及邻区下方410 km和660 km间断面的埋深和起伏形态特征.为增强对SS前驱波震相的识别,我们采用了时差校正和共反射点叠加分析.叠加结果显示,毗邻阿留...     

Anelastic structure of the upper mantle beneath the northern Philippine Sea

We investigated the upper mantle anelastic structure beneath the northern Philippine Sea region, including the Izu-Bonin subduction zone and the Shikoku Basin. We used regional waveform data from 69 events in the Pacific and the Philippine Sea slabs, recorded on F-net and J-array network broadband stations in western Japan. Using the S–P phase pair method, we obtained differential attenuation factors, δt^*, which represent the relative whole path Q. We conducted a tomographic inversion using 978 δt^* values to invert for a fine-scale (50–100 km) three-dimensional anelastic structure.

The results shows two high-Q regions (Q_P>1000) which are consistent with the locations of the Pacific and the Philippine Sea slabs. Also there is a low-Q (Q_P110) area extending to the deeper parts (350–400 km) of the model just beneath the old spreading center and the Kinan Seamount Chain in the Shikoku Basin. A small depth dependence of the laterally averaged Q_P was found, with values of 266 (0–250 km), 301 (250–400 km), and 413 (400–500 km).     

The lithosphere in the central-eastern Mediterranean area

The lithosphere beneath the central-eastern Mediterranean area has been investigated by the inversion of the regional dispersion relations derived from analysis of surface waves. It is possible to distinguish several types of crust with average S-wave velocities in the range 3.0–3.8 km/sec, and thicknesses varying from a minimum of about 30 km, which corresponds to the Apennines, Crete and Otranto Channel regions, to a maximum of about 51 km beneath the Ionian Sea, which can be considered as a submerged continent. Associated with these crustal features, large lateral variations have been detected in the lithosphere thickness, which varies from a minimum of about 30 km corresponding to the Tyrrhenian Sea and south of Crete to a maximum of about 130 km corresponding to south-eastern Alps and north-central Greece, while the sub-Moho S-wave velocity varies in the range 4.2–4.8 km/sec. The constraint furnished by our results to the geological-tectonic setting of the investigated area, characterized by the continent continent collision between Africa and Europe, is pointed out.Publication No. 405, P. F. Geodinamica, CNR, Roma, Italy.