Crustal transect across the North Atlantic

Two dimensional crustal models derived from four different ocean bottom seismographic (OBS) surveys have been compiled into a 1,580 km long transect across the North Atlantic, from the Norwegian Møre coast, across the extinct Aegir Ridge, the continental Jan Mayen Ridge, the presently active Kolbeinsey Ridge north of Iceland, into Scoresby Sund in East Greenland. Backstripping of the transect suggests that the continental break-up at ca. 55 Ma occurred along a west-dipping detachment localized near the western end of a ca. 300 km wide basin thinned to less than 20 km crustal thickness. It is likely that an east-dipping detachment near the present day Liverpool Land Escarpment was active during the late stages of continental rifting. A lower crustal high-velocity layer (7.2–7.4 km/s) interpreted as mafic intrusions/underplating, was present beneath the entire basin. The observations are consistent with the plume hypothesis, involving the Early Tertiary arrival of a mantle plume beneath central Greenland and focused decompression melting beneath the thinnest portions of the lithosphere. The mid-Eocene to Oligocene continental extension in East Greenland is interpreted as fairly symmetric and strongly concentrated in the lower crustal layer. Continental break-up which rifted off the Jan Mayen Ridge, occurred at ca. 25 Ma, when the Aegir Ridge became extinct. The first ca. 2 m.y. of oceanic accretion along the Kolbeinsey Ridge was characterized by thin magmatic crust (ca. 5.5 km), whereas the oceanic crustal formation since ca. 23 Ma documents ca. 8 km thick crust and high magma budget.     

南沙西部海域伸缩型右旋走滑双重构造系统及其动力学过程

万纳断裂带为一典型的右旋走滑系统,由其南段前锋的拉奈—沙捞越走滑－收缩叠瓦扇、北段尾端的南海西南次海盆西南端走滑－伸展叠瓦扇和中段的万安盆地走滑－拉分双重构造所组成,其动力主要源自中生代末以来华南—印支陆缘岩石圈的拆沉作用和南海海底扩张,它的走滑拉分作用直接导致了万安盆地的产生,对该海域油气等资源的形成与聚集起了重要的控制作用。     

Seismic amplitude tomography for crustal attenuation beneath China

Amplitude tomography reconstructs seismic attenuation directly from recorded wave amplitudes. We have applied the tomography to amplitude data reported in the 'Annual Bulletin of Chinese Earthquakes' and interpreted the regionally varying crustal attenuation in terms of tectonics. The seismic amplitudes were originally recorded for determining the M _L and M _S magnitudes. They generally correspond to the maximum amplitudes of the horizontal components of the short-period S waves and intermediate-period Rayleigh waves. Both sets of measurements are sensitive to crustal structure. The peak amplitudes from M _L amplitudes spread spherically with significant dispersion and scattering. M _S amplitudes show cylindrical spreading with little dispersion. Average crustal Q values for attenuation at 1 Hz are 737 and 505 for M _L and M _S, respectively, with substantial regional variations. Frequency dependence in the attenuation is also indicated. Regions with the lowest attenuation (high Q values) are beneath the south China Block, Sichuan Basin, Ordos Platform, the Daxinganling and the Korea Craton. These tend to be tectonically inactive regions, which are generally dominated by intrusive and cratonic rocks in the upper crust. Regions with the highest attenuation (low Q values) are beneath Bohai Basin, Yunnan, eastern Songpan-Ganzi Terrain, margins of the Ordos platform and the Qilian Shan. These are predominantly active basins, grabens and fold belts. The continental margin also highly attenuates both S and surface waves.     

Surface wave array tomography in SE Tibet from ambient seismic noise and two-station analysis – II. Crustal and upper-mantle structure

We determine the 3-D shear wave speed variations in the crust and upper mantle in the southeastern borderland of the Tibetan Plateau, SW China, with data from 25 temporary broad-band stations and one permanent station. Interstation Rayleigh wave (phase velocity) dispersion curves were obtained at periods from 10 to 50 s from empirical Green's function (EGF) derived from (ambient noise) interferometry and from 20 to 150 s from traditional two-station (TS) analysis. Here, we use these measurements to construct phase velocity maps (from 10 to 150 s, using the average interstation dispersion from the EGF and TS methods between 20 and 50 s) and estimate from them (with the Neighbourhood Algorithm) the 3-D wave speed variations and their uncertainty. The crust structure, parametrized in three layers, can be well resolved with a horizontal resolution about of 100 km or less. Because of the possible effect of mechanically weak layers on regional deformation, of particular interest is the existence and geometry of low (shear) velocity layers (LVLs). In some regions prominent LVLs occur in the middle crust, in others they may appear in the lower crust. In some cases the lateral transition of shear wave speed coincides with major fault zones. The spatial variation in strength and depth of crustal LVLs suggests that the 3-D geometry of weak layers is complex and that unhindered crustal flow over large regions may not occur. Consideration of such complexity may be the key to a better understanding of relative block motion and patterns of seismicity.     

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基于物元可拓的河南省柘城县浅层地下水质量综合评价

该文采用物元可拓法对2008年和2019年河南柘城县浅层地下水质量进行综合评价,并与综合评分法、单指标评价法的评价结果进行对比,分析物元可拓法的可靠性及水质变化趋势。研究表明:柘城县浅层地下水综合质量等级总体好于综合评分法和单指标评价法评价结果,绝大多数样品点综合质量等级优于Ⅲ类,Ⅳ类、Ⅴ类地下水质量影响因素主要为F-和总硬度;对比2008年与2019年水质变化,柘城县浅层地下水具有Ⅰ类水向Ⅱ类水、Ⅳ—Ⅴ类水向Ⅲ—Ⅳ类水变化的趋势,与2008年样品的物元可拓指数预测结果一致。物元可拓法将综合关联度与可拓指数相结合,既客观地反映地下水水质总体状况,又定性地分析样本水质的变化趋势,使评价结果更接近于实际情况,评价方法在水质评价领域具有应用价值和研究意义。     

清水沟北汊流路入海泥沙对东营港影响的数值分析

本文用平面二维潮流泥沙数学模型,模拟了清水沟流路北汊入海口海域的潮流泥沙扩散和河口淤积延伸方向等问题。在此基础上从流速场分布,泥沙扩散浓度分布及淤积厚度零米线的范围及河口淤积延伸方向等方面探讨了入海泥沙对东营港的影响。结论认为如果按此口门入海,入海泥沙不会对黄河海港产生直接淤积影响。     

一种基于激光点云数据的房屋轮廓线提取方法

提出了一种基于激光点云数据提取房屋轮廓线的方法.首先在屋顶激光点云数据中搜索平面距离最远的两个激光点,选择其一为起始点,根据相邻边缘点连线的一侧不存在激光点的原则,检测屋顶所有的边缘点;然后对边缘点分组,利用最小二乘直线拟合方法分别拟合各条轮廓线并进行规则化;最后,选择轮廓线最外侧的激光点,对各条轮廓线进行平移外扩,并通过相邻轮廓线相交确定屋顶角点的坐标.实验结果表明,与通常采用的对屋顶激光点云构建不规则三角网(TIN)获取边缘点的方法相比,该算法运行效率明显提高,检测到的屋顶边缘点数量更多,获取的屋顶轮廓线精度更高.     

The Mesoarchean emergence of modern-style subduction

Well-preserved volcanic sequences span the Paleoarchean to Neoarchean evolution of the Pilbara Craton, in northwestern Australia. This region provides the best physical evidence bearing on the stage of Earth's history when modern-style tectonic processes began. Paleoarchean assemblages in the eastern nucleus of the craton (the 3.51–3.24 Ga Pilbara Supergroup) show few features that can reasonably be interpreted as evidence for modern-style subduction processes. Incompatible trace element-enriched felsic volcanic horizons show geochemical evidence for the interaction between mafic magmas and crust, but this evidence, on its own, can equally well be interpreted in terms of either a subduction-enriched mantle source or local and limited assimilation of felsic crust into the voluminous tholeiitic magmas that dominate the Pilbara Supergroup. Viewed in context within the thick autochthonous and consistently upward-younging Pilbara Supergroup, these felsic units are most likely related to the same plume-dominated processes that formed the basalts that dominate the supergroup. It is very unlikely that modern-style plate tectonic processes played any role in the Paleoarchean evolution of the Pilbara Craton, although some form of non-uniformitarian (e.g. flat) subduction process may have operated.In stark contrast, the Mesoarchean units of the West Pilbara Terrane and the late-tectonic basins that cover that boundary between the West and East Pilbara Terranes, show clear evidence for modern-style convergent margin processes. Igneous rocks in this belt, which flanks the old eastern cratonic nuclei, have enriched geochemical signatures that cannot be accounted for by crustal contamination. This region is also characterised by a linear magmatic and structural fabric, by the presence of lithologically and geochronologically exotic belts, and by the presence of a broad belt of isotopically more juvenile crust. The collective strength of these arguments provides compelling evidence that a modern-style oceanic arc fringed the East Pilbara Terrane at 3.12 Ga and accreted to that terrane by 2.97 Ga. These assemblages mark the minimum age for the birth of modern-style plate subduction process.     

Submarine canyons on the north of Chiwei Island:influenced by recent extension of the southern Okinawa Trough

Based on new multibeam bathymetric data and about 300 km long single seismic profiles, three topographic units were identified： the canyons, fractural valley and submarine terrace on the north of Chiwei Island where is a structural transition zone between the southern trough and the middle trough. The Chiwei Canyon and the North Chiwei Canyon are two of the largest canyons in the East China Sea （ECS） slope. Topographic features and architectures of them are described. The study shows that both of them are originated along faults. The evolution and spatial distribution of topographic units in the study area are controlled mainly by three groups of faults which were formed and reactive in the recent extensional phase of Okinawa Trough. The Chiwei Canyon was initia- ted during the middle Pleistocene and guided by F4 that is a N--S trending fault on the slope and F1, a large NW--SE trending fault on the trough. The pathway migration from the remnant channel to the present one of Chiwei Canyon is the result of uplift of tilted fault block that is coupled to the recent extension movements of the southern trough. The submarine terrace is detached from the ECS slope by the NEE -trending fault. The North Chiwei Canyon, developing during the late Pleistocene, is guided by FS, a N-S trending fault, diverted and blocked by the submarine terrace.