Oligocene-early Miocene transgressive cover of the Betic-Rif Internal Zone. Revision of its geologic significance     

Francisco Serrano  Carlos Sanz de Galdeano  Khalil El Kadiri  Antonio Guerra-Merchán  Angel Carlos López-Garrido  Manuel Martín-Martín  Rachid Hlila 《Eclogae Geologicae Helvetiae》2006,99(2):237-253

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Contribution to a geodynamic reconstruction of the Anti-Atlas (Morocco) during Pan-African times with the emphasis on inversion tectonics and metallogenic activity at the Precambrian–Cambrian transition   总被引：3，自引：0，他引：3  

Dominique Gasquet  Gilles Levresse  Alain Cheilletz  Moulay Rachid Azizi-Samir  Abdellah Mouttaqi 《Precambrian Research》2005,140(3-4):157-182

New geochronological analyses (U–Pb SIMS zircon ages) have yielded ages of 552 ± 5 Ma for the Bou Madine rhyolitic dome (Ougnat, eastern Anti-Atlas), 543 ± 9 Ma for the Tachkakacht rhyolitic dyke (Saghro–Imiter, eastern Anti-Atlas), and 531 ± 5 Ma for the Aghbar trachytic sill (Bou Azzer, central Anti-Atlas). Inherited zircon cores from the Aghbar trachytic sill and from the Bou Madine rhyolitic dome have been shown to be of Statherian age (ca. 1600–1800 Ma) and Palæoproterozoic (>2100 Ma) age, respectively, suggesting that a significantly older protolith underlies the Pan-African rocks in the Central and Eastern Anti-Atlas. Granodiorites and rhyolites from the Saghro–Imiter area have similar low ⁸⁷Sr/⁸⁶Sr (0.702–0.706) and ¹⁴³Nd/¹⁴⁴Nd (0.5116–0.5119) initial ratios, suggesting a mixture of mantle and lower crust sources. This can also be inferred from the low ¹⁸⁷Os/¹⁸⁸Os ratios obtained on pyrite crystals from the rhyolites.A recently published lithostratigraphic framework has been combined with these new geochemical and geochronological data, and those from the literature to produce a new reconstruction of the complex orogenic front that developed at the northern edge of the Eburnian West African craton during Pan-African times. Three Neoproterozoic magmatic series can be distinguished in the Anti-Atlas belt, i.e., high-K calc-alkaline granites, high-K calc-alkaline to shoshonitic rhyolites and andesites, and alkaline-shoshonitic trachytes and syenites, which have been dated at 595–570, 570–545 and 530 Ma, respectively.The accretion of the Pan-African Anti-Atlas belt to the West African super continent (WAC) was a four-stage event, involving extension, subduction, moderate collision and extension. The calc-alkaline magmatism of the subduction stage was associated with large-scale base metal and gold mineralisation. Metallogenic activity was greatest during the final extensional stage, at the Precambrian–Cambrian boundary. It is characterised by world-class precious metal deposits, base–metal porphyry and SEDEX-type occurrences.  相似文献   

Influence of a Brazilian sewage outfall on the toxicity and contamination of adjacent sediments     

Abessa DM  Carr RS  Rachid BR  Sousa EC  Hortelani MA  Sarkis JE 《Marine pollution bulletin》2005,50(8):875-885

The submarine sewage outfall of Santos (SSOS) is situated in the Santos Bay (São Paulo, Brazil) and is potentially a significant source of contaminants to the adjacent marine ecosystem. The present study aimed to assess the influence of SSOS on the sediment toxicity and contamination at Santos Bay. At the disposal site, sediments tended to be finer, organically richer and exhibited higher levels of surfactants and metals, sometimes exceeding the “Threshold Effect Level” values. The SSOS influence was more evident toward the East, where the sediments exhibited higher levels of TOC, total S and metals during the summer 2000 sampling campaign. Sediment toxicity to amphipods was consistently detected in four of the five stations studied. Amphipod survival tended to correlate negatively to Hg, total N and % mud. This work provides evidence that the SSOS discharge affects the quality of sediments from Santos Bay, and that control procedures are warranted.  相似文献   

Unsteady convective atmospheric vortices     

Peter G. Bellamy-Knights  Rachid Saci 《Boundary-Layer Meteorology》1983,27(4):371-386

The convective atmospheric vortex models of Kuo (1966) and Kendall (1978) are extended to allow the formulation of similarity equations for the unsteady vortex arising from an ambient flow with unstable stratification and rotation. In particular, a new analytical solution is found which allows flow at large radius that is more physically realistic than in any earlier solutions. Additional unsteady solutions are obtained numerically but at large radius these are subject to the limitations of earlier solutions.  相似文献   

使用计算机作图显示三维地质表面     

Sow.  WTC 郭晓琰 《地球科学进展》1989,4(2):56-57

由于目前越来越多的地质信息由计算机来处理,计算机图形技术也越来越多的用来显示地质信息的几何状态。本文表明用于表面成像的计算机图形技术已经应用于许多地质学分支。 地质学是一门包括从原子到天体不同尺度自然结构的学科,对地质学家来说,无论尺度是单个矿物还是整个行星,使它们的三  相似文献   

地壳磁化强度模型和居里等温面   总被引：1，自引：0，他引：1       下载免费PDF全文

徐元芳 Barr.  DR 《地球物理学报》1997,40(4):481-486

利用卫星观测的长波磁异常，用等效偶极源方法推导了中国地区的视磁化强度分布．因卫星的高度远大于磁性地壳的厚度，将视磁化强度转换成磁化强度的垂直积分，它代表地壳内磁性物质的区域变化，利用视磁化强度与地表热流相应关系，计算了中国的新疆和东部一些地区居里等温面的深度．新疆地区的居里面深度为35-50km，其分布形态与塔里木盆地和准噶尔盆地的地貌比较相似；中国东部一些地区居里面深度在20-40km之间，与一些作者用航磁等数据得到的居里深度十分接近．  相似文献   

Coupled atmospheric and marine palaeoclimatic reconstruction for the last 35 ka in the Sele Plain–Gulf of Salerno area (southern Italy)     

Valentino Di Donato  Paola Esposito  Elda Russo-Ermolli  Arianna Scarano  Rachid Cheddadi   《Quaternary International》2008,190(1):146

Planktonic foraminifera and pollen data from core GNS84-C106 (Gulf of Salerno, Tyrrhenian Sea) were analysed through the Modern Analogue Technique, Constrained Cluster Analysis and relative variation biplots. A long period of mild climate, centred around 25 ka BP, is evident in both marine and continental reconstructions. The cooling phase from 17 to 14.7 ka BP, correlated to the H1 Heinrich event, is indicated by a sea surface temperature (SST) decrease, which roughly coincides with the cold-arid phase identified by annual and January temperatures. A rapid increase in atmospheric temperatures and precipitation, culminating at 13.8 ka BP, marks the BA cronozone. The corresponding increase in summer and winter SSTs, of 11 and 6.5 °C, respectively, occurred over 600 years. The beginning of the YD, centred around 12.5 ka BP, is marked by a decrease in summer and winter SSTs of, respectively, 4.5 and 3.5 °C in one century. The atmospheric evidence of the YD is primarily reflected in low January temperatures, reaching −6 °C, the lowest values ever experienced in the analysed time interval. The Late Glacial–Holocene transition is clearly recorded in both the continental and marine realms. From 11.5 to 9 ka BP, atmospheric temperatures record a period of substantial stability followed by a drop at 8.9 ka BP, which chronologically fall within the first RCC event (9–8 ka BP) of Mayewski et al. [2004. Holocene climate variability. Quaternary Research 62, 243–255], in correspondence with a phase of relatively high seasonality, indicated by foraminifera.  相似文献   

Shear Strength Response of a Geotextile-Reinforced Chlef Sand: A Laboratory Study     

Ismail Benessalah  Ahmed Arab  Pascal Villard  Khayra Merabet  Rachid Bouferra 《Geotechnical and Geological Engineering》2016,34(6):1775-1790

During the last earthquake that occurred in Chlef (El Asnam 1980, Algeria), a significant decrease in the shear strength has caused major damages to several civil and hydraulic structures (earth dams, embankments, bridges, slopes and buildings), especially for the saturated sandy soil of the areas near Chlef valley. This paper presents a laboratory study of drained compression triaxial tests conducted on sandy soil reinforced with horizontal layers of geotextile, in order to study the influence of geotextile layer characteristics both on shear stress–strain and on volumetric change–strain. Tests were carried out on medium and dense sand. The experimental programme includes some drained compression tests performed on reinforced sand samples, for different values of the geotextile layers number (N _g), of confining pressure (\( \sigma_{\text{c}}^{\prime } \)) and relative density (D _r). The test results have shown that the contribution of the geotextile at low values of the axial strain (ε ₁) is negligible, for higher values of (ε ₁); geotextile induces a quasi-linear increase in the deviator stress (q) and leads to an increase in the volume contractiveness within the reinforced samples. A negligible influence of geotextile layers number (N _g) on the stress–strain behaviour and the volumetric change has been shown, when normalized with N _g. The results indicate that the contribution of geotextile to the stress–strain mobilization increases with increasing confining pressure, while its contribution to the volume contraction decreases with the increase in the confining pressure.  相似文献   

Behavior of Loose Silty Sand of Chlef River: Effect of Low Plastic Fine Contents and Other Parameters     

Mohammed Chemmam  Mostefa Belkhatir  Rachid Bouferra 《Marine Georesources & Geotechnology》2016,34(4):384-394

This article presents a laboratory study of static behavior of silty-sand soils. The objective of this laboratory investigation is to study the effect of initial confining pressures and fines content on the undrained shear strength (known as liquefaction resistance) response, pore pressure, and hydraulic conductivity of sand–silt mixtures. The triaxial tests were conducted on reconstituted saturated silty-sand samples at initial relative density D_r = 15% with fines content ranging from 0 to 50%. All the samples were subjected to a range of initial confining pressures (50, 100, and 200 kPa). The obtained results indicate that the presence of low plastic fines in sand–silt mixture leads to a more compressible soil fabric, and consequently to a significant loss in the soil resistance to liquefaction. The evaluation of the data indicates that the undrained shear strength can be correlated to fines content (F_c), inter-granular void ratio (e_g), and excess of pore pressure (Δu). The undrained shear strength decreases with the decrease of saturated hydraulic conductivity and the increase of fines content for all confining pressures under consideration. There is a relatively high degree of correlation between the peak shear strength (q_peak) and the logarithm of the saturated hydraulic conductivity (k_sat) for all confining pressures.  相似文献   

Spatial and morphometric relationships of submarine landslides offshore west and southwest Iberia     

Gamboa  Davide  Omira  Rachid  Terrinha  Pedro 《Landslides》2022,19(2):387-405

Landslides - Submarine landslides are a ubiquitous geohazard in the marine environment and occur at multiple scales. Increasing efforts have been made during the last decade to catalogue and...  相似文献