Sur la présence d'une série molassique (de type série pourprée) au Sud-Est de l'Ahaggar (In Guezzam,Ahaggar, Algérie)Presence of molassic series (‘Série pourprée’ type) in the Southeast of the Ahaggar (In Guezzam,Ahaggar, Algeria)

In the western Ahaggar shield, the transition between the Precambrian and the Ordovician units is characterised by thick volcano-sedimentary series (‘Série pourprée’ of the Ahnet). This series, in part of Cambrian age, results from the demolition of the Panafrican belt. Similar series were known in grabens located between the West African craton and the Ahaggar, from the ‘Adrar des Iforas’, in the south, to the Ougarta belt, in the north. We describe in this study a new formation identical to the ‘Série pourprée’ of the Ahnet, but cropping out in the far Southeast of the Ahaggar (In Guezzam). This new datum improves the Panafrican belt configuration. To cite this article: H. Djellit et al., C. R. Geoscience 334 (2002) 789–794.     

Erzmikroskopische und mikroanalytische Untersuchungen an Manganerzkonkretionen aus dem pazifischen Ozean

Zusammenfassung Die Verteilung von Eisen, Mangan, Kupfer und Nickel in Manganerzkonkretionen wurde mit Hilfe der Elektronenstrahl-Mikroanalyse untersucht. Die einzelnen Manganknollen zeigen einen inhomogenen Aufbau aus Mn- und Fe-reichen Zonen, die sich erzmikroskopisch unterscheiden lassen. Auffallend sind die Korrelationen zwischen Mangan, Kupfer und Nickel auch im Mikronbereich. Die Zusammenhänge zwischen den spektralen Reflexionsfunktionen und der chemischen Zusammensetzung der Manganerzkonkretionen werden diskutiert.

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The distribution of iron, manganese, copper and nickel has been determinted in different zones of manganese nodules by electron probe analysis. A single nodule is characterized by the inhomogeneous distribution of these elements, and the correlation of Mn with Cu and Ni within certain zones. Concentrations of Mn, Cu and Ni of 41, 1.0 and 2.0 percent, respectively, have been found in layers containing 2.0 percent Fe only. On the other Land iron-rich layers are characterized by low contents of Mn, Cu and Ni. There is no considerable variation in the ratios of these elements with distance from the centre of an individual nodule. The correlation of reflectivity and chemical composition is emphasised.

     

Climate simulation over CORDEX Africa domain using the fifth-generation Canadian Regional Climate Model (CRCM5)

The new fifth-generation Regional Climate Model (CRCM5) was driven by ERA reanalyses for the period 1984–2008 over the African continent following the CORDEX experimental protocol. Overall the model succeeds in reproducing the main features of the geographical distribution and seasonal cycle of temperature and precipitation, the diurnal cycle of precipitation, and the West African Monsoon (WAM). Biases in surface temperature and precipitation are discussed in relation with some circulation defects noted in the simulation. In the African regions near the equator, the model successfully reproduces the double peak of rainfall due to the double passage of the tropical rainbelt, although it better simulates the magnitude and timing of the second peak of precipitation. CRCM5 captures the timing of the monsoon onset for the Sahel region but underestimates the magnitude of precipitation. The simulated diurnal cycle is quite well simulated for all of the regions, but is always somewhat in advance for the timing of rainfall peak. In boreal summer the CRCM5 simulation exhibits a weak cold bias over the Sahara and the maximum temperature is located too far south, resulting in a southward bias in the position of the Saharan Heat Low. The region of maximum ascent in the deep meridional circulation of the Hadley cell is well located in the CRCM5 simulation, but it is somewhat too narrow. The core of the African Easterly Jet is of the right strength and almost at the right height, but it is displayed slightly southward, as a consequence of the southward bias in the position of the Saharan Heat Low and the thermal wind relationship. These biases appear to be germane to the WAM rainfall band being narrower and not moving far enough northward, resulting in a dry bias in the Sahel.     

FLOW DIRECTION AND BRANCHING GEOMETRY AT JUNCTIONS IN DENDRITIC RIVER NETWORKS*

Automatic determination of flow direction from a partial view of a drainage network is a problem, notably in remote sensing. The main approach to determining which river link is pointing downstream uses spatial reasoning based on local features at junctions. Theoretical and empirical works on junction angles suggest that the two consecutive links which bound the most acute angle are the upstream links. This rule was tested using 2279 river junctions digitized from 19 dendritic drainage networks and was correct in nearly 90% of the cases. This result is better than that obtained from the decision table designed by Haralick, Wang et al. (1985) for the same purpose.     

Computation and visualisation of the accuracy of old maps using differential distortion analysis

The accuracy of old maps can hold interesting historical information, and is therefore studied using distortion analysis methods. These methods start from a set of ground control points that are identified both on the old map and on a modern reference map or globe, and conclude with techniques that compute and visualise distortion. Such techniques have advanced over the years, but leave room for improvement, as the current ones result in approximate values and a coarse spatial resolution. We propose a more elegant and more accurate way to compute distortion of old maps by translating the technique of differential distortion analysis, used in map projection theory, to the setting where an old map and a reference map are directly compared. This enables the application of various useful distortion metrics to the study of old maps, such as the area scale factor, the maximum angular distortion and the Tissot indicatrices. As such a technique is always embedded in a full distortion analysis method we start by putting forward an optimal analysis method for a general-purpose study, which then serves as the foundation for the development of our technique. Thereto, we discuss the structure of distortion analysis methods and the various options available for every step of the process, including the different settings in which the old map can be compared to its modern counterpart, the techniques that can be used to interpolate between both, and the techniques available to compute and visualise the distortion. We conclude by applying our general-purpose method, including the differential distortion analysis technique, to an example map also used in other literature.     

The Amsterdam-St. Paul volcanic province,and the formation of low Al tholeiitic andesites

The island of St. Paul in the Indian Ocean is located on the axis of the southeastern branch of the Indian Ocean Rise, as is the similar volcano of Amsterdam, 80 km to the north. Both volcanoes and possibly part of the local ridge are formed of a high-alumina plagioclase tholeiite making this a distinctive volcanic province. Lavas with Al₂O₂ contents of 12 to 22% are directly related by plagioclase fractionation. Residual rocks are a distinctive low silica, low alumina, potassic andesite with andesine and even olligoclase feldspar. Parallel lines of evolution exist in the sea floor plagioclase tholeiites, which differ only in slightly lower tenor of Sr and potassium group elements, and in the Steens Mountain tholeiites of Oregon, which differ in their significantly higher Sr and K group elements     

Second-order diffraction loads upon three-dimensional bodies

A non-linear theory is presented to derive second-order waveloads, in terms of the wave-steepness. All components of the second-order potential are included, which makes the solution valid for all range of wave frequencies. Numerical results are shown for a vertical cylinder.