Interprétation quantitative de l'anisotropie de susceptibilité magnétique

We propose a model in which the magnetic susceptibility anisotropy is considered as resulting from the distinct properties of two rock components. This model suggests that differences in the susceptibility ellipsoid can be principally related to mineralogical variations. Thus, some difficulties in the quantitative interpretation of anisotropy data may be explained. This model is consistent with a detailed analysis carried on Dalradian rocks from Scotland.A new methodology of anisotropy analysis is derived. It is based on the fact that, in specimens from a same sample, the susceptibility ellipsoid often varies, but relations nevertheless exist between values of the principal and mean susceptibilities. For each sample, measurements are made on 6–12 specimens. The study of the relations between principal and mean susceptibilities permits separation of the anisotropies of the rock components, the anisotropy determination appearing the best for the high susceptibility component.

Résumé

L'examen d'un modèle de roche, où l'anisotropie est considérée comme résultant des propriétés de deux constituants, montre que l'ellipsoïde de susceptibilité dépend fortement de la composition minéralogique. Ainsi s'expliquent certaines difficultés de l'interprétation quantitative des résultats d'anisotropie. Ce modèle est testé avec succès sur une série du Dalradien d'Écosse.Une nouvelle méthodologie d'analyse de l'anisotropie en découle. Elle est basée sur la constatation que l'ellipsoïde de susceptibilité présente souvent des fluctuations dans les différents spécimens taillés dans un même échantillon, et que des relations simples existent cependant entre les différentes valeurs des susceptibilités principales et moyenne. Pour chaque échantillon, les mesures sont faites sur 6–12 spécimens. L'étude des relations entre susceptibilités principales et moyenne permet une séparation des anisotropies des constituants de la roche, la détermination de l'anisotropie semblant d'autant meilleure que la susceptibilité est plus forte.     

Pétrologie de l'intrusion alcaline mésozoïque de la région d'Anemzi, Haut Atlas Central, Maroc

The Mesozoic Anemzi intrusion belongs to the peri-Atlantic Mesozoic alkali magmatism. It is composed of gabbros (with or without olivine), monzodiorites and quartz-bearing syenites. Dolerite dykes of alcaline affinity occur nearby. The alkaline rocks are intrusive into mid-Jurassic limestones. Liquidus phases crystallised following the characteristic order of alkali magmas, i.e. olivine + spinel, plagioclase, clinopyroxene and amphibole. Clinopyroxene and amphibole compositions are governed by Ti-tschermakite substitutions. Clinopyroxene and amphibole in syenites are enriched in aegirine and richterite components, respectively. The mineralogical and chemical evolution in the Anemzi intrusion is consistent with low-pressure fractional crystallisation from an alkali lava under low to moderate fO₂ (MW + [1 − 2 log units]). However, the parental liquid shows some geochemical characteristics of transitional magma, such as a high SiO₂ content (48 wt%) and low La/Yb_N ratio (5). Silica activity decreased from the parental magma (0.56) to 0.08 after fractionation of the gabbros and then increased to 1.0 at the end of the magmatic differentiation. Compared with rocks of similar ages, belonging to the peri-Atlantic Cretaceous alkaline magmatism, the Anemzi intrusion is distinguished by low REE contents ([La/Yb]_N = 6) and transitional basalt(s) as parental magma. This magma was emplaced through reactivation of Palæozoic fault systems.     

Le paléovolcanisme de la bordure occidentale de la boutonnière de Kédougou, Paléoprotérozoïque du sénégal oriental: incidences géotectoniques

The main petrographical and geochemical patterns (major, trace and rare earth elements) of the epimetamorphosed volcanic rocks from the Mako Supergroup (Birimian) from the central and southern part of the western border of the Kedougou-Kenieba Inlier (Palæoproterozoic) are presented. The petrographic patterns indicate a predominance of tholeiites with the middle and upper parts of the lithological sequence containing scarce andesite, rhyodacite and rhyolitic end members of calc-alkaline affinity. The observed geochemical trend is compared with the patterns from other Palæoproterozoic areas of the West African Craton, as well as recent volcanic rocks, whose geotectonic setting is well constrained. The evolution of the Kedougou Inlier corresponds to an ocean island-arc environment, in agreement with previous research on epimetamorphic volcanodetrital and sedimentary formations, and Birimian granitoids. This research underlines the importance of island-arc accretions in crust generation during Palæoproterozoic times in West Africa.     

Le magmatisme basique filonien néoprotérozoïque de la boutonnière de Zenaga, Anti-Atlas central, Maroc: pétrologie, géochimie et signification géodynamique

Before the Pan-African Orogeny, the Palæoproterozoic basement and its Neoproterozoic cover (limestones and quartzites) of the Zenaga Inlier were cross-cut by a swarm of doleritic dykes. They are more or less altered. The primary mineral assemblage consists of plagioclase, clinopyroxene, very rare orthopyroxene, ilmenite, apatite, micropegmatite and sometimes hornblende and biotite. Mineralogical and geochemical studies indicate that the dolerites are continental tholeiites. Two groups of dykes have been distinguished. Accordingly, rare earth elements, P₂O₅, Zr, Th, Ba and Sr contents are higher in group I than in group II, which is richer in V. Group I comprises the north-south and northwest-southeast swarms, while group II corresponds to northeast-southwest and east-west swarms, which were emplaced later. These geochemical variations may be explained by a higher degree of melting of the mantle source for the later group II. Doleritic dykes of Zenaga had been emplaced during an extensional episode, prior to Pan-African folding.     

Reconnaissance de la structure géologique du bassin de saïss occidental, Maroc, par sondages électriques

A geophysical study, based on 96 electrical resistivity measurements with a line length up to 4 km, was performed in the southern and southwestern parts of the Meknes Plateau, Morocco, which is a part of the Saiss Basin, located between the Rif Range to the north and the Middle Atlas Range to the south. This basin, whose maximum depth is 1.5km in the north, is filled with Triassic to Quaternary deposits overlying the Palæozoic basement and includes two main aquifers. The interpretation of the resistivity measurements, calibrated from deep boreholes, made it possible to obtain a new hydrogeological model for the Saiss Basin. The understanding of the basin structure is of primary importance for the water supply of this area, which has been affected by severe droughts in recent years.     

Les structures de la couverture Néoprotérozoïque terminal et Paléozoïque de la région de Tata, Anti-Atlas centre-occidental, Maroc: déformation polyphasée, ou interactions socle/couverture pendant l'orogenèse hercynienne?

The western Anti-Atlas was formed by a Precambrian basement in the core of anticlines, surrounded by a Neoproterozoic and Palæozoic cover. The structural study of the Tata regional rocks shows a heterogeneous deformation, characterised especially by two types of folds in two orthogonal directions: north-south to north-northeast-south-southwest-trending and east-west-trending.The north-south structures are present in all of the Palæozoic cover and belong to the major Variscan compression of Late Carboniferous age by a comparison of the other domains of the western Anti-Atlas. Alternatively, east-west folding is assigned only to the lower part of the cover and consists of a ductile heterogeneous deformation, especially marked at the basement-cover interface. These folds are associated with a subhorizontal cleavage, indicating a southern vergence of the structures. A discussion of the age and the tectonic style of these structures is proposed, as well as their significance within the Variscan belt along the northern margin of the West African Craton.     

Les rudistes du Crétacé moyen de la province Méditerranéenne occidentale. Evolution, paléoécologie, paléobiogéographie

Dans un premier temps on délimite les plates-formes à rudistes de la province méditerranéenne occidentale, au sein de laquelle on distingue, en fonction des particularités paléogéographiques locales, quatre sous-provinces. On souligne ensuite l'importance et la nature des événements paléogéographiques du Crétacé moyen qui ont joué un rôle dans la distribution et le développement des formations à rudistes; on distingue fondamentalement des événements eustatiques et tectoniques. Les événements biologiques sont caractérisés par un déclin ou une disparition des rudistes éocrétacés, un développement des Caprinidae et des Radiolitidae à l'Albo-Cénomanien, l'apparition des Hippuritidae et le déclin des Caprinidae au Turonien. On analyse les causes, les circonstances et les faits majeurs de l'évolution et de la paléoécologie des rudistes au cours de cette période.The rudistid platforms of the western Mediterranean embrace four palaeogeographical sub-provinces. The palaeogeographical events that decided the distribution and development of the rudistids are discussed. Of significance are eustatic and tectonic events.There was a decline or a disappearance of Early Cretaceous rudistids, a development of the Caprinidae and Radiolitidae around the Albian-Cenomanian, the appearance of the Hippuritidae and a decline of the Caprinidae in the Turonian. Factors of evolutionary and palaeoecological significance are reviewed.     

Évaluation de la qualité de l'eau par application de la méthode géoélectrique : exemple de la plaine d'El Mida–Gabes nord (Sud tunisien)

The Mida plain, which is part of the North Gabès region (southern Tunisia), is characterized by the deep sandy units of the ‘Continental intercalaire’ (CI) or the limestone of the Lower Senonian. A geophysical survey, by electrical sounding (ES), was undertaken in the studied region to better characterize the deep geological structure of this plain and therefore its aquifer resources potential. The analysis of the results shows that the prospected zone is characterized by the succession of several levels with contrasted resistivities, which are often affected by faults. Among these observed geoelectrical levels, the highly conductor one could host a saline aquifer. Another geoelectrical level corresponding to the resistant bedrock detected at Oudhref horst can contain better-quality water than that of the aquifer detected in the El Mida Graben. In this work, we tried to explain the origin of the salinity of this aquifer. Thus, we hypothesise about a contamination from Jebel Zemlet El Beida through a border fault and another one from the Sebkhet El Hamma. To cite this article: A. Mhamdi et al., C. R. Geoscience 338 (2006).     

Aspects géochronologiques(K-Ar)de l'activité volcanique dans l'île de moorea, Pacifique Central

Moorea volcanic island (Central Pacific) shows particular characteristics: it has been built by synchronous emissions (shown by radiometric K-Ar data) of basic volcanics (basalts and K hawaiites) and of very abundant evoluted lavas (K benmoreites), systematically aphyric, but containing ferromagnesian xenocrysts.Radiometric K-Ar data also underline the intricate and multistage character of the island building. Its genesis would be explained in the light of a fractional melting process.

Résumé

L 'fle volcanique de Moorea (Pacifique Central) présente la particularité d'avoir étéédifiée par la coémission (démontrée par les données radiométriques K-Ar) de roches basiques (basaltes et K hawaiites) et de roches évoluées très abondantes (K benmoréites), systématiquement aphyriques, mais contenant des xénocristaux ferromagnésiens.Les données radiométriques K-Ar soulignent aussi le caractère complexe de l'empilement volcanique construit en plusieurs stades. Sa genèse pourrait être expliquée par un processus de fusion fractionnée.     

Superposition de la tectonique éburnéenne et panafricaine dans les granitoïdes de la bordure nord du craton ouest africain, boutonniére de Zenaga, Anti-Atlas central, Maroc

The Zenaga Inlier shows a comprehensive record of the Eburnian and Pan-African Orogenies. The Eburnian is characterised by high-temperature regional metamorphism and complex magmatism. The early (Azguemerzi) granodiorite has an isotopic mantle signature and was emplaced diapirically during the Eburnian Orogeny causing local thermal metamorphism. The foliation observed in this granitoid is a result of the interference between its primary syn-emplacement foliation and the regional foliation under amphibolite-facies conditions. The northern part of Zenaga has been intruded by the leucocratic granites of Tazenakht. These granites are cut by mylonites and phyllonites, corresponding to the Pan-African shear zones and accompanied with sub-greenschist-facies metamorphism during the Pan-African Orogeny. The deformation was the result of a regional sinistral transpressive event. This study in the northern part of the West African Craton shows the superposition of the Pan-African on the Eburnian Orogeny and the presence of a major fault in the Anti-Atlas.     

L'érosion des chaînes de montagnes : influence de la sédimentation de piedmont

The applanation of mountain belts that results in peneplain is generally considered to be caused by the long-term activity of erosion. Peneplanation has been previously defined as the lowering of an elevated topography and the concomitant subduing of its relief. We propose a model following which piedmont sedimentation induces the base level rise, allowing applanation to develop at high elevation and resulting in an elevated ‘peneplain’. This model is illustrated by the morphological evolution of the southern flank of the Pyrenees during the Cainozoic. To cite this article: J. Babault, J. Van Den Driessche, C. R. Geoscience 337 (2005).     

Tableau de répartition stratigraphique des Grands Foraminifères caractéristiques du Crétacé moyen de la région Méditerranéenne

Le Groupe de Travail Européen des Grands Foraminifères présente un tableau de répartition stratigraphique de 42 espèces bien définies du Crétacé moyen de la région méditerranéenne. La répartition stratigraphique proposée pour chaque espèce est fondée soit sur des propres observations soit sur des données de la littérature supprimer et est contrôlée par la présence d'Ammonites ou de Foraminifères planctoniques.A summarizing account is presented of the deliberations of the research group for Large Foraminifers of the IGCP Project “Mid-Cretaceous Events”. Large Foraminifers are of incontestable value for dating carbonate platform sequences owing to the absence of many other diagnostic groups of organísms. A table of stratigraphical distributions for 42 species is presented.     

Série du marégraphe de Marseille : mesures de températures de surface de la mer de 1895 à 1956 : une correction

In 2007, a daily temperature series, which has been collected from 1885 to 1967 at the Marseilles tide recorder, was published here. Upon the available information provided by the data managing office, they were considered as sea-surface temperatures. Unfortunately they were not but air temperature inside the building. In the present paper, the authors correct this error by studying the true seawater temperature series which had been dug out after the publication of the precedent paper. The study of this slightly shorter series (1895–1956) leads to close conclusions. Along these 61 years, the warming trend of the surface seawater rises to +1.6 °C, to +1.3 °C if only January to May and December are selected and +2.2 °C during the June to November period. In the first half part of the series (1895–1925), the average value of each month distributions (except January) and annual minimal (except 1956) are significantly lower than along the second (1925–1956).     

Présentation d'un tableau de repérage palynostratigraphique dans le Crétace moyen de l'Europe occidentale

L'homogénéïté floristique de l'Europe de l'Ouest (boréale et mésogéenne) est remarquable au Crétacé moyen. Les limites des unités unités floristiques majeures sont controllées par des facteurs climatiques: la variation la plus importante étant située à la limite Cénomanien-Turonien.Western Europe (Boreal and Tethyan) was marked by floristic unity during the Mid-Cretaceous. The limits of the major floristic units were controlled by climatic factors, with the most accentuated variations at the Cenomanian-Turonian boundary. Stratigraphical accuracy is higher within basins than between geographical entities.     

Les granitoïdes néoprotérozoïques de Khzama, Anti-Atlas central, Maroc: marqueurs de l''évolution d''un magmatisme d''arc à un magmatisme alcaline

Petrological study and zircon typology provide important information that is related to the classification and genesis of Neoproterozoic granitoids in the Khzama area (northeast Siroua). The Pan-African granitoids show a transition from island-arc magmatism to alkaline magmatism. A space and time zonation of magmatism from the north to the south is evident. Early Pan-African granitoids were generated from various magma sources through different petrogenetic mechanisms. The first association corresponds to the low-K calc-alkaline plutons of Ait Nebdas, the second one correponds to high-K calc-alkaline post-collisional granites (Tamassirte-Tiferatine and Ifouachguel). Finally, shoshonitic magmatism (Irhiri) ends the magmatic evolution of the region. Thus, the late Pan-African granitic plutonism began with calc-alkaline associations and ended with K-alkaline magmatism in a transtensional setting, heralding the onset of the Moroccan Palæozoic cycle.     

Etude de l''acquisition d''éléments dissous par les eaux de lessivage des roches granitiques sous climat tempéré

Using chemical potential diagrams and budget calculations, we show that the acquisition of solutes by surface water in granitic rocks under temperate climate corresponds with transformation of primary minerals into kaolinite or halloysite. This mineral controls aluminium solubility.     

Les minerais de fer de l'Ordovicien inférieur du bassin de Bretagne-Anjou,France*

The iron resources of the ‘Bretagne-Anjou’ basin proceed essentially from four layers of sedimentary ore in the lower gres armoricain (Arenig). The more frequent paragenesis combine quartz, magnetite, siderite and silicates (bavalite, stilpnomelane). The hematite and pyrite parageneses are rarer. Two ore types were distinguished petrographically: one type with allochems and one without allochems. The study of both paragenesis and thickness variations of the ore layers enables one to reconstitute the sedimentation history: iron is dissolved on the emerged cadomian reliefs and transported to the sedimentary basin, which was shallow and strewn with islands and shoals. Then, according to local conditions, iron precipitated either under its oxydized forms (hematite, magnetite) or under its reduced forms (silicates, pyrite, siderite). The essential factors of parageneses differentiation were the depth of the basin, the water agitation and the organic matter content of the sediment. In some cases little concretions were formed in the sediment and gave rise to allochems which can be moved away later and deposited elsewhere.     

Prospection géoélectrique pour l'étude de l'aquifère thermal des calcaires récifaux, Hmeïma–Boujabeur (Centre ouest de la Tunisie)

The Hmeïma–Boujabeur zone is considered as an important geothermal province for Tunisia. The reef limestone, characterized by intense fracturation and important karstification, confers them the property to be a regional thermal aquifer. A geophysical study based on two complementary geoelectrical methods, the Electrical Sounding (ES) and Magneto-Telluric Sounding (MTS), turned out to be very efficient to provide a complete electrical image of the underground until about 1-km depth. The synthetic approach integrating all data allowed us to delimit the favourable area for thermal water exploitation. To cite this article: M. Gouasmia et al., C. R. Geoscience 338 (2006).