Phénomènes karstiques fossiles et actuels au sein des formations métamorphiques silico-alumineuses de la nappe pan-africaine de Yaoundé (Sud-Cameroun)Fossil and present-time karstic phenomena in silico-aluminous metamorphic formations of the Pan-African nappe of Yaoundé (South-Cameroon)

Numerous karstic features have been recognised in the non-carbonaceous micaschists and gneisses of the Yaoundé Pan-African nappe, south of Cameroon. It is shown that their formation was controlled by the structural features of the rocks. The wells and the pipes in the bedrock outcrops point out a current karstification process, resulting from the plagioclase dissolution by the acid rain waters. Hill wall alveoli and caves, of pre-Miocene age, are exhumed features that were done by dissolution in the aquiferous underground. To cite this article: J.-P. Vicat et al., C. R. Geoscience 334 (2002) 545–550.     

Hydro-geochemistry of groundwater and surface water in Dschang town(West Cameroon): Alkali and alkaline-earth elements ascertain lithological and anthropogenic constraints

This study focuses on the sources of alkali and alkaline-earth elements based on the geochemistry of groundwater and surface water in Dschang concerning environmental and anthropogenic constraints. A comprehensive set of 50 samples from groundwater and surface water were analyzed by ICPMS and processed by spatial interpolation in a GIS environment. The results highlight a geochemical anomaly at the center of the densely inhabited area subject to a profusion of open dumps discharges. This anomaly with the highest spatial contents of Be(Cs, Rb, Mg) suggests an anthropogenic source that demarcates with the lowest alkali and alkaline-earth elements on the peripheral area of Dschang. Other findings include lithological constraints with volcanic rocks being the main source compared to granitoid.The study points out good correlations between Be, Cs, Rb and Mg spatial distributions and physicochemical parameters of waters(K, EC, TDS), and inversely with the lowest p H. p H is established as the most functioning physico-chemical constraint of alkali and alkaline-earth mobility in Dschang. The p H lowest values within the geochemical anomaly also highlight the impact of human activities on water acidity, which later enhance elements mobility and enrichment. Despite low elements contents relative to WHO standards, our findings point out an example of anthropogenic impact on water geochemistry linked to solid waste pollution; it also demonstrates significant anthropogenic changes of environmental physicochemical parameters of prime importance in the mobility and distribution of elements in the study area.Similar assessments should be extended in major towns in Cameroon.     

Preface: Proceedings of the 5th International Symposium on Lithographic Limestone and Plattenkalk

Magmatic rocks from the pre-Mesozoic basements of the Sambuco and Maggia nappes have been dated by U–Pb zircon ages with the LA-ICPMS technique. Several magmatic events have been identified in the Sambuco nappe. The mafic banded calc-alkaline suite of Scheggia is dated at 540 Ma, an age comparable to that of mafic rocks in the Austroalpine Silvretta nappe. The Sasso Nero peraluminous augengneiss has an age of 480–470 Ma, like many other “older orthogneisses” in Alpine basement units. It hosts a large proportion of inherited zircons, which were dated around 630 Ma, a Panafrican age indicating the Gondwanan affiliation of the Sambuco basement. The calc-alkaline Matorello pluton yielded ages around 300 Ma, similar to numerous Late Carboniferous intrusions in other basement units of the Lower Penninic (Monte Leone, Antigorio, Verampio) and Helvetic domains (Gotthard and other External Crystalline Massifs). Associated lamprophyric dykes are slightly younger (300–290 Ma), like similar dykes sampled in gneiss blocks included in the sedimentary cover of the underlying Antigorio nappe (290–285 Ma). The Cocco granodiorite and Rüscada leucogranite, both intruding the basement of the neighbouring Maggia nappe, yielded ages of ca. 300–310 Ma, identical within errors to the age of the Matorello pluton. They are significantly older than former age determinations. This age coincidence, coupled with remarkable petrologic similarities between the Cocco and Matorello granodiorites, strongly suggests paleogeographic proximity of the Sambuco and Maggia nappes in Late Carboniferous times. In recent publications these two nappes have been interpreted as belonging to distinct Mesozoic paleogeographic domains: “European” for Sambuco and “Briançonnais” for Maggia, separated by the “Valais” oceanic basin. In this case, the similarity of the Matorello and Cocco intrusions would demonstrate the absence of any significant transcurrent movement between these two continental domains. Alternatively, according to a more traditional view, Sambuco and Maggia might belong to a single large Alpine tectonic unit.     

Archaean high-K granitoids produced by remelting of earlier Tonalite–Trondhjemite–Granodiorite (TTG) in the Sangmelima region of the Ntem complex of the Congo craton,southern Cameroon

We present a geochemical and isotopic study that, consistent with observed field relations, suggest Sangmelima late Archaean high-K granite was derived by partial melting of older Archaean TTG. The TTG formations are sodic-trondhjemitic, showing calcic and calc-alkalic trends and are metaluminous to peraluminous. High-K granites in contrast show a potassic calc-alkaline affinity that spans the calcic, calc-alkalic, alkali-calcic and alkalic compositions. The two rock groups (TTG and high-K granites) on the other hand are both ferroan and magnesian. They have a similar degree of fractionation for LREE but a different one for HREE. Nd model ages and Sr/Y ratios define Mesoarchaean and slab-mantle derived magma compositions respectively, with Nb and Ti anomalies indicating a subduction setting for the TTG. Major and trace element in addition to Sr and Nd isotopic compositions support field observations that indicate the derivation of the high-K granitic group from the partial melting of the older TTG equivalent at depth. Geochemical characteristics of the high-K granitic group are therefore inherited features from the TTG protolith and cannot be used for determining their tectonic setting. The heat budget required for TTG partial melting is ascribed to the upwelling of the mantle marked by a doleritic event of identical age as the generated high-K granite melts. The cause of this upwelling is related to linear delamination along mega-shear zones in an intracontinental setting.     

Estuarine conservation and restoration: the Somme and the Seine case studies (English Channel, France)

Megatidal estuaries such as the Seine and the Somme (North-Western France) are rather well delimited and human impacts on them are well understood. Since the middle of the 19th Century, there has been a slow but irreversible degradation of the state of these English Channel estuaries. However, current conservation and restoration strategies tend to freeze habitats in a particular state, their status being defined, most often, through a patrimonial or utilitarian approach. Connectedness between biotopes (sensu habitat+community) has a tendency to be neglected, especially with regard to main ecological gradients, i.e., salinity. In this paper, evaluation methodologies are proposed with the intention of assessing changes to ecosystem functions, under anthropogenic disturbance, controlled or otherwise. The Seine (a heavily industrialised ecosystem) is compared to the Somme (considered here for its pseudo-natural features) in order to discriminate between oceanic processes (siltation and plugging of estuaries) and anthropogenic influences. Preservation and restoration of habitats rely on a robust scientific methodology. The multi-scale approach adopted in the projects presented here relies on sensitive socio-ecological assessment procedures, tools for evaluating ecological quality, and well-built monitoring programmes based upon pertinent indicators. Such managerial tools were used to refine strategies and make them compatible with the sustainable co-development of resources in a European context. This paper demonstrates how scientists were able to acquire and apply knowledge in the field of rehabilitation and restoration. Jointly with managers and policy-makers, they have brought scientific information and socio-economics together in order to answer questions about the restoration of sites or habitats and to anticipate future propositions in the spirit of Integrated Coastal Zone Management (ICZM).     

Seismic anisotropy of the Pacific Ocean inferred from long-period surface waves dispersion

The dispersion of surface (Rayleigh and Love) waves in the period range 40–300 s along a large number of paths, allows the estimation of both the azimuthal anisotropy and the shear-wave polarization anisotropy. The regional dispersion is determined, taking into account simultaneously its dependence with age and an azimuthal factor. The Pacific Ocean has been divided into 5 regions for Rayleigh waves and into 3 regions for Love waves. This partition discriminates the regions of extreme age which show a fast variation of dispersion with age, from the regions of intermediate age where the variation is weak. A variation of ～ 2% of Rayleigh-wave group velocity with the azimuth of the path, measured with respect to the direction of spreading is displayed, up to very long-period. On the contrary, the azimuthal anisotropy for Love waves is difficult to resolve. For Rayleigh waves, the present-day direction of plate motion seems to agree best with the direction of maximum velocity. On the other hand, the isotropic inversion of the regional dispersion curves indicates, except for young regions, a discrepancy between Rayleigh-wave and Love-wave models. With this hypothesis, SH-velocities are higher than SV-velocities for the regions older than 23 Ma, down to a depth of 300 km, which is indicative of the presence of polarization anisotropy. The latter, very weak for the young part of the ocean, increases with age and reaches 7%, for the oldest region.     

The pre-Cretaceous deep-seated tectonics of the Abukuma massif and its place in the structural framework of Japan

The structures and microstructures of the Takanuki and Hitachi areas in the Abukuma massif, Northeast Japan are described. In the Takanuki area, the basic Gosaisho series thrusts the pelitic Takanuki ones in a HP metamorphic context. The nappe structure is afterwards refolded by a migmatitic dome: the Samegawa dome, in a HT metamorphic context. Microtectonic analysis shows that the nappe was transported from south to north along the stretching lineation. Geometric features suggest that the Samegawa dome was emplaced by diapirism. The role of the thrust surface as an instable interface promoting the doming is emphasized. The Hitachi metamorphic rocks composed of basic schist, limestone and sandstone shist thrust the pelitic rocks of the western Hitachi gneisses. As for the Takanuki area, the thrusting occurred in ductile synmetamorphic conditions with a north or northeastward displacement. Owing to lithologic, petrologic, structural similitudes, the nappe of the Hitachi metamorphic rocks and that of the Gosaisho series are unified into a unique nappe with a northward motion. The emplacement occurred between late Permian and late Cretaceous likely in late Jurassic. The allochthonous units of the Abukuma massif are correlated with the Green Schist nappe described in Southwest Japan, since they are surrounded by the same zones, namely the Tanba zone and the Kurosegawa-Kitakami one. Moreover both in Southwest and Northeast Japan, the emplacement of the Green Schist nappes is due to a shear deformation inducing rotational structures along the stretching lineation indicating the same sense of transport, that is eastward in Southwest Japan and northward in Northeast Japan, owing to the late bending of the Japanese Islands. The late Jurassic nappe structure is obliquely overprinted by a HT metamorphism, Ryoke in Southwest Japan, Abukuma in Northeast Japan, and afterwards cut by late faults as the Median Tectonic Line or the Tanakura fault, giving rise to the present complexity.     

Potential Impact of Climate Change on Vegetation in the European Alps: A Review

Based on conclusions drawn from general climatic impact assessmentin mountain regions, the review synthesizes results relevant to the European Alps published mainly from 1994 onward in the fields of population genetics, ecophysiology, phenology, phytogeography, modeling, paleoecology and vegetation dynamics. Other important factors of global change interacting synergistically with climatic factors are also mentioned, such as atmospheric CO₂ concentration, eutrophication, ozone or changes in land-use. Topics addressed are general species distribution and populations (persistence, acclimation, genetic variability, dispersal, fragmentation, plant/animal interaction, species richness, conservation), potential response of vegetation (ecotonal shift – area, physiography – changes in the composition, structural changes), phenology, growth and productivity, and landscape. In conclusion, the European Alps appear to have a natural inertia and thus to tolerate an increase of 1–2 K of mean air temperature as far as plant species and ecosystems are concerned in general. However, the impact of land-use is very likely to negate this buffer in many areas. For a change of the order of 3 K or more, profound changes may be expected.