Paleomagnetism and potassium-argon age of the Messejana dike (Portugal and Spain): angular limitation to the rotation of the Iberian Peninsula since the Middle Jurassic

A paleomagnetic and potassium-argon dating investigation has been carried out on a 530-km-long dike system which transects the western Iberian Peninsula in a northeasterly direction. The K-Ar age determinations were made on mineral separates exclusively. They range between 160 and 200 Ma and the authors suppose that this reflects the actual time interval of the intrusion, in accord with previous results. The paleomagnetic pole derived from 12 sites regularly distributed along the dike (71°N, 236°E) coincides well with other Mesozoic paleomagnetic poles from the western Africa. A contemporaneous pole from stable Europe is tentatively deduced from African and North American Late Triassic/Early Jurassic poles using different reconstruction models around the North Atlantic Ocean. The divergence between this pole and the Iberian pole corresponds to the result obtained for Permian poles.     

Recuperated optically stimulated luminescence dating of middle-size quartz grains from the Palaeolithic site of Bonneval (Eure-et-Loir,France)

The site of Bonneval (France) exhibits a 6 m thick stratigraphical sequence in which Middle and Lower Palaeolithic artefacts were discovered. The stratigraphy of the site consists of an alternation of loess-like silt layers deposited along glacial stages, and paleosoil horizons corresponding to the vegetal stabilization of the area during interglacials. These layers hence provide past environmental informations and can be used as stratigraphical markers and compared with well dated sequences of Northern France. Eight sediment samples from this Middle and Upper Pleistocene sequence were dated using the recuperated optically stimulated luminescence (Re-OSL) signal of quartz grains. The middle-size fraction (40–63 μm) shows a good reproducibility of the OSL signals which allows the application of a multiple-aliquot additive-dose approach for D_e determination. For all samples, the growth curves are linear up to an accumulated dose of 3000 Gy. The Re-OSL ages range from ～100 to ～450 ka and are in good agreement with the independent chronological information; they establish when handaxe industries have been made during the Middle and Upper Pleistocene in the Paris Basin.     

Long-period Love wave overtone data in North America and the Pacific Ocean: new evidence for upper mantle anisotropy

Two Tonga-trench earthquakes recorded in North America on WWSSN long-period instruments have been processed by making use of spatial analysis techniques to retrieve the phase velocities of Love wave modes. Dispersion data up to the third overtone at periods longer than 30 s have been obtained for two broad areas: the Pacific Ocean and North America. It is found that the observed Love wave phase velocities are not in agreement with the values predicted from the previously published models inferred from Rayleigh wave data. Phase velocity discrepancies as large as 3–4% in the Pacific Ocean and 2% in North America are observed up to the second overtone. Preliminary direct computations, which have been performed in the framework of transversely isotropic models with the axis of symmetry oriented vertically, suggest that the cause for this departure from isotropy probably extends to depths > 200 km.     

Joint Electrical and Time Domain Electromagnetism (TDEM) Data Inversion Applied to the Super Sauze Earthflow (France)

In order to evaluate the risk of an earthflow to evolve abruptly into torrential surge, knowledge of its internal structure is necessary. This study deals with the internal structure of the Super Sauze earthflow developed in black marls in the southern French Alps. Difficulties in this study area are a rough topography, surface heterogeneities and a large thickness variability of the earthflow mass. These conditions hamper the application of geotechnical methods as a preferred investigation mean. Moreover, they pose problems to geophysical investigations and their interpretation.This paper shows the advantage offered by the joint inversion of Time Domain ElectroMagne-tism data (TDEM) and data obtained from Direct Current soundings (DC). The results of the joint inversions are checked using geotechnical data. The internal structure of the earthflow interpreted on the basis of joint inversion data is comparable to that obtained from geotechnical results. Moreover, contrary to separate electrical and TDEM inversions, a satisfactory joint inversion model can be derived without supplying additional a priori information.     

Tracing and dating recent chemical transfers in weathering profiles by trace-element geochemistry and U---U---Th disequilibria: the example of the Kaya lateritic toposequence (Burkina-Faso)

The paper presents a synthesis of the approach recently developed to constrain the nature and the age of recent element mobility in weathering profiles. The approach relies on a comparison of variations of trace elements and U-series disequilibria in weathering profiles. It is exemplified in the case of old lateritic profiles from the Kaya toposequence, Burkina Faso. Trace elements and ²³⁸U---²³⁴U---²³⁰Th disequilibria have been analysed in whole rock samples from two pits located in very contrasted topographical positions. Trace element data show that the whole toposequence is marked by an intense chemical remobilization, including uranium, from the cap to the lower part of the profiles. ²³⁸U---²³⁴U---²³⁰Th disequilibrium data outline that all the levels of the profiles are affected by recent U---Th fractionations, and that each level of the toposequence is marked by U gains and losses. The chronological approach developed in this paper leads to an age of about 400 kyr for the dismantling of the iron cap, and ages ranging from 0 to 400 kyr for U accumulation in the pink clay horizon of the profiles. The depth repartition of U accumulation in the profile implies that the remobilisation processes in this toposequence varied through time, on time constants similar to those of climatic variations. To cite this article: F. Chabaux et al., C. R. Geoscience 335 (2003).     

Spatial versus temporal patterns in fish assemblages of a tropical estuarine coastal lake: The Ebrié Lagoon (Ivory Coast)

The fish assemblages of the Ebrié lagoon (Ivory Coast) were sampled by experimental fishing over the entire lagoon using a purse seine net. The sampling was conducted in the two main hydroclimatic seasons for this ecosystem, i.e. in the dry season (March–April) and in the wet season (August–September). The results obtained showed a fish assemblage organized around a consistently occurring group of twenty species. When analysed in terms of ecological categories, the seasonal influence led to a cycle in the assemblages from freshwater to marine around this permanent species pool, with a seasonal renewal of the assemblage. At the scale of the lagoon, there were variations in the composition of the assemblages that clearly distinguished the western part from the eastern one. The limit was situated at the Vridi canal, a wide artificial channel permanently connecting the lagoon to the sea. To the west, the assemblage was characterised by a strong spatial uniformity and low seasonal variability. To the east, the assemblage formed two different entities; one assemblage with pronounced freshwater affinities occurring in a side arm and the other assemblage with great seasonal variability under the alternating influence of seawater in the dry season and freshwater in the wet season. This part of the lagoon functioned somewhat like a typical estuary.     

U---Pb, Sr and Nd evidence for grenvillian and latest proterozoic tectonothermal activity in the spitsbergen caledonides, arctic ocean

Within the Caledonian complexes of northwestern Spitsbergen, high PT formations provide U---Pb zircon ages of 965±1 Ma of a metagranite and 955±1 Ma of a corona gabbro, indicating the influence of Grenvillian activity in the area. Various isotopic systems suggest that these rocks were partially derived by reworking of ancient crust (as old as Archaean). Eclogites and felsic agmatite indicate latest Proterozoic magmatic or metamorphic events (625₋₅⁺² and 661±2 Ma, respectively) by U---Pb zircon dating. The eclogitic metamorphism age is not fully constrained and ranges between 540 and 620 Ma; this occurred prior to the superimposed Caledonian metamorphism, indicated by a part of the K---Ar and Rb---Sr mineral cooling ages. The new data and other evidence of Precambrian tectonothermal activity on Svalbard suggest that the Early Palaeozoic and Late Proterozoic successions exposed elsewhere on Svalbard may also be underlain by Grenvillian or older basement rocks. Relationships to other Grenvillian and older terrains in the Arctic are reviewed.     

Indices de confinement dans les dépôts lacustres holocènes de Sarliève (Limagne,France)

The autochthonous magnesian carbonates and the gypsum depicted in the Holocene deposits from the ‘Marais de Sarliève’ (Limagne, France) result from an alkaline, magnesian and sulphated composition of waters. This is also recorded by the composition of algal micro- and macrophytes. This may be explained by the dissolved fluxes from the Oligocene catchment and from a restriction. During the historical period, the lake evolved towards a lesser restriction. To cite this article: J.-G. Bréhéret et al., C. R. Geoscience 335 (2003).     

Contrasted Late Glacial and Holocene hydrology of Sarliève paleolake (France) from sediment geometry and detrital versus biochemical composition

Since the end of the Last Glacial Maximum, hydrology in Europe has been influenced by both climate changes, and since Neolithic times, an increase in human activity. Paleohydrological reconstructions, especially from lake studies, can help identify the respective impact of these two factors. The present work focuses on a lacustrine geosystem, the Sarliève paleolake in the Massif Central (France), in an unusually dry, temperate area. The lake sediment geometry (core drillings, geotechnical methods), and the geochemical and mineralogical characterization of the catchment rocks and soils, and of the lacustrine deposits, indicate major variations in paleohydrology during the last 12,000 years as dated by ¹⁴C, palynology and tephrochronology. In addition, a model quantifying detrital versus biochemical lacustrine components was developed to identify hydrological trends. The data show that the Sarliève area was characterized mainly by remarkably dry conditions, hence sharpening the climatic trends at middle latitudes in Western Europe. Three main hydrological phases are distinguished since the Late Glacial: (1) 13.7–7.5 ka cal BP, a dominant dry climate, with a peak at ca. 8 ka cal BP, leading to a lowstand in water level and unusual mineral authigenesis, zeolite then dolomite, constituting up to 60% of the lacustrine sediments; (2) 7.5 to ca. 5.3 ka cal BP, repeated short-duration hydrological alternations that could have been climate-driven: lowstands in water level with up to 60% biochemical minerals versus higher water levels with <10% biochemical minerals; (3) 5.3 ka cal BP to the Middle Ages (i.e. beginning in the 5th century AD), a hydrological trend towards perennial high water level, with mainly detrital sediments, probably linked to climate evolution, except periods of obvious human-driven drying during the last two millennia.