Structural and geochemical data on the Rio Magno Unit: evidence for a new 'Apenninic' ophiolitic unit in Alpine Corsica and its geodynamic implications

The Rio Magno Unit (RMU) tectonically overlies the Schistes Lustrés units in south-eastern Alpine Corsica. It is represented by an ophiolitic sequence, showing remarkable differences with respect to the commonly recognized Corsican ophiolites. This unit can be distinguished from the Schistes Lustrés by the lack of HP–LT metamorphism, reflecting its different geodynamic setting, although both were involved early in the same tectonic events. Similarly, the RMU can be distinguished from the Balagne Unit by the presence of normal-MORB basalts and the scarcity of continent-derived sedimentary input, testifying to a different oceanic palaeogeographical setting. Moreover, the petrochemical and stratigraphic features of the RMU ophiolitic sequence show close analogies with the Internal Ligurides of the Northern Apennines. The RMU represents the first record of a nonmetamorphic 'Apenninic'-type ophiolitic unit in Alpine Corsica, supporting the hypothesis that the Alpine Corsica – Northern Apennine system represents a double-vergent accretionary wedge.     

Geochemical influences on H40/1 bacteriophage inactivation in glaciofluvial sands

Geochemical heterogeneities may cause spatial variations in virus inactivation rates resulting from interactions with minerals leading to differences in natural disinfection capacity within an aquifer. Column studies investigating the interaction of the bacteriophage H40/1 with natural sands sampled from the Kappelen test site (Kappelen), Bern, Switzerland indicated that inactivation rates are higher for adsorbed bacteriophages than for those suspended in groundwater. Moreover, breakthrough curves obtained from field-based tracer tests at Kappelen indicated that the adsorbed H40/1 is inactivated in-situ at comparable rates. Statistical analyses of mineralogical data failed to demonstrate significant spatial variations in aquifer composition either across the site or with depth. In contrast hydrochemical analyses of groundwater samples collected at Kappelen demonstrated that iron-reducing groundwater occurs below aerobic waters. Tracer breakthrough curves indicate that H40/1 survival is not affected by variable redox conditions. Investigation results suggest that spatial geochemical variability does not significantly affect H40/1s inactivation rate at Kappelen.     

Dynamics of Small Earth-Approachers on Low-Eccentricity Orbits and Implications for Their Origins

The population of Near-Earth Asteroids (NEAs) appears to be overabundant at sizes smaller than 50m, compared to a power-law extrapolation from kilometer-sized objects. Several of these small NEAs are also concentrated on low-eccentricity orbits, where a few larger Earth-crossers are observed, and are called Small Earth-Approachers (SEAs). Their source region as well as the dynamical mechanisms involved in their transport close to the Earth on low-eccentricity orbits have not yet been determined. In this paper, we present our numerical and statistical study of the production and dynamical evolution of these SEAs. We first show that three main sources of Earth-crossers which are, according to recent simulations, the 3/1 and ₆ resonances in the main belt, and the Mars-crosser population, are not able to produce as many bodies on SEAs-like orbits compared to other Earth-crossing orbits as has been inferred from observations. From these sources, SEAs-like orbits are reached through the interplay of two required mechanisms: secular resonances and planetary close approaches. However, the time spent on these orbits remains smaller than 1 Myr as confirmed by the study of the evolutions of 11 observed SEAs which also reveal the action of various mechanisms such as close approaches to planets and/or secular resonances. Therefore, our results present some mechanisms which can be responsible for their production but none that would preserve the lifetime of the SEAs sufficiently to enhance their abundance relative to other Earth-crossing orbits at the level observed. The overabundance of the SEA population, if real, remains a problem and could be related to the influence of collisional disruption and tidal splitting of Earth-crossers, as well as to observational biases that might account for a discrepancy between theory and observation.     

Rhine flood deposits recorded in the Gallo-Roman site of Oedenburg (Haut-Rhin, France)

From the first to the fourth century AD, the Gallo-Roman town of Oedenburg developed in the alluvial landscape of the southern Upper Rhine Graben. Throughout this period, the landscape mosaic, composed of palaeochannels, stable palaeoislands and river terraces, continued to evolve. A district of this town, situated on a lateral Rhine channel system, was archaeologically excavated. Large-scale excavation and cross-section analysis provide evidence of changing fluvial conditions during the period under study. At about AD 20 or earlier, this lateral part of the floodplain, affected by very fine sedimentation, was occupied by moribund marshy palaeochannels. When the first Gallo-Roman settlers occupied the site, they filled parts of the channels with woven brushwood in order to create an efficient circulation surface. The sedimentary infill of this palaeochannel records four different flood deposits interlayered with dated anthropogenic units (pavements, road embankments, and other structures). Archaeological analysis and dendrochronological dating indicate that these four flooding events occurred during a short time period between AD 20 and AD 145/146. These geoarchaeological observations focus on floods that do not seem to have significantly affected human occupation in this part of the Rhine floodplain. These results are set in the broader context of the Rhine catchment and the Alps.     

An inter-comparison of regional climate models for Europe: model performance in present-day climate

The analysis of possible regional climate changes over Europe as simulated by 10 regional climate models within the context of PRUDENCE requires a careful investigation of possible systematic biases in the models. The purpose of this paper is to identify how the main model systematic biases vary across the different models. Two fundamental aspects of model validation are addressed here: the ability to simulate (1) the long-term (30 or 40 years) mean climate and (2) the inter-annual variability. The analysis concentrates on near-surface air temperature and precipitation over land and focuses mainly on winter and summer. In general, there is a warm bias with respect to the CRU data set in these extreme seasons and a tendency to cold biases in the transition seasons. In winter the typical spread (standard deviation) between the models is 1 K. During summer there is generally a better agreement between observed and simulated values of inter-annual variability although there is a relatively clear signal that the modeled temperature variability is larger than suggested by observations, while precipitation variability is closer to observations. The areas with warm (cold) bias in winter generally exhibit wet (dry) biases, whereas the relationship is the reverse during summer (though much less clear, coupling warm (cold) biases with dry (wet) ones). When comparing the RCMs with their driving GCM, they generally reproduce the large-scale circulation of the GCM though in some cases there are substantial differences between regional biases in surface temperature and precipitation.     

Caractérisation des ignimbrites néogènes du bassin d'Arequipa,Pérou

Four ignimbritic units have filled the Arequipa Basin and outcrop around the Chachani Volcano. (1) The oldest Río Chili ignimbrite is 13.33 Ma old; (2) the most widespread La Joya ignimbrite is 4.9 Ma old; (3) the Arequipa Airport ignimbrite (1.65 Ma) flowed from an area buried beneath Chachani; (4) the Yura Tuffs, 1.02 Ma old, are restricted to the west of Chachani. All are calc-alkaline rhyolites with plagioclase, biotite, quartz, sanidine, and opaques, but the Río Chili and La Joya ignimbrites also contain amphibole. Trace elements of the older ignimbrites reflect stronger crustal influence. To cite this article: P. Paquereau et al., C. R. Geoscience 337 (2005).     

Late Cenozoic geodynamic evolution of eastern Indonesia

This paper presents an internally and globally consistent model of plate evolution in eastern Indonesia from Middle Miocene to Present time. It is centered on the Banda Sea region located in the triple junction area between the Pacific–Philippine, Australia and South–East Asia plates. The geological and geophysical data available from Indonesia were until recently insufficient to define a unique plate tectonic model. In this paper, the new data taken into account clearly restrict the possible interpretations. Owing to a great number of geological, geophysical and geochemical studies, the major plate boundaries (the Sunda–Banda subduction zone to the south, the Tarera–Aiduna Fault zone and the Seram Thrust to the east, and the Sorong Fault zone and Molucca Sea collision zone to the north) are now clearly identified. The age of the major tectonic structures is also better known. Geodetic measurements well constrain the Present time plate kinematics. We also consider the deformation history within eastern Indonesia, where numerous short-lived microplates and their related microcontinents successively accreted to the Asiatic margin. Moreover, magnetic anomalies identification of the North and South Banda Sea basins allows a precise kinematic reconstruction of the back-arc opening. We used the Plates software to test the coherency of our model, presented as a series of 4 plate reconstruction maps from 13 Ma to the present. Finally, the origin of oceanic domains restored by our reconstruction is discussed.     

Révision stratigraphique de l’Ile de Timor (Indonésie orientale)

Resume. De nombreuses échelles stratigraphiques de l’?le de Timor ont vu le jour car les corrélations stratigraphiques étaient rendues difficiles par la complexité tectonique de l’?le et par la variété des faciès. Dès les premiers travaux, plusieurs séries ont été distinguées, regroupées en trois unités litho-structurales principales: unité allochtone, sub-autochtone et autochtone. Les données stratigraphiques et géodynamiques récentes montrent que l’?le de Timor est en fait constituée par six unités litho-structurales, chacune témoignant d’un épisode majeur de l’histoire géologique de l’Ile. Cette histoire consiste d’abord à la séparation, au Jurassique, d’un bloc issu du continent de Gondwana (unité para-allochtone) qui est entré en collision avec l’arc volcanique asiatique à l’Oligo-Miocène (unités allochtone et sub-allochtone). Cet ensemble a ensuite été séparé de l’Asie lors de l’ouverture de la mer de Banda sud, au Miocène supérieur (unité sub-autochtone) pour finalement entrer en collision avec la marge nord australienne au cours du Pliocène moyen (plateforme australienne et groupe de Kolbano). Timor est depuis incorporée à la marge nord australienne (unité autochtone). Ce travail présente les différentes échelles stratigraphiques de chaque nouvelle unité, des propositions de corrélations et une comparaison avec les ?les voisines de Sumba et de Sulawesi pour aboutir à une proposition d’évolution géodynamique. Manuscrit re?u le 13 février 2004 Révision acceptée le 14 février 2005