An early Palaeozoic supra-subduction lithosphere in the Variscides: new evidence from the Maures massif

Petrographic and geochemical studies of peridotites and melagabbros from the Maures massif (SE France) provide new constraints on the Early Palaeozoic evolution of the continental lithosphere in Western Europe. Peridotites occur as lenses along a unit rooted in the main Variscan suture zone. They are dominantly spinel peridotites and minor garnet–spinel peridotites. Spinel peridotites represent both residual mantle and ultramafic cumulates. Mantle-related dunites and harzburgites display high temperature textures, with olivine (Mg#_0.90), orthopyroxene (Mg#_0.90) and spinel (TiO₂ < 0.2%; Cr#_0.64–0.83) compositions typical of fore-arc upper mantle. Ultramafic cumulates are dunite adcumulates, harzburgite heteradcumulates and mesocumulates, melagabbro heteradcumulates and amphibole peridotites, with olivine (Mg#_0.85–0.89), orthopyroxene (Mg#_0.86–0.89) and Cr-spinel (TiO₂ = 0.5–3.3%; Cr#_0.7–0.98) compositions typical of ultramafic cumulates. Cr-spinel compositions of both spinel peridotite types suggest their genesis in a supra-subduction zone lithosphere. Core to rim zoning in spinel is related to the incomplete influence of regional metamorphism and serpentinisation. The covariation of major and minor elements with Al₂O₃ for cumulates is consistent with igneous processes involving crystal accumulation. Both mantle and cumulate dunites and harzburgites have U-shaped REE patterns and extremely low trace element contents, similar to peridotites from modern fore-arc peridotites (South Atlantic) and from ophiolites related to supra-subduction zones (Semail, Cyclops, Pindos, Troodos). Melagabbros also have U-shaped REE patterns similar to xenoliths from the Philippine island arc, but also similar to intrusive ultramafic cumulates from the Semail nappe of Oman related to a proto-subduction setting. A wehrlite has a REE pattern similar to that of amphibole peridotites reflecting metasomatism of clinopyroxene-bearing peridotites due to subduction-related fluids. The Maures spinel peridotites and melagabbros are therefore interpreted as the lowermost parts of a crustal sequence and minor residual mantle of lithosphere generated in a supra-subduction zone during Early Palaeozoic time. Garnet–spinel peridotites are chemically close to melagabbros, but have recorded high pressure metamorphism before their retrogression similar to spinel peridotites into amphibolites to greenschists facies metamorphism. They indicate burial to mantle depths of the margin of the supra-subduction lithosphere during the Early Palaeozoic continental subduction. Both peridotite types were exhumed during the Upper Palaeozoic continental collision. Comparable observations from other Variscan-related peridotites, in particular of the Speik complex of the Autroalpine basement, and a common age for the subduction stage allow extension of these regional conclusions to a broad area sharing the Cambrian suture zone, extending from the Ossa-Morena to the Bohemian massif.     

Toward an early warning system for dengue prevention: modeling climate impact on dengue transmission

Dengue fever is the most prevalent mosquito-borne viral disease of humans in tropical lands. As an efficient vaccine is not yet available, the only means to prevent epidemics is to control mosquito populations. These are influenced by human behavior and climatic conditions and thus, need constant effort and are very expansive. Examples of succeeded prevention are rare because of the continuous reintroduction of virus or vector from outside, or growing resistance of mosquito populations to insecticides. Climate variability and global warming are other factors which may favour epidemics of dengue. During a pilot study in Claris EC project, a model for the transmission of dengue was built, to serve as a tool for estimating the risk of epidemic transmission and eventually forecasting the risk under climatic change scenarios. An ultimate objective would be to use the model as an early warning system with meteorological forecasts as input, thus allowing better decision making and prevention.     

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.     

Origine mantélique des benmoreites de Moorea (Iles de la Societé, Pacifique)

Lavas from Moorea form two groups of about the same volumetric importance: a basalt hawaiite group and a mugearite benmoreite trachyte group. Although more differentiated, the rocks of the second group cannot be related to the first by simple fractional crystallization. A model of fractional fusion or the same mantle source may explain the relationship between the two groups. The primary magmas generated by fractional fusion were subsequently modified by low pressure fractionation.     

Rare earth geochemistry of fused ophiolitic and alpine lherzolites

Partial fusion hypotheses have been proposed for the origin of lherzolite-harzburgite alpine peridotite associations. Analyzed lherzolites from Othris, Ronda, Lanzo and Beni Bouchera, have light REE depleted to chondritic REE abundances, and clinopyroxenes contain most of the REE relative to depleted olivine and orthopyroxene. Variation in the level of REE enrichment within these lherzolites indicates mantle heterogeneity probably caused by partial melting processes. The Beni Bouchera spinel lherzolite and the Othris plagioclase lherzolite are the best candidates for relatively undepleted mantle based on REE studies. Fractional fusion calculations (15–25%) reveal that partial melts have REE characteristics somewhat similar to oceanic tholeiites. Conversely, computed source peridotites from oceanic tholeiites (Schilling, 1975) are similar to the alpine lherzolites reported here. Alpine lherzolites are, however, depleted in trace elements (K, Rb, Sr and Ba, Menzies and Murthy 1976). Since the lherzolites have an undepleted major, minor and REE chemistry close to that of pyrolite, the lost trace element-rich fraction must represent a small degree of melting. It is proposed that alpine lherzolites are residue left after the loss of a nephelinitic/alkalic fraction, ([Ce/Yb]_N=2.0–4.01) representing a small degree of partial fusion. This labile fraction may have existed as an intergranular phase or hydrous mineral prior to melting.     

A computer simulation study of OH defects in Mg2SiO4 and Mg2GeO4 spinels

Classical atomistic simulation techniques have been used to investigate the energies of hydrogen defects in Mg₂SiO₄ and Mg₂GeO₄ spinels. Ringwoodite (γ-Mg₂SiO₄) is considered to be the most abundant mineral in the lower part of the transition zone and can incorporate large amounts of water in the form of hydroxyls, whereas the germanate spinel (γ-Mg₂GeO₄) corresponds to a low-pressure structural analogue for ringwoodite. The calculated defect energies indicate that the most favourable mechanisms for hydrogen incorporation are coupled either with the reduction of ferric iron or with the creation of tetrahedral vacancies. Hydrogen will go preferentially into tetrahedral vacancies, eventually leading to the formation of the hydrogarnet defect, before associating with other negatively charged point defects. The presence of isolated hydroxyls is not expected. The same trend is observed for germanate, and thus γ-Mg₂GeO₄ could be used as a low-pressure analogue for ringwoodite in studies of water-related defects and their effect on physical properties.     

Calibration framework for a Kalman filter applied to a groundwater model

The paper presents a novel approach to the setup of a Kalman filter by using an automatic calibration framework for estimation of the covariance matrices. The calibration consists of two sequential steps: (1) Automatic calibration of a set of covariance parameters to optimize the performance of the system and (2) adjustment of the model and observation variance to provide an uncertainty analysis relying on the data instead of ad-hoc covariance values. The method is applied to a twin-test experiment with a groundwater model and a colored noise Kalman filter. The filter is implemented in an ensemble framework. It is demonstrated that lattice sampling is preferable to the usual Monte Carlo simulation because its ability to preserve the theoretical mean reduces the size of the ensemble needed. The resulting Kalman filter proves to be efficient in correcting dynamic error and bias over the whole domain studied. The uncertainty analysis provides a reliable estimate of the error in the neighborhood of assimilation points but the simplicity of the covariance models leads to underestimation of the errors far from assimilation points.     

Plate Motion of India and Interseismic Strain in the Nepal Himalaya from GPS and DORIS Measurements

We analyse geodetically estimated deformation across the Nepal Himalaya in order to determine the geodetic rate of shortening between Southern Tibet and India, previously proposed to range from 12 to 21 mm yr^?1. The dataset includes spirit-levelling data along a road going from the Indian to the Tibetan border across Central Nepal, data from the DORIS station on Everest, which has been analysed since 1993, GPS campaign measurements from surveys carried on between 1995 and 2001, as well as data from continuous GPS stations along a transect at the logitude of Kathmandu operated continuously since 1997. The GPS data were processed in International Terrestrial Reference Frame 2000 (ITRF2000), together with the data from 20 International GNSS Service (IGS) stations and then combined using quasi- observation combination analysis (QOCA). Finally, spatially complementary velocities at stations in Southern Tibet, initially determined in ITRF97, were expressed in ITRF2000. After analysing previous studies by different authors, we determined the pole of rotation of the Indian tectonic plate to be located in ITRF2000 at 51.409±1.560° N and ?10.915±5.556°E, with an angular velocity of 0.483±0.015°. Myr^?1. Internal deformation of India is found to be small, corresponding to less than about 2 mm yr^?1 of baseline change between Southern India and the Himalayan piedmont. Based on an elastic dislocation model of interseismic strain and taking into account the uncertainty on India plate motion, the mean convergence rate across Central and Eastern Nepal is estimated to 19±2.5 mm yr^?1, (at the 67% confidence level). The main himalayan thrust (MHT) fault was found to be locked from the surface to a depth of about 20 km over a width of about 115 km. In these regions, the model parameters are well constrained, thanks to the long and continuous time-series from the permanent GPS as well as DORIS data. Further west, a convergence rate of 13.4±5 mm yr^?1, as well as a fault zone, locked over 150 km, are proposed. The slight discrepancy between the geologically estimated deformation rate of 21±1.5 mm yr^?1 and the 19±2.5 mm yr^?1 geodetic rate in Central and Eastern Nepal, as well as the lower geodetic rate in Western Nepal compared to Eastern Nepal, places bounds on possible temporal variations of the pattern and rate of strain in the period between large earthquakes in this region.