Evidence for pre-orogenic,Early Devonian rifting in the Variscan belt: stratigraphy and structure of the Palaeozoic cover of the Mauges Unit (Upper Allochthon,Armorican massif,France)     

Céline Ducassou  Michel Ballèvre  Hubert Lardeux  Cécile Robin 《International Journal of Earth Sciences》2011,100(7):1451-1475

The Palaeozoic sediments from the Mauges Unit (Armorican Massif, France) are the best-preserved pre-orogenic sequences belonging to the Upper Allochthon. Two coherent sequences are identified. The southern unit (Chateaupanne Unit) represents the cover of the Proterozoic basement and consists of Ordovician sediments unconformably overlain by Emsian carbonates followed by Emsian to earliest Eifelian immature sandstones. The northern unit (Tombeau Leclerc Unit) consists of an Hirnantian to Emsian condensed sequence, in reverse position, that has been thrust over the southern unit. The Devonian unconformity is interpreted as evidence for an Early Devonian extension, recorded by normal faults affecting both the Early Devonian limestones and the underlying Ordovician series. This crustal extension, recorded here for the first time, is possibly related to the opening of a back-arc basin (Saint-Georges-sur-Loire) associated with the subduction of an ocean located further south (Galicia-Brittany-Massif Central Ocean).  相似文献   

Electron precipitation and mass motion in the 1991 June 9 white-light flare     

J. -F. De La Beaujardière  R. C. Canfield  T. R. Metcalf  E. Hiei  T. Sakurai  K. Ichimoto 《Solar physics》1994,151(2):389-392

We use H line profiles as a diagnostic of mass motion and nonthermal electron precipitation in the white-light flare (WLF) of 1991 June 9 01:34 UT. We find only weak downflow velocities (10 km s^–1) at the site of white-light emission, and comparable velocities elsewhere.We also find that electron precipitation is strongest at the WLF site. We conclude that continuum emission in this flare was probably caused by nonthermal electrons and not by dynamical energy transport via a chromospheric condensation.  相似文献   

Distribution of carbon in a tropical hypersaline estuary,the Casamance (Senegal,West Africa)     

J. Pagès  J. Lemoalle  B. Fritz 《Estuaries and Coasts》1995,18(3):456-468

The Casamance estuary, on the coast of Senegal, is an inverse hypersaline estuary: salinity increases landward, and dry season salinity values are up to 172 psu due to the evaporation of seawater. Dissolved inorganic carbon (DIC) concentrations decreased landward as a negative linear function of salinity. Thermodynamic modelling and the absence of CaCO₃ in the sediments indicate that this loss of DIC was not due to calcite precipitation in the main water body. The innermost, almost landlocked, waters contained high phytoplankton biomass (50–300 μg chl I^?1) and high concentrations of allochthonous dissolved organic carbon. Photosynthetic uptake of DIC and subsequent particulate organic carbon sedimentation is proposed as hypothetical explanation of the relationship between DIC and salinity; localized overheating in shallow waters might also be involved.  相似文献   

Water quality in rivers of western Switzerland: Application of an adaptable index based on benthic invertebrates   总被引：1，自引：0，他引：1  

Claude Lang  Geneviève l'Eplattenier  Olivier Reymond 《Aquatic Sciences - Research Across Boundaries》1989,51(3):224-234

Water quality was estimated from 205 samples of benthic invertebrates collected between 1982 and 1986 in 51 rivers of western Switzerland (canton of Vaud). Each sample consisted of the combined list of taxa resulting from one spring sample pooled with one summer sample. Water quality was indicated by total number of taxa and number of taxa intolerant of pollution: i. e. Heptageniidae, Plecoptera, and Trichoptera with a case. Six classes of values were delimited for each of these two variables by cluster analysis. Values from zero to five were attributed to each class. The index of water quality was computed by adding these two values in each sample. According to this index, good water quality was indicated by 42% of samples. This index can be adapted to other rivers because its components are easily modified.  相似文献   

Constraints on the formation of cyclic units in ultramafic zones of large basaltic chambers     

Geneviève Brandeis 《Contributions to Mineralogy and Petrology》1992,112(2-3):312-328

Petrological models for the formation of cyclic units in ultramafic zones at the base of large mafic layered intrusions are still few and simple. In this study, we develop simple physical constraints, such as the volume balance and density relationships between the various liquids involved. We consider the formation of an entire ultramafic zone, made of N cyclic units due to N successive reinjections. We tackle the following problems. Are all the injections of the same chemical composition? Are the N injections responsible for the formation of the whole magma chamber? Two end-member models are examined. In the first one, the chamber grows with each injection and does not lose any magma (variable volume model, VV). In the second one, the chamber initially forms with a large volume of magma and remains at a constant volume with subsequent reinjections of small volumes balanced by eruptions of equal volume (fixed volume model, FV). Several scenarios for the formation of a cyclic unit are envisioned. Petrological data such as thicknesses of the cumulate layers and their compositions, together with the fractionation density of the cumulate phases, allow the calculation of the density evolution, as a function of the initial density only. Thus, the model yields constraints on the density of the initial liquid, and hence on its chemical composition. The volumes of the injected magmas are calculated as well as the evolution of the densities of the different involved liquids. We apply the calculation to the specific case of the Ultramafic Series of the Stillwater complex. The multiple reinjection models require that the initial liquids densities vary between 2.71 and 2.72 g/cm³, corresponding to MgO-rich liquids for which there is no field evidence, either in dikes or in chilled margins. They also require that the source produces liquids of different chemical composition. Thus, the evolution of the igneous system requires another, larger reservoir where differentiation takes place prior to injection into the shallower one. Contrary to the multiple reinjection models, a closed system model where crystallization proceeds in an isolated bottom layer does not require any specific value of density for the liquids, provided, of course, that it is of basaltic composition. On the negative side, such a model is not supported by any physical processes which would account for the formation of cyclic units.  相似文献   

The dynamic cusp at low altitudes: a case study utilizing Viking,DMSP-F7, and Sondrestrom incoherent scatter radar observations     

J. Watermann  O. de la Beaujardière  D. Lummerzheim  J. Woch  P. T. Newell  T. A. Potemra  F. J. Rich  M. Shapshak 《Annales Geophysicae》1994,12(12):1144-1157

Coincident multi-instrument magnetospheric and ionospheric observations have made it possible to determine the position of the ionospheric footprint of the magnetospheric cusp and to monitor its evolution over time. The data used include charged particle and magnetic field measurements from the Earth-orbiting Viking and DMSP-F7 satellites, electric field measurements from Viking, interplanetary magnetic field and plasma data from IMP-8, and Sondrestrom incoherent scatter radar observations of the ionospheric plasma density, temperature, and convection. Viking detected cusp precipitation poleward of 75.5○ invariant latitude. The ionospheric response to the observed electron precipitation was simulated using an auroral model. It predicts enhanced plasma density and elevated electron temperature in the upper E- and F-regions. Sondrestrom radar observations are in agreement with the predictions. The radar detected a cusp signature on each of five consecutive antenna elevation scans covering 1.2 h local time. The cusp appeared to be about 2○ invariant latitude wide, and its ionospheric footprint shifted equatorward by nearly 2○ during this time, possibly influenced by an overall decrease in the IMF B_z component. The radar plasma drift data and the Viking magnetic and electric field data suggest that the cusp was associated with a continuous, rather than a patchy, merging between the IMF and the geomagnetic field.  相似文献   

A comparison between transilient turbulence theory and the exchange coefficient model approaches     

J. Cuxart  P. Bougeault  P. Lacarrère  J. Noilhan  M. R. Soler 《Boundary-Layer Meteorology》1994,67(3):251-276

The transilient turbulence model of Stull and Driedonks (1987) and the exchange coefficient model of Bougeault and Lacarrère (1989) are intercompared on four convective days of the HAPEX-MOBILHY experiment. We show that the numerical formulations of the models are very close to each other, despite their totally different theoretical backgrounds. Both models describe the mixing effect of the turbulence via matrices having the same general characteristics. The results of the models are also found to be very similar in general, although the transilient approach requires more computations. Some systematic differences are noted and discussed in detail. We argue that similar work should be conducted on non-convective cases before deciding on the superiority of one or the other approach.  相似文献   

Petrogenesis of the Swartruggens and Star Group II kimberlite dyke swarms,South Africa: constraints from whole rock geochemistry   总被引：1，自引：0，他引：1  

Nancy Coe  Anton le Roex  John Gurney  D. Graham Pearson  Geoff Nowell 《Contributions to Mineralogy and Petrology》2008,156(5):627-652

Thirty-seven samples from the Swartruggens and Star Group II kimberlite dyke swarms, emplaced through the Kaapvaal craton, have been analysed for their major and trace element and Sr, Nd and Hf isotope compositions. The samples are all MgO-rich (~12–35 wt%) with high Mg# (0.72–0.90) and Ni (~610–2700 ppm) contents. The kimberlites are strongly enriched in incompatible elements (Zr = 140–668 ppm; La = 124–300 ppm; Nb = 68–227 ppm; Ba = 1500–7000), and have high and variable chondrite normalised La/Yb ratios (Swartruggens = 94 ± 21; Star = 202 ± 36). ⁸⁷Sr/⁸⁶Sr (0.70718–0.71050) ratios are elevated, whereas εNd (−11.95 to −7.84) and ¹⁷⁶Hf/¹⁷⁷Hf ratios (0.282160–0.282564) are low. Inter- and intra-dyke compositional variation is significant, and there are systematic differences between the kimberlites found at the two localities. Intra-locality differences can largely be attributed to a combination of the effects of alteration, crustal contamination, macrocryst entrainment and phenocryst fractionation. There is some evidence for distinct parental magmas formed through variable and low degrees (0.5–2%) of partial melting, as illustrated by crossing rare earth element patterns. The Star kimberlites have derived from a less radiogenic source, with higher LREE enrichment than the Swartruggens kimberlites. Inferred primary magmas at each locality have high Mg# (~0.83), are Ni-rich (850–1220 ppm) and are strongly enriched in incompatible elements. Calculated mantle source compositions are strongly enriched in incompatible elements (La/Yb_n ~ 10–50), but refractory in terms of Mg# and Ni contents. Incompatible element ratios such as Ba/Nb (>13.5), La/Nb (> 1.1) and Ce/Pb (< 22) are unlike those characteristic of Group I kimberlites or ocean island basalts, but indistinguishable from calc-alkaline magmas. Taken together with extremely low εNd and εHf, these compositional characteristics are used to argue for derivation of these Group II kimberlite magmas from the deep subcontinental lithospheric mantle, metasomatised during the Proterozoic by calc-alkaline fluids/melts.  相似文献   

The Effect of pH on Electro-osmotic Flow in Argillaceous Rocks     

Mohamed Younes Ahmed  Said Taibi  Hanène Souli  Jean-Marie Fleureau 《Geotechnical and Geological Engineering》2013,31(4):1335-1348

The use of electro-osmosis in geotechnical engineering has been considered since 1930. Its application has been tested in several sectors like the fight against rising damp in porous materials, the consolidation of soils as well as the remediation of the soils contaminated by oil and heavy metals, etc. The paper presents an experimental study of the electrokinetic phenomena resulting from the application of an electric field to argillaceous sandstones. The electroosmotic tests required the development of a completely new experimental device composed of a confining cell, of a system of measurement and control of the pH of the electrolyte near the electrodes, of a system of control and measurement of the voltage and current at the edges of the sample, and of a system of measurement of inlet (anodic) and outlet (cathodic) flows. Various boundary conditions are considered with and without control of pH. In addition, a micro structural modelling of the electrokinetic phenomena makes it possible to quantify the electro osmotic conductivity with a good agreement with experiments.  相似文献   

Combined U/Pb and (U–Th)/He geochronometry of basalt maars in Western Carpathians: implications for age of intraplate volcanism and origin of zircon metasomatism     

Vratislav Hurai  Martin Danišík  Monika Huraiová  Jean-Louis Paquette  Alexander Ádám 《Contributions to Mineralogy and Petrology》2013,166(4):1235-1251

The age of intraplate volcanism in northern Pannonian Basin of Carpathians is revisited using a combination of zircon U/Pb, zircon (U–Th)/He and apatite (U–Th)/He dating techniques, complemented by electron microprobe (EMP) characterisation of dated minerals. A total of six maar structures and diatremes in the South-Slovakian Volcanic Field (SSVF) were dated and the obtained new ages yielded the following key findings: Two isolated maars in SE part indirectly dated by geomorphologic constraints to Late Pleistocene are actually of Pliocene (2.8 ± 0.2 Ma) and Late Miocene (5.5 ± 0.6 Ma) ages. In contrast, two maars in NW part of the study area are of Late Pliocene age (4.1 ± 0.4 and 5.2–5.4 Ma), younger than the Late Miocene age (~6.5 Ma) inferred previously from K/Ar data on the proximal basaltic lava flows. These maars therefore belong to the second volcanic phase that was previously identified only in SE part of the SSVF. In the light of the new geochronologic data, it seems likely that the Pliocene phreatomagmatic eruptions may have occurred along extension-related, NW- and NE-trending orthogonal faults. EMP analyses and imaging revealed an extensive syn- and post-growth metasomatic replacement by dissolution-reprecipitation in the majority of zircons. Abundant silicate melt inclusions in porous metasomatised parts of the zircons are diagnostic of magmatic rather than hydrothermal metasomatism. Consistent ages of the metasomatised and non-metasomatised zones do not indicate disturbance of the U–Pb system during the metasomatism. Enrichment in U and Th loss in the metasomatised zircons are diagnostic of an increasing oxygen fugacity triggered by degassing of the volatile residual melt during the final stages of alkali basalt fractionation. Rare zircon-to-baddeleyite transformation was probably connected with lowered silica activity in carbonated basaltic magmas in south-eastern part of the study area.  相似文献