Tectonique cassante et champ de contraintes tertiaire en avant des Alpes orientales: le Jura souabe

Résumé La reconnaissance de la fracturation dans le Jura souabe, et son analyse qualitative et quantitative (par traitement informatique) a permis de distinguer cinq épisodes tectoniques cassants, caractérisés successivement par: une distension N-S, des compressions N-S et SW-NE, une distension E-W et une compression ESE-WNW. Ces états de contrainte au sein de la plateforme européenne, en avant de la zone de collision Afrique-Europe, traduisent les mouvements relatifs de ces plaques au cours du Tertiaire.

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Scouting of fracturation in Swabian Jura, and qualitative and quantitative (by informatical treatment) analysis allow to distinguish five breacking tectonic episodes caracterized by: (1) a N-S distension, (2) N-S and (3) NE-SW compressions, (4) a E-W distension and (5) a ESE-WNW compression. These stress fields inside the european platform, in front of collision zone Africa-Europa, attest relative motions of these plates during Tertiary.

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Zusammenfassung Im Schwäbischen Jura lassen die qualitative und die quantitative Aufnahme der Frakturierung sowie ihre EDV-Analyse fünf aufeinander folgende Phasen der tektonischen Beanspruchung (tectonique cassante) erkennen: (1) Zerrung N-S, (2) Kompression N-S und (3) SW-NE, (4) Zerrung E-W, (5) Kompression ESE-WNW. Diese Spannungszustände in der Kollisions-zone von Afrika und Europa vorgelagerten europäischen Plattform geben ein Bild der Relativbewegungen dieser beiden Kontinentalplatten im Tertiär.

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EDV 5 : 1. N-S, 2. N-S 3. SW-NE, 4. E-W, 5. ESE-WNW. .

     

Sulfur speciation and associated trace metals (Fe,Cu) in the pore waters of Great Marsh,Delaware

The pore waters of sediments from a salt marsh along the Delaware estuary have been analyzed for sulfur species and associated trace metals. Since the sediment interface is usually in contact with the atmosphere, the sulfur species are dependent on the production of hydrogen sulfide by sulfate reduction and subsequent oxidation by diffusing oxygen. The most important species observed are hydrogen sulfide, polysulfide ions and thiosulfate. Secondary reactions of hydrogen sulfide and polysulfides with decomposing organic matter yield significant concentrations of both thiols and organic polysulfides. Upon isolation of the sediment from the atmosphere due to tidal inundation, bacterial sulfate reduction becomes the dominant process. This results in the reduction of the polysulfides in agreement with thermodynamic predictions, and suggests that the redox couple sulfide/polysulfide is a good redox indicator under such reducing environments.The concentrations of trace elements Cu and Fe in the pore waters are mainly controlled by sulfide formation. Calculations show that copper is strongly complexed probably with organo-sulfur ligands. Iron might be complexed as such sulfur species to a much lesser extent than copper.     

On the velocity of jets powering hot spots similar to those in Cygnus A

Hot spots similar to those in the radio galaxy Cygnus A can be explained by the strong shock produced by a supersonic but classical jet \(\left( {u_{jet}< c/\sqrt 3 } \right)\) . The high integrated radio luminosity (L?2×10⁴⁴ erg s^?1) and the strength of mean magnetic field (B?2×10^?4 G) suggest the hot spots are the downstream flow of a very strong shock which generates the ultrarelativistic electrons of energy ?≥20 MeV. The fully-developed subsonic turbulence amplifies the magnetic field of the jet up to 1.6×10^?4 G by the dynamo effect. If we assume that the post-shock pressure is dominated by relativistic particles, the ratio between the magnetic energy density to the energy density in relativistic particles is found to be ?2×10^?2, showing that the generally accepted hypothesis of equipartition is not valid for hot spots. The current analysis allows the determination of physical parameters inside hot spots. It is found that:

1. The velocity of the upstream flow in the frame of reference of the shock isu ₁?0.2c. Radio observations indicate that the velocity of separation of hot spots isu _sep?0.05c, so that the velocity of the jet isu _jet=u ₁+u _sep?0.25c.
2. The density of the thermal electrons inside the hot spot isn ₂?5×10^?3 e ^? cm^?3 and the mass ejected per year to power the hot spot is ?4M ₀yr^?1.
3. The relativistic electron density is less than 20% of the thermal electron density inside the hot spot and the spectrum is a power law which continues to energies as low as 30 MeV.
4. The energy density of relativistic protons is lower than the energy density of relativistic electrons unlike the situation for cosmic rays in the Galaxy.

     

Mn-oxide: a major source of easily mobilisable Co and Ni under reducing conditions in New Caledonia FerralsolsLes oxydes de Mn : une source majeure de Co et Ni facilement mobilisables en conditions réductrices dans les Ferralsols de Nouvelle-Calédonie

Chemical and bacterial reduction and dissolution of Fe and Mn-oxide and the concomitant solubilisation of Co and Ni were studied in a surface horizon of a New-Caledonia Ferralsol. Chemical extractions showed that Mn and Co were in a large part associated in Mn-oxides. The main part of Ni was associated with goethite, but a very small fraction was also associated with Mn-oxides. Anaerobic reducing bacterial activity was responsible for Fe solubilisation at a smaller extent than for Mn solubilisation and consequently for associated metal release. Submicroscopic investigations revealed the presence of a Mn-oxide containing Co, Ni and Al, close to a lithiophorite–asbolane mixed-layers Mn-oxide, which can be considered as a main source of easily available metals in this soil. To cite this article: C. Quantin et al., C. R. Geoscience 334 (2002) 273–278.     

Premières mesures françaises de gravimétrie absolue dans les régions polaires Antarctique (terre Adélie) et Arctique (Spitzberg)First French measurements of absolute gravity in the Antarctic (Terre Adélie) and Arctic (Spitsbergen) polar regions

This study is devoted to the first French determinations of absolute gravity in the Antarctic (Dumont-d'Urville, Terre Adélie) and Arctic (Ny-Alesund, Spitsbergen) polar regions. The measurements in Dumont-d'Urville were performed in 2000 with the help of the FG5#206 absolute gravimeter belonging to the French geodetic community since beginning 1997; they show a strong noise that causes an uncertainty close to 11 μGal in the determination of the mean gravity value, which will be compared, to a new determination planned for 2005. The Ny-Alesund measurements show on the contrary a noise that, although very variable in time, leads to a gravity determination of better than 5 μGal. The comparison of our value with a previous one two years before suggests a gravity decrease of about 4 μGal which is related to the vertical uplift measured by Very Large Baseline Interferometry (VLBI) at the same site. However, the uncertainty does not allow discriminating between height-to-gravity conversion factors originating from different models. To cite this article: J. Hinderer et al., C. R. Geoscience 334 (2002) 819–826.     

Les « Sables fauves », témoins de mouvements tectoniques dans le bassin d'Aquitaine au Miocène moyenThe ‘Sables fauves’, showing tectonic deformations in the Aquitaine Basin during Middle Miocene

New data about the ‘Sables fauves’ clearly show two different kinds of organisations: stratigraphic superposition of Langhian and Serravallian units to the south and embankment of Serravallian sediments in Langhian deposits to the north. The result consists in two different palaeogeographic schemes. The Langhian Sea is widely extended, forming the ‘Lectoure Gulf’, whereas the Serravallian Sea is much more reduced, with valleys fillings to the north and a little gulf to the south. This involves a tectonic activity along a structure which orientation is N120°E (‘flexure de la Douze’) and a relative rising of the northern part, of about 30 m. To cite this article: P. Gardère et al., C. R. Geoscience 334 (2002) 987–994.     

A Semi-Evolutive Partially Local Filter for Data Assimilation

The singular evolutive extended Kalman (SEEK) filter has been proposed recently by Pham et al. (1997) for data assimilation into numerical oceanic models. This filter has been applied in different realistic ocean frameworks and has provided satisfactory results ( Pham et al., 1997; Verron et al., 1998). However, the SEEK filter remains expensive in real operational assimilation. To reduce cost and obtain a better representativity, we introduce the idea ‘local correction basis'. Such basis however cannot be made to evolve according to the model without destroying its locality property. Therefore we shall keep this basis fixed and we augment it by a few global basis vectors which evolve. The resulting semi-evolutive partially local filter is much less costly to implement than the SEEK filter and yet can yield better results. In the first application, validation twin experiments are conducted in a realistic setting of the OPA model over the tropical Pacific Ocean.