Paleomagnetic data bearing on the age of high terrace deposits (Durance sequence) in Alpine valleys of Southeastern France     

Michel Dubar  Franois Semah 《Quaternary Research》1986,25(3)

In the Durance valley, high terrace levels are stepped below the young. Villafranchian surface and separated from the middle and low terraces by a step 80 m high. A paleomagnetic study was made of a key bed of freshwater silt on the high terrace, that contains archaic faunal and floral elements. The section contains two sedimentary cycles with opposite polarities, which are therefore placed on either side of the Brunhes-Matuyama boundary. By comparison with the northern European climato-stratigraphy, this sequence is correlated with the base of the Cromerian complex. Taking into account the morphological relations to the upland morainic system, this youngest high terrace is assigned a Günz age.  相似文献   

Crustal scale balanced cross-sections of the Pyrenees; discussion     

Michel Seguret  Marc Daignieres 《Tectonophysics》1986,129(1-4)

From surface and subsurface data, line-length and area balancing were used to construct four balanced and restored sections of the Pyrenees. In the Mesozoic cover, a thin-skinned tectonic model is used. In the basement an anticlinal stack geometry is applied for the foreland part of the thrust nappes. We present and discuss three possible models for the deep structures of the belt: a thin-skinned tectonic model, a thick-skinned tectonic model and an inhomogeneous strain model. The thrusts steepen downwards and the displacements die out in ductile deformation deep in the section. Therefore, we use the inhomogeneous strain model and we equal-area balance the surface of the continental crust.Hanging-wall sequence diagrams are constructed taking into account (1) the strong N-S thickness variations of the Mesozoic cover related to the Cretaceous drift of Spain and (2) the related crustal thinning of the North Pyrenean Zone superimposed upon a previous late Hercynian rise of the lower crust.The Moho step at the vertical of the North Pyrenean Fault results from the thinning of the North Pyrenean Zone. The thickening of both the Axial Zone and the North Pyrenean Zone during the Eocene compressional event preserved the step geometry.Calculated values of the minimum shortening range from 55 km in the western part of the belt to 80 km in the eastern part. Most of the shortening occurs south of the North Pyrenean Fault in the eastern part (Axial Zone) and north of the North Pyrenean Fault in the western part (Labourd thrust).  相似文献   

A chlorite solid solution geothermometer the Los Azufres (Mexico) geothermal system   总被引：24，自引：1，他引：24  

Michel Cathelineau  David Nieva 《Contributions to Mineralogy and Petrology》1985,91(3):235-244

Chlorite constitutes a major hydrothermal alteration product of metamorphism of andesites, in the active geothermal system of Los Azufres (Mexico). Electron microprobe analyses performed on a set of crystals from each sample show wide variations in composition. Correlation coefficients among chemical constituents were calculated. It is shown that the tetrahedral charge is positively correlated with the octahedral vacancy and negatively with the iron content, and there is almost no correlation with the octahedral aluminium and magnesium content. A procedure is proposed to select end-members and substitution vectors, and to give a general formula for these chlorites.Their formation temperatures are estimated with great accuracy, combining results of microthermometric data on fluid inclusions from gangue minerals of chlorites (quartz, calcite), direct measurements in wells (Kuster equipment), and chemical geothermometers. Correlations between chlorite compositions, range and nature of site occupancy, and temperature are good. Formation temperatures of chlorites range from 130° C to 300° C. As no other thermodynamic parameter varies significantly in the studied field (composition of the host rocks, nature of the geothermal fluids, pressure, ...), these variations of site occupancy (mainly Al(IV) and the octahedral occupancy (6-Al(VI)-(Mg+Fe(2+)) = VAC) are considered mainly as temperature dependent.Molar fractions of each end-member show very different variations with increasing temperature: X-kaolinite decreases, and X-chamosite increases, while X-talc-3 brucite does not show significant change. From these data, activity coefficients and standard state chemical potential of major components, and molar free energy formation of chlorite have been calculated for each temperature of crystallisation.  相似文献   

Past Antarctic Peninsula climate (1850–1980) deduced from an ice core isotope record     

Alberto J. Aristarain  Jean Jouzel  Michel Pourchet 《Climatic change》1986,8(1):69-89

A detailed climatic study of the Antarctic Peninsula from 1850 to 1980 has been carried out through the analysis of deuterium content in the snow layers of Dalinger Dome (James Ross Island, Antarctic Peninsula). It is based on the high correlation found between mean deuterium contents at this site and temperature data from stations within this region going back as far as April 1903 for the Argentine Orcadas station.The strong correlation between isotopes and temperatures first reveals a 1956 isotope reference for the region considered. Secondly, the isotope-temperature gradient is estimated at 4.5%. °C^–1 for deuterium.After checking that the major temperature anomalies on the Antarctic Peninsula recorded since 1904 (according to available data) correspond to annual mean stable isotope peaks at Dalinger Dome, the amplitude of four prior anomalies are estimated in °C. Finally, a cooling of about 2 °C since 1850 is suggested for the region.  相似文献   

Book reviews     

Bert Bolin  G. Shaw  Michel M. Verstraete  M. J. Salinger 《Climatic change》1986,9(3):363-366

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Correspondence analysis in the study of lithogeochemical data: General strategy and the usefulness of various data-coding schemes     

Michel Mellinger 《Journal of Geochemical Exploration》1984,21(1-3)

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Rare earths in barites: distribution and effects on aqueous partitioning     

François Guichard  Thomas M. Church  Michel Treuil  Henri Jaffrezic 《Geochimica et cosmochimica acta》1979,43(7):983-997

A large variety of barites collected from marine and continental environments was analyzed by neutron activation for the rare-earth elements (REE) La, Ce, Sm, Eu and Dy. Relative to chondrites, all barites show a decrease of the lighter REE from La toward Eu. The abundance and distribution of rare earths in barites show a distinction of barite types. Deep-sea barites have large REE concentrations as do other authigenic deep-sea minerals and display the chondrite normalized Eu minimum, but not the negative Ce anomaly, of sea water. Other barites, mostly on land, some hydrothermal, and others of shallow marine origin, display lower total Ree concentrations. Chondrite normalized positive Eu anomalies are displayed by those varieties of reducing sedimentary and metamorphic origin.Distribution of REE in barite can be attributed both to crystallographic constraints of substitution, and to solution complexing of REE in the precipitating medium. Plots of rare earth partitioning versus effective ion size suggest that the decreasing enrichment toward Eu for all barite types is caused by crystallographic constraints due to contraction of the substituting REE ion sizes relative to the size of the host Ba ion. Solution effects on REE substitution in barite can be evaluated by writing solid solution distribution equations based on mass action of REE and Ba sulfates and the lanthanide (Ln) solution species Ln (CO₃)^?5₄), LnSO⁺₄, LnCl⁺² and LnF⁺². Under normal sea water conditions, solution complexing plays a minor role. However, increased alkalinities of reducing sediments and increased brine chlorinities could cause significant complexing and deplete REE heavier than Eu. Besides Dy in barites, this could be true for aqueous precipitation of REE in general.  相似文献   

Coupes sismiques des structures superficielles dans les petites antilles—I: Guadeloupe     

Jacques Dorel  Sylvain Eschenbrenner  Michel Feuillard 《Pure and Applied Geophysics》1979,117(5):1050-1069

Summary In 1976 and 1977, seismic profiles were carried out in Guadeloupe. Two profiles were established in the area of La Soufriére volcano and one profile through the northern part of Guadeloupe and southern part of Grande Terre. The two first profiles were occupied from 1 to 30 km and the third profile between 5 and 50 km.The interpretation shows that the superficial structures are characterized by a three-layers model: the compressional velocity is about 2.7 to 3.0 km/s down to a depth from 1 to 3 km. Below this, the velocity is between 4.0 and 4.5 km/s in a layer whose thickness varies from 1 to 2.5 km. Under this layer we find a 6.0–6.1 km/s layer which is one of the two known crustal layer under Lesser Antilles. The boundary between the old and new are which form the Lesser Antilles arc, is marked by a thicker layer of sediments on the eastern flank of recent volcanic chain.

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A re-appraisal of the use of trace elements to classify and discriminate between magma series erupted in different tectonic settings   总被引：2，自引：0，他引：2  

David A. Wood  Jean-Louis Joron  Michel Treuil 《Earth and Planetary Science Letters》1979,45(2):326-336

The hygromagmatophile element composition of basic lavas from several tectonic environments are compared with the estimated composition of the primordial mantle. The observed variations are used to subdivide mid-ocean ridge basalts (MORB) into two main types according to the tectonic character of the ridge segment from which they were erupted. Ridge segments with positive residual gravity, depth and heat flow anomalies erupt E-type MORB which are predominantly enriched in the more hygromagmatophile elements, but also include magma types which are depleted in most of these elements. Both enriched and depleted E-type MORB can be distinguished from the basalts erupted at normal ridge segments (N-type MORB) by their La/Ta ratios (in E-type MORB La/Ta ～10, in N-type MORB La/Ta is ～15) and by Hf/Ta ratios (in E-type MORB Hf/Ta> 7, in N-type MORB Hf/Ta> 7). E-type MORB can be distinguished from the basalts erupted at ocean islands by their higher Hf/Ta ratios (>2). A Th-Hf-Ta triangular diagram is used to discriminate between the different ocean floor basalts as well as those erupted at destructive plate margins, which are depleted in Ta and Nb. This diagram can also distinguish between silicic lavas from the different tectonic environments as well as identifying lavas that have been contaminated with continental crust.  相似文献   

Large intensity changes of the non-dipole field during a polarity transition     

Michel Prévot 《Physics of the Earth and Planetary Interiors》1977,13(4):342-345

Directional and paleointensity data for the Steens Mountain geomagnetic polarity transition do not agree with the two simplest models of reversals: rotation of undiminished dipole or gradual diminution followed by change in polarity of the dipole moment. Instead, large and rapid changes in the intensity and direction of the field occur, probably as a result of non-dipole variations.  相似文献