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951.
Triassic basalt of the Middle Atlas has been subject to metamorphic transformation then weathering. Occurrence in both metabasalt and saprolite of ubiquitous clay minerals, such as smectite and mixed layers chlorite–smectite, makes it difficult to distinguish between the two alteration facies and explains the interest of complementary sources of information. In the Bhallil weathering profile, petrographical and mineralogical analyses of primary igneous minerals and their alteration products coupled with Fe oxidation state determination in clay fractions allow to identify three alteration facies: (i) metamorphic basalt, where iron occurs mainly as the ferrous form; (ii) the lower part of saprolite, where iron is partially oxidized to its ferric form; (iii) the upper part of saprolite, where iron is completely oxidized. To cite this article: A. Dekayir et al., C. R. Geoscience 334 (2002) 877–884. 相似文献
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954.
Christina Widiwijayanti Christel Tiberi Christine Deplus Michel Diament Valentin Mikhailov Rmy Louat 《Tectonophysics》2004,386(3-4):203-222
In order to better understand the tectonic framework of the Northern Molucca Sea area, we inverted satellite and sea-surface gravity data into an iterative scheme including a priori seismological and geological data. The resulting 3-D density model images the various tectonic units from the surface down to 40 km. We proceed to various tests to assess the stability and robustness of our inversion. In particular, we performed an offset and average smoothing method to properly refine our results. The resulting model shows a striking vertical regularity of the structures through the different layers, whereas the density contrasts appear strongly uneven in the horizontal direction.The density model emphasizes the complexity of the upper lithospheric structure in the northern Molucca Sea, which is clearly dominated by the interaction between ophiolitic ridges, sedimentary wedges and rigid blocks of the Philippine Sea Plate. It also provides new, hard information that can be used in discussion of the evolution of the region.Large density variations are concentrated in the central part of northern Molucca Sea and dominate the upper lithospheric. North–south trending density structures along the Central Ridge and west dipping thrust faults on the western side of the region are clearly imaged. In the eastern part of the region, we distinguish several blocks, especially the Snellius Plateau which seems to be split into two parts. We interpret this as an oceanic plateau associated with thicker crust that previously belonged to the Philippine Sea Plate. This crust is now trapped between the Molucca Sea complex collision zone and the Philippine Trench, due to the development of a new subduction zone in its eastern side. 相似文献
955.
Michel Cathelineau Serge Fourcade Stephane Buschaert Davy Rousset Alain Meunier Marc Javoy 《Geochimica et cosmochimica acta》2004,68(11):2529-2542
Isotopic age determination of fluid events in basement fractures is of prime importance for the reconstruction of paleo-hydraulic regimes. However, material from fracture or wall-rock is not generally easily dated. In the case of fractures filled by minerals such as phyllosilicates, it can be expected that subsequent fluid circulation may cause partial alteration or recrystallization of the previous phyllosilicates, disturbing the isotope signature. Using a specific methodology, including the extraction process without contamination of the finest (<0.2 μm) authigenic fractions, identification of clay fractions by X-ray diffraction (XRD), scanning electron microscopy, and electron microprobe, analysis of stable isotope composition (D/H, δ18O) and K-Ar dating, fluid event ages can be estimated. This methodology was applied to the northwestern margin of the French Massif Central, where basinal brines have interacted with the sedimentary cover (infra-Toarcian formations) and the fractured Hercynian crystalline basement. During periods of unknown age, precipitates from brines sealed most basement fractures as carbonates, and a recrystallization of early Hercynian phengite (+chlorite) into illite and illite/smectite minerals occurred in the earlier fractures.XRD patterns show that the clay fraction is dominated by illite and ordered mixed-layer minerals having a relatively high amount of illite (I-S R3). In the shallowest samples beneath the paleo-surface, a late assemblage is identified as a mixture of I-S minerals, a part of them being characterized by significant amounts of swelling layers (I-S R0).The δD values of the clay fractions are fairly constant around −50 ± 10‰, but δ18O values of the same clay fractions display a wide range from 8‰ to 18‰ (SMOW). The study of the size fractions between 2 and 0.2 μm reveals a correlation between K-Ar ages and δ18O values, which cannot be related to a change in the relative amounts of clay minerals. Deep fractures (at depths of 570 and 923 m) are characterized by δ18O values around 10.3 ± 0.4‰ SMOW and old ages ranging from 253 to 272 Ma. The finest fractions (<0.2 μm) of clays located at shallow level (<300 m) beneath the paleo-surface yield the highest δ18O values (15.7-18.2‰) and the youngest K-Ar dates (188 to 198 Ma). These dates are likely to record the extensional episode and thermal anomaly related to the rifting of central Atlantic Ocean (c. 190 Ma). The other important thermal/extensional episode which affected the Aquitaine Basin, namely the opening of the Gascogne gulf at c. 120 Ma, is not detected by the K-Ar method on the clay fractions. We believe that resetting of clay-type K-Ar ages linked to recrystallization was favoured in the upper part of the basement where the ancient (Hercynian) clay minerals were destabilized by subaerial low-temperature alteration during Permo-Triassic times.From a methodologic point of view, the analytical characterization of several size fractions of clays from reactivated fractures, using both stable isotope geochemistry and K-Ar isotope geochemistry, can be considered as a powerful tool for dating fluid events when no suitable material for conventional methods is available. 相似文献
956.
Recent studies of lake-level fluctuations during the last deglaciation in eastern France (Jura Mountains and Pre-Alps) and on the Swiss Plateau show distinct phases of higher water level developing at the beginning and during the latter part of Greenland Stade 1 (i.e., Younger Dryas event) and punctuating the early Holocene period at 11,250-11,050, 10,300-10,000, 9550-9150, 8300-8050, and 7550-7250 cal yr B.P. The phases at 11,250-11,050 and 8300-8050 cal yr B.P. appear to be related to the cool Preboreal Oscillation and the 8200 yr event assumed to be associated with deglaciation events. A comparison of this mid-European lake-level record with the outbursts from proglacial Lake Agassiz in North America suggests that, between 13,000 and 8000 cal yr B.P., phases of positive water balance were the response in west-central Europe to climate cooling episodes, which were induced by perturbation of the thermohaline circulation due to sudden freshwater releases to oceans. This probably was in response to a southward migration of the Atlantic Westerly Jet and its associated cyclonic track. Moreover, it is hypothesized that, during the early Holocene, varying solar activity could have been a crucial factor by amplifying or reducing the possible effects of Lake Agassiz outbursts on the climate. 相似文献
957.
958.
Jean-Claude André Michel Déqué Philippe Rogel Serge Planton 《Comptes Rendus Geoscience》2004,336(6):491-503
The main features of the heat wave that has affected western Europe during the summer 2003 are first recalled, with particular concern to analogies and differences with previous warm-summer episodes. The skills of the various operational and pre-operational seasonal forecasting models in use at that time are evaluated. None of them has been able to consistently forecast the heat wave three month in advance. The meteorological and climatic situation prevailing during spring 2003 is then analysed. A strong sea-surface-temperature (SST) anomaly is found to have developed over North Atlantic a few months before the heat wave. The particular mechanism at work to explain the occurrence of the heat wave is not a ‘simple downwind influence’ of the SST anomaly. It very likely involves complex ocean–atmosphere interaction phenomena of a kind that is not properly taken into account in today's seasonal forecasting models. To cite this article: J.-C. André et al., C. R. Geoscience 336 (2004). 相似文献
959.
The Kibaran belt that extends all over the central Africa, from the Katanga to the southern Uganda, straddles the African great lakes area. The Kibaran stratigraphy is one of the most debated questions. Some geologists favoured two different systems: the Ruzizian system (1800–2000 Ma) and the Burundian or Kibaran system (1600–960 Ma). Some others favoured a single Burundian system with large areas granitised and metamorphosed during several Burundian tectonic events and with a lot of sedimentary fold belts of the Burundian age. Recent geological data and new remote sensing interpretations allow us to favour the second hypothesis but with a major disconformity within the Burundian system which is separating the Upper and the Lower Burundian. To cite this article: M. Villeneuve, J. Chorowicz, C. R. Geoscience 336 (2004). 相似文献
960.
The flooding of the abandoned iron mines of Lorraine leads to a degradation of the groundwater quality. Laboratory experiments allowed us to build a kinetic chemical model based on simple chemical mechanisms. During mining operations, pyrite oxidation and carbonate dissolution lead to the precipitation of gypsum. The local decrease of pH favours local dissolutions of minerals and releases ions that are fixed on cationic exchange sites. During the flooding of the mine workings, the dissolution of the newly precipitated gypsum, the precipitation of carbonates and cationic exchange reactions are responsible for the increase of the concentrations of sulphate, magnesium, sodium, potassium and strontium in water. Thereafter, these concentrations would decrease with the natural leaching of the mining reservoir. To cite this article: P. Collon et al., C. R. Geoscience 336 (2004). 相似文献