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841.
Using energy and entropy constraints applicable to the Earth's core, the heat flow at the core–mantle boundary (CMB) needed to sustain a given total dissipation in the core can be computed. Reasonable estimates for the present Joule dissipation in the core gives a present heat flow of 6 to 10 TW at the CMB. Palaeointensity data acquired from rocks younger than 3.5 Ga provide support that the Joule dissipation in the core before inner core crystallization was between today's value and four times lower than today. Prior to inner core crystallization (around 1 Ga), the magnetic field was maintained by thermal convection driven by core cooling, and our calculations of the two extreme cases predict that the heat flow at the CMB at that time was either 14 to 24 TW in the case of constant dissipation, or essentially the same as today in the lower field intensity case. 相似文献
842.
Carlos Marı́n Jesús Galindo-Zaldı́var Luı́s Roberto Rodrı́guez-Fernández 《Comptes Rendus Geoscience》2003,335(2):255-264
The Campo de Dal??as, located between the central and eastern Betic Cordilleras, shows an evolution determined by the overprinting of two main stress fields since Pliocene times. The first of these develops hybrid and tensional joint sets up to Pleistocene (100 000 yr) and is characterized by NNW–SSE horizontal trend of compression and an ENE–WSW horizontal extension. The second stress field has prolate to triaxial extensional ellipsoids, also with ENE–WSW horizontal extension, and continues to be active today. The most recent stresses produce the reactivation of previous joints as faults whose trends are comprised mainly from N120°E to N170°E and have a normal and transtensional regime, with dextral or sinistral components. The palaeostress evolution of this region is similar to that undergone by other basins of the Eastern Betic Cordilleras, although the Pliocene–Pleistocene transcurrent deformations in the Campo de Dal??as only develop joints and not strike-slip faults. 相似文献
843.
The Bolcana ore deposit (Metaliferi Mountains, western Romania) is a porphyry ore deposit with associated epithermal veins. On the basis of different parageneses, four alteration types were distinguished: potassic, phyllic, argillic and propylitic. The mineralogical and geochemical data and estimated crystallisation temperatures of alteration minerals indicate an evolution of the system from an early period of porphyry type mineralisation (Cu+Au) to a late period of low-sulphidation epithermal mineralisation (Au+base metal). To cite this article: V. Milu et al., C. R. Geoscience 335 (2003). 相似文献
844.
Morteza Khalatbari-Jafari Thierry Juteau Hervé Bellon Hashem Emami 《Comptes Rendus Geoscience》2003,335(12):917-929
New field and laboratory studies on the ophiolite of Khoy (northwestern corner of Iran) lead to the discovery that there are not one, but two ophiolitic complexes in the Khoy area: (1) an old, poly-metamorphic ophiolite, whose oldest metamorphic amphiboles yielded a Lower Jurassic apparent 40K–40Ar age, and whose primary magmatic age should logically be pre-Jurassic (Upper-Triassic?); (2) a younger non metamorphic ophiolite of well dated Upper Cretaceous age. To cite this article: M. Khalatbari-Jafari et al., C. R. Geoscience 335 (2003). 相似文献
845.
IR spectra of clay samples can be collected in transmission through oriented deposits onto glass slide and are similar to the IR spectra of the same samples obtained by traditional way (KBr pellets). As examples, it is shown that it is possible to differentiate dickite from kaolinite and smectites of various chemistries, as easily by both ways. It is thus possible to use oriented clay deposits onto glass slide for both XRD and IR studies and to easily take benefit of the complementarity of both techniques. To cite this article: S. Petit et al., C. R. Geoscience 335 (2003). 相似文献
846.
Juan Morales Avto Goguitchaichvili Edgardo Cañon-Tapia Raquel Negrete 《Comptes Rendus Geoscience》2003,335(14):995-1004
From a large collection (more than 300 oriented cores) of Baja California Mio-Pliocene volcanic units, sampled for magnetostratigraphy and tectonics, 46 samples were selected for Thellier paleointensity experiments because of their low viscosity index, stable remanent magnetization and close to reversible continuous thermomagnetic curves. 19 samples, coming from 4 individual basaltic lava flows, yielded reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 3.6 to 6.2 ×1022 A m2. Our results, although not numerous, are of high technical quality and comparable to other paleointensity data recently obtained on younger lava flows. The NRM fractions used for paleointensity determination range from 38 to 79% and the quality factors vary between 4.8 and 16.7, being normally greater than 5. The combination of Baja California data with the available comparable quality Plio-Plesitocene paleointensity results yields a mean VDM of 6.3 ×1022 A m2, which is almost 80% of the present geomagnetic axial dipole. Reliable paleointensity results for the last 5 Ma are still scarce and of dissimilar quality, which makes it hard to draw any firm conclusions regarding the Pliocene and Early/Middle Pleistocene evolution of the geomagnetic field. To cite this article: J. Morales et al., C. R. Geoscience 335 (2003). 相似文献
847.
Reza Sheikholeslami Hervé Bellon Hachem Emami Mossaı̈eb Sabzehei Alain Piqué 《Comptes Rendus Geoscience》2003,335(13):981-991
The metamorphic rocks of the Neyriz area (Sanandaj–Sirjan zone) represent a Palaeozoic sequence, the upper part of which being palaeontologically dated from the Carboniferous and the Permian. Field structural analysis of the whole sequence, detailed in laboratory by microstructural one and 40K–40Ar dating carried on separated minerals, lead to establish that the whole sequence, from gneisses to Permian rocks, has suffered a unique synmetamorphic deformation, of variable intensity, marked by a foliation. Isotopic ages measured on extracted amphiboles and micas, clustered in four groups between 300 and 60 Ma, show the successive stages of their slow exhumation, which ended by the end of the Cretaceous. To cite this article: R. Sheikholeslami et al., C. R. Geoscience 335 (2003). 相似文献
848.
Robert A. Berner 《Comptes Rendus Geoscience》2003,335(16):1173-1177
The rise of large vascular plants during the mid-Paleozoic brought about a major increase in the rates of weathering of silicate minerals that induced a drop in the level of atmospheric CO2 and contributed, via the atmospheric greenhouse effect, to global cooling and the initiation of the most long lived and a really extensive glaciation of the past 550 million years. Sedimentary burial of the microbiologically resistant remains of the plants resulted during the Permo-Carboniferous in both further lowering of CO2 and in elevation of atmospheric O2. Evidence of changes in CO2 and O2 are provided by mathematical models, studies of paleosols, fossil plants, fossil insects, and the effects of modern plants on silicate weathering, and by laboratory studies of the effects of changes in O2 on plants and insects. To cite this article: R.A. Berner, C. R. Geoscience 335 (2003). 相似文献
849.
The Cabo de Gata Níjar Natural Park in Spain presents certain unique characteristics which make it of great geological, ecological
and environmental interest. As a prior step to the environmental study of the park and the impact produced by the mining activities
carried out up until ten years ago, the mining and waste-tip areas were defined and a lithological discrimination carried
out. For this purpose, various remote-sensing techniques were employed to create high-resolution image documents on which
the geological and environmental mapping of the area would be based, with special attention being paid to the areas of former
mining activity because of its implications for the local environment. The merging of remote-sensing images with different
spatial/spectral resolutions has become a highly useful tool, due to their increasing availability. Such is the case of the
SPOT panchromatic and Landsat Thematic Mapper multispectral images, which are two of the most commonly used images in geological
and environmental studies based on remote sensing. Of the numerous techniques described for merging remote sensing image data,
this paper analyses those based on Intensity-Hue-Saturation, Principal Component Analysis, Spherical Coordinates, High-Pass
Filters and Colour Normalized transform, using SPOT panchromatic and Landsat TM data. The research goals were (1) to analyse
and compare some of the most commonly used methods; (2) to select the most appropriate method for a detailed geological-environmental
study, and (3) to apply the methodology to the Cabo de Gata-Níjar area, the site of one of the most distinctive natural parks
in the country.
Received: 29 November 1996 · Accepted: 29 July 1997 相似文献
850.
Jean-Philippe Bellot 《Comptes Rendus Geoscience》2004,336(1):67-74
A new geological map of the Palaeozoic units from the Porquerolles Island provides evidence for two main units joined along a north–south-trending shear zone: a western unit of schist, sandstone and microconglomerates, an eastern unit of schist including calcareous rocks, metabasite and various types of mineralisations. The shear zone has experienced a main strike-slip, sinistral movement in a ductile regime, and a late westward normal movement from ductile to brittle regimes. It crosscuts an early thrust fault, marked by penetrative syn-metamorphic deformation and the abundance of quartz rods. According to geometrical relationships, the regional conic fold is interpreted as a large-scale dragging of foliation related to strike-slip faulting. Combined folding and wrenching point out a main transpressional tectonics related to the continental collision dated Middle Visean in the central and eastern parts of the Variscan Maures massif. To cite this article: J.-P. Bellot, C. R. Geoscience 336 (2004). 相似文献