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251.
Guillaume Martelet Nicole Debéglia Catherine Truffert 《Comptes Rendus Geoscience》2002,334(7):449-454
For regional and national study purposes, there is a high need for updating the terrain corrections (TC) in the French gravity database. We have recomputed the TC for all the French gravity stations from 50 m out to a distance of 167 km. We compute the TC with a flat-top-prism algorithm and three DEM with grid spacing of 50, 250 and 1000 m, used in the zones 53 m/3 km, 3 km/10 km et 10 km/167 km, respectively. Analysing the DEM/station Δz and comparing our results to the ones previously obtained in the Alps area, we estimate the accuracy of our TC to be better than 1 mGal. To cite this article: G. Martelet et al., C. R. Geoscience 334 (2002) 449–454. 相似文献
252.
253.
Solar and anthropogenic imprints on Lake Masoko (southern Tanzania) during the last 500 years 总被引:1,自引:0,他引:1
Yannick Garcin David Williamson Laurent Bergonzini Olivier Radakovitch Annie Vincens Guillaume Buchet Joël Guiot Simon Brewer Pierre-Etienne Mathé Amos Majule 《Journal of Paleolimnology》2007,37(4):475-490
The Masoko crater-lake in southern Tanzania provides a continuous record of environmental changes covering the last 500 years.
Multi-proxy studies were performed on a 52 cm sediment core retrieved from the deepest part of the lake. Magnetic, organic
carbon, geochemical proxies and pollen assemblages indicate a dry climate during the ‘Little Ice Age’ (AD 1550–1850), confirming
that the LIA in eastern Africa resulted in marked and synchronous hydrological changes. However, the direction of response
varies between different African lakes (low versus high lake-levels), indicating strong regional contrasts that prevent the
clear identification of climate trends over eastern Africa at this time. Inferred changes in Masoko lake-levels closely resemble
the record of solar activity cycles, indicating a possible control of solar activity on the climate in this area. This observation
supports previous results from East African lakes, and extends this relationship southward. Finally, anthropogenic impact
is observed in the Masoko sediments during the last 60 years, suggesting that human disturbance significantly affected this
remote basin during colonial and post-colonial times. 相似文献
254.
Prosper Andrianjakavah Stefano Salvi Didier Béziat Damien Guillaume Michel Rakotondrazafy Bernard Moine 《Mineralium Deposita》2007,42(4):385-398
The Maevatanana deposits consist of gold-bearing quartz–sulphide veins crosscutting banded iron formation (BIF) within a metamorphosed
2.5 Ga greenstone belt. The host rocks are dominated by a sequence of migmatites, gneisses, amphibolites, magnetite-rich quartzites
and soapstones, intruded by large granitoid batholiths (e.g. the 0.8 Ga Beanana granodiorite). In the mineralised rocks, pyrite
is the dominant sulphide, in addition to accessory chalcopyrite and galena. Outside the immediate ore zone, the BIF is dominated
by quartz + magnetite ± hematite, accompanied by cummingtonite, albite and biotite. Gold occurs as globular grains (usually
<500 μm) within quartz crystals close to the sulphides and as invisible inclusions within pyrite and chalcopyrite (up to 2,500 ppm
Au content). Fluid inclusion textural and microthermometric studies indicate heterogeneous trapping of a low-salinity (∼3.6 wt.%
eq. NaCl) aqueous fluid coexisting with a carbonic fluid. Evidence for fluid-phase immiscibility during ore formation includes
variable L/V ratios in the inclusions and the fact that inclusions containing different phase proportions occur in the same
area, growth zone, or plane. Laser Raman spectroscopy confirms that the vapour phase in these inclusions is dominated by CO2 but shows that it may contain small amounts of CH4 (<1 mol%), H2S (<0.05 mol%) and traces of N2. Fluid inclusion trapping conditions ranged from 220 to 380°C and averaged 250°C. Pressure was on the order of 1–2 kbar.
The abundant CO2 and low salinity of the inclusions suggest a metamorphic origin for the fluid. Likewise, the presence of H2S in the fluid and pyritisation of the wall-rock indicate that gold was likely transported by sulphide complexing. Fluid immiscibility
was probably triggered by the pressure released by fracturing of the quartzites during fault movements due to competence differences
with the softer greenstones. Fracturing greatly enhanced fluid circulation through the BIF, allowing reaction of the sulphide-bearing
fluids with the iron oxides. This caused pyrite deposition and concomitant Au precipitation, enhanced by fluid phase separation
as H2S partitioned preferentially into the carbonic phase. 相似文献
255.
Abderrazak El Harti Abderrazak Bannari El Mostafa Bachaoui El Mostafa Aarab Guillaume Girouard Abderrahmen El Ghmari 《Comptes Rendus Geoscience》2004,336(14):1311-1320
The geodynamic model of the Palaeozoic basin opening of central Jebilet has been unknown before this study in spite of the abundance in the geological studies carried out in the studied sector using conventional methods. This is due to the scarcity of the key beds and synsedimentary structures. Using the synergy between the image data of the Landsat satellite TM sensor and the ground data, we have highlighted, herein, new structural data allowing the design of a new model of the Palaeozoic basin opening of central Jebilet. This opening could have been made according to dextral submeridian transverse faults with the individualization of subequatorial normal faults. To cite this article: A. El Harti et al., C. R. Geoscience 336 (2004). 相似文献
256.
Guillaume Dencausse Michel Arhan Sabrina Speich 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(11):1406-1421
Using weekly sea surface height data, Agulhas rings from the period October 1992 to December 2006 are detected and tracked, from their formation dates and throughout the Cape Basin. While 102 of them formed at the Agulhas Current retroflection, their subsequent subdivisions and junctions led to 199 trajectories. The rings geographical probability of presence shows two maxima. One, related to numerous ring passages, lies in the submarine bight formed by the Erica seamount, the Schmitt-Ott seamount, and the northeastern tip of the Agulhas Ridge. The other one, to be ascribed to topographic blocking of the eddies, is southeast of the latter obstacle. On the basis of topographic effects three routes for Agulhas rings are distinguished, a Northern route for rings that enter the south-Atlantic northeast of the Erica seamount, a Central one for those passing westward between this seamount and the tip of the Agulhas Ridge, and a Southern one farther south. Despite its bathymetric obstacles, the central route is the dominant one, both in terms of percentage of eddy crossings at its definition segment, and in terms of conveyed volume transport. Specific behaviours of rings along each route are described, referring to observations in previous studies. Some rings from the Northern route interact with the flow regime of the South African continental slope. The southernmost trajectories of the Central route are thought to settle the location of the climatological Subtropical Front in that region. The rings of the Southern route experience important core property alteration as they transit through the subantarctic domain. 相似文献
257.
AbstractThe ability of hydrological models to deal with changing conditions should not be taken for granted: it is an unfortunate but well-known problem of hydrology that the model structure and/or parameters optimized for certain conditions may not be transferable in time. Consequently, it is essential that, for application under changing conditions (e.g. in climate change impact studies), models be thoroughly assessed for their extrapolation capacity using adequate protocols. This editorial provides an overview of the Special Issue of Hydrological Sciences Journal compiled after a workshop on this theme held during the General Assembly of the International Association of Hydrological Sciences (IAHS) in Gothenburg (Sweden) in 2013. The Workshop participants had been invited to apply a calibration and evaluation protocol to their own models on a given set of changing basins. The results show that this protocol is an appropriate and instructive way of assessing the suitability of hydrological models to be applied under changing conditions. This special issue also includes papers following alternative testing methodologies, as well as an opinion paper on the definition of non-stationarity. 相似文献
258.
Combined hillslope diffusion and sediment transport simulation applied to landscape dynamics modelling 总被引:1,自引:0,他引:1 下载免费PDF全文
We present a new numerical approach for simulating geomorphic and stratigraphic processes that combines open‐channel flow with non‐uniform sediment transport law and semi‐empirical diffusive mass wasting. It is designed to facilitate modelling of surface processes across multiple space‐ and time‐scales, and under a variety of environmental and tectonic conditions. The physics of open‐channel flow is primarily based on an adapted Lagrangian formulation of shallow‐water equations. The interaction between flow and surface geology is performed by a non‐uniform total‐load sediment transport law. Additional hillslope processes are simulated using a semi‐empirical method based on a diffusion approach. In the implementation, the resolution of flow dynamics is made on a triangulated grid automatically mapped and adaptively remeshed over a regular orthogonal stratigraphic mesh. These new methods reduce computational time while preserving stability and accuracy of the physical solutions. In order to illustrate the potential of this method for landscape and sedimentary system modelling, we present a set of three generic experiments focusing on assessing the influence of contrasting erodibilities on the evolution of an active bedrock landscape. The modelled ridges morphometrics satisfy established relationships for drainage network geometry and slope distribution, and provide quantitative information on the relative impact of hillslope and channel processes, sediment discharge and alluviation. Our results suggest that contrasting erodibility can stimulate autogenic changes in erosion rate and influence the landscape morphology and preservation. This approach offers new opportunities to investigate joint landscape and sedimentary systems response to external perturbations. The possibility to define and track a large number of materials makes the implementation highly suited to model source‐to‐sink problems where material dispersion is the key question that needs to be addressed, such as natural resources exploration and basin analysis. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
259.
Douwe J.J. van Hinsbergen Guillaume Dupont-Nivet Radoslav Nakov Karen Oud Christian Panaiotu 《Earth and Planetary Science Letters》2008,273(3-4):345-358
The region located between the Carpathian–Balkan and Aegean arcs, the Moesian Platform and Bulgarian Rhodope, is generally assumed to have been stably attached to the East European craton during the Cenozoic evolution of these arcs. The kinematic evolution of this region is, however, poorly constrained by paleomagnetic analysis. In this paper we provide new paleomagnetic data (800 volcanic and sedimentary samples from 12 localities) showing no significant post-Eocene rotation of the Moesian platform and Rhodope with respect to Eurasia, therefore confirming the stability of this region. We compare this result to a provided review of paleomagnetic data from the South Carpathians (Tisza block) and the Aegean region. The Tisza block underwent 68.4 ± 16.7° of middle Miocene ( 15–10 Ma) clockwise rotation with respect to the Moesian Platform, in line with previous rotation estimates based on structural geology. The stability of the Moesian platform during middle Miocene eastward emplacement of the Tisza block into the Carpathian back-arc supports dextral shear along the Southern Carpathians recorded by 13–6 Ma clockwise strike-slip related rotations in foreland deposits. The new reference direction for the Moesian platform and Rhodope allows accurate quantification of the rotation difference with the west Aegean domain at 38.0 ± 7.2° occurring between 15 and 8 Ma. To accommodate this rotation, we propose that the pivot point of the west-Aegean rotation was located approximately in the middle of the rotating domain rather than at the northern tip as previously proposed. This new scenario predicts less extension southeast of the pivot point, in good agreement with estimates from Aegean structural geology. Northwest of the pivot point, the model requires contraction or extrusion that can be accommodated by the coeval motion of the Tisza Block around the northwestern edge of the Moesian platform. 相似文献
260.
Cindy Luisier Lukas Baumgartner Guillaume Siron Torsten Vennemann Martin Robyr 《Geostandards and Geoanalytical Research》2019,43(4):635-646
Five new natural white mica reference materials (RMs) were developed for in situ H2O content analyses by secondary ion mass spectrometry at the SwissSIMS laboratory of Lausanne University, Switzerland. The white mica reference materials cover a large part of the natural muscovite–phengite compositional range and are therefore suitable as reference materials for the analysis of natural rocks as well as individual minerals. The independent H2O content of the reference materials UNIL_WM1 to UNIL_WM5 was obtained by thermal conversion elemental analyser and corresponds to 4.35 ± 0.02, 4.33 ± 0.03, 4.30 ± 0.07, 4.50 ± 0.02 and 4.42 ± 0.11 (% m/m, ± 1s), respectively. SIMS determinations of H2O content revealed a matrix effect correlated to the FeO content of white mica. The compositional range in FeO of the reference materials that were calibrated for H2O determination is from 1.13% to 3.67% m/m. No crystallographic orientation dependency was observed at the level of homogeneity of these reference materials. An analytical precision of 0.02% to 0.08% m/m (1SE) is expected for the final uncertainty on measurements of unknown white micas in natural samples. 相似文献