ESTIMATION DE LA POROSITE DE QUELQUES CHONDRITES PAR ANALYSE D'IMAGES DE LEURS SECTIONS POLIES     

M. Christophe Michel-Lvy 《Meteoritics & planetary science》1978,13(3):305-309

An eventual correlation between petrologic type and porosity can only be found if measurements are made on unshocked or slightly shocked chondrites. The main advantage of the method (described earlier in this journal) is that it offers the possibility of correlating on a very small scale porosity with structural and petrographical features along with punctual chemical and isotopic data. Moreover, the method is relatively unpolluting and non destructive, and requires very small amounts of matter. Porosity measurements have been carried out on polished thin section of five H chondrites, after sample impregnation with a fluorescent resin. The microscope images were analysed with a Quantimet. Two types of information have been derived. First, the macroporosity of the material ranges up to 13% in Sena (H5) but falls to 4% in samples that have undergone slight shock or compression. Second, approximately 25% by volume of the stones consist of zones of great permeability which were fissured or filled with loose debris. This has been observed in all cases but the well-recrystallised Kernouve (H6) stone. Additional measurements are required to correlate petrology and porosity on a statistically significant basis.  相似文献   

Biological Evaluation of Marine ­Protected Area: Evidence of Crowding Effect on a Protected Population of Queen Conch in the Caribbean     

Christophe Béné  & Alexander Tewfik 《Marine Ecology》2003,24(1):45-58

Abstract This study provides a first evaluation of the biological impact of a marine fishery reserve on the stock of queen conch (Strombus gigas) in the Turks and Caicos Islands. The density and the shell length of the population living in the reserve are compared with those of the individuals living in the surrounding fished areas. The results show that the adult density is six times higher in the reserve than in the fished areas. The shell length analysis shows that both adults and juveniles are significantly smaller in the reserve than in the fished area. This unexpected result suggests the existence of a crowding effect (i. e. a high density‐induced reduction in growth rate) within the ­reserve. It is hypothesised that this crowding effect is due to the superimposition of two factors leading to very high density values in the reserve: (a) the reduced fishing mortality following the creation of the reserve, (b) the existence of natural barriers that impede the emigration of adults outside the reserve. These results are then discussed in relation to current considerations on marine fisheries reserves.  相似文献   

Microseismicity, tectonics and seismic potential in southern Caribbean and northern Venezuela     

Omar J. Pérez  Carlos Sanz  Gabriela Lagos 《Journal of Seismology》1997,1(1):15-28

Approximately one thousand microearthquakes with body-wave magnitude mb have been located in northern Venezuela and the southern Caribbean region (9–12° N; 64–70° W) since the installation in 1980 of the Venezuelan Seismological Array, together with forty events of mb 4, one of them with surface-wave magnitude Ms 6. Focal depths are in the range of 0 to <15 km. This geologically complex region is part of the boundary between the Caribbean and the South American Plates. Epicentral locations indicate that this E–W oriented portion of the boundary is formed by two 400 km long subparallel fault zones: San Sebastián fault zone (SSF), 20 km north of Caracas along the coast; and La Victoria fault zone (LVF), 25 km south of the city. They are clearly delineated by the microseismicity. New composite focal mechanism solutions (CFMS) along these faults show right-lateral strike-slip (RLSS) motion on nearly E–W oriented fault planes. NW-striking subsidiary active faults occur in the region and intercept the two main E–W fault zones. These interceptions show high levels of microearthquake activity and seismic moment release when compared to other portions of both, the main and subsidiary faults. New CFMS at those fault crossing sites show NW-striking RLSS motion and normal faulting, in an en-echelon-like structural behavior. Geological data and quantitative comparisons with other transcurrent plate boundaries in the world suggest that the rate of plate motion in this area is on the order of 20 mm/y. Several moderate and large shocks have occurred along the SSF and LVF since 1640, including an Ms 7.6 event in 1900 on SSF. Although the region may be relatively far from a repeat of this earthquake, seismicity data indicate that strong shocks could take place along segments of the seismically active faults identified in this study.  相似文献   

Trajectories of the local centroids in a two-component stellar system near the sun     

R. Cubarsi  J. Sanz  J. M. Juan 《Astrophysics and Space Science》1990,170(1-2):197-203

In a stellar system in nonsteady states and according to Chandrasekhar's axially-symmetric model, the general trajectory of a local centroid is determined. In the neighbourhood of the Sun, a two-component stellar system is adopted to explain the kinematic behaviour of the stellar samples. Depending on the kinematic parameters of each subsystem, the trajectories of every local subcentroid are shown.Life is long for the wise stars but short for the giddy ones Some Seneca-like words about De breviate vitaePaper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.  相似文献   

Inverse modeling for seawater intrusion in coastal aquifers: Insights about parameter sensitivities,variances, correlations and estimation procedures derived from the Henry problem     

Esteban Sanz  Clifford I. Voss 《Advances in water resources》2006

Inverse modeling studies employing data collected from the classic Henry seawater intrusion problem give insight into several important aspects of inverse modeling of seawater intrusion problems and effective measurement strategies for estimation of parameters for seawater intrusion. Despite the simplicity of the Henry problem, it embodies the behavior of a typical seawater intrusion situation in a single aquifer. Data collected from the numerical problem solution are employed without added noise in order to focus on the aspects of inverse modeling strategies dictated by the physics of variable-density flow and solute transport during seawater intrusion. Covariances of model parameters that can be estimated are strongly dependent on the physics. The insights gained from this type of analysis may be directly applied to field problems in the presence of data errors, using standard inverse modeling approaches to deal with uncertainty in data.  相似文献   

Structural patterns in river network organization at both infra‐ and supra‐basin levels: the case of a granitic relief     

Christophe Cudennec  Youssef Fouad 《地球表面变化过程与地形》2006,31(3):369-381

Within a landform, the channelized water path from any point to the corresponding outlet is split into successive components within the Strahler ordering scheme. The probability density functions (pdf) of the length L of the whole channelized path and of the lengths of the components are studied as multi‐level structural functions. We have considered a granitic area and studied both its main basin and the set of its 48 constituent basins. With respect to the main basin, the pdf of the component lengths exhibit a strong scaling property, except for the highest orders, due to a hierarchical constraint; hence, the pdf of sum L has no particular shape. We have nevertheless identified an underlying structural pattern at particular infra‐ and supra‐basin levels, where the hierarchical constraint is weaker. This identification process entails noting structurally emerging patterns based on multi‐level variables and distributions, which satisfy the general self‐similarity of networks. The fairly good fit of an analytical gamma law with most of these emerging patterns can prove to be a positive step towards both a general modelling approach to the geomorphometric functions and a stronger geomorphological basement of hydrological transfer functions. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

On the northward shift of the West African monsoon     

Romain Ramel  Hubert Gallée  Christophe Messager 《Climate Dynamics》2006,26(4):429-440

The regional climate model (MAR) is used to perform a simulation of the year 1992 over West Africa. It is shown that MAR is able to simulate the main features of the rainy regime over West Africa and especially the discontinuous seasonal progression of the West African Monsoon along the year. One particular feature that is reasonably well reproduced is the abrupt shift of the rain band from 5° to 10°N at the end of June (also called “monsoon jump”). This study suggests that such a phenomenon is associated with the shift of the Saharan heat low between two favourite positions: one being over the Sahelian area (10–15°N) and the other over the Saharan area (20–25°N). These two favourite locations of the heat low are linked to the spatial distribution of surface albedo over West Africa that drives the spatio-temporal location of the surface temperature maxima. A detailed analysis of this “monsoon jump” is performed and the causes of the strong decrease in precipitation that precedes the northward shift of the rain band are also investigated.  相似文献   

Influence of observed and RCM-simulated precipitation on the water discharge over the Sirba basin, Burkina Faso/Niger     

Christophe Messager  Hubert Gallée  Olivier Brasseur  Bernard Cappelaere  Christophe Peugeot  Luc Séguis  Michel Vauclin  Romain Ramel  Gilles Grasseau  Laurent Léger  Denis Girou 《Climate Dynamics》2006,27(2-3):199-214

The forcing of a hydrologic model (ABC) by both observed and simulated precipitation from a regional climate model (MAR) has been performed over the Sirba watershed (39,000 km²) located in the Sahelian region. Two aspects have been more specifically examined: the spatial and temporal representations of precipitation. The comparison between simulated and observed discharges—using observed rainfall datasets as forcing of the hydrologic model—has shown that the representation of daily precipitation (which is mainly convective in the Sahelian region) was not sufficiently accurate to correctly simulate the hydrologic response of the watershed. Since this response drives the soil water budget and consequently the amount of evaporation in forthcoming coupling experiments, it is thus necessary to develop more realistic infra-daily precipitation associated with convective events. A new temporal disaggregation scheme has been then developed. Considering observed as well as simulated precipitation fields, this method has significantly improved the simulated discharge at the catchment outlet. The major role played by the temporal component compared to spatial component of the precipitation has been then underlined. In addition, the present study shows the unsuitability of the simulated precipitation from the RCM to directly force a hydrologic model at infra daily timescale even if the cumulative amount and the main features of the precipitation seasonal cycle are well simulated.  相似文献   

A strike-slip fault corridor within the Alpujarra Mountains (Betic Cordilleras,Spain)     

C. Sanz De Galdeano  J. Rodriguez-Fernandez  A. C. Lopez-Garrido 《International Journal of Earth Sciences》1985,74(3):641-655

In a long »corridor« in the internal zone of the Betic Cordilleras, bounded to the north by the Sierra Nevada and to the south by the Sierras Lujar, Contraviesa and Gador, two discernible bands of strike-slip faults exist: These bands lie in an approximately E-W direction and are well exposed throughout and beyond the 85 km long belt.The characteristic features of strike-slip faults, such as cataclastic rocks composed of variously textured fault breccia, which also include fragments of rocks up to several metres in size, incipient foliation, and striation demonstrate the right-lateral nature of the displacement. There are also fractures which run in a NW-SE and NNE-SSW direction, and apparently originated after formation of the E-W faults. The magnitude of lateral displacements is difficult to calculate. Evidence of considerable vertical movement can be found locally.These strike-slip displacements have essentially occurred since the middle Miocene period and might still be taking place.

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Zusammenfassung In einem langen Korridor in der Internzone der Betischen Kordilleren, der im Norden durch die Sierra Nevada und im Süden von den Sierren Lujar, Contraviesa und Gador begrenzt wird, sind zwei Streifen mit Blattverschiebungen zu beobachten. Sie streichen etwa E-W und sind gut entlang des 85-km-langen Gürtels aufgeschlossen.Die charakteristischen Merkmale von Blattverschiebungen wie etwa kataklastische Gesteine mit verschiedenen texturierten Verwerfungsbrekzien, welche auch bis zu mehreren Metern große Gesteinsblöcke enthalten, beginnende Schieferung und Harnischbildung, zeigen die dextrale Natur der Bewegung. Daneben treten NW-SE und NNE-SSW streichende Störungen auf, die offensichtlich nach der Bildung der E-W verlaufenden Blattverschiebungen entstanden sind. Das Ausmaß des lateralen Versatzes ist schwer abzuschätzen.Diese Bewegungen finden seit dem mittleren Miozän statt und sind möglicherweise immer noch aktiv.

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Résumé Dans un long couloir situé dans la zone interne des Chaînes Bétiques et limité au Nord par la Sierra Névada et au Sud par les Sierras de Lujar, Contraviesa et Gador, s'observent deux trains de failles de décrochements. Ces trains ont une direction à peu près E-W et sont bien exposés tout au long des 85 Km de cette zone.Les traits caractéristiques des décrochements, tels que cataclasites, brèche de faille avec fragments de roches jusqu'à plusieurs mètres de longueur, schistosité naissante et striation, démontrent la nature dextre du déplacement. Il existe aussi des fractures de direction NW-SE et NNW-SSW qui se sont vraisemblablement formées postérieurement aux failles E-W. La valeur du déplacement latéral est difficile à estimer. On rèléve localement des indices de mouvements verticaux importants.Ces décrochements se sont produits essentiellement à partir du Miocène moyen et peuvent encore se poursuivre aujourd'hui.

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La matrice noire et blanche de la chondrite de Tieschitz (H3)     

Mireille Christophe Michel-Levy 《Earth and Planetary Science Letters》1976,30(1):143-150

Finely divided products, making a black carbonaceous crust, have coated the chondrules in space. After their agglomeration, a white silica and alumina rich deposit has accumulated between them. Crystalline debris that are found outside the chondrules must not be confounded with the black and white products, which are the only constituents of the low-temperature matrix. These debris are broken chondrules but do not necessarily result from shock effects. Many of them are indeed very fragile, because they are very porous: they contain primary cavities (voids in the crystals, druses) and secondary ones because of selective leaching of part of the glass. The chemical elements leached out of the chondrules may have been reconcentrated in the matrix.  相似文献