L'hétérogénéité du manteau supérieur à l'aplomb du volcan de Nyos (Cameroun) révélée par les enclaves ultrabasiques

Volcanic deposits from the lake Nyos (Cameroon) contain ultrabasic xenoliths: lherzolites, harzburgites and websterites, sometimes containing amphibole. Major and trace element compositions of clinopyroxene and amphibole allow the recognition of an old depletion event followed by two episodes of incompatible-element enrichment. One is high in light rare-earth elements, while the other one is low in these elements, but high in U and Th. Their relative chronology is not yet established. To cite this article: R. Temdjim et al., C. R. Geoscience 336 (2004).     

Sub-ice geology inland of the Transantarctic Mountains in light of new aerogeophysical data

The Transantarctic Mountains are a major geologic boundary that bisects the Antarctic continent, separating the low-lying, tectonically active terrains of West Antarctica from the East Antarctic craton. A new comprehensive aerogeophysical data set, extending 1150 km from the Ross Sea into the interior of East Antarctica provides insights into the complex structure inland of the Transantarctic Mountains. Geophysical maps, compiled from 21 000 km of gravity, magnetic and subglacial topography data, outline the boundaries of several geologic and tectonic segments within the survey area. The coherent pattern in magnetic data and mesa topography suggests a subglacial extent of the Transantarctic Mountains 400–500 km inland the last exposed rock outcrops. We estimate the maximum thickness of a potential sediment infill in the Wilkes Subglacial Basin to be less than 1 km, based on gravity modeling and source depth estimates from magnetic data. The coherent nature of the potential field and topography data, together with the northwest–southeast trends, define the Adventure Subglacial Trench and the Resolution Subglacial Highlands as a tectonic unit. The crustal structure and the strong similarity of the observed gravity with fold-and-thrust belts suggest a compressional scenario for the origin of the Adventure Subglacial Trench and the Resolution Subglacial Highlands. The complexity and apparent structural control of the Wilkes Subglacial Basin raise the issue of what influence pre-existing structures may have played in the formation of the Transantarctic Mountains system. The previous hypothesis of a thermal boundary beneath the mountains is difficult to reconcile with our new gravity data. The apparent difficulties to match our new data with certain key aspects of previous models suggests that a reassessment of the existing uplift models is necessary. We have modeled the prominent gravity anomaly over the Transantarctic Mountains with thicker crust.     

Development of an earthquake loss model for Turkish catastrophe insurance

Following the devastating Kocaeli and Düzce earthquakes of August andNovember 1999, the Turkish Government was faced with an enormousfinancial burden as a result of its statutory obligation to cover the full costsof rebuilding. In order to offset this liability in the future – which has hadan adverse effect on the Government's economic programme – acompulsory earthquake insurance scheme has been introduced for allhouseholders in Turkey. A key element for successful implementation ofthis novel and ambitious programme is the transfer of the earthquake riskabsorbed by the Turkish Catastrophe Insurance Pool (TCIP) to theinternational reinsurance market. An earthquake loss model, described inthis paper, has been developed for the TCIP to serve as a basis for thedecision-making process with respect to the pricing of its insurance policy,risk control, the purchase of reinsurance, and the transfer of seismic risk.Sample results of the loss calculations are presented.     

Diagenesis in carbonate sediments: A review

Reading the old textbooks of more than twenty-five years ago reveals the enormous progress made in our understanding since those days. Advances have been made along several related paths, particularly through the study of texture and structure with the microscope, the examination of Recent marine sediments and the application of the principles of solution chemistry. As a result we can now relate many diagenetic products with distinctive textures or structures to one of, say, four main types of aqueous solution. Sea water yields characteristic growths of aragonite and Mg-calcite whose fossilized calcitic remains we can yet recognise in the ancient. Fresh water gives rise to other individual calcite textures whose imprint also we can distinguish in the old rocks. Solution extruded at depth from compacting clays produce even other calcite textures which we are now beginning to understand. Certain mixtures of sea water with fresh water provide not only beach rocks with aragonite and Mg-calcite cements but dolomites through the Dorag process. Clearly influential in the control of marine precipitation we find organic compounds such as the humic acids.Superimposed on these products are the results of pressure solution. Clues to the action of processes in the distant past remain in the form of traces of magnesium, strontium or uranium and of relic isotopic ratios of oxygen and carbon, indicators of the composition of long vanished solutions. We think more clearly that before about rates of processes, duration of exposure to diagenetic environments, the balance between relatively closed and open chemical systems, the movements and mixing of ground waters and their chemical changes. In these and other ways carbonate diagenesis has come of age.

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Zusammenfassung Liest man alte Lehrbücher, die vor mehr als 25 Jahren erschienen sind, so erkennt man, welcher Fortschritt auf dem Gebiet der Karbonatdiagenese seither zu verzeichnen ist. Diese Fortschritte sind auf verschiedenen benachbarten Wegen erreicht worden, speziell durch Gefügeuntersuchungen mit dem Mikroskop, durch das Studium rezenter mariner Sedimente und durch die Anwendung der Prinzipien der Elementreaktionen in Lösungen. Als Ergebnis können wir heute viele diagenetische Stadien mit bestimmten Gefügemerkmalen und in erster Annäherung mit 4 Lösungsmilieus in Verbindung bringen. Meereswasser bewirkt typisches Wachstum von Aragonit und Hoch-Mg-Calcit, die wir in den fossilen calcitischen Gefügen wiedererkennen können. Süßwasser bewirkt auf der anderen Seite ein typisches Calcitwachstum, das wir ebenfalls in alten Gesteinen wiederfinden können. Lösungen, die aus kompaktierenden tonhaltigen Sedimenten aufsteigen, erzeugen calcitische Zementationsgefüge, die wir heute zu verstehen gelernt haben. Eine Mischung von Meereswasser und Süßwasser ist nicht nur für die beach-rock Zementation mit Aragonit und Hoch-Mg-Calcit verantwortlich, sie bewirkt auch Dolomitisierung nach dem Dorag-Modell.Organische Komponenten wie etwa Huminsäuren haben einen deutlichen Einfluß auf die Diageneseprozesse unter marinen Bedingungen. Den Diageneseprodukten werden schließlich die Einwirkungen der Drucklösung überlagert. Als Schlüssel zum Ablauf der Diagenese vergangener Zeiten verbleiben Spurenelemente wie Magnesium, Strontium und Uran sowie das reliktische Isotopenverhältnis von Sauerstoff und Kohlenstoff erhalten; sie markieren die Zusammensetzung längst vergangener Lösungen. Wir können heute besser die Anteile verschiedener Diageneseprozesse, die Dauer der Wirkung bestimmter Prozesse, den Ausgleich zwischen geschlossenen und offenen chemischen Systemen, die Bewegungen und Mischungen sowie chemischen Veränderungen des Grundwassers abschätzen.

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Résumé Parmi les vieux manuels agés de vingt-cinq ans ou davantage, on est frappé d'un coup d'il du progrès réalisé, surtout par l'étude des textures et des structures au microscope, par l'examen des sédiments marins actuels et par l'application de la chimie aqeuse. Conséquement on peut aujourd'hui lier les produits diagénétiques avec quatre solutions principaux. L'eau de mer donne les accroissements typiques en aragonite et en calcite-hautement magnésienne, reconnaisables aux calcaires anciens. L'eau douce fait surgir des textures individuelles en calcite. Aux profondeurs plus grands, les solutions exprimées des argiles y peuvent encore provoquer d'autres textures en calcite. L'eau de mer et l'eau douce, melées ensembles, y donnent naissance aux beach-rocks et aux dolomies. Franchement influents, aux précipitations marines, sont les composés organiques. Tout peut Être subi de la pression-solution. Données de la diagenèse ancienne restent telles que les traces du magnésium, du strontium et de l'uranium et des niveaux en ¹⁸O et ¹³C, témoins des compositions des solutions depuis longtemps disparues. On pense plus clairement qu'autrefois des taux des processus, des durées d'exposition aux milieux diagénétiques, du bilan entre les systèmes relativement fermés ou ouverts, des mouvements et des mélanges des eaux sousterraines et de leures changements chimiques. Dans telles mesures, l'étude de la diagenèse carbonatée a atteint sa majorité.

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Measured oxygen fugacities of the Angra dos Reis achondrite as a function of temperature

Measurements of the oxygen fugacity (?O₂) as a function of temperature (T) were made on an interior bulk sample of the cumulate achondrite, Angra dos Reis. Data clustered between the?O₂-T relationship of the iron-wüstite assemblage and 1.2 log atm units above iron-wüstite. Interpretation of the data indicates that, throughout most of the cooling history of the meteorite, ?O₂ values were defined by equilibria involving iron-bearing species at values close to the ?O₂ of the assemblage iron-wüstite. Measured ?O₂ data are compatible with crystallization and cooling at pressures greater than 50 bars.     

Seismic design spectra and mapping procedures using hazard analysis based directly on oscillator response

The calculation of design spectra for building sites threatened by seismic ground motion is approached by considering the maximum responses of linearly elastic oscillators as indicators of ground motion intensity. Attenuation functions describing the distribution of response as a function of earthquake magnitude and distance are derived using 68 components of recorded ground motion as data. With a seismic hazard analysis for several hypothetical building sites, the distributions of maximum oscillator responses to earthquakes of random magnitude and location are calculated, and spectra are drawn to indicate the maximum responses associated with specified probability levels. These spectra are compared to design spectra calculated from published methods of amplifying peak ground motion parameters. The latter spectra are found to be inconsistent in terms of risk for building sites very close and very far from faults. A ground motion parameter defined to be proportional to the maximum response of a 1 Hz, 2 per cent damped linearly elastic oscillator is investigated; this parameter, in conjunction with peak ground acceleration, is found to lead to risk-consistent design spectra. Through these two parameters, a design earthquake magnitude and design hypocentral distance are defined, for a specified building site and risk level. The use of these parameters in the seismic hazard mapping of a region is illustrated.     

Reduction of mare basalts by sulfur loss

Metallic Fe content and S abundance are inversely correlated in mare basalts. Either S volatilization from the melt results in reduction of Fe²⁺ to Fe⁰ or else high S content decreases Fe⁰ activity in the melt, thus explaining the correlation. All considerations favor the model that metallic iron in mare basalts is due to sulfur loss. The Apollo 11 and 17 mare basalt melts were probably saturated with S at the time of eruption; the Apollo 12 and 15 basalts were probably not saturated.Non-mare rocks show a positive correlation of S abundance with metallic Fe content; it is proposed that this is due to the addition of meteoritic material having a fairly constant Fe⁰/S ratio. If true, metallic Fe content or S abundance in non-mare rocks provides a measure of degree of meteoritic contamination.     

The dissolution kinetics of biogenic calcium carbonates in seawater

Dissolution rate as a function of degree of undersaturation was measured on shells of individual species of coccoliths and foraminifera, various size fractions of sediment from the Ontong-Java Plateau and the Rio Grande Rise, a collection of large pteropods, and on synthetic calcite and aragonite powder.Results of the study indicate that all biogenic and synthetic calcium carbonate follows the rate law $\text{R\% = k\%(1 ? Ω)}{}^{\mathrm{n}}$ where $\text{Ω  [Ca}{}^{\mathrm{2+}}\text{][CO}{}_3{}^{\mathrm{2?}}\text{]/K'}{}_{\mathrm{sp}}$ and K'_sp is the apparent solubility product of calcite or aragonitic seawater. In the case of all calcite samples, n_c = 4.5, while for aragonitic samples n_a = 4.2. The ‘rate constant’, k%, varies widely between samples and in many cases is inversely correlated with grain size. However, the individual species of coccoliths, E. huxleyi and C. neohelis, which were cultured in the laboratory appear not to follow this rule, with dissolution rates an order to magnitude lower than expected.     

Sensitivity to space and time resolution of a hydrological model using digital elevation data

The space and time resolutions used for the input variables of a distributed hydrological model have a sufficient impact on the model results. This resolution depends on the required accuracy, experimental site and the processes and variables taken into account in the hydrological model. The influence of space and time resolution is studied here for the case of TOPMODEL, a model based on the variable contributing area concept, applied to an experimental 12 km² catchment (Coët-Dan, Brittany, France) during a two month winter period. A sensitivity analysis to space and time resolution is performed first for input variables derived from the digital elevation data, secondly for the optimized values of the TOPMODEL parameters and finally for modelling efficiency. This analysis clearly shows that a relevant domain of space and time resolutions where efficiency is fairly constant can be defined for the input topographic variables, as opposed to another domain of larger resolutions that induces a strong decrease of modelling efficiency. It also shows that the use of a single set of parameters, defined as mean values of parameters on this relevant domain of resolution, does not modify the accuracy of modelling. The sensitivity of the parameters to space and time resolution allows the physical significance of the parameter values to be discussed.     

The multivariate controls of hydraulic geometry: A causal investigation in terms of boundary shear distribution

Most downstream hydraulic geometry exponents have been found to be very close to the classic values reported by Leopold and Maddock (1953). These have been viewed as the simplified cases to general trends because the hydraulic geometry of alluvial channels is actually the product of ‘multivariate controls’ (Richards, 1982). This paper is an attempt to develop a soundly based foundation for the explanation of the physical mechanisms of these controls. A quantitative relationship between channel shape and boundary shear distribution developed from experimental flume results is found to be applicable in some instances to alluvial channels, particularly to stable canals. On the basis of this relationship, it is shown that downstream hydraulic geometry is determined not only by flow discharge, but also by channel slope, channel average roughness and sediment composition of the channel boundary. This is strongly supported by our analysis of 529 observations from both stable canals and natural rivers in the U.S.A. and the U.K. The difference between regime relations in canals and the hydraulic geometry of rivers appears to be caused mainly by channel slope and average roughness, which can be regarded as constants only in stable canals. The close relationship between discharge and channel average roughness observed in canals is not repeated in natural channels, partly because of the variety of flow values used to define the channel-forming discharge. Furthermore, it is indicated that the effects of the sediment composition of the channel boundary on hydraulic geometry are significant and need further investigation.