Analyse quantitative du rifting et de la relaxation thermique de la partie occidentale de la marge transformante nord-africaine: le Rif externe (Maroc)

Résumén

Un résumé de la cinématique du Rif externe central et occidental, de la période de rifting aux événements tectoniques récents, est proposé. L’évolution du Rif externe est abordée sur le plan quantitatif par le biais des principaux mécanismes physiques qui ont pu affecter cette partie de la marge nord-africaine durant le Jurassique, le Crétacé et le Paléocène (processus de dilatation thermique lors du rifting et de contraction thermique lors du refroidissement de la lithosphère, effondrement par flexure de la marge nord-africaine lors de la collision). Les géométries et densités obtenues sont comparées avec les données gravimétriques et de sismique réfraction.

Un des aspects du modèle de Wernicke (1985), la dissymétrie du rifting de la marge africaine, semble se vérifier par l’analyse et à la modélisation de la subsidence. Mais celle-ci montre l’existence d’une fusion adiabatique tardive du manteau supérieur dont le modèle de cisaillement simple ne tient pas compte. Le soulèvement de la marge à la fin du rifting ne serait pas dû uniquement à une dilatation thermique car les données de terrain montrent des discordances angulaires importantes sur la marge africaine. Ce soulèvement est attribué localement à la poussée des rides médio-océaniques naissantes de l’Atlantique et du sillon transformant nord-africain.

La modélisation de la subsidence tectonique permet d’expliquer l’absence d’intumescence sous le Rif. Elle est attribuée aux taux d’amincissement crustaux élevés durant les phases d’extension triasico-jurassiques. Cette approche quantitative confirme l’existence d’un bassin intra-continental avorté au sud-ouest et d’une marge continentale sensu stricto au nord-est. Si l’océanisation paraît plus tardive dans la partie rifaine du sillon transformant nord-africain (Callovien) que dans l’Atlantique Central (Bajocien), la subsidence post-rift ne se distingue pas de celle d’une marge passive classique.     

Methodology of seasonal morphological modelisation for nourishment strategies on a Mediterranean beach

A modified 2DH morphodynamical model was employed to simulate the evolution of large-scale features with major implications for beach nourishment. The study was focused on modelling the evolution of material artificially placed in different parts of the profile, extracting or adding material to the natural bars, and quantifying how the profile responds to different wave climates and nourishment placements. The simulated results were compared with field data from a Mediterranean beach.     

Contrasting regional discharge evolutions in the Amazon basin (1974–2004)

Former hydrological studies in the Amazon Basin generally describe annual discharge variability on the main stem. However, the downstream Amazon River only represents the mean state of the Amazonian hydrological system. This study therefore uses a new data set including daily discharge in 18 sub-basins to analyze the variability of regional extremes in the Amazon basin, after recalling the diversity of the hydrological annual cycles within the Amazon basin. Several statistical tests are applied in order to detect trends and breaks in the time series. We show that during the 1974–2004 period, the stability of the mean discharge on the main stem in Óbidos is explained by opposite regional features that principally involve Andean rivers: a decrease in the low stage runoff, particularly important in the southern regions, and an increase in the high stage runoff in the northwestern region. Both features are observed from the beginning of the nineties. These features are also observed in smaller meridian sub-basins in Peru and Bolivia. Moreover we show that the changes in discharge extremes are related to the regional pluriannual rainfall variability and the associated atmospheric circulation as well as to tropical large-scale climatic indicators.     

Issues in characterizing heterogeneity and connectivity in non-multiGaussian media

The performances of kriging, stochastic simulations and sequential self-calibration inversion are assessed when characterizing a non-multiGaussian synthetic 2D braided channel aquifer. The comparison is based on a series of criteria such as the reproduction of the original reference transmissivity or head fields, but also in terms of accuracy of flow and transport (capture zone) forecasts when the flow conditions are modified. We observe that the errors remain large even for a dense data network. In addition some unexpected behaviours are observed when large transmissivity datasets are used. In particular, we observe an increase of the bias with the number of transmissivity data and an increasing uncertainty with the number of head data. This is interpreted as a consequence of the use of an inadequate multiGaussian stochastic model that is not able to reproduce the connectivity of the original field.     

Challenges for integrated water resources management

During the past century, while world population tripled, the use of water increased sixfold. Irrigation accounts for 70% of global water withdrawals, industry for 20%, and municipal use for 10%. To meet the water resources challenges, a series of transitions is under way, with major implications for water management. The present paper underlines the major issues and the new role of engineers in charge of development and project management.     

Uniwavelength lidar sensitivity to spherical aerosol microphysical properties for the interpretation of Lagrangian stratospheric observations

The determination of stratospheric particle microphysical properties from multiwavelength lidar, including Rayleigh and/or Raman detection, has been widely investigated. However, most lidar systems are uniwavelength operating at 532 nm. Although the information content of such lidar data is too limited to allow the retrieval of the full size distribution, the coupling of two or more uniwavelength lidar measurements probing the same moving air parcel may provide some meaningful size information. Within the ORACLE-O3 IPY project, the coordination of several ground-based lidars and the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) space-borne lidar is planned during measurement campaigns called MATCH-PSC (Polar Stratospheric Clouds). While probing the same moving air masses, the evolution of the measured backscatter coefficient (BC) should reflect the variation of particles microphysical properties. A sensitivity study of 532 nm lidar particle backscatter to variations of particles size distribution parameters is carried out. For simplicity, the particles are assumed to be spherical (liquid) particles and the size distribution is represented with a unimodal log-normal distribution. Each of the four microphysical parameters (i.e. log-normal size distribution parameters, refractive index) are analysed separately, while the three others are remained set to constant reference values. Overall, the BC behaviour is not affected by the initial values taken as references. The total concentration (N₀) is the parameter to which BC is least sensitive, whereas it is most sensitive to the refractive index (m). A 2% variation of m induces a 15% variation of the lidar BC, while the uncertainty on the BC retrieval can also reach 15%. This result underlines the importance of having both an accurate lidar inversion method and a good knowledge of the temperature for size distribution retrieval techniques. The standard deviation (σ) is the second parameter to which BC is most sensitive to. Yet, the impact of m and σ on BC variations is limited by the realistic range of their variations. The mean radius (r_m) of the size distribution is thus the key parameter for BC, as it can vary several-fold. BC is most sensitive to the presence of large particles. The sensitivity of BC to r_m and σ variations increases when the initial size distributions are characterized by low r_m and large σ. This makes lidar more suitable to detect particles growing on background aerosols than on volcanic aerosols.     

Absence of overall feedback in a benthic estuarine community: A system potentially buffered from impacts of biological invasions

Species introductions are among the most dramatic human-induced impacts on aquatic and terrestrial ecosystems around the world. Stability patterns of an estuarine benthic community were investigated through guild interaction models representing the community before and after human-mediated species invasions. The study area was Yaquina Bay, a developed estuary on the central Oregon coast, U.S., where at least 12 species of nonindigenous invertebrates have been inadvertently introduced. Three of the introduced species (the polychaetes Hobsonia florida and Pseudopolydora kempi and the cumacean Nippoleucon hinumensis) are probably among the 10 most abundant invertebrate species in the intertidal benthic community. To estimate effects and potential risks of species introductions on the native community we constructed 2 types of community models based on functional-group interactions, namely, activity guild models and trophic guild models. In both cases we observed that overall feedback has a strong tendency towards zero in pre-invasion and post-invasion models. We generated 12,000 random models of similar size and could not detect this tendency. We suggest that the weak or absent overall feedback in this community may be an ecological property and not an intrinsic property of large systems as such. The reduced response to input from either invertebrate invasions or removal of native top predators, may to some extent buffer the community from such impacts. Alternative guild models suggested increased risk of stability decline in the invaded community even after accounting for potential complexity effects on stabllity. Further species introductions in this intermediately invaded estuary should be avoided.     

Modeling Permeability Changes Caused by Hydrothermal Circulation

A new model for simulating porosity and permeability changes caused by sedimentary reservoirsdiagenesis is presented. Permeability is computed from changes in the mineral volume fractionsresulting from precipitation and dissolution of the rock-forming mineral as fluid flows throughvariable salinity and temperature fields. Its evolution is controlled by a power—law relationship,in which a weighting coefficient is assigned to clay minerals. This approach allows theincorporation of the widely observed influence of clay content on the porosity—permeabilityrelationship. A synthetic example is set up to analyze the sensitivity of the results to a set offour controlling parameters: the effect of the clay-weighting coefficient compared to the effectof the salinity gradient, temperature gradient, and exponent coefficient of the permeabilityevolution law. Using a large range of values for these parameters, the results show that theirinfluence is of equivalent magnitude in terms of permeability evolution rate. It also seemsthat the value of the clay-weighting coefficient affects the evolution trend: permeability mayincrease when the porosity decreases (and vice versa). The model is compared to the classicalapproach for which permeability is a function of porosity change only. Results display thestrong influence of even low values of the clay-weighting coefficient on the permeabilitychange. Consequently, the specific influence of mineral transfers on pore structure changes isa key parameter for modeling permeability changes and cannot be bypassed by the use ofsimple porosity—permeability evolution law.