Jurassic penetrative deformation and Cenozoic uplift in the Central High Atlas (Morocco): A tectonic model. structural and orogenic inversions

The Central High Atlas is characterized by a relatively mild deformation, with a local development of a more or less penetrative cleavage in the axial part of the belt. Locally, this cleavage and the plutonic bodies which outcrop in the core of most of the north-east trending anticlines (ridges »with intrusions«) are overlain by a disconformable sequence that comprises red clastic deposits which are capped by Cenomano-Turonian limestones. The age of these red deposits are bracketted between Bathonian to Mid-Cretaceous. Therefore, the spatial relationship of the cleavage with mafic intrusions, in the axial part of the Central High Atlas, suggests that it developed contemporaneously with the emplacement of magmas, probably during Mid-Jurassic times. During this period, the structural inversion occurred, from a transtensional regime to a transpressive regime.There are few indications that an important shortening persisted within the axial part of the Central High Atlas after the end of the Jurassic. Along the northern and the southern borders of the belt, however, Cretaceous and Cenozoic strata were deposited. Their sedimentary facies bear witness of successive stages of the Atlas evolution. Especially, the rapidly coarsening up of the clastic deposits since the Oligocene in these marginal basins indicates the onset of the rising up of the belt, i. e. the orogenic inversion.

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Zusammenfassung Der zentrale Hohe Atlas ist eine intrakontinentale Gebirgskette, die sich im Bereich der zwischen dem Sahara-Plateau und dem marokkanischen Variszikum gelegenen Trias- und Jurabecken herausbildete. Dieses junge Gebirge, dessen höchste Erhebungen über 4000 Meter hoch sind, ist durch einen relativ geringen Verkürzungsgrad gekennzeichnet. Axial tritt jedoch eine Schieferung auf im Bereich der N80° E ausgerichteten antiklinalen Hauptfalten sowie rund um die N45° E verlaufenden »Intrusions-Falten« und die Gangfelder. Die räumlichen Beziehungen dieser Schieferung zu den synsedimentären Faltungsstrukturen und den Gängen legen die Vermutung nahe, daß ihre Hausbildung während des Doggers mit dem Aufstieg und dem Eindringen der Plutone in das Sedimentgestein zeitlich zusammenfiel. In diesem Zeitraum erfolgte eine Strukturumkehrung, die das allmähliche Verschließen der Becken und das Zustandekommen der Schieferung bewirkte.Im Laufe des Tertiärs wurde nach und nach das gesamte Atlasgebiet einer etwa N-S gerichteten Einspannung ausgesetzt. Mit der Ablagerung immer gröberer Sedimente in den letzten Randbekken setzte schließlich im Oligozän die orogene Inversion ein, die das Gebirgsrelief bedingt hat.

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Résumé Le Haut Atlas central est une chaîne intracontinentale qui s'est développée à l'emplacement de bassins subsidents triasiques et jurassiques installés en bordure de la plate-forme saharienne et de la chaîne hercynienne marocaine. Cette chaîne récente qui culmine à plus de 4000 mètres, est caractérisée par un taux de raccourcissement modeste. Cependant, dans son axe, une déformation pénétrative s'observe au droit des rides anticlinales majeures N 80° E, et autour des »rides avec intrusions« N 45° E et des champs filoniens. Les relations spatiales de cette schistosité avec les structures plicatives synsédimentaires et les intrusions nous conduisent à considérer son développement comme globalement contemporain de la mise en place des plutons dans la couverture sédimentaire. L'âge des dépôts détritiques, discordants sur les roches intrusives et sur le Lias schistosé, étant estimé callovien ou wealdien, la déformation synschisteuse est par conséquent d'âge postbathonien et antécrétacé. C'est pendant cette période que se manifeste, avec le passage d'un régime tectonique contrôlé par le jeu de décrochements senestres E-W à un autre dominé par le fonctionnement de décrochements senestres NE-SW en compression, une inversion structurale responsable de la fermeture progressive des bassins et du développement de la schistosité.Au cours du Tertiaire, une compression subméridienne a progressivement affecté tout le domaine atlasique. Et c'est à partir de l'Oligocène, avec l'arrivée de dépôts détritiques de plus en plus grossiers dans les bassins résiduels bordiers, que débute l'inversion orogénique, responsable du relief de la chaîne.

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Natural speciation of Mn, Ni, and Zn at the micrometer scale in a clayey paddy soil using X-ray fluorescence, absorption, and diffraction

The natural speciation of Mn (0.19 g/kg), Ni (46 mg/kg), and Zn (42 mg/kg) in the argillic horizon (120 cm depth, pH = 5.6) of an Ultisol from a paddy soil in northern Taiwan was investigated by advanced X-ray synchrotron techniques. Microchemical associations were imaged by synchrotron-based X-ray microfluorescence, host minerals were identified by standard and micrometer-resolved X-ray diffraction, and the local coordination environment of Mn, Ni, and Zn was probed using extended X-ray absorption fine structure (EXAFS) spectroscopy on a powdered sample and a soil thin section, and polarized EXAFS spectroscopy on a highly textured self-supporting clay film from the <2 μm fraction of the soil. Manganese was concentrated in Fe-Mn soft mottles (44.4 g/kg) as turbostratic hexagonal birnessite and lithiophorite having Mn³⁺/Mn⁴⁺ atomic ratios of ∼20% and 50%, respectively. Quantitative analysis of high-order scattering paths of the EXAFS spectrum for natural and synthetic lithiophorite revealed that Mn³⁺ and Mn⁴⁺ are ordered in the layer. A structural model is proposed, in which Mn⁴⁺ and Mn³⁺ are ordered similarly to Al and Li in the layer, with Mn³⁺ cations being surrounded by six Mn⁴⁺, and Mn⁴⁺ cations by three Mn³⁺ and three Mn⁴⁺. Similar cation ordering in the manganese and aluminum layers likely provides a more homogeneous local balance of the excess and deficit of charges in each layer and increases the stability of lithiophorite. Ni (r = 0.70 Å) substitutes for Mn (r(Mn⁴⁺) = 0.54 Å, r(Mn³⁺) = 0.65 Å) in the manganese layer in the natural lithiophorite. In contrast, Zn (r = 0.74 Å) fills vacant sites in the gibbsitic layer of natural lithiophorite, in a similar manner as lithium (r = 0.74 Å) in synthetic lithiophorite. The partitioning of Ni and Zn between the two layers is a result of the general preference of Ni, whose size is intermediate between those of Mn³⁺ and Li⁺, for slightly smaller sites. In contrast with nickel, which is detected only where there is lithiophorite, the Zn-lithiophorite association found in Fe-Mn mottles is not representative of the bulk soil. The combined use of X-ray diffraction, and powder and polarized EXAFS spectroscopy revealed that Zn is predominantly bound to hydroxy-Al interlayers sandwiched between 2:1 vermiculite layers in the fine soil matrix. The incorporation of Zn in the gibbsitic layer of both lithiophorite and vermiculite helps increase the stability of these minerals by providing positive charge to balance the negative charge from the 2:1 phyllosilicate layer and the layer of lithiophorite. This binding environment for zinc is probably the main mechanism by which zinc is sequestered in acidic to near-neutral aluminum-rich clayey soils.     

Constraints on the ore fluids in the Sando Alcalde AuAg epithermal deposit,southwestern Peru: fluid inclusions and stable isotope data

The Sando Alcalde ore deposit (southwestern Peru) has been studied in order to characterize the physicochemical parameters of the ore fluids and to determine the fluid process (mixing or boiling) which involves the precious metal mineralization. Mineralogy, ${\delta }^{18}{\mathrm{O}}_{\mathrm{quartz}}$ isotopic values and fluid inclusion data give arguments in favour of a boiling phenomenon. This conclusion corroborates fluid inclusion studies previously performed in this area on the low-sulphidation epithermal deposits of Arcata, Orcopampa and Apacheta, where boiling has been described as the main factor for ore deposition. To cite this article: A.-S. André-Mayer et al., C. R. Geoscience 337 (2005).     

Total atmospheric ozone measured by ground based high resolution infrared spectra-comparison with dobson measurements

Total atmospheric ozone has been measured at the “Observatoire de Haute-Provence”, Chiran Station, 1905 m high with a ground based, high resolution i.r. spectrometer, using the Sun as a source. Results are compared with a Dobson spectrophotometer which has been calibrated and intercompared with a standard. Total ozone values found with Dobson are 4% lower than values given by i.r. measurements. In addition, a negative correlation between total ozone and tropopause height has been found.     

Climate change impacts on groundwater resources: modelled deficits in a chalky aquifer,Geer basin,Belgium

An integrated hydrological model (MOHISE) was developed in order to study the impact of climate change on the hydrological cycle in representative water basins in Belgium. This model considers most hydrological processes in a physically consistent way, more particularly groundwater flows which are modelled using a spatially distributed, finite-element approach. Thanks to this accurate numerical tool, after detailed calibration and validation, quantitative interpretations can be drawn from the groundwater model results. Considering IPCC climate change scenarios, the integrated approach was applied to evaluate the impact of climate change on the water cycle in the Geer basin in Belgium. The groundwater model is described in detail, and results are discussed in terms of climate change impact on the evolution of groundwater levels and groundwater reserves. From the modelling application on the Geer basin, it appears that, on a pluri-annual basis, most tested scenarios predict a decrease in groundwater levels and reserves in relation to variations in climatic conditions. However, for this aquifer, the tested scenarios show no enhancement of the seasonal changes in groundwater levels.

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Resumen Se ha desarrollado un modelo hidrológico integrado (MOHISE) para estudiar el impacto del cambio climático en el ciclo hidrológico de cuencas representativas en Bélgica. Este modelo considera todos los procesos hidrológicos de forma coherente, especialmente en relación con los flujos de aguas subterráneas, que son modelados por medio de un enfoque de elementos finitos espacialmente distribuidos. Gracias a esta herramienta numérica precisa, y tras una calibración y validación detalladas, se puede obtener interpretaciones cuantitativas de los resultados del modelo del acuífero. Considerando escenarios de cambio climático IPCC, se ha aplicado el enfoque integrado a la evaluación del impacto de dicho cambio climático en el ciclo hidrológico de la cuenca del Geer. Se describe los detalles y resultados del modelo de las aguas subterráneas en términos del impacto del cambio climático en la evolución de las reservas de los acuíferos. Los resultados preliminares indican que es posible esperar déficits de aguas subterráneas en un futuro en Bélgica.

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Résumé Un modèle hydrologique intégré (MOHISE) a été développé afin détudier limpact du changement climatique sur le cycle hydrologique de bassins versants représentatifs de Belgique. Ce modèle prend en compte tous les processus hydrologiques dune manière physiquement consistante, plus particulièrement les écoulements souterrains qui sont modélisés par une approche spatialement distribuée aux éléments finis. Grâce à cet outil numérique précis, après une calibration et une validation détaillées, des interprétations quantitatives peuvent être réalisées à partir des résultats du modèle de nappe. Considérant des scénarios de changements climatiques de lIPCC, lapproche intégrée a été appliquée pour évaluer limpact du changement climatique sur le cycle de leau du bassin du Geer en Belgique. Le modèle de nappe est décrit en détail et les résultats sont discutés en terme dimpact du changement climatique sur lévolution des réserves souterraines. Les premiers résultats indiquent que des déficits deau souterraine peuvent apparaître dans le futur en Belgique.

     

Geological interpretation of combined SEABEAM,GLORIA and seismic data from Porto and Vigo Seamounts,Iberian continental margin

Long-range sidescan sonar (GLORIA) data over Porto and Vigo Seamounts collected in 1978 has been re-interpreted in the light of SEABEAM bathymetric surveys conducted in 1982. The application of quantitative bathymetric information enables the interpreter to allow for artefacts inherent in the GLORIA data and to separate topography-related primary backscattering variations on the sonographs from more subtle changes that result from textural, slope and outcrop effects. The distinctions are made easier when slant-range corrected GLORIA data are available.Use of the combined survey data to precisely locate seismic profiling tracks and to identify likely areas of outcrop has allowed refined geological maps of the seamounts to be drawn and regional fault trends detected. The overall outline of the seamounts appears strongly fault-controlled.Porto and Vigo Seamounts are made up of the same geological formations and have had a similar structural history since their uplift as continental fault blocks in the Late Cretaceous to Middle Eocene period. Ravines that dissect the presumably lithified scarps bounding the seamounts may be relict features but still appear to control sediment input to gulley and channel systems in the surrounding topography. Sedimentary ridges associated with the seamounts represent anomalously thick sequences of post-Eocene material and probably result from interaction of downslope sedimentary processes and contour-following boundary currents.Contribution No. 274 from the Groupe d'Etude de la Marge Continentale (ERA 605).     

Chemical speciation of arsenic in different marine organisms: Importance in monitoring studies

Arsenic is a widely distributed element in the marine environment. Inorganic and organic compounds have extremely different toxicological effects, and their characterization is thus of great utility when monitoring and assessing the impact of arsenic pollution. In this study both the levels of total arsenic and its chemical speciation were analyzed in several marine organisms collected from Cienfuegos Bay (Cuba) following an episode of acute As-contamination. Fish from the more impacted site were characterized by elevated concentrations of arsenic (up to 500 microg/g d.w.) and inorganic species represented the predominant forms in muscle tissues of these organisms.     

Pre-explosive conduit conditions of the 1997 Vulcanian explosions at Soufrière Hills Volcano,Montserrat: II. Overpressure and depth distributions

A type example of Vulcanian eruptive dynamics is the series of 88 explosions that occurred between August and October 1997 at Soufrière Hills volcano on Montserrat Island. These explosions are interpreted to be caused by the pressurization of a conduit by a shallow highly crystalline and degassed magma plug. We test such an interpretation by combining the pressures and porosities of the pre-explosive magma column proposed by Burgisser et al. (2010, doi:10.1016/j.jvolgeores.2010.04.008) into a physical model that reconstructs a depth-referenced density profile of the column for four mechanisms of pressure buildup. Each mechanism yields a different overpressure profile: 1) gas accumulation, 2) conduit wall elasticity, 3) microlite crystallization, and 4) magma flowage. For the first three mechanisms, the three-part vertical layering of the conduit prior to explosion was spatially distributed as a dense cap atop the conduit with a thickness of a few tens of meters, a transition zone of 400–700 m with heterogeneous vesicularities, and, at greater depth, a more homogeneous, low-porosity zone that brings the total column length to ~ 3.5 km. A shorter column can be obtained with mechanism 4: a dense cap of less than a few meters, a heterogeneous zone of 200–500 m, and a total column length as low as 2.5 km. Inflation/deflation cycles linked to a periodic overpressure source offer a dataset that we use to constrain the four overpressure mechanisms. Magma flowage is sufficient to cause periodic edifice deformation through semi-rigid conduit walls and build overpressures able to trigger explosions. Gas accumulation below a shallow plug is also able to build such overpressures and can occur regardless of magma flowage. The concurrence of these three mechanisms offers the highest likelihood of building overpressures leading to the 1997 explosion series. We also explore the consequences of sudden (eruptive) overpressure release on our magmatic columns to assess the role of syn-explosive vesiculation and pre-fragmentation column expansion. We find that large shallow overpressures and efficient syn-explosive vesiculation cause the most dramatic pre-fragmentation expansion. This leads us to depict two end-member pictures of a Vulcanian explosion. The first case corresponds to the widely accepted view that the downward motion of a fragmentation front controls column evacuation. In the second case, syn-explosive column expansion just after overpressure release brings foamed-up magma up towards an essentially stationary and shallow fragmentation front.