Caractérisation minéralogique et microtexturale du vieillissement anthropique du minerai de fer lorrainMineralogical and microtextural characterisation of the anthropic-origin ageing of iron ore in Lorraine (France)

The abandoned pillars in iron mines in Lorraine show that the oolitic ore underwent several chemical-mineralogical transformations. The first one is a rapid alteration of the diagenetic siderite and berthierine cement promoted by a bacterial flora. The oolites of goethite pack down and get loaded by deviatory-type stresses; they desquamate, get deformed and become powdery; the ore looses its cohesion. A second step, featuring recrystallisation–neoformation of berthierine in the previously formed micro-cavities, then the formation of a cavernous goethite–hematite crust, leads to a low cohesion to the crumbled ore; this anthropic-origin ageing may be one cause of mine collapsing. To cite this article: G. Dagallier et al., C. R. Geoscience 334 (2002) 455–462.     

Geometrical relationships between unconformities and subsequent folding: the Arro fold system (southern Pyrenees)Relations entre discordances et plissement dans le système de plis et chevauchements d'Arro (bassin de Graus-Tremp,Pyrénées)

The Arro system is an oblique fold system involving the Eocene sediments of the Graus-Tremp basin. It consists of westward-verging folds, trending NNW–SSE, some of them related with thrusts, in piggyback sequence. Seismic profiles allow to infer the geometry of structures at depth: folds and thrusts are ‘decolled’ over an unconformity between turbiditic and platform sediments. Re-activation of bedding surfaces by folding in the underlying units resulted in folding and thrusting in the upper series. To cite this article: A.M. Casas et al., C. R. Geoscience 334 (2002) 765–772.     

Méthodologie de l'extraction des hydrocarbures aromatiques polycycliques. Application à des sédiments de la lagune de Bizerte (Tunisie)Methodology of extraction of polycyclic aromatic hydrocarbons. Application to sediment from the Bizerte Lagoon (Tunisia)

The marine environment is very complex, with several important chemical, biological, and sedimentological interactions. Sediments constitute a reservoir for numerous pollutants, including polycyclic aromatic hydrocarbons (PAHs). PAHs are known by their mutagenic and/or carcinogenic effect. This study evaluates contamination levels in Bizerte Lagoon with respect to PAHs. The extraction efficiency of PAHs from sediment has been evaluated using Soxhlet or ultrasonic procedures, based on the recovery of an internal standard (9,10-dihydroanthracene). Several PAHs were found, including sixteen that are reported in the priority list of pollutants of the US Environmental Protection Agency (EPA). PAHs analysis was carried out by GC–FID and GC–MS for 16 samples sampled in two seasons: summer and winter. Results permit an assessment of PAHs pollution in the Bizerte Lagoon. To cite this article: N. Mzoughi et al., C. R. Geoscience 334 (2002) 893–901.     

Réflexions sur la formation des bandes de Forbes : l'instabilité de la fusion de la glace saleReflections on the formation of Forbes ogives: the instability of fusion of dirty ice

Forbes ogives show alternations of dark (ice + mineral dust) and light bands at the surface of certain glaciers. We propose to understand their origin by the influence of the content of mineral matter on the lowering of the temperature and pressure of ice fusion and upon the increase of fusion velocity. We are then in an unstable situation: a local increase in the mineral content being induced by the fusion process, which in turn increases; this creates a dark band. The movement of the glacier cannot keep up with the fusion: pressure is lowered again below the fusion point, and a white band is formed. To cite this article: B. Guy et al., C. R. Geoscience 334 (2002) 1061–1070.     

Structure of the Mérida Andes, Venezuela: relations with the South America–Caribbean geodynamic interaction

For over 50 years, several models based on diverse geologic concepts and variable quality of data have been proposed to explain the major structure and history of the Mérida Andes (MA), in western Venezuela. Lately, this chain growth and associated flexural basins deepening have been related to incipient type-A subductions of either polarity, accounting for the across-chain asymmetry. However, these recent models have not well integrated the present tectonically active setting driven by neighboring major plate interactions. At present, this chain exhibits ongoing strain partitioning where cumulative right-lateral slip along chain axis is as much as half of, or about the same, as the transverse shortening since late Miocene, thus implying that the NNE-directed Maracaibo block extrusion with respect to the South America (SA) plate is not a secondary feature. Consequently, this paper discusses some limitations exhibited by the SE-directed continental subduction models—Maracaibo crust underthrusting the Mérida Andes—in the light of available geological and geophysical data. Besides, it is herein proposed that the Mérida Andes structuration is related to a NW-directed, gently dipping, incipient type-A subduction, where chain growth and evolution are similar to those of a sedimentary accretionary wedge (i.e., Barbados), but at crustal scale and with ongoing strain partitioning. This continental subduction is the SE portion of a major orogenic float that also comprises the Perijá range and the Santa Marta block.     

Crustal make-up of the northern Andes: evidence based on deep crustal xenolith suites, Mercaderes, SW Colombia

Samples of the deep crust and upper mantle in the Northern Andes occur as abundant xenoliths in the Granatífera Tuff, a late Cenozoic vent in the Mercaderes area of SW Colombia. The lower crustal assemblage includes granulites, hornblendites, pyribolites, pyroxenites and gneisses; mafic rocks predominate, but felsic material is also common. P–T conditions for the pyribolite assemblages (i.e. Hbl+Fs/Scp+Grt+Cpx+Qtz±Bt), which are the best constrained, are 720–850 °C and 10–14 kbar, consistent with a deep-to-lower crustal origin. A notable feature of this xenolith suite is that it is dominated by hornblende. However, mineral reactions within the suite show that there is a transition from amphibolite to granulite facies, and there is a probable restite–melt relationship represented within the suite. However, the latter appears to be dominated by hornblende and garnet.The mafic rocks mostly lack the high Cr and Ni that would be expected of cumulates. Neither do they possess the positive Sr and Eu anomalies that would be consistent with resite or cumulate models for the lower crust. They bear greatest similarity to oceanic basalts (s.l.). The Rb contents of the xenoliths, whether mafic or silicic, are very low, and the more silicic members of the suite tend to have small positive Sr and Eu anomalies, which are transitional to adakitic compositions. The Sr isotopic compositions of the xenoliths lie between 0.704 and 0.705; however, the Nd isotopic compositions are much more variable, indicating considerable long-term heterogeneity. Few of the xenoliths can be compositionally recognised as metasedimentary; however, a sedimentary component is evident in the Pb isotopic compositions. Within these constraints, our favoured model is a deep crust formed by basaltic components (subduction–accretion?), and minor sediment, which is subject to an increase in thermal gradient to produce the granulites, any melting being dominated by hornblende-out reactions involving garnet. However, there is no evidence of any pervasive crustal melting, leading to the conclusion that the voluminous Andean magmatism arises from the mantle wedge.     

The influence of tectonically derived relief and climate on the extent of the last Glaciation east of the Patagonian ice fields (Argentina, Chile)

Two large ice fields between 46°30′ and 51°30′S cover the Patagonian Andes. The North and South Patagonian Ice Fields are separated by the transandine depth line at 47°45′ to 48°15′S. Canal and Río Baker run through this depression. The two ice fields are generally considered relics of a continuous ice cap, which covered the entire Patagonian Andes from 39° to 52°S and extended far into the eastern foreland of the Andes. This assumption is not correct for the 200-km-long section of the Andes between Lago Pueyrredón (Lago Cochrane in Chile) (47°15′S) and Lago San Martín (Lago O'Higgins in Chile) (48°45′S). The lack of a continuous ice cap extending far into the east is caused by the transandine depth line, playing a crucial role in the fluvial erosion and the glacial scouring of this tectonic zone. This depression formed a river system (e.g. Río Baker, Río Bravo and Río Mayer) that drains towards the west. Reconstruction of the maximum glacial advance of the last ice age shows that the eastern outlet glaciers of the two ice fields between Lago San Martín and Lago Pueyrredón did not drain towards the east, but rather followed the general gradient of the transandine depth line. In this area the eastern flank of the Andes between Monte San Lorenzo (3770 m) and Sa. de Sangra (2155 m) supported valley glaciers, which were independent of the expanding ice fields. Only a few valley glaciers advanced towards the Patagonian Meseta. The terminal moraines of these glaciers were erroneously interpreted as the eastern edge of a continuous ice cap. North of 47°30′S the outlet glaciers of the NPI advanced 200 km during the LGM and the late glacial advances nearly reached to 71°W. In contrast, south of 49°S glacier expansion was comparatively less: The LGM is situated only 85–115 km east of the present margins of the large outlet glaciers (O'Higgins, Viedma, and Upsala), and no late glacial advance reached 72°W. These considerable differences of glacier expansion were influenced by the northward migration of the westerly precipitation belt during glacial cycles. There is tentative evidence that the glaciers advanced three times in the period from 14 000 to 9 500 ¹⁴C years BP.     

Evolution of the West Andean Escarpment at 18°S (N. Chile) during the last 25 Ma: uplift, erosion and collapse through time

The geological record of the Western Andean Escarpment (WARP) reveals episodes of uplift, erosion, volcanism and sedimentation. The lithological sequence at 18°S comprises a thick pile of Azapa Conglomerates (25–19 Ma), an overlying series of widespread rhyodacitic Oxaya Ignimbrites (up to 900 m thick, ca. 19 Ma), which are in turn covered by a series of mafic andesite shield volcanoes. Between 19 and 12 Ma, the surface of the Oxaya Ignimbrites evolved into a large monocline on the western slope of the Andes. A giant antithetically rotated block (Oxaya Block, 80 km×20 km) formed on this slope at about 10–12 Ma and resulted in an easterly dip and a reversed drainage on the block's surface. Morphology, topography and stratigraphic observations argue for a gravitational cause of this rotation. A “secondary” gravitational collapse (50 km³), extending 25 km to the west occurred on the steep western front of the Oxaya Block. Alluvial and fluvial sediments (11–2.7 Ma) accumulated in a half graben to the east of the tilted block and were later thrust over by the rocks of the escarpment wall, indicating further shortening between 8 and 6 Ma. Flatlying Upper Miocene sediments (<5.5 Ma) and the 2.7 Ma Lauca–Peréz Ignimbrite have not been significantly shortened since 6 Ma, suggesting that recent uplift is at least partly caused by regional tilting of the Western Andean slope.     

Distribution d'un élément trace (Cr) dans un sol développé sur roches métamorphiques : variabilité à l'échelle d'un versantDistribution of chromium in soils developed into metamorphic rocks: variability at the hillslope scale

The influence of pedogenetic factors onto spatial variability of chromium in soil surface horizon was studied in a metamorphic area (Massif Central, France) at the hillside scale. A grid survey was performed. Surface soil horizons were sampled at each point and analysed in Cr. Results show the determinant influence of the rock type on Cr content distribution along the slope. However, surface horizons of these soils exhibit a lower Cr content heterogeneity than the deeper horizons. This was explained by soils' material erosion and deposition along the slope. Tillage and hedges also influence the Cr distribution in surface horizon along the slope. To cite this article: S. Salvador-Blanes et al., C. R. Geoscience 334 (2002) 51–58     

Évolution temporelle et architecture interne d'un banc sableux estuarien : la Longe de Boyard (littoral atlantique,France)

The evolution and the internal architecture of an estuary type sand ridge has been studied with a set of bathymetric data recorded during the last two centuries and with a dense grid of recent very high resolution seismic profiles. Bathymetric data of the so-called, Longe de Boyard sand ridge, displays sand losts due to wave and tide erosion. Internal geometry, through seismic profile analysis, indicates two main phases of deposition recording both, a recent high energy environment and an older low energy one, respectively. Such an evolution is believed to record changes in sedimentation processes mainly related to the end of the Holocene transgression (8 000–5 000 yr BP). To cite this article: É. Chaumillon et al., C. R. Geoscience 334 (2002) 119–126.