Failles normales Paléocène à Lutétien en zone sud-pyrénéenne (Aragon,Espagne) et flexuration de la plaque ibérique

Close to the South Pyrenean thrust front, synsedimentary normal faults develop in the ‘Garumnian’ continental deposits (Maastrichtian to Palaeocene) and in the carbonate platform during the Lutetian. During the Lutetian, this deformation is accompanied by a change in the sedimentation characterized by the deposit of two series of laminated limestones associated to monogenetic breccias. These normal faults would have formed at the bending of the Iberian plate subducting under Europe. They may also be considered as the starting point for the megaturbidites that deposited further north in the Eocene turbiditic basin. To cite this article: Y. Hervouët et al., C. R. Geoscience 337 (2005).     

Spatial and temporal relationships between compression, strike-slip and extension in the Central Venezuelan Andes: Clues for Plio-Quaternary tectonic escape

The geometry of tectonic structures, attributed to the Neogene–Quaternary time interval, is described in the active setting of the Venezuelan Andes. Our methodology is based on the analysis of radar satellite and Digital Elevation Model imagery, complemented by structural fieldwork and the compilation of seismotectonic data to make a structural analysis on a regional scale. Radar images provide first class data for morphostructural analysis in areas of dense vegetation and frequent cloud covering, like the Venezuelan Andes. We focused our analysis in the Burbusay–Río Momboy and Boconó faults corner located in the central part of the belt.We have described three stages of deformation during the Neogene–Quaternary. The first one, Mio-Pliocene in age, is a NW–SE compression responsible for the uplift of the Venezuelan Andes. The second tectonic stage corresponds to a strike-slip regime of deformation marked by shearing along the Boconó, Burbusay and Valera faults, which separates two triangular wedges in the larger Trujillo block. This strike-slip faulting-dominated compressional-extensional tectonic regime allowed the Trujillo crustal block to move towards the NE. Wrenching has therefore started at some point between the Pliocene and the Quaternary. These two tectonic events are consistent with ongoing strain partitioning in the Venezuelan Andes. The third stage corresponds to extensional deformation limited to the Trujillo block and is still active today. Extension is associated with the motion of crustal blocks moving relative to each other, probably above the upper-lower crust boundary. Such extensional deformation can be understood considering that the crust extends and stretches at the same time as it moves towards the NE. The combination of both horizontal lateral motion and extension is characteristic of a tectonic escape process. The northeastward escape of the Trujillo block, which belongs to the larger North Andes block, occurs as a result of the combination of the NW–SE intracontinental convergence between the South-American plate and the Maracaibo block, and the presence to the north of the Caribbean oceanic plate considered as a free boundary. We have showed that the kinematics of the Caribbean plate offers not only a favorable environment, but may also be considered as the driving force of the tectonic escape of the North Andes block.     

New biostratigraphical data for the Burdigalian Montchaibeux Member at the locality Courrendlin-Solé (Canton of Jura,Switzerland)

Geological surveys were carried out in the Miocene deposits at the place known as “En Solé” east of the village Courrendlin (Delémont Basin, Canton of Jura, Switzerland). This resulted in the discovery of new Miocene small mammal assemblages. The association of the rodents Megacricetodon aff. collongensis and Melissiodon sp. allows to biochronostratigraphically correlate for the first time the so-called “Rote Mergel des Mont Chaibeux” representing the lower part of the Montchaibeux Member of the Bois de Raube Formation to the regional M. collongensis–Keramidomys interval zone (MN 4; early Miocene).     

Karst system developed in salt layers of the Lisan Peninsula,Dead Sea,Jordan

The Lisan Peninsula, Jordan, is a massive salt layer accumulated in the inner part of the Dead Sea’s precursory lakes. This tongue-shaped, emergent land results in a salt diapir uplifted in the Dead Sea strike-slip regional stress field and modified by the water level fluctuations of the last lake during the Holocene. These two elements, associated with dissolution caused by rainfall and groundwater circulation, resulted in an authentic karst system. Since the 1960s, the Dead Sea lowering of 80 cm to 1 m per year caused costly damages to the industrial plant set up on the peninsula. The Lisan karst system is described in this article and the components of the present dynamic setting clarified.     

A TEM study of exsolution in Ca-rich pyroxenes from the Paris and Renazzo chondrites: Determination of type I chondrule cooling rates

We conducted a transmission electron microscope study of the exsolution microstructures of Ca-rich pyroxenes in type I chondrules from the Paris CM and Renazzo CR carbonaceous chondrites in order to provide better constraints on the cooling history of type I chondrules. Our study shows a high variability of composition in the augite grains at a submicrometer scale, reflecting nonequilibrium crystallization. The microstructure is closely related to the local composition and is thus variable inside augite grains. For compositions inside the pyroxene miscibility gap, with a wollastonite (Wo) content typically below 40 mole%, the augite grains contain abundant exsolution lamellae on (001). For grain areas with composition close to Wo₄₀, a modulated texture on (100) and (001) is the dominant microstructure, while areas with compositions higher than Wo₄₀ do not show any exsolution microstructure development. To estimate the cooling rate, we used the spacing of the exsolution lamellae on (001), for which the growth is diffusion controlled and thus sensitive to the cooling rate. Despite the relatively homogeneous microstructures of augite grains with Wo < 35 mole%, our study of four chondrules suggests a range of cooling rates from ~10 to ~1000 °C h⁻¹, within the temperature interval 1200–1350 °C. These cooling rates are comparable to those of type II chondrules, i.e., 1–1000 °C h⁻¹. We conclude that the formation of type I and II chondrules in the proto-solar nebula was the result of a common mechanism.     

Evaporation and Sublimation of Boric Acid: Application for Boron Purification from Organic Rich Solutions

We report in this paper a series of experiments whose purpose is to test the long held idea of boron volatility when neutral to acidic solutions containing traces of boron are evaporated. Boron recoveries have been measured precisely by isotopic dilution and boron isotopic ratios have also been determined. Most of the evaporations have been conducted at 60-65 °C. It was found that no loss of boric acid occurs when c. 1 μg of B is evaporated in water, HCl, HF and acetone solutions. At the same time, we did not observe any associated B isotopic fractionation. In contrast to previous studies on B volatilisation, it was not found necessary to use mannitol to prevent B losses during the evaporation of acidic solutions. Important losses of boron were observed from solutions of methanol, ethanol and organic-rich natural solutions. In the presence of methanol and ethanol, a volatile methyl (or ethyl) borate is likely to be formed. In contrast to these studies, we show that boron is highly volatile when the dried residues of an evaporation are heated further. The sublimation of boron that then occurs can be moderated by the use of mannitol, but a slight increase of the temperature allows B volatilisation. Despite the substantial losses of B by sublimation, no B isotopic fractionation was observed at 60-65 C. The property of boron to sublime was used to separate B from an organic matrix. A miniaturized sublimation apparatus, comprising a 5 ml Teflon beaker, and the associated microsublimation technique are described. The 100% boron recovery and the absence of B isotopic fractionation make this method suitable for extracting B from organic-rich samples.     

Contribution of long-term hydrothermal experiments for understanding the smectite-to-chlorite conversion in geological environments

The smectite-to-chlorite conversion is investigated through long-duration experiments (up to 9 years) conducted at 300 °C. The starting products were the Wyoming bentonite MX80 (79 % smectite), metallic iron and magnetite in contact with a Na–Ca chloride solution. The predominant minerals in the run products were an iron-rich chlorite (chamosite like) and interstratified clays interpreted to be chlorite/smectite and/or corrensite/smectite, accompanied by euhedral crystals of quartz, albite and zeolite. The formation of pure corrensite was not observed in the long-duration experiments. The conversion of smectite into chlorite over time appears to take place in several steps and through several successive mechanisms: a solid-state transformation, significant dissolution of the smectite and direct precipitation from the solution, which is over-saturated with respect to chlorite, allowing the formation of a chamosite-like mineral. The reaction mechanisms are confirmed by X-ray patterns and data obtained on the experimental solutions (pH, contents of Si, Mg, Na and Ca). Because of the availability of some nutrients in the solution, total dissolution of the starting smectite does not lead to 100 % crystallization of chlorite but to a mixture of two dominant clays: chamosite and interstratified chlorite/smectite and/or corrensite/smectite poor in smectite. The role of Fe/(Fe + Mg) in the experimental medium is highlighted by chemical data obtained on newly formed clay particles alongside previously published data. The newly formed iron-rich chlorite has the same composition as that predicted by the geothermometer for diagenetic to low-grade metamorphic conditions, and the quartz + Fe-chlorite + albite experimental assemblage in the 9-year experiment is close to that fixed by water–rock equilibrium.     

The dependence of albite feldspar dissolution kinetics on fluid saturation state at acid and basic pH: Progress towards a universal relation

Here we report on two separate ongoing, multi-year investigations on the dependence of the dissolution rate (R) of albite feldspar on fluid saturation state, as defined by the Gibbs free energy of reaction (ΔG_r) for dissolution. The investigations are based on dissolution at pH 9.2, 150 °C and pH 3.3, 100 °C. Both studies reveal that the R–ΔG_r relation is highly non-linear and sigmoidal. The kinetic data from the first study, being the most complete, can be fitted with a sigmoidal rate curve that is composed of two separate, parallel rate laws that represent distinct mechanisms of dissolution. The switch between one dominant mechanism and the other may be controlled by a critical free energy. The fact that in both studies the same type of sigmoidal R–ΔG_r relation exists for dissolution at different pH and temperature condition suggests that this behavior may be universal for albite and other feldspars. Moreover, the experimental data contradict the commonly used R–ΔG_r relation that is loosely based on transition state theory (TST). This has important implications with respect to the accuracy of geochemical codes that model water–rock interactions at near-equilibrium conditions.     

In-Street Wind Direction Variability in the Vicinity of a Busy Intersection in Central London

We present results from fast-response wind measurements within and above a busy intersection between two street canyons (Marylebone Road and Gloucester Place) in Westminster, London taken as part of the DAPPLE (Dispersion of Air Pollution and Penetration into the Local Environment; www.dapple.org.uk ) 2007 field campaign. The data reported here were collected using ultrasonic anemometers on the roof-top of a building adjacent to the intersection and at two heights on a pair of lamp-posts on opposite sides of the intersection. Site characteristics, data analysis and the variation of intersection flow with the above-roof wind direction (θ _ref ) are discussed. Evidence of both flow channelling and recirculation was identified within the canyon, only a few metres from the intersection for along-street and across-street roof-top winds respectively. Results also indicate that for oblique roof-top flows, the intersection flow is a complex combination of bifurcated channelled flows, recirculation and corner vortices. Asymmetries in local building geometry around the intersection and small changes in the background wind direction (changes in 15- min mean θ _ref of 5°–10°) were also observed to have profound influences on the behaviour of intersection flow patterns. Consequently, short time-scale variability in the background flow direction can lead to highly scattered in-street mean flow angles masking the true multi-modal features of the flow and thus further complicating modelling challenges.