Neoproterozoic subductions and differential exhumation of western Hoggar (southwest Algeria): new structural, petrological and geochronological evidence

The western terranes exposed east of the Pan-African suture in western Hoggar (southwest Algeria), are reexamined in the light of new structural, petrologic and by the ⁴⁰Ar/³⁹Ar laser probe data on metamorphic micas and amphiboles. To the north, the Tassendjanet nappe includes the Paleoproterozoic basement, its Mesoproterozoic cover and mafic rocks representing the roots of a ca. 680 Ma arc overlain by Late Neoproterozoic andesites and volcanic greywackes. The nappe preserved at rather shallow crustal level in the east was emplaced southward (D_1a) to southeastward (D₂). In the south, two metamorphic suites are distinguished. The Tideridjaouine–Tileouine high-pressure metamorphic belt (T=550–600 °C, P=1.4–1.8 GPa) represents a slab of subducted continental material exposed along the western edge of the In Ouzzal granulite unit interpreted as a microcontinent. Differential exhumation of tectonic slices from the high-pressure belt occurred around 615–600 Ma through a system of west-directed recumbent folds (D_1b). The Egatalis high grade belt in the west was intruded by syn-metamorphic gabbro–norite bodies. It includes unretrogressed low-pressure granulite facies rocks (T around 750–800 °C, P0.45 GPa) cooled at a rate of 15°/m.y. between 600 and 580 Ma, and followed by the emplacement of several late-kinematic granitic plutons. Final exhumation of the low-pressure, high-temperature metamorphic rocks, that are not found as pebbles in the molasse, took place in the Late Cambrian. The early and relatively fast cooling of the high-pressure and high-temperature metamorphic rocks of the southern part of the Tassendjanet terrane is at variance with the slow cooling of central Hoggar where repeated magmatic activity as young as Late Cambrian occurred [Lithos 45 (1998) 245].     

Synthesis,crystallographic data,solubility and electrokinetic properties of meta-zeunerite,meta-kirchheimerite and nickel-uranylarsenate

{M[UO₂¦AsO₄]₂ · nH₂O} with M=Cu²⁺, Co²⁺, Ni²⁺ has been synthesized from reagent grade chemicals and by ion exchange of trögerite {HUO₂AsO₄ · 4 H₂O}. Synthetic meta-zeunerite (M=Cu²⁺), meta-kirchheimerite (M=Co²⁺) and nickel-uranylarsenate are all tetragonal. The cell parameters determined from Guinier-Hägg diffraction data for {Cu[UO₂¦AsO₄]₂ · 8 H₂O} are a=b=7.10 Å and c=17.42 Å, with Z=2 and the measured density 3.70 g cm^?3. The cell parameters for {Co[UO₂¦AsO₄]₂ · 7 H₂O} and {Ni[UO²¦AsO⁴]² · 7 H₂O} are a=b=20.25 Å and c=17.20 Å, with Z=16 and the measured density 3.82 and 3.74 g cm^?3, respectively. The solubility products for synthetic Cu-, Co- and Ni-uranylarsenate at 25° C are 10^?49.20, 10^?45.34 and 10^?45.10, respectively. The zeta-potential remains negative between pH=2 and pH=9 and is strongly affected by the presence of different cations.     

Amphibolite facies conditions in the oceanic crust: example of amphibolitized flaser-gabbro and amphibolites from the Chenaillet ophiolite massif (Hautes Alpes,France)

Petrological and mineralogical data on amphibolitized gabbros from an Alpine ophiolite massif (Chenaillet Massif, France) are presented. Comparison with metagabbros dredged from the ocean floor shows that synkinematic amphibolite facies conditions may be reached in gabbros after their initial crystallization in the vicinity of the ridge. It is suggested that sub-horizontal plastic flow took place in the gabbroic layer near the axis of a slowly spreading ocean ridge before the intrusion of diabase dykes. This thermo-tectonic regime which at the Chenaillet produced flaser-gabbros and layers of foliated amphibolites with brown hornblende and pargasite, probably also affected most of the other ophiolitic gabbros of the Piemont zone prior to the low-temperature/high-pressure Alpine metamorphism.     

Influence of morphological changes on suspended sediment dynamics in a macrotidal estuary: diachronic analysis in the Seine Estuary (France) from 1960 to 2010

Ocean Dynamics - Estuaries are subject to extensive morphological changes through human activities, such as deepening and narrowing via dredging and channelization. The estuary sediment load,...     

The AMANDES tidal model for the Amazon estuary and shelf

The AMANDES project aims to study transports from the Andean mountains to the Atlantic Ocean through the Amazon system. This requires realistic estuarine modelling in this area strongly forced by tides and river discharge. As none of the existing models for this region would fit the actual needs of the project, a specific new generation model has been implemented.The model is based on the hydrodynamic finite element model T-UGOm. In a first step, we limit our investigations to tidal dynamics. As the Amazon estuary is a very shallow macro-tidal area, it is necessary to improve the available bathymetries and to develop a precise bottom friction parametrisation.In this paper, we discuss the implementation of a high resolution regional model. This allows us to develop a precise and accurate tidal model: for instance, the overall root mean square error on complex differences is reduced from 54 cm in a standard model to 27 cm in our best model. Such precise and accurate tidal modelling is a prerequisite for modelling particle transport.     

Numerical modeling of bedload and suspended load contributions to morphological evolution of the Seine Estuary (France)

This numerical modeling study (i) assesses the influence of the sediment erosion process on the sediment dynamics and subsequent morphological changes of a mixed-sediment environment, the macrotidal Seine estuary, when non-cohesive particles are dominant within bed mixtures (non-cohesive regime), and (ii) investigates respective contributions of bedload and suspended load in these dynamics. A three dimensional (3D) process-based morphodynamic model was set up and run under realistic forcings (including tide, waves, wind, and river discharge) during a 1-year period. Applying erosion homogeneously to bed sediment in the non-cohesive regime, i.e., average erosion parameters in the erosion law (especially the erodibility parameter, E₀), leads to higher resuspension of fine sediment due to the presence of coarser fractions within mixtures, compared to the case of an independent treatment of erosion for each sediment class. This results in more pronounced horizontal sediment flux (two-fold increase for sand, +30% for mud) and erosion/deposition patterns (up to a two-fold increase in erosion over shoals, generally associated with some coarsening of bed sediment). Compared to observed bathymetric changes, more relevant erosion/deposition patterns are derived from the model when independent resuspension fluxes are considered in the non-cohesive regime. These results suggest that this kind of approach may be more relevant when local grain-size distributions become heterogeneous and multimodal for non-cohesive particles. Bedload transport appears to be a non-dominant but significant contributor to the sediment dynamics of the Seine Estuary mouth. The residual bedload flux represents, on average, between 17 and 38% of the suspended sand flux, its contribution generally increasing when bed sediment becomes coarser (can become dominant at specific locations). The average orientation of residual fluxes and erosion/deposition patterns caused by bedload generally follow those resulting from suspended sediment dynamics. Sediment mass budgets cumulated over the simulated year reveal a relative contribution of bedload to total mass budgets around 25% over large erosion areas of shoals, which can even become higher in sedimentation zones. However, bedload-induced dynamics can locally differ from the dynamics related to suspended load, resulting in specific residual transport, erosion/deposition patterns, and changes in seabed nature.     

Leaching of coal-mine tips (Nord-Pas-de-Calais coal basin,France) and sulphate transfer to the chalk aquifer: example of acid mine drainage in a buffered environment

In northern France, the weathering and oxidation of pyrite-containing coal mine spoils can lead to sulphate enrichment of the underlying chalk aquifer. Two sites have been studied: in a free water-table zone and in a confined-aquifer zone. Solid-fraction analyses have shown a release of carbon and sulphur from the waste dump surfaces. The groundwater isotopic analysis (notably ³⁴S) showed the existence of two sulphate sources (mine spoils and gypsum contained in Cenozoic formations). This study also saw evidence of bacterial sulphate reduction in the confined zone as a result of the release of carbon from mine spoils. The effects of acid mine drainage were the presence of secondary minerals as small jarosite dots (cm) present at depth, gypsum or ferrihydrite present on the bottom of temporary ponds, and an increase in sulphate content of groundwaters sampled downstream of the waste dumps. This acid mine drainage is buffered during its infiltration into the chalk aquifer. This is evidenced by the increase in magnesium, calcium, bicarbonate or strontium, resulting of carbonate digestion downstream of the sites. No significant leaching of metals, even those associated with the sulphide fraction, was seen at the two studied sites.     

Modelling the global ocean tides: modern insights from FES2004

During the 1990s, a large number of new tidal atlases were developed, primarily to provide accurate tidal corrections for satellite altimetry applications. During this decade, the French tidal group (FTG), led by C. Le Provost, produced a series of finite element solutions (FES) tidal atlases, among which FES2004 is the latest release, computed from the tidal hydrodynamic equations and data assimilation. The aim of this paper is to review the state of the art of tidal modelling and the progress achieved during this past decade. The first sections summarise the general FTG approach to modelling the global tides. In the following sections, we introduce the FES2004 tidal atlas and validate the model against in situ and satellite data. We demonstrate the higher accuracy of the FES2004 release compared to earlier FES tidal atlases, and we recommend its use in tidal applications. The final section focuses on the new dissipation term added to the equations, which aims to account for the conversion of barotropic energy into internal tidal energy. There is a huge improvement in the hydrodynamic tidal solution and energy budget obtained when this term is taken into account.     

Adoption of flood-related preventive behaviours by people having different risks and histories of flooding

Natural Hazards - The increase in the frequency of floods, which is a projected consequence of climate change, can have wide-ranging health and economic impacts. To cope with these floods and to...     

Particle transport mechanics and induced seismic noise in steep flume experiments with accelerometer-embedded tracers

Recent advances in fluvial seismology have provided solid observational and theoretical evidence that near-river seismic ground motion may be used to monitor and quantify coarse sediment transport. However, inversions of sediment transport rates from seismic observations have not been fully tested against independent measurements, and thus have unknown but potentially large uncertainties. In the present study, we provide the first robust test of existing theory by conducting dedicated sediment transport experiments in a flume laboratory under fully turbulent and rough flow conditions. We monitor grain-scale physics with the use of ‘smart rocks’ that consist of accelerometers embedded into manufactured rocks, and we quantitatively link bedload mechanics and seismic observations under various prescribed flow and sediment transport conditions. From our grain-scale observations, we find that bedload grain hop times are widely distributed, with impacts being on average much more frequent than predicted by existing saltation models. Impact velocities are observed to be a linear function of average downstream cobble velocities, and both velocities show a bed-slope dependency that is not represented in existing saltation models. Incorporating these effects in an improved bedload-induced seismic noise model allows sediment flux to be inverted from seismic noise within a factor of two uncertainty. This result holds over nearly two orders of magnitude of prescribed sediment fluxes with different sediment sizes and channel-bed slopes, and particle–particle collisions observed at the highest investigated rates are found to have negligible effect on the generated seismic power. These results support the applicability of the seismic-inversion framework to mountain rivers, although further experiments remain to be conducted at sediment transport near transport capacity. © 2018 John Wiley & Sons, Ltd.