L'écaille des Gaux (Ardennes beiges): un exemple d'évolution tectono-sédimentaire complexe lors du développement d'un pli de propagation

The kinematics of internal structuration in the Gaux thrust sheet (Charleroi Coal basin; Belgium) is explained with the help of a geometric model. This model is controlled by the distribution of Landelies red breccia outcrops that was syntectonically deposited, probably during the Namurian. We suggest that the tectono-sedimentary evolution was controlled by (1) the growth of a Fault-propagation fold, (2) the formation of collapse structures and (3) localized interbed extension.

---

L'écaille des Gaux (Ardennes beiges): un exemple d'évolution tectono-sédimentaire complexe lors du développement d'un pli de propagation

     

RT sculptoris: An important star for low-mass close binary research

TheUBV light curves of Duerbeck and Karimie (1979) for the southern eclipsing binary RT Sculptoris were analysed using information limit optimizing computer programs. We decide in favour of the transit primary minimum hypothesis. The system is found to be semi-detached, but in the rare primary contact configuration. This implies a fast mass transfer process. Our derived picture of the system broadly corroborates that of Hilditch and King (1986), but our analysis includes a detailed treatment of the hot spot. We also report on recent spectroscopic radial velocity data obtained at Mt. Stromlo, Canberra. The system provides an important case study for low mass close binary research.     

Modelling stalagmite growth and δC as a function of drip interval and temperature

Growth and stable isotope composition of stalagmites are affected by climate changes. To understand the underlying mechanisms, we developed a time dependent multi-box model that describes stalagmite growth and stable isotope fractionation of carbon under disequilibrium conditions. The model takes variations of the drip interval, temperature and the amount of mixing between the impinging drops and the solution on top of the stalagmite into account, which allows to quantify the influence of these parameters. To calculate the variations of δ¹³C, the multi-box model was calibrated by comparison with an existing growth model. The results show that drip interval, temperature and the mixing coefficient do have a significant influence on δ¹³C. However, considering the higher natural variability of the drip interval, this parameter might have the largest influence.     

Teleseismic waveform modelling with a one-way wave equation

It is now widely accepted that elastic properties of the continental lithosphere and the underlying sublithospheric mantle are both anisotropic and laterally heterogeneous at a range of scales. To fully exploit modern three-component broad-band array data sets requires the use of comprehensive modelling tools. In this work, we investigate the use of a wide-angle, one-way wave equation to model variations in teleseismic 3-D waveforms due to 2-D elastic heterogeneity and anisotropy. The one-way operators are derived based on a high-frequency approximation of the square-root operator and include the effects of wave propagation as well as multiple scattering. Computational cost is reduced through a number of physically motivated approximations. We present synthetic results from simple 1-D (layer over a half-space) and 2-D (subduction zone) models that are compared with reference solutions. The algorithm is then used to model data from an array of broad-band seismograph stations deployed in northwestern Canada as part of the IRIS-PASSCAL/LITHOPROBE CANOE experiment. In this region radial-component receiver functions show a clear continental Moho and the presence of crustal material dipping into the mantle at the suture of two Palaeo-Proterozoic terranes. The geometry of the suture is better defined on the transverse component where subduction is associated with a ∼10 km thick layer exhibiting strong elastic anisotropy. The modelling reproduces the main features of the receiver functions, including the effects of anisotropy, heterogeneity and finite-frequency scattering.     

Influence of observed and RCM-simulated precipitation on the water discharge over the Sirba basin, Burkina Faso/Niger

The forcing of a hydrologic model (ABC) by both observed and simulated precipitation from a regional climate model (MAR) has been performed over the Sirba watershed (39,000 km²) located in the Sahelian region. Two aspects have been more specifically examined: the spatial and temporal representations of precipitation. The comparison between simulated and observed discharges—using observed rainfall datasets as forcing of the hydrologic model—has shown that the representation of daily precipitation (which is mainly convective in the Sahelian region) was not sufficiently accurate to correctly simulate the hydrologic response of the watershed. Since this response drives the soil water budget and consequently the amount of evaporation in forthcoming coupling experiments, it is thus necessary to develop more realistic infra-daily precipitation associated with convective events. A new temporal disaggregation scheme has been then developed. Considering observed as well as simulated precipitation fields, this method has significantly improved the simulated discharge at the catchment outlet. The major role played by the temporal component compared to spatial component of the precipitation has been then underlined. In addition, the present study shows the unsuitability of the simulated precipitation from the RCM to directly force a hydrologic model at infra daily timescale even if the cumulative amount and the main features of the precipitation seasonal cycle are well simulated.     

Estimating the uncertainty of coral isochron U–Th ages

Recently, alternative models to estimate the age of diagenetically altered fossil reef corals have been presented based on either redistribution of U or its immediate daughters ²³⁴Th and ²³⁰Th. Here, we present three methods to estimate the uncertainty of ages derived using an amended version of our coral isochron method [Scholz et al., 2004. U-series dating of diagenetically altered fossil reef corals. Earth and Planetary Science Letters 218, 163–178], which is based on addition/loss of U. The obtained uncertainties are substantially larger than those previously published and should, in general, be more reliable. The isochron method yields larger uncertainties than alternative models based on Th redistribution due to -recoil processes. However, comparison of model open-system ages based on such redistribution of U-series daughters for different sub-samples from an individual coral specimen shows that the smaller errors derived with these models cannot account for the observed variability. We recognise that none of the available models is applicable to all corals, probably reflecting different diagenetic processes even in different sub-samples from one coral specimen. To better understand the diagenetic processes and precisely constrain the uncertainties of the ages derived from diagenetically altered corals, the application of all available models is recommended.     

Experimental study of bubble coalescence in rhyolitic and phonolitic melts

We have experimentally studied the process of bubble coalescence in rhyolite and phonolite melts of natural composition. The experiments involved decompression of water-saturated melts equilibrated at pressures and temperatures from 100 to 150 MPa and 775 to 840 °C in vertically oriented, rapid-quench capable, cold seal pressure vessels. One type of experiments (rhyolite MCR-100, 120, 150 and phonolite LSP-120 series') approximates a “static” bubble coalescence case, where we held the decompressed samples for ∼5 seconds to 4320 minutes (3 days) before quenching. The second type (rhyolite LPC-100 series) replicates an “expanding” bubble coalescence environment, where we continually decompressed the experiments at a rate of 0.5 MPa/s, examining samples quenched at ending pressures between 10 and 80 MPa. Our “static” case (MCR-100, 120, and 150, and LSP-120) results show significant increases in the modal bubble sizes and in the sizes of the largest bubbles, corresponding to measurable broadening in the size distributions. Their bubble number densities (N_V) decrease as a function of hold time at their final pressures (P_F), and can be fit well by power law functions. Our “expanding” case experiments (LPC-100) show a significant drop in N_V during the duration of the experiments that can be fit by an exponential equation as N_V vs. either time or P_F. Average estimates of bulk coalescence rates indicate a ∼1 order of magnitude drop in N_V for “static” case rhyolites in a 2-3 day period, and ∼2 orders of magnitude for phonolites within a 3 day period. Despite a ∼2 order of magnitude difference in viscosity, coalescene in the phonolite is only slightly faster than the rhyolite. The “expanding” case experiments show a ∼1 order of magnitude drop in N_V over 180 seconds. Thus, N_V's decrease 4 orders of magnitude faster in expanding vs. static bubbly rhyolite melts. Our results imply that significant bubble coalescence can occur in rhyolite magmas at relatively fast (∼20 m/s) ascent rates in the conduit. Thus, bubble interconnectivity, leading to high permeability, is possible during ascent. Bubble coalescence may occur during second boiling in magma bodies that are stalled in the crust. The timescales over which this occurs is much faster than the estimated rise rates for bubbles to reach the top of the magma chamber.     

Heterogeneous bubble nucleation on Fe-Ti oxide crystals in high-silica rhyolitic melts

The nucleation of H₂O bubbles in magmas has been proposed as a trigger for volcanic eruptions. To determine how bubbles nucleate heterogeneously in silicate melts, experiments were carried out in which high-silica rhyolitic melts were hydrated at 740-800°C and 50-175 MPa, decompressed by 20-70 MPa, and held at the lower pressures for ≥10 s before being quenched. The hydration conditions were subliquidus, and all samples contain blocky magnetite + needle-shaped hematite ± plagioclase. Magnetite is abundant at 800°C and high pressures, whereas hematite becomes more abundant at lower temperatures and pressures. Bubbles nucleated in a single event in all samples, with the number density (N_T) of bubbles varying between 2 × 10⁷ and 1 × 10⁹ cm⁻³. At low degrees of supersaturation, one to a few bubbles nucleate on faces of magnetite, but at medium to high degrees of supersaturation, multiple bubbles nucleate on single magnetite grains. On hematite, one to a few bubbles nucleated at the ends of the needle-shaped crystals at medium supersaturations, but formed along their entire lengths at high supersaturations. N_T increases as water diffusivity decreases, indicating that the number of bubbles nucleated is influenced by their growth, which depletes the melt with respect to H₂O and lowers supersaturation. If volcanic eruptions are triggered by bubble formation in magmas stored in shallow-level magma chambers, then the supersaturations needed for heterogeneous nucleation suggest that only small amounts of crystallization are needed, whereas homogeneous nucleation is unlikely to trigger eruptions.