Acceleration of buoyancy-driven fractures and magmatic dikes beneath the free surface

By performing buoyancy-driven fracture experiments in brittle gelatin we observe that the ascent velocity of a fracture containing a finite volume of fluid increases when approaching the free surface. We theoretically describe this free-surface effect and quantify it by introducing an effective depth-dependent fracture toughness and developing an ascent model on the basis of linear fracture mechanics. We develop a successful inversion approach and resolve the actual and critical fracture length and the ascent velocity far away from the free surface from the observation of the fracture tip migration alone. Other parameters, as the fluid volume included in the fracture and the in situ fracture toughness, can be derived. Applying the model and inversion to the 1998 eruption at Piton de la Fournaise, Reunion Island, reveals estimates of the length and critical length of the feeding magma batch, the magma batch volume and the in situ fracture toughness. It further indicates that the ascent velocity of the magma batch was probably much smaller at greater depths and that the batch might have been initiated several months or years before the eruption.     

Thermal ion measurements on board Interball Auroral Probe by the Hyperboloid experiment

Hyperboloid is a multi-directional mass spectrometer measuring ion distribution functions in the auroral and polar magnetosphere of the Earth in the thermal and suprathermal energy range. The instrument encompasses two analyzers containing a total of 26 entrance windows, and viewing in two almost mutually perpendicular half-planes. The nominal angular resolution is defined by the field of view of individual windows 13° × 12.5°. Energy analysis is performed using spherical electrostatic analyzers providing differential measurements between 1 and 80 eV. An ion beam emitter (RON experiment) and/or a potential bias applied to Hyperboloid entrance surface are used to counteract adverse effects of spacecraft potential and thus enable ion measurements down to very low energies. A magnetic analyzer focuses ions on one of four micro-channel plate (MCP) detectors, depending on their mass/charge ratio. Normal modes of operation enable to measure H⁺, He⁺, O⁺⁺, and O⁺ simultaneously. An automatic MCP gain control software is used to adapt the instrument to the great flux dynamics encountered between spacecraft perigee (700 km) and apogee (20 000 km). Distribution functions in the main analyzer half-plane are obtained after a complete scan of windows and energies with temporal resolution between one and a few seconds. Three-dimensional (3D) distributions are measured in one spacecraft spin period (120 s). The secondary analyzer has a much smaller geometrical factor, but offers partial access to the 3D dependence of the distributions with a few seconds temporal resolution. Preliminary results are presented. Simultaneous, local heating of both H⁺ and O⁺ ions resulting in conical distributions below 80 eV is observed up to 3 Earths radii altitudes. The thermal ion signatures associated with large-scale nightside magnetospheric boundaries are investigated and a new ion outflow feature is identified associated to the polar edge of the auroral oval. Detailed distribution functions of injected magnetosheath ions and ouflowing cleft fountain ions are measured down to a few eVs in the dayside.     

Le passage Ordovicien–Silurien et la partie inférieure du Silurien (Sud-Est du Massif armoricain, France)The Ordovician–Silurian transition and the lower part of the Silurian (southeastern Armorican Massif, France)

A new section in the Silurian graptolitic ‘phtanites’ (black cherts) of Les Fresnaies at Chalonnes-sur-Loire (SE Armorican Massif) shows for the first time that these rocks (1) succeed conformably to Uppermost Ordovician (Hirnantian) glaciomarine deposits and (2) contain successive graptolite assemblages that characterise the base of the Silurian, the whole Rhuddanian and Aeronian stages and the lower part of the Telychian. To cite this article: J.M. Piçarra et al., C. R. Geoscience 334 (2002) 1177–1183.     

Sedimentary fabric characterized by X-ray tomography: A case-study from tsunami deposits on the Marquesas Islands,French Polynesia

X-ray tomography is used to analyse the grain size and sedimentary fabric of two tsunami deposits in the Marquesas Islands (French Polynesia, Pacific Ocean) which are particularly exposed to trans-Pacific tsunamis. One site is located on the southern coast of Nuku Hiva Island (Hooumi) and the other one is on the southern coast of Hiva Oa Island (Tahauku). Results are compared with other techniques such as two-dimensional image analysis on bulk samples (particle analyser) and anisotropy of magnetic susceptibility. The sedimentary fabric is characterized through three-dimensional stacks of horizontal slices (following a vertical step of 2·5 mm along the cores), while grain-size distribution is estimated from two-dimensional vertical slices (following a step of 2 mm). Four types of fabric are distinguished: (a) moderate to high angle (15 to 75°); (b) bimodal low-angle (<15°); (c) low to high angle with at least two different orientations; and (d) dispersed fabric. The fabric geometry in a tsunami deposit is not only controlled by the characteristics of the flow itself (current strength, flow regime, etc.) but also sediment concentration, deposition rate and grain-size distribution. There is a notable correlation between unimodal high-angle fabric – type (a) – and finely-skewed grain-size distribution. The two tsunami deposits studied represent two different scenarios of inundation. As demonstrated here, X-ray tomography is an essential method for characterizing past tsunamis from their deposits. The method can be applied to many other types of sediments and sedimentary rocks.     

应用多种数据融合方法对多时相SPOT图像处理提取三峡工程大坝区地面动态变化信息

三峡库区三斗坪大坝区及其周围地区在1994到1998年期间，由于大坝的施工地面发生了巨大变化。这些变化在1990、1997和1998年获取的SPOT遥感图像上都有明显的记录，其中一些地面变化细节在分辨率10m的全波段（P）图像更为清楚。这里利用三种方法（高通滤波算法（HPF algorithm，Chavezet al．1991）、RWM—ARSIS方法（Ranchin and Wald，1999）和P＋XS算法（Anonymous 1986））对三峡工程三斗坪大坝区的多时相高空间分辨率的SPOT图像（获取时闻为1990，1997和1998年，包括全波段P图像和多波段XS图像）进行融合处理，在提取测区地面动态变化信息的同时，从理论和实际应用的角度探讨了不同数据融合方法的效果。     

Coordinated monitoring of the eccentric O-star binary Iota Orionis: optical spectroscopy and photometry

With the objective of investigating the windwind collision phenomenon and supporting contemporaneous X-ray observations, we have organized a large-scale, coordinated optical monitoring campaign of the massive, highly eccentric O9 III+B1 III binary Iota Orionis. Successfully separating the spectra of the components, we refine the orbital elements and confirm the rapid apsidal motion in the system. We also see strong interaction between the components during periastron passage and detect phase-locked variability in the spectrum of the secondary star. However, we find no unambiguous signs of the bow shock crashing on the surface of the secondary, despite the predictions of hydrodynamic simulations. Combining all available photometric data, we find rapid, phase-locked variations and model them numerically, thus restricting the orbital inclination to 50° i 70°.     

Assessment of river flow with significant lateral inflow through reverse routing modeling

The discharge hydrograph estimation in rivers based on reverse routing modeling and using only water level data at two gauged sections is here extended to the most general case of significant lateral flow contribution, without needing to deploy rainfall–runoff procedures. The proposed methodology solves the Saint‐Venant equations in diffusive form also involving the lateral contribution using a “head‐driven” modeling approach where lateral inflow is assumed to be function of the water level at the tributary junction. The procedure allows to assess the discharge hydrograph at ends of a selected river reach with significant lateral inflow, starting from the stage recorded there and without needing rainfall data. Specifically, the MAST 1D hydraulic model is applied to solve the diffusive wave equation using the observed stage hydrograph at the upstream section as upstream boundary condition. The other required data are (a) the observed stage hydrograph at the downstream section, as benchmark for the parameter calibration, and (b) the bathymetry of the river reach, from the upstream section to a short distance after the downstream gauged section. The method is validated with different flood events observed in two river reaches with a significant intermediate basin, where reliable rating curves were available, selected along the Tiber River, in central Italy, and the Alzette River, in Luxembourg. Very good performance indices are found for the computed discharge hydrographs at both the channel ends and along the tributaries. The mean Nash‐Sutcliffe value (NS_q) at the channel ends of two rivers is found equal to 0.99 and 0.86 for the upstream and downstream sites, respectively. The procedure is also validated on a longer stretch of the Tiber River including three tributaries for which appreciable results are obtained in terms of NS_q for the computed discharge hydrographs at both the channel ends for three investigated flood events.     

Hillslope response to knickpoint migration in the Southern Appalachians: implications for the evolution of post‐orogenic landscapes

The southern Appalachians represent a landscape characterized by locally high topographic relief, steep slopes, and frequent mass movement in the absence of significant tectonic forcing for at least the last 200 Ma. The fundamental processes responsible for landscape evolution in a post‐orogenic landscape remain enigmatic. The non‐glaciated Cullasaja River basin of south‐western North Carolina, with uniform lithology, frequent debris flows, and the availability of high‐resolution airborne lidar DEMs, is an ideal natural setting to study landscape evolution in a post‐orogenic landscape through the lens of hillslope–channel coupling. This investigation is limited to channels with upslope contributing areas >2.7 km², a conservative estimate of the transition from fluvial to debris‐flow dominated channel processes. Values of normalized hypsometry, hypsometric integral, and mean slope vs elevation are used for 14 tributary basins and the Cullasaja basin as a whole to characterize landscape evolution following upstream knickpoint migration. Results highlight the existence of a transient spatial relationship between knickpoints present along the fluvial network of the Cullasaja basin and adjacent hillslopes. Metrics of topography (relief, slope gradient) and hillslope activity (landslide frequency) exhibit significant downstream increases below the current position of major knickpoints. The transient effect of knickpoint‐driven channel incision on basin hillslopes is captured by measuring the relief, mean slope steepness, and mass movement frequency of tributary basins and comparing these results with the distance from major knickpoints along the Cullasaja River. A conceptual model of area–elevation and slope distributions is presented that may be representative of post‐orogenic landscape evolution in analogous geologic settings. Importantly, the model explains how knickpoint migration and channel–hillslope coupling is an important factor in tectonically‐inactive (i.e. post‐orogenic) orogens for the maintenance of significant relief, steep slopes, and weathering‐limited hillslopes. Copyright © 2011 John Wiley & Sons, Ltd.