Strain localization in sand: an overview of the experimental results obtained in Grenoble using stereophotogrammetry

Experimental results are presented from the extensive program of drained plane strain compression tests on sand carried out in Grenoble over the last two decades. Systematic analysis of photographs of the deforming specimen allowed for measuring deformations and determining strain fields throughout the test, that is: prior to, at, and after the onset of strain localization. The principles, details and accuracy of the procedure are described, as well as its suitability to properly depict the patterns of deformation. Findings concerning the occurrence and progression of strain localization are discussed. The issues of shear band orientation and thickness are addressed, as well as temporary and persistent complex localization patterns, and the volumetric behaviour inside a band after its formation. The influence of such variables as initial state of the sand (effective stress and relative density), specimen size and slenderness, as well as grain size, is discussed. Copyright © 2004 John Wiley & Sons, Ltd     

Runoff responses to afforestation in a watershed of the Loess Plateau,China

Afforestation has been suggested as a means of improving soil and water conservation in north‐western China, especially on the Loess Plateau. Understanding of the hydrological responses to afforestation will help us develop sustainable watershed management strategies. A study was conducted during the period of 1956 to 1980 to evaluate runoff responses to afforestation in a watershed on the Loess Plateau with an area of 1·15 km², using a paired watershed approach. Deciduous trees, including locust (locusta L.), apricot (praecox L.) and elm (ulmus L.), were planted on about 80% of a treated watershed, while a natural grassland watershed remained unchanged. It was estimated that cumulative runoff yield in the treated watershed was reduced by 32% as a result of afforestation. A significant trend was also observed that shows annual runoff reduction increases with the age of the trees planted. Reduction in monthly runoff occurred mainly from June to September, which was ascribed to greater rainfall and utilization by trees during this period. Afforestation also resulted in reduction in the volume and peak flow of storm runoff events in the treated watershed with greater reduction in peak flow. Copyright © 2003 John Wiley & Sons, Ltd.     

Thermoluminescence of the lunar surface

Observations of the lunar luminescence are reported for a dozen of specific Moon features using the line-depth method with a high resolution spectroscopic technique. The data indicate a variation of the Moon proper emission as a function of the phase angle which is interpreted as a proof of the thermoluminescent origin of this emission.     

La Formation glaciaire de la Mambéré (République Centrafricaine): Reconstitution paléogéographique et implications à l'échelle du Paléozoique africain

The Mambéré Formation constitutes a horizontal unit located in the western and southwestern part of the Central African Republic. Stratigraphical and sedimentological study provides strong argument to attribute these deposits a glacial origin. A palaeogeographical reconstruction has been outlined in order to precise the age of the formation. Two main categories of glacial deposits have been recognized:

glaciogenic deposits made of basal tills (with facetted pebbles) and flow tills (with flattened blunt pebbles);

reworked glacial deposits formed of sandstones and conglomeratic sandstones, in continuous beds, lenses or isolated blocks, together with siltstones and bedded sandstones.

The southerly provenance of the detrital material is demonstrated by quartz grain surface analysis and heavy-mineral study. This material results essentially from the dismantling of the Precambrian Schistoquartzitic Complex and secondarily from the Granitogneissic Complex. According to the palaeomagnetic polar paths and the migration of the glacial centers over the African continent during the Palaeozoic, the Mambéré Formation may be attributed a Lower Silurian age by reference to similar formations observed in Cameroon or a Lower Carboniferous age by comparison with the glacial formations reported from Niger and Egypt.     

Modelling weathering processes at the catchment scale: The WITCH numerical model

A numerical model of chemical weathering in soil horizons and underlying bedrock (WITCH) has been coupled to a numerical model of water and carbon cycles in forest ecosystems (ASPECTS) to simulate the concentration of major species within the soil horizons and the stream of the Strengbach granitic watershed, located in the Vosges Mountains (France). For the first time, simulations of solute concentrations in soil layers and in the catchment river have been performed on a seasonal basis. The model is able to reproduce the concentrations of most major species within the soil horizons, as well as catching the first-order seasonal fluctuations of aqueous calcium, magnesium and silica concentrations. However, the WITCH model underestimates concentrations of Mg²⁺ and silica at the spring of the catchment stream, and significantly underestimates Ca²⁺ concentration. The deficit in calculated calcium can be compensated for by dissolution of trace apatite disseminated in the bedrock. However, the resulting increased Ca²⁺ release yields important smectite precipitation in the deepest model layer (in contact with the bedrock) and subsequent removal of large amount of silica and magnesium from solution. In contrast, the model accurately accounts for the concentrations of major species (Ca, Mg and silica) measured in the catchment stream when precipitation of clay minerals is not allowed. The model underestimation of Mg²⁺ and H₄SiO₄ concentrations when precipitation of well crystallized smectites is allowed strongly suggests that precipitation of well crystallized clay minerals is overestimated and that more soluble poorly crystallized and amorphous materials may be forming. In agreement with observations on other watersheds draining granitic rocks, this study indicates that highly soluble trace calcic phases control the aqueous calcium budget in the Strengbach watershed.     

Behaviour of a rupture of the 21 June 2000 earthquake in South Iceland as revealed in an asphalted car park

A major earthquake (M=6.6) occurred on 21 June 2000, in South Iceland. This paper presents an unusual example of left-lateral strike-slip displacement recorded in a newly asphalted car park surface through a mechanically consistent pattern of open fissures and pressure ridges resulting from simple shear and rotation. Measurement of these features allows accurate reconstruction of the local deformation. The behaviour of the asphalt layer resembles that of analogue physical models, especially in terms of rotations induced by shear deformation. It is finally shown that through a wide range of scales some basic patterns associating rotation and opposite senses of strike-slip exist in the South Iceland Seismic Zone.     

An X-ray absorption fine structure and nuclear magnetic resonance spectroscopy study of gallium-silica complexes in aqueous solution

The influence of aqueous silica on gallium(III) hydrolysis in dilute (2 × 10⁻⁴ ≤ m_Ga ≤ 5 × 10⁻³) and moderately concentrated (0.02 ≤ m_Ga ≤ 0.3) aqueous solutions was studied at ambient temperature, using high resolution X-ray absorption fine structure (XAFS) and nuclear magnetic resonance (NMR) spectroscopies, respectively. Results show that, in Si-free acidic solutions (pH < 3), Ga is hexa-coordinated with oxygens of H₂O molecules and/or OH groups in the first coordination sphere of the metal. With increasing pH, these hydroxyl groups are progressively replaced by bridging oxygens (-O-), and polymerized Ga-hydroxide complexes form via Ga-O-Ga chemical bonds. In the 2.5-3.5 pH range, both XAFS and NMR spectra are consistent with the dominant presence of the Ga₁₃ Keggin polycation, which has the same local structure as A1₁₃. Under basic pH (pH > 8), Ga exhibits a tetrahedral coordination, corresponding to Ga(OH)₄⁻ species, in agreement with previous NMR and potentiometric studies. Major changes in Ga hydrolysis have been detected in the presence of aqueous silica. Ga is tetra-coordinated, both in basic and acid (i.e., at pH > 2.7) Si-bearing solutions (0.01 ≤ m_Si ≤ 0.2), and forms stable gallium-silicate complexes. In these species, Ga binds via bridging oxygen to 2 ± 1 silicons, with an average Ga-Si distance of 3.16 ± 0.05 Å, and to 2 ± 1 silicons, with an average Ga-Si distance of 3.39 ± 0.03 Å. These two sets of Ga-Si distances imply the formation of two types of Ga-silicate aqueous complex, cyclic Ga-Si_2-3 species (formed by the substitution of Si in its tri-, tetra- or hexa-cyclic polymers by Ga atoms), and chainlike GaSi_2-4 species (similar to those found for A1), respectively. The increase in the number of Si neighbors (a measure of the complex concentration and stability), in alkaline media, with increasing SiO₂(aq) content and decreasing pH is similar to that for A1-Si complexes found in neutral to basic solutions. At very acid pH and moderate silica concentrations, the presence of another type of Ga-Si complex, in which Ga remains hexa-coordinated and binds to the silicon tetrahedra via the GaO₆ octahedron corners, has also been detected. These species are similar to those found for Al³⁺ in acid solutions. Thus, as for aluminum, silicic acid greatly hampers Ga hydrolysis and enhances Ga mobility in natural waters via the formation of gallium-silicate complexes.     

Comparison between the Chile and Mexico triple junction areas substantiates slab window development beneath northwestern Mexico during the past 12-10 Myr

When combined with the Miocene-Recent volcanic record of Baja California, a parallel drawn between the Chile and Mexico triple junction areas substantiates slab window development beneath northwestern Mexico during the past 12-10 Myr. The slab-free zone manifestations challenge the notion that ridge subduction has not occurred beneath the southern Baja California peninsula. The geochemically distinctive rocks from the Santa Clara volcanic field of west-central Baja California, including coeval adakites and niobium-enriched basalt, are commonly inferred to signal partial melting of the subducting plate at shallow depths and relatively high temperatures, before slab dehydration occurs. Such PT conditions for slab melting have only been observed in association with spreading-ridge subduction. We propose that slab window development beneath southern Baja California and mainland Mexico (30° to 18°N) resulted from subduction of the East Pacific rise.