Field‐based and spectral indicators for soil erosion mapping in semi‐arid mediterranean environments (Coastal Cordillera of central Chile)

The Coastal Cordillera of central Chile is naturally sensitive to soil erosion due to moderate to steep slopes, intense winter rains when the vegetation cover is scarce, and deeply weathered granitic rocks. In 1965, 60 per cent of its surface was moderately to very severely eroded. Today this process is still largely active, but no data are currently available to evaluate the real extent, distribution and severity of soil degradation on a regional scale. This information is vital to support efficient soil conservation plans. A multi‐scale approach was implemented to produce regional land degradation maps based on remote sensing technologies. Fieldwork has shown that the surface colour or ‘redness’ and the density of coarse fragments are pertinent erosion indicators to describe a typical sequence of soil degradation in the context of mediterranean soil developed on granitic materials and micaschists. Field radiometric experiments concluded that both factors influence the reflectance of natural surfaces and can be modelled using radiometric indices accessible from most satellites operating in the optical domain, i.e. redness index and brightness index. Finally the radiometric indices were successfully applied to SPOT images to produce land degradation maps. Only broad classes of erosion status were discriminated and the detection of the degradation processes was only possible when most of the fertile layer had already been removed. This technology provides decision‐making information required to develop regional soil conservation plans and to prioritize actions between catchment areas, especially in vast inter‐tropical regions where spatialized data are not always readily available. Copyright © 2006 John Wiley & Sons, Ltd.     

Wet Snow Avalanche Deposits in the French Alps: Structure and Sedimentology

We analyse the morphology and sedimentology of 25 dirty snow avalanche deposits in the French Alps. The deposits typically have either a snow-ball structure or a massive structure with sliding planes. The snow balls show a longitudinal and a vertical sorting that reflects a sieve effect, similar to that observed in other rapid inertial granular flows. The massive type results from snow compaction when the avalanche is channelled by a gully or when it reaches the distal part of the scree. Velocity decrease and compaction limit the deformation to a zone at the base of the snow mass and cause the formation of distinctive sliding planes. These appear as smooth recrystallised surfaces due to local melt from frictional heating. The flow can be assimilated to a frictional granular flow. No systematic variation of size and shape of the rock debris has been observed along the profiles in both types of deposit. The distribution of rock debris and its fabric suggest that the clasts are transported passively and do not undergo any sorting during displacement. Snow melt after avalanching causes a redistribution of rock debris particularly when the snow thickness is important. This redistribution does not generate new sedimentological characteristics such as enhanced sorting or fabric.     

Geomorphic and dendroecological impacts of slushflows in central Gaspé Peninsula (Québec, Canada)

Slushflows are rare phenomena in southeastern Canada. Here we report for the first time the occurrence of slushflows in a subalpine environment in eastern Canada (Mt. Albert, Gaspé Peninsula, Québec). Because nothing is known of their frequency- magnitude in the area, we reconstructed the chronology of slushflow events over the past century using dendrogeomorphic techniques based on impact scars, reaction wood and traumatic resin ducts. Slushflows contributed to the formation of a tongue-shaped accumulation of 17900 m² at the outlet of a firstorder drainage basin. The slushflow boulder tongue was composed of heterogeneous-sized, angular and unoriented clasts, which are markedly different from the sediments of an adjacent alluvial fan. Although movements were initiated above the subalpine forest limit, slushflows induced forest fragmentation along the treed slope. Three slushflow events were identified over the past century, in 1925, 1964 and 1988, respectively, which indicate exceptional initiation conditions and considerable geomorphic activity of individual events.     

Practical method for drawing a VGP path

A statistical method is proposed for the smoothing of polar wander paths and for giving their confidence limits by the mean of successive ellipses. The method is fully parametrical and is based on the relations between the inertia matrix and the parameters of the Fisher distribution, from which a bivariate form is deduced. An elementary tensorial calculation gives the parameters of the confidence ellipse around a vectorial weighted mean, for any unimodal set of vectors. This model can also be used for other statistical tests, wherever the rotational symmetry hypothesis is not consistent (i.e. the fold test).     

Modelling the relationship between magnetic fabric and strain in polymineralic rocks

We present a model that is applicable to the relatively frequent case of rocks in which the magnetic fabric is dominated by uniaxial paramagnetic minerals, and in which the deformation (pure shear) corresponds to the March-Fernandez model. Borradaile et al.'s procedure for the isolation of the magnetic fabric of monocrystals of anisotropic components allows us to obtain the magnetic properties equivalent to those of all the component minerals if they should be artificially aligned. In the case of real component minerals with similar shape parameters, χ (dependent on dimension ratio), these properties will correspond to those of a theoretical equivalent mineral. Therefore, finite strain and orientation tensors may be determined from magnetic fabric measurements of such polymineralic rocks.     

Fluid composition and evolution in coesite-bearing rocks (Dora-Maira massif,Western Alps): implications for element recycling during subduction

Fluid inclusions and F, Cl concentration of hydrous minerals were analysed in the coesite-pyrope quartzite, the interlayered jadeite quartzite and their country-rock gneiss from the Dora-Maira massif using a combination of microthermometry, Raman spectrometry, synchrotron X-ray microfiuorescence and electron microprobe analysis. Three populations of fluid inclusions were recognized texturally and can be related to distinct metamorphic stages. A low-salinity aqueous fluid occurs in the retrogressed country gneiss and as late secondary inclusions in jadeite quartzite and chloritized pyrope. An earlier secondary population is found in matrix quartz of the jadeite- and pyro-pe-quartzites. This population can be related to the early decompression and so to incipient breakdown of garnet into phlogopite-bearing assemblages. The inclusion fluid is highly saline (up to 84 wt% equivalent NaCl) and contains Na, Ca, Fe, Cu and Zn as major cations. In pyrope quartzite, additional K was found in these brines, which locally coexist with CO₂-rich inclusions. The oldest fluid inclusions are preserved in kyanite grains included in fresh pyrope and in pyrope itself. In pyrope, all inclusions have decrepitated and contain magnesite, an Mg-phosphate, sheet-silicate(s), a chloride and an opaque phase, with no fluid preser ved. In contrast, the kyanite inclusions in pyrope preserve primary H₂O-CO₂ low-salinity fluid inclusions, probably owing to the low compressibility of the kyanite inclusions and host garnet. In spite of in-situ re-equilibration, these inclusions can be interpreted as relics of the dehydration fluid that attended pyrope growth. These correlations between textural and chemical fluid inclusion data and metamorphic stages are consistent with the fluid composition calculated from the halogen content of different generations of phlogopite and biotite. The preservation of different fluid compositions, both in time and space, is evidence for local control and possibly origin of the fluids, in agreement with isotopic data. These results, in particular the absence of CO₂ in the jadeite quartzite, are best interpreted in terms of a fluid-melt system evolution. With increasing metamorphism, partitioning of H₂O, Na, Ca, Fe and heavy metals into melt (jadeite quartzite) and Mg, Na/K, F, CO₂ and P(?) into a residual aqueous fluid can account for depletion in Na, Ca and Fe of the pyrope quartzite. During the retrograde path, a _{H 2 O} rose as melt crystallized, generating the two populations of hypersaline and water-rich fluids that were highly reactive to pyrope. The process of fluid-melt interaction envisioned here coupled with models of melt extraction in subduction zones provides an attractive opportunity for the instantaneous ( < 1 Ma) and selective transport of elements between a downgoing slab and the overlying mantle wedge.     

The product spectra of gravity and barometric pressure in Europe

A total of 111 000 hourly values of gravity and barometric pressure from stations in Europe is analysed. The data consist of two sets of records from Brussels, an early set of 36 000 h length and a more recent set of 21 000 h length, a set of records from Bad Homburg of 24 000 h length, and a set of records from Strasbourg of 30 000 h length. All of the gravity measurements were made with similar superconducting instruments and the pressure data were recorded simultaneously at each superconducting gravimeter site. The four sets of records have different time bases, and to bring out common features and suppress individual station systematic errors, the product spectrum is introduced. Spectral density estimates are first computed for a common spectral window for each record, and the product spectrum is formed by multiplying individual spectral estimates across records. The cumulative distribution function is found for the product spectrum and confidence intervals are calculated from it by iteration. The product spectrum in gravity reveals a triplet of resonances in the subtidal band which are shown by an automated computer search to be uniquely associated with the translational modes of the solid inner core. The product spectrum in barometric pressure clearly reveals the first 10 solar heating tides in the atmosphere, but otherwise does not show common features with the gravity product spectrum. In particular, the triplet of resonances in the subtidal band of the gravity product spectrum do not show up in the product spectrum of barometric pressure, climinating the atmosphere as their source.