L'événement intraviséen dans la zone moldanubienne de la chaîne varisque d'Europe: les données des formations volcano-sédimentaires dévono-dinantiennes du Massif Central Français,des Vosges du Sud (France) et de la Forêt Noire (R.F.A.)

Resumé L'existence d'une phase tectonique intraviséenne dans la chaîne varisque d'Europe a été admise par divers auteurs. Le présent travail apporte des précisions sur la nature de cet événement dans le Nord du Massif Central, les Vosges et la Forêt Noire, c'est à dire dans la zone interne varisque. Des données nouvelles concernent les Vosges et la Forêt Noire. Dans le domaine étudié, cet événement géodynamique n'est pas phcatif et se caractérise par une période d'instabilité au cours du Viséen supérieur qui conduit à d'importants phénomènes de resédimentation, résultats de l'érosion de paléoreliefs. Cette période d'instabilité se prolonge au cours du Viséen supérieur où une emersion généralisée se produit dans la zone moldanubienne. Dans les zones externes septentrionales, la déformation synmétamorphique qui se déplace vers l'extérieur de la chaîne est contemporaine de cet événement. Dans le massif armoricain (zone externe), il se traduit par des mouvements décrochants. Cet événement s'explique par un épaississement crustal, conséquence directe de la collision varisque et se traduit par un soulèvement qui précède la phase de déformation paroxysmale d'âge tardi-viséen et carbonifère supérieur.

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The existence of an Intravisean tectonic phase in the variscan belt of Europe is admitted by some authors. The present work specifies the characteristics of such an event in the northern part of the French Massif Central, in the Vosges mountains and in the Black Forest, i.e. in the internal variscan zone. New data are given for the Vosges and the Black Forest. In the considered domain the geodynamic event does not have a folding effect. It leads to a period of instability during Late Visean, inducing important resedimentation processes as a result of erosion of the paleoreliefs. A generalized emersion of the Moldanubian Zone occurs at the same time. This event coincides with the migration toward the North of the syn-metamorphic deformation in the extern parts of the belt. In the Armorican Massif (external zone) strike slip faults are active during the same period. That event can be explained by the crustal thickening resulting from the variscan collision. The uplift preceds the paroxysmal Late Visean and Upper Carboniferous tectonic phase.

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Zusammenfassung Verschiedene Autoren vertreten die Meinung, daß das mitteleuropäische Variszikum innerhalb des Visé von einer tektonischen Phase betroffen wurde. Die gegenwärtige Arbeit soll eine genauere Vorstellung von den Ereignissen geben, welche sich um diese Zeit im nördlichen Zentralmassiv, den Vogesen und dem Schwarzwald abspielten. Innerhalb des angeführten Bereiches läßt sich nirgends eine Faltung beobachten. Es handelt sich vielmehr um ein geodynarmsches Ereignis, welches sich im obereren Visé als eine Periode der Instabilität mit sehr beachtlichen Erosionen einerseits und ebenso bedeutenden Resedimentationen andererseits äußert. Diese Zeit der Instabilität mündet noch im Laufe des obersten Visé in einer allgemeinen Heraushebung des Moldanubikums. Das Intra-Visé Ereignis ist zeitgleich mit der nordwärtigen Wanderung der synmetamorphen Deformation in den externen Zonen. Im armorikanischen Massiv fällt es anscheinend mit bedeutenden Blattverschiebungen zusammen. Offensichtlich steht hinter den Vorgängen als Ursache eine relativ plötzliche Krustenverdickung im Zusammenhang mit der variszischen Kollision. Die Heraushebung geht der paroxysmalen Deformation im späten Visé bzw. frühen Oberkarbon voraus.

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The problem of regionalization among geographers: the case of SwitzerlandDie mühe der geographen mit der regionalisierung am beispiel der SchweizA propos de la problématique de la régionalisation chez les géographes: le cas de l'espace suisse

The lack of any unified view amongst geographers in their conceptions of the articulation of Swiss space is evidence, in the dialectical sense, of the complexity of the reality. It is also, however, a sympton of methodological uncertainty. The purpose of this article is an analysis of this malaise, usually passed over in silence.The evolution of geography during the last hundred years cannot be easily defined, but against a general trend opposed approaches have always been present.The manner of conceiving Swiss space is still affected by the changes in the physiognomy of the country, but the analysis of reality and reality itself are out of phase.It appears possible that the presuppositions admitted by authors or the intellectual paths followed by them in regionalisation should be considered and treated as one of the fundamentals of all studies concerned with the organisation of space.     

Water-assisted migmatization of metagraywackes in a Variscan shear zone, Aiguilles-Rouges massif, western Alps

Migmatitic rocks developed in metagraywackes during the Variscan orogeny in the Aiguilles-Rouges Massif (western Alps). Partial melting took place 320 Ma ago in a 500 m-wide vertical shear zone. Three leucosome types have been recognised on the basis of size and morphology: (1) large leucosomes > 2 cm wide and > 40 cm long lacking mafic selvage, but containing cm-scale mafic enclaves; (2) same as 1 but with thick mafic selvage (melanosome); (3) small leucosomes < 2 cm and < 40 cm) with thin dark selvages (stromatic migmatites). Types 1 + 2 have mineralogical and chemical compositions in keeping with partial melting experiments. But Type 3 leucosomes have identical plagioclase composition (An_19–28) to neighbouring mesosome, both in terms of major- and trace-elements. Moreover, whole-rock REE concentrations in Type 3 leucosomes are only slightly lower than those in the mesosomes, unlike predicted by partial melting experiments. The main chemical differences between all leucosome types can be related to the coupled effect of melt segregation and late chemical reequilibration.

Mineral assemblages and thermodynamic modelling on bulk-rock composition restrict partial melting to  650 °C at 400 MPa. The large volume of leucosome (20 vol.%) thus generated requires addition of 1 wt.% external water. Restriction of extensive migmatization to the shear zone, without melting of neighbouring metapelites, also points to external fluid circulation within the shear zone as the cause of melting.     

Structural development of the Tso Morari ultra-high pressure nappe of the Ladakh Himalaya

A continental subduction-related and multistage exhumation process for the Tso Morari ultra-high pressure nappe is proposed. The model is constrained by published thermo-barometry and age data, combined with new geological and tectonic maps. Additionally, observations on the structural and metamorphic evolution of the Tso Morari area and the North Himalayan nappes are presented. The northern margin of the Indian continental crust was subducted to a depth of > 90 km below Asia after continental collision some 55 Ma ago. The underthrusting was accompanied by the detachment and accretion of Late Proterozoic to Early Eocene sediments, creating the North Himalayan accretionary wedge, in front of the active Asian margin and the 103–50 Ma Ladakh arc batholith. The basic dikes in the Ordovician Tso Morari granite were transformed to eclogites with crystallization of coesite, some 53 Ma ago at a depth of > 90 km (> 27 kbar) and temperatures of 500 to 600 °C. The detachment and extrusion of the low density Tso Morari nappe, composed of 70% of the Tso Morari granite and 30% of graywackes with some eclogitic dikes, occurred by ductile pure and simple shear deformation. It was pushed by buoyancy forces and by squeezing between the underthrusted Indian lithosphere and the Asian mantle wedge. The extruding Tso Morari nappe reached a depth of 35 km at the base of the North Himalayan accretionary wedge some 48 Ma ago. There the whole nappe stack recrystallized under amphibolite facies conditions of a Barrovian regional metamorphism with a metamorphic field gradient of 20 °C/km. An intense schistosity with a W–E oriented stretching lineation L₁ and top-to-the E shear criteria and crystallization of oriented sillimanite needles after kyanite, testify to the Tso Morari nappe extrusion and pressure drop. The whole nappe stack, comprising from the base to top the Tso Morari, Tetraogal, Karzok and Mata–Nyimaling-Tsarap nappes, was overprinted by new schistosities with a first N-directed and a second NE-directed stretching lineation L₂ and L₃ reaching the base of the North Himalayan accretionary wedge. They are characterized by top-to-the S and SW shear criteria. This structural overprint was related to an early N- and a younger NE-directed underthrusting of the Indian plate below Asia that was accompanied by anticlockwise rotation of India. The warping of the Tso Morari dome started already some 48 Ma ago with the formation of an extruding nappe at depth. The Tso Morari dome reached a depth of 15 km about 40 Ma ago in the eastern Kiagar La region and 30 Ma ago in the western Nuruchan region. The extrusion rate was of about 3 cm/yr between 53 and 48 Ma, followed by an uplift rate of 1.2 mm/yr between 48 and 30 Ma and of only 0.5 mm/yr after 30 Ma. Geomorphology observations show that the Tso Morari dome is still affected by faults, open regional dome, and basin and pull-apart structures, in a zone of active dextral transpression parallel to the Indus Suture zone.     

Site effect and damage distribution in Pujili (Ecuador) after the 28 March 1996 earthquake

The 28 March 1996 earthquake (Mw=5.7) produced extensive damage in Pujili, a small town located in the central part of the Inter-Andean valley of Ecuador. Variations in the damage rate of adobe constructions throughout the city let us suppose a ground motion amplification related to site effects. A site effect study using the H/V spectral ratio confirmed the good agreement with geological formations and showed that a second peak in some soil response appears at sites that are located over a zone of alluvial deposits, in the most damaged area of the city. This second peak is in the frequency range 5–7 Hz, close to the natural response of adobe building frequency. Thus, the second peak seems to increase the damage rate of adobe buildings and may be related to superficial alluvial deposits of a river, acting independently of the rest of the soil column.     

Conditional Wavelet Technique Applied to Aircraft Data Measured in the Thermal Internal Boundary Layer During Sea-Breeze Events

We describe a wavelet-based technique to determine the spectral turbulentcontribution to the vertical flux of sensible heat in a position-wavelength representation. This technique combines awavelet transform (Morlet wavelet) with conditional sampling. We apply this methodto aircraft datacollected during a sea-breeze circulation (BEMA97 experiment) with heterogeneousturbulence conditions horizontally and vertically as well. The turbulent fluxes are analysed with the conditional wavelet techniqueas a function of the wavelength and the horizontal distance.The turbulent processes within the thermal internal boundary layer associatedwith the sea breeze are clearly identified. The results exhibit the wavelength bands through which the upward flux (originating fromthe surface) and the downward flux (originating from the top of theboundary layer) are important.     

Relative current effect on short wave growth

Ocean Dynamics - Short waves growth is characterized by nonlinear and dynamic processes that couple ocean and atmosphere. Ocean surface currents can have a strong impact on short wave steepness and...     

Causes and consequences of bimodal grain-size distribution of tephra fall deposited during the August 2006 Tungurahua eruption (Ecuador)

The violent August 16–17, 2006 Tungurahua eruption in Ecuador witnessed the emplacement of numerous scoria flows and the deposition of a widespread tephra layer west of the volcano. We assess the size of the eruption by determining a bulk tephra volume in the range 42–57 × 10⁶ m³, which supports a Volcanic Explosivity Index 3 event, consistent with calculated column height of 16–18 km above the vent and making it the strongest eruptive phase since the volcano’s magmatic reactivation in 1999. Isopachs west of the volcano are sub-bilobate in shape, while sieve and laser diffraction grain-size analyses of tephra samples reveal strongly bimodal distributions. Based on a new grain-size deconvolution algorithm and extended sampling area, we propose here a mechanism to account for the bimodal grain-size distribution. The deconvolution procedure allows us to identify two particle subpopulations in the deposit with distinct characteristics that indicate dissimilar transport-depositional processes. The log-normal coarse-grained subpopulation is typical of particles transported downwind by the main volcanic plume. The positively skewed, fine-grained subpopulation in the tephra fall layer shares close similarities with the elutriated co-pyroclastic flow ash cloud layers preserved on top of the scoria flow deposits. The area with the higher fine particle content in the tephra layer coincides with the downwind prolongation of the pyroclastic flow deposits. These results indicate that the bimodal distribution of grain size in the Tungurahua fall deposit results from synchronous deposition of lapilli from the main plume and fine ash elutriated from scoria flows emplaced on the western flank of the volcano. Our study also reveals that inappropriate grain-size data processing may produce misleading determination of eruptive type.     

Biogeochemistry and dynamics of settling particle fluxes at the Antikythira Strait (Eastern Mediterranean)

For the first time, a 12-month trap experiment was conducted on both sides of the strait between Crete and Antikythira Island (Eastern Mediterranean Sea) from June 1994 to June 1995 as part of the PELAGOS experiment. Analyses of major chemical constituents, including carbohydrates and stable lead isotopes and Scanning Electron Microscope studies were performed on the trap samples. Total mass fluxes varied between 1 and 1273 mg m⁻² d⁻¹. The lowest fluxes observed were in summer and autumn 1994, when stratification of the water column was at its deepest. In general, mass fluxes exhibited very low values throughout this experiment confirming the strong oligotrophy of this area. The mean contents of the major constituents (carbonates, opal, lithogenic fraction) were quite similar during the survey and between traps, with the exception of organic carbon contents, which were highest (7–10%) in summer 1994, i.e. during the period of lowest mass fluxes. During the first 6-month deployment (summer–autumn 1994) there was an important mass flux peak, which was depleted in organic carbon, at the Ionian near-bottom trap. This event coincided with a violent wind episode, which may have caused the resuspension of particles, which were then transported down the steep continental slope on the Ionian side of the strait. A smaller peak in mass flux occurred at the Aegean near-bottom trap, coincident with rainfall. Both these events indicate that environmental factors can control flux variations in an oligotrophic environment. During the second 6-month deployment (winter–spring 1995) there was another important increase in mass fluxes, which occurred at all three traps, although in the Ionian traps mass flux peaks were delayed by one to two sampling intervals. The distance between the two mooring sites gives a rough estimate of a minimum horizontal advection speed of 2 cm s⁻¹ for this particulate transfer from the Aegean to the Ionian area. This estimate is in good agreement with the measured current velocities.     

Pre-eruptive physical conditions of El Reventador volcano (Ecuador) inferred from the petrology of the 2002 and 2004–05 eruptions

A petrological study of the eruptive products of El Reventador allowed us to infer the magmatic processes related to the 2002 and 2004–05 eruptions of this andesitic stratovolcano. On November 3, 2002, El Reventador experienced a highly explosive event, which was followed by emplacement of two lava flows in November–December 2002. Silica contents range from 62 to 58 wt.% SiO₂ for the November 3 pyroclastic deposits to 58–56 and 54–53 wt.% SiO₂ for the successive lava flows. In November 2004 eruptive activity resumed supplying four new lava flows (56–54 wt.% SiO₂) between November 2004 and August 2005.