Die Existenz,Beschaffenheit und Bedeutung einer Zwischenschicht zwischen Gletschereis und Felsuntergrund

Zusammenfassung Fast unter allen Gletschern, einschließlich des Eisschildes auf Grönland, ist durch seismische Messungen eine Zwischenschicht zwischen Eis und Fels festgestellt worden. Sie ist auch in den Vorfeldern der Gletscher unter fluviatilen Ablagerungen vorhanden. Seismische Wiederholungsmessungen in Abständen von 25 bis 30 Jahren haben ergeben, daß sich die Morphologie der Oberkante der Zwischenschicht durch Sedimentation und Erosion beachtlich verändert. Es ist naheliegend, daß man sich die Zwischenschicht als gefrorene Grundmoräne vorstellt. Gegen diese Anschauung spricht aber die Existenz derselben in den Vorfeldern der Gletscher.Die beobachteten Geschwindigkeiten seismischer Wellen in der Zwischenschicht mit Werten zwischen 4000 und 5000 m/s können auch in verwitterten Gesteinen auftreten. Die erwähnten Veränderungen der Gestalt der Eisunterkante in relativ kurzer Zeit ist jedoch mit einer Deutung der Zwischenschicht als verwitterter Fels nicht in Einklang zu bringen.Die Verfasser nehmen an, daß die Zwischenschicht aus sehr dicht gepackten Sedimenten besteht. Mit dieser Annahme lassen sich alle Beobachtungsergebnisse erklären.

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Seismic measurements have shown nearly under all glaciers, including the iceshields of Greenland, a band between ice and rockbasement. This band exists also under the fluviatil sediments in the forefield of the glaciers. Seismic remeasurements in an interval of about 25–30 years have shown, that the morphology of the surface of the band has been heavily changed by sedimentation and erosion.It is evident that one imagines the band to be a frozen ground moraine. But the existence of the band in the forefield of glaciers demonstrates that this opinion must be wrong.The measured velocities of seismic waves between 4000 and 5000 m/s are possible also in weathered rocks. The above mentioned changes of the contour of the bottom of the ice during a relatively short period however cannot be reconciled with the interpretation of the band as weathered rock.The authors assume that the band consists of very densely packed sediments. This assumption explains all results of the observations.

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Résumé Presque sous tous les glaciers inclusivement l'Inlandeis du Groenland on a constaté par des mesurages séismiques une couche intermédiaire entre la glace et le roc. Cette couche existe aussi sous les sédiments fluviatils dans les glacis des glaciers. Pendant un intervalle de 25 à 30 ans des remesurements séismiques ont prouvé que la morphologie du bord supérieur de la couche intermédiaire a été considérablement changée par la sédimentation et l'érosion.On est bien tenté de croire, que la couche intermédiaire fût une moraine inférieure glacée. Mais l'existence de cette couche intermédiaire dans les glacis des glaciers démontre que cette opinion est incorrecte.Les vitesses des ondes séismiques mesurées de 4000 à 5000 m/s dans la couche intermédiaire se trouvent aussi dans des rocs décomposés. Mais les changements de la forme du bas de la glace pendant une période relativement courte ne peuvent pas être mis d'accord avec l'interprétation de la couche intermédiaire comme roc décomposé.Les auteurs supposent, que la couche intermédiaire se compose de sédiments très denses. Cette opinion prouve toutes les résultats d'observation.

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Middle Miocene peralkaline ignimbrites in the Hermosillo region (Sonora, Mexico): Geodynamic implications

Scattered ignimbritic mesas crop out in the Hermosillo region (Sonora, Mexico). These rocks that have been dated at 12.5 Ma (Middle Miocene) have the petrography and chemical characteristics of comendites. Such a flare-up of peralkaline acidic volcanism, after a long period of subduction-related arc volcanism, emphasises an important change in the source of volcanism. It corresponds to the latest stage of continental extension prior to the marine invasion and the development of spreading centres in the Gulf of California. To cite this article: J. Vidal Solano et al., C. R. Geoscience 337 (2005).     

Geochemical characteristics of Yamadağı volcanics in central east Anatolia: an example from collision-zone volcanism

Neogene Yamadağı volcanic rocks consist of basaltic trachyandesite, trachyandesite, andesite, and dacite. The major- and trace-element chemistry indicates that the lavas are dominantly calc-alkaline and mildly alkaline in character, sodic in nature, and intermediate to acidic in composition. REE and trace-element patterns of volcanic rocks are similar to those typical of within plate magmatics. Volcanic rocks have low ⁸⁷Sr/⁸⁶Sr (0.70389–0.70633) and high ¹⁴³Nd/¹⁴⁴Nd ratios (0.51267–0.51276) and mostly plot within the mantle array of the isotope ratio diagram. The linear correlations among ⁸⁷Sr/⁸⁶Sr−¹⁴³Nd/¹⁴⁴Nd, SiO₂−⁸⁷Sr/⁸⁶Sr and SiO₂−¹⁴³Nd/¹⁴⁴Nd isotope ratios in the volcanics suggest that fractional crystallization combined with minor assimilation was an important process within the collision zone.     

Multi-chronometric ages and origin of Archaean tonalitic gneisses in Finnish Lapland: A case for long crustal residence time

The Tojottamanselkä gneisses of the Koitelainen region, northern Finland, have been dated by the Sm-Nd and the common Pb methods. The Sm-Nd data of seven samples from a small area (100 m × 100 m) define an isochron ofT=3.06±0.12 (2) Ga, with correspondingI _Nd=0.50848±9 (2), or _Nd(T)=–3.7±1.8. This age is in good agreement with the zircon U-Pb discordia age (3.1 Ga) reported by Kröner et al. (1981) and is interpreted as the time of magmatic emplacement. The distinctly negative _Nd(T) value is found for the first time for Archean tonalitic gneisses and implies derivation of these magmas by remelting of continental material with a long (200–500 Ma) crustal residence time. A few samples, on the other hand, possess _Nd(T) values close to zero, hence they are thought to be derived by partial melting of basaltic sources with nearchondritic REE distribution patterns.Common Pb isotopic data yield an isochron age of 2.64±0.24 (2) Ga which is in agreement, within error limit, with the published Rb-Sr isochron age of 2.73±0.24 Ga (Kröner et al. 1981). The age of ca. 2.7 Ga is interpreted as the time of regional metamorphism during which both Pb and Sr isotopes were rehomogenised.The tonalitic gneisses have highly fractionated REE patterns with (La/Yb)_N ratios varying from 9 to 43. Like most Archean gneisses of TTG composition (tonalite-trondhjemite-granodiorite), they could be derived by partial melting of crustal sources of basaltic to granodioritic compositions. Direct derivation by melting of mantle peridotites is excluded.The present geochemical study indicates that the Tojottamanselkä gneisses have had a very complex history that involved multi-stage development. Together with the published age data for the basement gneisses and greenstone belts of eastern central Finland (Vidal et al. 1980; Martin et al. 1983a), we conclude that the Archean crustal development in Finland started at least 3.5 Ga ago and passed through a series of magmatic and metamorphic events at 3.1, 2.85, 2.65 and 2.5 Ga before the final intrusions of K-rich granites about 2.4 Ga ago.     

Étude expérimentale des transformations de phase dans un gradient thermique : application au granite de Soultz-sous-Forêts,France

The circulation of an injected fluid in the deep granite of the Soultz-sous-Forêts HDR (‘Hot Dry Rock’) site causes important textural and mineralogical modifications. In order to determine the potential crystallization sequences along the fluid pathway, several experiments under thermal gradient were conducted at 600 bar (between 300 and 200 °C). After 40 days, the observed sequences of the newly formed products along the thermal gradient are always as follows, from high to low temperatures: quartz, feldspars, and finally interstratified clays, this implying a significant mobility of aluminium in the solution. To cite this article: A. Baldeyrou et al., C. R. Geoscience 335 (2003).