Crustal thickening and crustal extension as controls on the evolution of the drainage network of the central Swiss Alps between 30 Ma and the present: constraints from the stratigraphy of the North Alpine Foreland Basin and the structural evolution of the Alps

The combined information about sedimentary petrography from the North Alpine Foreland Basin and structural geology from the Alps allows a qualitative reconstruction of the drainage network of the central Swiss Alps between 30 Ma and the present. This study suggests that crustal thickening and crustal thinning significantly controlled the location of the drainage divide. It also reveals the possible controls of crustal thickening/thinning on the change of the orientation of the drainage network from across-strike between 30 and 14 Ma to along-strike thereafter. Initial crustal thickening in the rear of the wedge is considered to have formed the drainage divide between north and south at 30 Ma. Because the location of crustal thickening shifted from east to west between ≈30–20 Ma, the catchment areas of the eastern dispersal systems reached further south than those of the western Alpine palaeorivers for the same time slice. Similarly, the same crustal dynamics appear to have controlled two phases of denudation that are reflected in the Molasse Basin by petrographic trends. Uplift in the rear of the wedge caused the Alpine palaeorivers to expand further southward. This is reflected in the foreland basin by increasing admixture of detritus from structurally higher units. However, tectonic quiescence in the rear of the wedge allowed the Alpine palaeorivers to cut down into the Alpine edifice, resulting in an increase of detritus from structurally lower units. Whereas uplift in the rear of the wedge was responsible for initiation of the Alpine drainage systems, underplating of the external massifs some 50 km further north is thought to have caused along-strike deviation of the major Alpine palaeorivers. Besides crustal thickening, extension in the rear of the wedge appears to have significantly controlled the evolution of the drainage network of the western Swiss Alps. Slip along the Simplon detachment fault exposed the core of the Lepontine dome, and caused a 50-km-northward shift of the drainage divide.     

Durbachites–Vaugnerites – a geodynamic marker in the central European Variscan orogen

Durbachites–Vaugnerites are K–Mg‐rich magmatic rocks derived from an enriched mantle source. Observed throughout the European Variscan basement, their present‐day geographical distribution does not reveal any obvious plate‐tectonic context. Published geochronological data show that most durbachites–vaugnerites formed around 335–340 Ma. Plotted in a Visean plate‐tectonic reconstruction, the occurrences of durbachites–vaugnerites are concentrated in a hotspot like cluster in the Galatian superterrane, featuring a distinctive regional magmatic province. Reviewing the existing local studies on Variscan durbachite–vaugnerite rocks, we interpret their extensive appearance in the Visean in terms of two factors: (i) long‐term mantle enrichment above early Variscan subduction systems; and (ii) melting of this enriched subcontinental mantle source during the Variscan collision stage due to thermal anomalies below the Galatian superterrane, possibly created by slab windows and and/or the sinking of the subducted Rheic slab into the mantle. The tectonic reorganization of Europe in the Late Palaeozoic and during the Alpine orogeny has torn apart and blurred this marked domain of durbachites–vaugnerites.     

A novel sampling technique for image sharpening in infrared array cameras

A new nondestructive readout scheme of the array will be presented which allows first order wavefront corrections of images degraded by atmospheric seeing. During the stare time the integration ramp of the detector signal is sampled every 100 msec. A regressional fit of the sampled data points yields the slope of the integration ramp which is proportional to the flux received by a detector pixel. To this readout mode which is commonly used for IR arrays a small software module can be added to compensate the image motion of the observed object by shifting the nondestructively sampled images. This has the same effect as a tip tilt correction by an active optical element - but without the extra complexity of such a device. First results obtained at the telescope are presented.     

Timing and mechanisms of Central Himalayan exhumation: discriminating between tectonic and erosion processes

The ability to deduce exhumation mechanisms from thermochronological data is hampered by the fact that assumptions on the thermal state of the lithosphere have to be made. Additional argumentation is generally required to discriminate between erosion-controlled and tectonically induced exhumation. This problem can be overcome by studying the spatial distribution of zircon and apatite (U-Th)/He and fission track data. In this work the variation of four different low temperature isotopic systems generating age trends along a sampling line is used to infer mechanisms of Quaternary exhumation in the Central High Himalayan Metamorphic Belt. Observed zircon age trends with southwards increasing cooling ages (from 0.5 to 1.7 Ma) are attributed to tectonically induced exhumation. The uniform apatite cooling ages clustered c. 0.5 Ma are attributed to erosion.     

Structural change associated with the incommensurate-normal phase transition in akermanite, Ca2MgSi2O7, at high pressure

The structural changes associated with the incommensurate (IC)-normal (N) phase transition in akermanite have been studied with high-pressure single-crystal X-ray diffraction up to 3.79?GPa. The IC phase, stable at room pressure, transforms to the N phase at ～1.33?GPa. The structural transformation is marked by a small but discernable change in the slopes of all unit-cell parameters as a function of pressure. It is reversible with an apparent hysteresis and is classified as a tricritical phase transition. The linear compressibility of the a and c axes are 0.00280(10) and 0.00418(6)?GPa^?1 for the IC phase, and 0.00299(11) and 0.00367(8)?GPa^?1 for the N phase, respectively. Weighted volume and pressure data, fitted to a second-order Birch-Murnaghan equation of state (K′≡4.0), yield V₀=307.4(1)?ų and K₀=100(3)?GPa for the IC phase and V₀=307.6(2)?ų and K₀=90(2)?GPa for the N phase. No significant discontinuities in Si–O, Mg–O and Ca–O distances were observed across the transition, except for the Ca–O1 distance, which is more compressible in the IC phase than in the N phase. From room pressure to 3.79?GP the volume of the [SiO₄] tetrahedron is unchanged (2.16?ų), whereas the volumes of the [MgO₄] and [CaO₈] polyhedra decrease from 3.61 to 3.55(1)?ų and 32.8 to 30.9(2)?ų, respectively. Intensities of satellite reflections are found to vary linearly with the isotropic displacement parametr of Ca and the librational amplitude of the [SiO₄] tetrahedron. At room pressure, there is a mismatch between the size of the Ca cations and the configuration of tetrahedral sheets, which appears to be responsible for the formation of the modulated structure; as pressure increases, the misfit is diminished through the relative rotation and distortion of [MgO₄] and [SiO₄] tetrahedra and the differential compression of individual Ca–O distances, concurrent with a displacement of Ca along the (110) mirror plane toward the O1 atom. We regard the high-pressure normal structure as a result of the elimination of microdomains in the modulated structure.     

Organic carbon isotope geochemistry of clayey deposits and their associated porewaters, southern Alberta

The organic carbon cycle of slowly permeable, clayey glacial till deposits in the Western Interior Great Plains, southern Alberta, was investigated by examining the relationship between solid organic matter (SOM) in the till sediments and dissolved organic carbon (DOC) in the till porewaters. Geochemically, the tills can be divided into two distinct zones: an upper oxidized (low SOM) till zone, and a lower unoxidized (high SOM) till zone. Till porewaters in both zones are characterized by high DOC contents. Radiocarbon dating and comparison of SOM and DOC fractions suggest DOC in the deep unoxidized zone originated during deglaciation, and is probably representative of groundwater ages in this till zone. In the oxidized zone, DOC originates from variable mixtures of soluble organic matter emplaced during deglaciation, and Cretaceous age coal fragments in this till zone. SOM in the upper till zone was mainly oxidized to CO₂ gas during lowered water table conditions of the Altithermal climatic period. The subsurface production of fossil CO₂ gas has serious implications for using the conventional dissolved inorganic carbon (DIC) ¹⁴C groundwater dating method in these clayey till porewaters.     

Satellite pictures of the snow-covered alps during April 1960

Summary From April 2 to 7, 1960, the meteorological satellite Tiros I televised the Alps and the surrounding area. The Alpine mountains were snow covered but the river valleys and lakes were readily discernible in the pictures under clear sky conditions. The pictures show marked differences when the area was overcast as compared to times when the area was clear or partly cloudy. These differences are discussed with the aid of satellite pictures and surface cloud cover maps.

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Zusammenfassung Vom 2. bis 7. April 1960 hat der meteorologische Satellit Tiros I Fernsehaufnahmen der Alpen und der umliegenden Gebiete gemacht. Die Alpenberge waren damals schneebedeckt, dagegen waren die Flußtäler und Seen bei Schönwetter in den Bildern deutlich zu erkennen. Die Bilder zeigen markante Unterschiede, wenn das Gebiet von Wolken bedeckt war, im Vergleich zu den Bildern bei wolkenlosem oder nur teilweise wolkigem Wetter. Diese Unterschiede werden an Hand von Satellitenaufnahmen und von nach Bodenbeobachtungen entworfenen Bewölkungskarten diskutiert.

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Résumé Le satellite météorologique Tiros I a effectué du 2 au 7 avril 1960 des prises de vue télévisées des Alpes et des régions avoisinantes. Les montagnes étaient alors recouvertes de neige, mais les vallées et les lacs sont, par ciel serein, nettement visibles sur les images. Ces images montrent des différences sensibles si les diverses régions ont un ciel couvert ou, au contraire, serein ou peu nuageux. On discute alors ces différences tant au moyen des images produites par le satellite que par des cartes de la nébulosité basées sur des observations au sol.

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With 6 Figures

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Dedicated toDr. W. Mörikofer on the occasion of his 70th birthday.