Observations of the optical spectrum of the dayside magnetospheric cleft aurora

Optical spectra of the cleft aurora in the region 5000–8500 Å were measured in December, 1977 at Cape Parry, N.W.T. A Michelson interferometer was used at a resolution of 10 cm^?1. The auroral features observed were OI (5577, 6300-64, 7774, 8446 Å), OII (7319-30 Å), NI (5200 Å), Hα, O₂ atm (1,1), some weak N₂1P bands and possibly some Meinel bands of N₂⁺. In addition, nightglow emissions of Na and OH were observed. Theoretical predictions of the OI and NI emission rates using the model of Link et al. (1980) fit the observed rates reasonably well if a 40 eV Maxwellian incident electron spectrum is assumed. The predicted rates for OII exceed the observed value by a factor of 4. It is suggested that the ionization cross-section may be over-estimated.     

Geometry and kinematics of early Alpine nappes in a Briançonnais basement (Ambin Massif,Western Alps)

Petrological and structural observations from the Ambin pre-alpine basement dome and from its Briançonnais and Piedmont covers show an early D1 nappe-forming event overprinted by a major D2 (+?D3) ductile shearing deformation. The D1 event is characterised by garnet-blueschist facies metamorphic assemblages retrogressed to greenschist facies conditions during D2 then D3 stages near the top of the dome. North-verging D1 structures preserved in the core of the dome are consistent with alpine evolutionary models, in which exhumation of HP–LT metamorphic alpine rocks occurs initially in a north–south direction. To cite this article: J. Ganne et al., C. R. Geoscience 336 (2004).     

Geodynamic evolution of the Pan-African orogenic belt: A new interpretation of the Hoggar shield (Algerian Sahara)

The Pan-African orogenic belt of Hoggar, 800 km wide, represents an extremely tightened complex and composite mobile zone. In the western part, the earlier thick meta-sedimentary units and alkaline-peralkaline intrusives, both of middle Proterozoic age, account for the early mobility of a N-S trending ensialic domain. Later, large scale upper-mantle contribution was responsible for many magmatic complexes of basic to ultrabasic rocks intruded at c. a. 800 m. y. in cratonic platform sediments 1000 to 800 m. y. old. A very important volume of volcanoclastic deposits, andesites to dacites, and widespread calc-alkaline batholiths are supposed to derive from two assemblages of island-arc type upon basic crust and of Andean type upon granulite basement and this newly accreted material 800 to 650 m. y. old, may be related to subduction zones dipping East. A cryptic suture ist postulated along the margin of the West African craton. The central Hoggar is mainly composed of pre-Pan-African gneisses belonging to the Eburnean cycle andpro parte formed during a Kibaran cycle, and ensialic upper Proterozoic schist belts, which were subjected at varying degrees to the Pan-African deformation and metamorphism. Eastern Hoggar includes also pre-Pan-African gneisses and granites, and the narrow ensialic upper Proterozoic Tiririne belt formed during the late Pan-African along a N-S trending major shear zone. High structural level nappes, recumbent folding, anatexis and syn-kinematic granites emplaced at c. a. 650 m. y. define the early Pan-African, related to crustal thickening and crustal melting during the early stage of a continental collision. The late Pan-African E-W compression produced in the whole shield N-S trending folds geometrically linked to a mega-system of shear belts and strike-slip faults, which dissect the earlier edifices into N-S trending branches and blocks of many hundred kilometres lateral displacement. This pattern is the result of an extreme E-W tightening of the belt between two rigid plates: the West African craton and the East Saharian craton colliding with a sinistral confrontation. During this stage (600–550 m. y.) an unquantified E-W shortening may have been absorbed by lateral movements along conjugate shear zones with a main sinistral component. Mineral ages and diagenesis of molassic deposits point out to a post-Cambrian uplift of the belt.

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Zusammenfassung Der panafrikanische Orogen-Gürtel des Hoggar-Gebirges in Südalgerien erstreckt sich über eine Breite von ca. 800 km und entspricht einer extrem eingeengten, aus komplexen geologischen Einheiten aufgebauten mobilen Zone. In seinem westlichen Teil zeugen bereits die untersten mächtigen metasedimentären Ablagerungen sowie die alkalischen und hyperalkalischen Intrusionen des mittleren Proterozoikums von der frühen Mobilität eines N-S streichenden ensialischen Bereiches. Vor ca. 800 M. J. wurden die etwa 800–1000 M. J. alten Kratonsedimente von großen Mengen basischen und ultrabasischen Materials aus dem oberen Erdmantel intrudiert. Volumenmäßig noch bedeutender sind Vulkanite und Vulkanoklastite von andesitischer bis rhyodazitischer Zusammensetzung sowie zahlreiche batholitische Intrusivkörper kalkalkalischer Natur. Als Ursprung dieser Gesteine wird eine Zone vom Typus Inselbogen auf basischer (ozeanischer) Kruste sowie ein Orogen vom andinen Typus auf granulitischem Untergrund in Betracht gezogen. Dieses der Kruste vor ca. 650–800 M. J. neu zugeführte Material steht möglicherweise in Zusammenhang mit nach Osten einfallenden Subduktionszonen, und wir nehmen daher die Existenz einer tief abgetragenen Sutur am Ostrand des westafrikanischen Kratons an.Der zentrale Hoggar besteht hauptsächlich aus prä-panafrikanischen Gneisen, die dem eburnischen und teilweise auch dem kibaridischen Zyklus angehören; ferner enthält er schmale ensialische, mit spätproterozoischen Schiefern aufgefüllte Grabenzonen. Diese verschiedenen Elemente werden in unterschiedlichem Maße von der panafrikanischen Tektogenese und Metamorphose beansprucht.Der östliche Hoggar besteht ebenfalls aus Gneisen und Graniten prä-panafrikanischen Alters, wogegen die schmale Kette des Tiririne-Gebirges während der Endphase der panafrikanischen Faltung entlang einer N-S streichenden bedeutenden Scherzone gebildet wurde.Deckentektonik, liegende Falten, Anatexis sowie ca. 650 M. J. alte syntektonische Granitoide charakterisieren die frühe Phase der panafrikanischen Orogenese im Hoggar und können als Resultat von Krustenverdickung und Krustenaufschmelzung infolge von Plattenkollision interpretiert werden. Die spät-panafrikanische Tektonik beruht auf einer O-W gerichteten Einengung der gesamten Hoggar-Gebirgskette, wodurch N-S streichende Falten entstanden, die geometrisch mit einem Mega-System von Scherzonen und Brüchen in Verbindung stehen. Dabei wurden die alten Massive in langgestreckte N-S orientierte Blöcke zerschnitten, bei denen seitliche Verschiebungen bis zu mehreren hundert km vorkommen. Dieses Strukturbild ist das Ergebnis eines äußerst starken O-W gerichteten Zusammenschubs der mobilen Zone zwischen zwei starren Platten: dem westafrikanischen Kraton und dem Ostsahara-Kraton, die unter Linksdrehung aufeinander stießen. Während dieser Phase (550–600 M. J.) trat ferner eine quantitativ noch nicht abschätzbare O-W-Verkürzung der Gebirgskette ein, die möglicherweise durch seitliche Verschiebung entlang konjugierter Scherzonen mit dominierender Linkskomponente absorbiert wurde. Mineralalter sowie Diagenese-Alter der panafrikanischen Molasseablagerungen stellen die letzte post-orogene Hebung im Hoggar deutlich in das Post-Kambrium.

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Résumé La chaine Pan-Africaine affleure sur 800 km de large au Hoggar. Elle représente une zone mobile complexe et composite extrêmement raccourcie. Dans la partie Ouest, les premiers dépôts méta-sédimentaires épais, ainsi que des intrusions alcalines et hyperalcalines d'âge Protérozoique moyen témoignent de la mobilité précoce d'un domaine sud-méridien ensialique. Plus tard vers 800 Ma, la contribution à grande échelle du manteau supérieur est remarquable sous la forme d'intrusions basiques et ultrabasiques mises en place dans des sédiments cratoniques déposés entre 1000 et 800 Ma. Plus importants encore en volume, les dépôts volcano-détritiques, les roches volcaniques allant des andésites aux rhyo-dacites et de nombreux batholites calco-alcalins peuvent provenir de zones de type arc insulaire établies sur croûte basique et de type cordillére andine sur croûte granulitique. Ce matérial témoigne d'importants phénomènes d'accrétion continentale probablement liés à des zones de subduction pentées à l'Est ayant fonctionné entre environ 800 et 650 Ma. La bordure du craton Ouest-Africain correspondrait à une suture océanique. Le Hoggar central est principalement constitué par des gneiss pré-Pan-Africains appartenant à l'Eburnéen, et en partie au cycle «Kibarien» ainsi que par d'étroits sillons ensialiques de Protérozoique supérieur. Les déformations et le métamorphisme Pan-Africain affectent ces unités de manière variable. Le Hoggar oriental comprend des gneiss et granites d'âge pré-Pan-Africain, et l'étroite chaîne de Tiririne plissée au Pan-Africain tardif est controlée par une zone majeure de cisaillement. Chevauchements, plis couchés, anatexie et granites syn-tectoniques mis en place à 650 Ma sont en relation avec les premiers stades de collision continentale pendant le Pan-Africain précoce, engendrant un épaississement crustal important et des fusions anatectiques profondes. Le Pan-Africain tardif implique une compression globale E-W de la chaîne qui a produit des plis Nord-Sud géométriquement liés à un méga-systéme de cisaillements et de décrochements, qui découpent les édifices antérieurs en branches et blocs longitudinaux Nord-Sud avec des déplacements latéraux de plusieurs centaines de kilométres. Cette disposition résulte d'un serrage E—W extrême de la chaîne entre deux plaques rigides: le craton Ouest-Africain et le craton Est-Saharien qui s'affrontaient avec une composante sénestre. Au cours de ce stade (600–550 Ma) un raccourcissement E-W encore non quantifié a aussi été absorbé par le jeu de mouvements latéraux le long des zones de cisaillement conjuguées. Les âges des minéraux, et ceux de la diagenése des dépôts molassiques soulignent l'âge post-Cambrien du dernier soulévement de la chaîne.

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- Hoggar 800 , . . 800–1000 , 800 , . - . , . , 650–800 , - , . Hoggar . . - , , , ; , , - . - . Hoggar - , - , - . , , 650 - Hoggar , . - Hoggar, - , , -, . , -, . , : , . — 550 600 , , - , , , . - - Hoggar .

     

Geochemistry of early mesozoic tholeiites from Morocco

Mesozoic dolerites from two areas of Morocco, the High Atlas fold belt between Marrakech and Demnat and the Anti-Atlas belt in the area of Foum Zquid, are most high-Ti quartz-normative tholeiites whichi in many respects resemble Mesozoic dolerite dikes from eastern North America. The dolerites display a wide range of major and trace element compositions, some of which are due to fractional crystallization. The doleritic sequences from High Atlas also show vertical stratigraphic zonation which is characterized by a progressive depletion of lithophile elements toward the top. This trend together with regularities of trace element ratio variations are indicative of a dynamic melting of an initially homogeneous source. It is suggested that the continental upper mantle source for dolerites of Morocco was enriched in several incompatible elements in comparison with the upper mantle source for ocean floor tholeiites.     

From the simplest chemical system to the natural one: garnet peridotite barometry

The available experimental data on garnet-bearing-assemblages for synthetic chemical systems (MAS, FMAS, CMAS) have been used to calibrate consistent models for the Al-solubility in orthopyroxene coexisting with garnet, on the basis of equilibrium reaction Py(opx) ? Py(gt). The alternative reaction En(opx)+MgTs(opx) ? Py(gt) is discarded as it yields larger a-posteriori uncertainties. To provide a reliable equation, directly applicable to natural garnet lherzolites, each successive synthetic-system calibration is tested against Mori and Green's (1978) natural-system reequilibration data. For the MAS system, an ideal solution model with constant ΔH°, ΔV° and ΔS° based on 12-oxygen structural formulae for aluminous pyroxenes yields the best fit (GPa, K), $${\text{25,134 + 9,941 }}P - 23.177{\text{ }}T{\text{ + }}RT{\text{ ln (}}X_{{\text{Al}}}^{TB'} {\text{) = 0}}$$ . The MAS synthetic-system calibration can be directly applied to the FMAS system by adding an empirical correction term (20,835 [X _Fe ^gt ]²) independent of either pressure and temperature. However, this correction term is not important because of the limited Fe content of mantle peridotites. When calcium is added to the MAS system, the equilibrium constant is calculated as: $$K_{{\text{CMAS}}} = {{[(1 - X_{{\text{Ca}}}^{M2} )^2 (X_{{\text{Al}}}^{TB'} )]} \mathord{\left/ {\vphantom {{[(1 - X_{{\text{Ca}}}^{M2} )^2 (X_{{\text{Al}}}^{TB'} )]} {[(1 - X_{{\text{Ca}}}^X )^3 (X_{{\text{Al}}}^Y )^2 ]}}} \right. \kern-\nulldelimiterspace} {[(1 - X_{{\text{Ca}}}^X )^3 (X_{{\text{Al}}}^Y )^2 ]}}$$ where M2 and TB′ are pyroxene sites and X and Y are garnet sites. Up to 5 GPa, X _Ca ^X ～ and the CMAS experimental data agree well with the MAS model, but for Yamada and Takahashi's (1983) higher pressure experiments (up to 10 GPa), this no longer holds. Indeed, the garnet solid solution does not behave ideally and an asymmetric regular solution model is needed for application to the deepest natural samples available (>7GPa). Calibration based on new high pressure data yields, $$\begin{gathered} \Delta G_{{\text{CMAS}}}^{XS} = (X_{{\text{Ca}}}^X )(1 - X_{{\text{Ca}}}^X )(0.147 - X_{{\text{Ca}}}^X ) \hfill \\ {\text{ }} \cdot {\text{(6,440,535 - 1,490,654 }}P{\text{)}} \hfill \\ \end{gathered}$$ . According to tests of the inferred solution model, the CFMAS system is a good analogue of natural systems in the pressure, temperature and composition ranges covered by the natural-system reequilibration data (up to 1,500° C and 4 GPa). Simultaneous application of this thermobarometer and of the two-pyroxene mutual solubility thermometer (Bertrand and Mercier 1985) to the phases of the garnet-peridotite xenoliths from Thaba Putsoa, Lesotho, yields a refined paleogeotherm for southern Africa strongly contrasting with previous results. The “granular” nodules yield a thermal gradient of about 8 K/km characteristic of a lithospheric-type environment, whereas the “sheared” ones show a lower gradient of about 1 K/km. This is a typical geotherm expected for a steady thermal state with an inflexion point at the depth of about 160 km corresponding to the lithosphere/asthenosphere boundary.     

Potential Role of Solar Variability as an Agent for Climate Change

Numerical experiments have been carried out with a two-dimensional sector averaged global climate model in order to assess the potential impact of solar variability on the Earth's surface temperature from 1700 to 1992. This was done by investigating the model response to the variations in solar radiation caused by the changes in the Earth's orbital elements, as well as by the changes intrinsic to the Sun. In the absence of a full physical theory able to explain the origin of the observed total solar irradiance variations, three different total solar irradiance reconstructions have been used. A total solar irradiance change due to the photospheric effects incorporated in the Willson and Hudson (1988) parameterization, and the newly reconstructed solar total irradiance variations from the solar models of Hoyt and Schatten (1993) and Lean et al. (1995). Our results indicate that while the influence of the orbital forcing on the annual and global mean surface temperature is negligible at the century time scale, the monthly mean response to this forcing can be quite different from one month to another. The modelled global warming due to the three investigated total solar irradiance reconstructions is insufficient to reproduce the observed 20th century warming. Nevertheless, our simulated surface temperature response to the changes in the Sun's radiant energy output suggests that the Gleissberg cycle (88 years) solar forcing should not be neglected in explaining the century-scale climate variations. Finally, spectral analysis seems to point out that the 10- to 12-year oscillations found in the recorded Northern Hemisphere temperature variations from 1700 to 1992 could be unrelated to the solar forcing. Such a result could indicate that the eleven-year period which is frequently found in climate data might be related to oscillations in the atmosphere or oceans, internal to the climate system.     

Functioning and precipitation-displacement modelling of rainfall-induced deep-seated landslides subject to creep deformation

We propose an approach to study the hydro-mechanical behaviour and evolution of rainfall-induced deep-seated landslides subjected to creep deformation by combining signal processing and modelling. The method is applied to the Séchilienne landslide in the French Alps, where precipitation and displacement have been monitored for 20 years. Wavelet analysis is first applied on precipitation and recharge as inputs and then on displacement time-series decomposed into trend and detrended signals as outputs. Results show that the detrended displacement is better linked to the recharge signal than to the total precipitation signal. The infra-annual detrended displacement is generated by high precipitation events, whereas annual and multi-annual variations are rather linked to recharge variations and thus to groundwater processes. This leads to conceptualise the system into a two-layer aquifer constituted of a perched aquifer (reactive aquifer responsible of high-frequency displacements) and a deep aquifer (inertial aquifer responsible of low-frequency displacements). In a second step, a new lumped model (GLIDE) coupling groundwater and a creep deformation model is applied to simulate displacement on three extensometer stations. The application of the GLIDE model gives good performance, validating most of the preliminary functioning hypotheses. Our results show that groundwater fluctuations can explain the displacement periodic variations as well as the long-term creep exponential trend. In the case of deep-seated landslides, this displacement trend is interpreted as the consequence of the weakening of the rock mechanical properties due to repeated actions of the groundwater pressure.