Insolation,CO2 et précipitations en période interglaciaire

Climate models of different complexity confirmed that the long-term variations of the astronomical parameters are driving the climatic variations during the Quaternary (and other geological times) at the timescale of tens to hundreds of thousands of year. An Earth system Model of Intermediate Complexity (LLN 2-D NH) [Gallée et al., J. Geophys. Res. 96 (1991) 13161] was able to simulate long-term climatic variations such as the recurrence of glacial interglacial cycles during the Quaternary. This model is used here to simulate the future climate. In most of the hypotheses, the model simulates an extended present interglacial covering the next 50 000 years. A regional atmospheric model (MAR) [Gallée, Mon. Weather Rev. 123 (7) (1995) 2051–2069; Gallée, Schayes, Bound-Lay. Meteorol. 59 (1994) 141–161] allows the simulation of the climatic changes at a more local scale, in particular in the case of an increase in atmospheric CO₂ concentration and of a change in the astronomical forcing. To cite this article: M.-F. Loutre, A. Berger, C. R. Geoscience 337 (2005).     

Evolution diagénétique dans la craie pélagique dolomitisée du crétacé supérieur du bassin de Paris (région de Provins, France)

Two deep scientific boreholes, named Poigny 701 and Sainte-Colombe 702, located in the Paris Basin near Provins (Seine-et-Marne, France), recovered a complete Upper Cretaceous chalk succession. A correlation between the boreholes lithostratigraphy, reflexion seismic profiles and diagenetic patterns shows that major velocity variations measured in the seismic reflection profiles correspond to dolomitized chalk intervals. Dolomitisations occurred during early and burial diagenesis. The understanding of these complex diagenetic events has an important economic consequence on the static correction of the chalk formation in the Paris Basin. Optimisation of petroleum prospecting below chalk cover is thus possible. The chalk series of the 701 and 702 boreholes range from the Cenomanian to the upper Campanian. In this succession, micrite has a primarily biogenic origin; it consists of pelagic organisms, indicative of warm seawater with values around 25°C. Several hiatuses occur in the 701 borehole. These hiatuses indicate the existence of particular hydrodynamic conditions. Deep-water channels were locally recognised in the Paris Basin as in Normandy and Picardy. In the 702 borehole, massive dolomitisation affected the upper meters of sediment below the sea floor. This early phase of massive dolomitisation was induced by slow circulation of a magnesium-rich seawater mass, along the seawater/sediment interface. Thus, this area was a zone of intense marine circulation between the North-West infralittoral and the South-West bathyal domains, across the London-Paris Basin. Magnesium-rich seawater had as origin the recrystallisation of the biogenic peri-plateform carbonates. During both dolomitisation and dissolution of the calcite matrix of the massive dolomite, the calcite had cemented chalk around the massive dolomite body. After deposition, the chalk series was progressively compacted and lithified by burial calcite cement. During the late Campanian-Maastrichtian, the burial compaction of the chalk and thermal gradient reached their maximum, as compressive stresses from the Pyrenean orogenesis affected the Paris Basin. At this time, compaction of the massive dolomite induced the expulsion of magnesian-rich fluids into the underlying already compacted chalk series. In 702 borehole, a diffuse dolomitisation then affected strongly underlying the chalk series. In 701 borehole, this diffuse dolomitisation affected slightly the lower half of the chalk series. Laterally, dolomitisation decreases gradually and affected a only smaller thickness of the chalk series, disappearing laterally. During the progressive emergence of the Paris Basin, from the Paleogene to the Quaternary, the chalk series were partially invaded by continental fresh water. Thus partial dedolomitisation affected the massive dolomite, whereas total dedolomitisation affected only the upper first meters of it.Manuscrit reçu le 20 juin 2003 Révision acceptée le 9 septembre 2004     

Structures et cinématique liées á la mise en place des péridotites de Ronda (Cordilléres Bétiques,Espagne)

The interaction of the Mid-Atlantic Ridge with the North Atlantic Mantle Plume has produced a magmatic plateau centred about Iceland. The crust of this plateau is 30 km thick on average. This abnormal thickness implies that, unlike other slow-spreading ridges, addition of magmatic material to the crust is not balanced by crustal stretching. The thermal effect of the plume also reduces the strength of the lithosphere. Both mechanisms affect the rifting process in Iceland. A structural review, including new field observations, demonstrates that the structure of the Iceland plateau differs from that of other slow-spreading oceanic ridges. Lithospheric spreading is currently accommodated in a 200 km wide deformation strip, by the development of a system of half-grabens controlled by growth faults. Similar extinct structures, with various polarities, are preserved in the lava pile of the Iceland plateau. These structures are identified as lithospheric rollover anticlines that developed in hanging walls of listric faults. We introduce a new tectonic model of accretion, whereby the development of the magmatic plateau involved activation, growth and decay of a system of growth fault/rollover systems underlain by shallow magma chambers. Deactivation of a given extensional system, after a lifetime of a few My, was at the expense of the activation of a new, laterally offset, one. Correspondingly, such systems formed successively at different places within a 200 km wide diffuse plate boundary. Unlike previous ones, this new model explains the lack of an axial valley in Iceland, the dip pattern of the lava pile, the complex geographical distribution of ages of extinct volcanic systems and the outcrops of extinct magma chambers.     

La couverture océanique des ultrabasites de Lanzo (Alpes occidentales) : arguments lithostratigraphiques et pétrologiques

Résumé

De nombreuses analogies entre les évolutions pétro-logiques et métamorphiques des ultrabasites de Lanzo et des ophiohtes liguro-piémontaises ont été soulignées récemment. Ces analogies concernent: la nature des ultrabasites (Iherzolites à plagioclase peu appauvries), la présence de gabbros et de filons basaltiques superficiels intrusifs dans les péridotites, l’existence de reliques de paragenèses métamorphiques de haute température attribuables à un épisode océanique, l’empreinte du métamorphisme alpin de haute pression. Ces faits ont conduit certains auteurs à proposer que les péridotites de Lanzo représentent un fragment du plancher océanique téthysien.

Nous avons tenté de vérifier cette hypothèse en recherchant la couverture océanique (volcanique et/ou sédimentaire) des péridotites de Lanzo, dans la partie occidentale du massif où des pincées de métabasites et de schistes lustrés s.l. sont visibles associées aux serpentinites.

Aux environs du village de Richiaglio, à l’Est de la cicatrice de Viù, les successions lithostratigraphiques reconstituées comprennent schémati-quement : les ultrabasites serpentinisées du corps de Lanzo, des métabasites, des quartzites manganésifères (comparables à celles du Mont Viso par exemple), et des calcschistes. Ces successions s’apparentent à celles décrites dans les couvertures d’autres ophiolites piémontaises, ce qui conduit à proposer que ces ensembles métavolcaniques et Dimetaires dimentaires représentent des résidus de la couverture océanique originelle des péridotites. L’analyse des paragenèses éclogitiques des métasédiments et des métabasites permet de montrer que l’évolution P,T alpine de ces lambeaux est compatible avec celle du corps de Lanzo. Cette interprétation et ses implications sont discutées.     

Résumé

Ohne Zusammenfassung     

Le mécanisme de la précipitation du Co3 Ca et la Détermination de L'épaisseur du Dépôt annuel de la vase dans le lac de Neuchâtel

Sans résumé     

Résumés

Sans résumé