Influence de la subsidence sur la sédimentation

Résumé Des seuils peu profonds et stables, balayés par des courants (stabiliseuils) séparaient en France pendant le Jurassique supérieur des aires subsidentes où s'accumulaient les sédiments; des récifs coralliens pouvaient s'y développer lorsque la subsidence était comprise entre 30 et 70 m. par zone paléontologique, et constituer alors de véritables barrières: la localisation des régions où la subsidence optima était réalisée est par conséquent un élément essentiel de l'abondance et de la répartition des sédiments calcaires.Les variations locales de la subsidence constituent donc un facteur important de la sédimentation, non seulement au point de vue quantitatif, mais encore en ce qui concerne la nature des dépôts.     

Analytic signals for multivariate data

Hilbert transformation is a standard tool in univariate signal-processing. It leaves the information content unaffected for, apart from a change of sign, the iterated transform reproduces the original data. As we know from Fourier transforms, such alternative representations of the same information, notwithstanding their theoretical equivalence with the data, can assist powerfully with extracting and interpreting that information. Although the extension to multivariate data is not so obvious for Hilbert as for Fourier transforms, Nabighian gave a treatment of the bivariale situation in 1984. Fueter, some 50 years earlier, had worked on an analogous extension problem, seeking to generalize complex function theory. On comparing these two developments we learn that, although Nabighian's transforms fit naturally into Fueter's theory, they are only one among many alternative possibilities. This paper presents a general theory, of higher dimensional Hilbert transforms and analytic signals, applicable to data of all dimensions less than eight. The change-of-field-direction fillers used in geophysical data processing are shown to arise as special situations.     

Monochromatic observations of three nebular objects in the direction of NGC 1714

Using a CCD as detector, we have observed three nebulosities not yet listed in catalogues; they all present a planetary nebula morphology and are situated in the immediate neighbourhood (less than 3 arc min to the East) of NGC 1714. Observations have been made through interference filters centered on the main emission lines of these objects.The observations were made at the E.S.O. in La Silla (Chile).European Southern Observatory, La Silla, Santrago de Chile.     

A simple method of computing the variation of annual precipitation over mountainous terrain

A method for computing the distribution of annual precipitation for mountainous areas is presented. As a pilot study, the South Thompson River basin in British Columbia is examined. In addition to data from climatological (valley) stations, annual precipitation is estimated for snow course (mountain) locations from the water equivalent of the snowpack on April 1.An equation for the dependence of precipitation amount on orography is derived from simple physical considerations. Regression equations based on this are then computed and the calculated precipitation is checked with runoff data. A basic predictor is the mean vertical water vapor transport through a horizontal unit area due to flow over undulating terrain. This depends on the average slope of a region 3 deg long by 2 deg lat centered on each grid point. The transport at the earth's surface has a simple correlation coefficient with the combined valley and mountain precipitation data of 0.76. When the transport is computed at the mean cloud base (taken as 1800 m above sea-level), the correlation coefficient rises to 0.87.     

Geophysics of the Quebec Appalachians

This paper deals with a synthesis of the geophysics of the Quebec Appalachians. An emphasis is given to the regional geophysical surveys for the final geological interpretation. Regional, residual and downward continued aeromagnetic (AM) data yield information on the tectonic history of the region. The main features are pattern recognition of folded and particularly faulted structures and the boundaries of the tectonic domains. Three main fault systems have been identified and classified using AM information. In addition, depths to the crystallise basement and to the Cambro-Ordovician magnetic layers were obtained for sectors of the Gulf of St. Lawrence-Gaspé region. Nine main geological features are extracted from the processed AM data. Detailed geological modelling of some of the identified structures (e.g. nappes) are presented.Five main geological features are recognized from the gravity data. These features are correlated with the ones obtained from AM data whenever possible. Geological models based on information from surface geology, rock density contrasts and occasional contraints from regional seismic reflection data are presented. The boomerang shape of the Sutton-Bennett anticlinorium and the planar or curved slab-like shape of the obducted ophiolite bodies (upper mantle proto-oceanic floor) may be considered as the significant features.Regional reflection seismic data give a clear geological picture of three (out of four) distinguishable zones of the Quebec Appalachians, that is: autochthonous, parautochthonous, and transition zone: parautochthonous-allochthonous(corresponding approximately to the external domain). The pattern recognition of the allochthonous (approximate internal domain) is somewhat more difficult to decipher from the reflection seismic data. Intraplate seismicity and seismotectonics indicate the presence of four zones of high seismicity in southern Quebec. In almost all zones, the seismotectonic history goes back to Late Precambrian (Hadrynian) times.The scarce and scattered paleomagnetic data of geological formations located on the platform and the external domain of the Quebec Appalachians and in the internal domain are compared with those belonging to the stable North American plate. The paleomagnetic results indicate that the angular difference between the poles from the platform and external domain is large at Cambrian times (40°) and small at Devonian times (5°-10°). This suggests a fast rate of motion and accretion of the internal domain relative to the North American Plate in the Cambrian—Lower Devonian time span (550→-385 Ma).     

Sur le mecanisme de formation de la schistosite dans l'himalaya

After recalling briefly the major geological characters of the Nepal Himalayas and the global evolution of the Himalayan chain we stress the importance of three dominant and remarkably constant tectonic features in this chain: (1) a huge overthrust is seen everywhere to entail a visible throw of more than 100 km; (2) a shallow dipping cleavage develops over an observable thickness in excess of 15 km; and (3) in the underthrusting slab the stretching lineation keeps a similar direction constantly perpendicular to the local trend of the chain.We suggest that these features, especially the cleavage, can be explained in terms of a simple mechanical model, namely simple shear on a crustal scale. This clearcut case could be used as a model for the origin of flattish cleavages in many orogens. After discussing other modes of formation for such cleavages (diapiric intrusion, extension) we show that the Himalayan model could apply to other chains.     

The subalkaline potassic magmatism of the Ballons massif (Southern Vosges,France): shoshonitic affinity

A systematic geochemical study of the Ballons plutonic complex (Southern Vosges, France) indicates that the border facies of the massif are composed of two continuous differentiation series ranging from gabbros to quartz monzonites and very similar to the shoshonitic associations (high K₂O/Na₂O ratios as well as high Ba, Sr, U and Th contents). The existence of a geochemical discontinuity between the two series and the monzogranite, which is the principal component of the massif, argues against the monzogranite being directly related to the two differentiation series by fractional crystallization. The zoned structure of the monzogranite can be attributed to in situ crystal fractionation.