Stars and bars     

Johan H Knapen 《Astronomy& Geophysics》2005,46(6):6.28-6.33

Bars are a common feature of disc galaxies and, as such, must be taken into account in understanding the evolution of structure within galaxies, argues Johan H Knapen.  相似文献   

Caledonide development offshore–onshore Svalbard based on ocean bottom seismometer, conventional seismic, and potential field data     

Asbjrn Johan Breivik  Rolf Mjelde  Paul Grogan  Hideki Shimamura  Yoshio Murai  Yuichi Nishimura 《Tectonophysics》2005,401(1-2):79-117

The western Barents Sea and the Svalbard archipelago share a common history of Caledonian basement formation and subsequent sedimentary deposition. Rock formations from the period are accessible to field study on Svalbard, but studies of the near offshore areas rely on seismic data and shallowdrilling. Offshore mapping is reliable down to the Permian sequence, but multichannel reflection seismic data do not give a coherent picture of older stratigraphy. A survey of 10 Ocean Bottom Seismometer profiles was collected around Svalbard in 1998. Results show a highly variable thickness of pre-Permian sedimentary strata, and a heterogeneous crystalline crust tied to candidates for continental sutures or major thrust zones. The data shown in this paper establish that the observed gravity in some parts of the platform can be directly related to velocity variations in the crystalline crust, but not necessarily to basement or Moho depth. The results from three new models are incorporated with a previously published profile, to produce depth-to-basement and -Moho maps south of Svalbard. There is a 14 km deep basement located approximately below the gently structured Upper Paleozoic Sørkapp Basin, bordered by a 7 km deep basement high to the west, and 7–9 km depths to the north. Continental Moho-depth range from 28 to 35 km, the thickest crust is found near the island of Hopen, and in a NNW trending narrow crustal root located between 19°E and 20°E, the latter is interpreted as a relic of westward dipping Caledonian continental collision or major thrusting. There is also a basement high on this trend. Across this zone, there is an eastward increase in the V_P, V_P/V_S ratio, and density, indicating a change towards a more mafic average crustal composition. The northward basement/Moho trend projects onto the Billefjorden Fault Zone (BFZ) on Spitsbergen. The eastern side of the BFZ correlates closely with coincident linear positive gravity and magnetic anomalies on western Ny Friesland, apparently originating from an antiform with high-grade metamorphic Caledonian terrane. A double linear magnetic anomaly appears on the BFZ trend south of Spitsbergen, sub-parallel to and located 10–50 km west of the crustal root. Based on this correlation, it is proposed that the suture or major thrust zone seen south of Svalbard correlates to the BFZ. The preservation of the relationship between the crustal suture, the crustal root, and upper mantle reflectivity, challenges the large-offset, post-collision sinistral transcurrent movement on the BFZ and other trends proposed in the literature. In particular, neither the wide-angle seismic data, nor conventional deep seismic reflection data south of Svalbard show clear signs of major lateral offsets, as seen in similar data around the British Isles.  相似文献   

On the vertical structure of the Rhine region of freshwater influence   总被引：1，自引：0，他引：1  

Gerben J. de Boer  Julie D. Pietrzak  Johan C. Winterwerp 《Ocean Dynamics》2006,56(3-4):198-216

An idealised three-dimensional numerical model of the Rhine region of fresh water influence (ROFI) was set up to explore the effect of stratification on the vertical structure of the tidal currents. Prandle’s dynamic Ekman layer model, in the case of zero-depth-averaged, cross-shore velocities, was first used to validate the response of the numerical model in the case of barotropic tidal flow. Prandle’s model predicted rectilinear tidal currents with an ellipse veering of up to 2%. The behaviour of the Rhine ROFI in response to both a neap and a spring tide was then investigated. For the given numerical specifications, the Rhine plume region was well mixed over the vertical on spring tide and stratified on neap tide. During spring conditions, rectilinear tidal surface currents were found along the Dutch coast. In contrast, during neap conditions, significant cross-shore currents and tidal straining were observed. Prandle’s model predicted ellipse veering of 50%, and was found to be a good indicator of ellipticity magnitude as a function of bulk vertical eddy viscosity. The modelled tidal ellipses showed that surface currents rotated anti-cyclonically whereas bottom currents rotated cyclonically. This caused a semi-diurnal cross-shore velocity shearing which was 90° out of phase with the alongshore currents. This cross-shore shear subsequently acted on the horizontal density gradient in the plume, thereby causing a semi-diurnal stratification pattern, with maximum stratification around high water. The same behaviour was exhibited in simulations of a complete spring–neap tidal cycle. This showed a pattern of recurring stratification on neaps and de-stratification on springs, in accordance with observations collected from field campaigns in the 1990’s. To understand the increase in ellipticities to 30% during neaps and the precise shape of the vertical ellipse structure, stratification has to be taken into account. Here, a full three-dimensional numerical model was employed, and was found to represent the effect of de-coupling of the upper and lower layers due to a reduction of mixing at the pycnocline.  相似文献   

Ottar Basin, SW Barents Sea: a major Upper Palaeozoic rift basin containing large volumes of deeply buried salt   总被引：1，自引：0，他引：1  

Asbjørn Johan Breivik  Steinar Thor Gudlaugsson  Jan Inge Faleide 《Basin Research》1995,7(4):299-312

Seismic mapping and gravity modelling of the Ottar Basin - a little studied Upper Palaeozoic graben in the south-western Barents Sea - demonstrates the presence of a major rift basin with large accumulations of unmobilized salt. Buried beneath thick, flat-lying Mesozoic strata, the NE-trending fault-bounded basin is at least 170 km long, varies in width between 50 and 80 km and coincides with a negative gravity anomaly of more than — 10 mgal. Seismic observations show that the south-western part is a half-graben tilted to the north-west whereas the north-eastern part appears to be more symmetric in shape. A large mass deficiency in the north-eastern part of the basin, indicated by a gravity anomaly of more than — 30 mgal, makes it necessary to postulate large amounts of salt within the basin. The preferred gravity model shows a total basin depth of 9.5 km, basin relief of 4.2 km and a salt volume of 6800 km³ corresponding to a 2.4-km-thick salt layer. Similar basin depths, but only 500–600 km³ of salt, are indicated beneath the Samson Dome in the south-western part of the basin. Unlike salt bodies in other Barents Sea basins, the thick salt deposit in the north-eastern part of the Ottar Basin is relatively unaffected by halokinesis. Interfingering of different basin facies, lack of tectonic reactivation of the basin and a relatively late differential loading by protruding cover strata probably explain these differences in development. The large size and voluminous salt deposits establish the Ottar Basin as one of the major Barents Sea evaporite basins and an important structural component of the Upper Palaeozoic rift system.  相似文献   

Morphology and stratigraphy from several deglaciations in the Transtrand Mountains,western Sweden     

Johan Kleman  Ingmar BorgstrM  Ann-Marie Robertsson  Marianne Lillieskld 《第四纪科学杂志》1992,7(1):1-17

  相似文献   

Measuring coastal cliff retreat in the Kongsfjorden area, Svalbard, using terrestrial photogrammetry     

Bjørn Wangensteen  Trond Eiken  Rune S. Ødegård  & Johan Ludvig Sollid 《Polar research》2007,26(1):14-21

  相似文献   

Accessory minerals of the Cínovec (Zinnwald) granite cupola, Czech Republic: indicators of petrogenetic evolution     

Z. Johan  V. Johan 《Mineralogy and Petrology》2005,83(1-2):113-150

Summary A fully cored drillhole was drilled to 1596m by the Czech Geological Survey in 1961–1963 in the central part of the Cínovec (Zinnwald) granite cupola. Two types of granite were intersected: zinnwaldite granite (ZG), observed down to a depth of 730m, and protolithionite granite (PG), occurring to the end of the hole. The core was used to study the distribution and chemistry of: zircon, thorite, xenotime, monazite, bastnäsite, synchysite, REE oxyfluorides and hydroxyfluorides. Zircon occurs throughout the drillcore; it is strongly hydrated and fluorinated with about 18.5wt.% H₂O content in the apical part of the cupola. Its F-content reaches 2.41wt.%. Within the PG, the F concentration in zircon is low. Zircon is poor in Th and U and its HfO₂ contents vary from 1.01 to 5.24wt.%. Thorite is common in the PG, becoming rare in the ZG. It is strongly hydrated (up to 14wt.% H₂O) and fluorinated (up to 2.04wt.% F). Extensive solid solution between ThSiO₄ and YPO₄ was observed. Xenotime is strongly hydrated (up to 16wt.% H₂O), but its F content is low (<0.31wt.%). Two types of monazite were identified: Th-rich (up to 9.3wt.% ThO₂) in the ZG, and Th-poor (<2.5wt.% ThO₂) in the PG. Monazite remained stable during the hydration and fluorination process. Its REE chondrite-normalized distribution patterns show negative anomalies for La and Nd and a pronounced negative anomaly for Eu. Chemical compositions of several REE oxyfluorides and hydroxyfluorides were studied. REE fluorocarbonates are represented by bastnäsite and synchysite. Bastnäsite is abundant in the ZG. Its chondrite-normalized REE patterns are characterized by an important negative Eu anomaly and downward kinks at La and Nd. Synchysite-(Ce) and synchysite-(Y) are particularly well developed in the deeper parts of the cupola, and exhibit REE distribution patterns characterized by a weak negative Eu anomaly (synchysite-(Ce)), or a weak positive Eu anomaly (synchysite-(Y)).The distribution of accessory minerals reveals five major evolution stages: (1) Early magmatic crystallization of albite and orthoclase. (2) A late magmatic stage comprising protolithionite, quartz, accessory zircon, thorite, xenotime and monazite. (3) Interaction of this magmatic association with a fluid phase rich in F, CO₂ and H₂O, leading to the transformation protolithionite zinnwaldite and to the remobilisation of Nb, Ta, Ti, W, Sn. Accessory minerals formed during stage (2) were hydrated and fluorinated, except monazite. (4) The transfer of volatiles into the apical part of the cupola followed by the opening of the magmatic system generated microgranites and hydrolysis-type reactions leading to the appearance of REE oxyfluorides and hydroxyfluorides. (5) A late CO₂- and F-rich fluid phase was responsible for the deposition of REE fluorocarbonates. Monazite and xenotime became unstable in the apical part of the cupola. An influx of fluids with high Ca-activity occurred late during stage (5) and led to the formation of synchysite, and finally to the extensive precipitation of fluorite.  相似文献   

Colloidal rare earth elements in a boreal river: Changing sources and distributions during the spring flood     

Karen Andersson  David Turner  Björn Stolpe  Johan Ingri 《Geochimica et cosmochimica acta》2006,70(13):3261-3274

Variations in the physico-chemical speciation of the rare earth elements (REE) have been investigated in a subarctic boreal river during an intense spring flood event using prefiltered (<100 μm) samples, cross-flow (ultra)filtration (CFF), flow field-flow fractionation (FlFFF), and diffusive gradients in thin films (DGT). This combination of techniques has provided new information regarding the release and transport of the REE in river water. The colloidal material can be described in terms of two fractions dominated by carbon and iron, respectively. These two fractions, termed colloidal carrier phases, showed significant temporal changes in concentration and size distribution. Before the spring flood, colloidal carbon concentrations were low, the colloids being dominated by relatively large iron colloids. Colloidal concentrations increased sharply during the spring flood, with smaller carbon colloids dominating. Following the spring flood, colloidal concentrations decreased again, smaller carbon colloids still dominating. The REE are transported mainly in the particulate and colloidal phases. Before the spring flood, the REE composition of all measured fractions was similar to local till. During the spring flood, the REE concentrations in the colloidal and particulate fractions increased. The increase was most marked for the lighter REE, which therefore showed a strong enrichment when normalized to local till. Following the spring flood, the REE concentrations decreased again and reverted to a distribution similar to local till. These changes in the concentration and distributions of carbon iron and REE are interpreted in terms of changing hydrological flow paths in soil and bedrock which occur during the spring flood.  相似文献   

Local mobility and the spatial dynamics of measles in a rural area of Kenya     

Alan Ferguson  Johan Leeuwenburg 《GeoJournal》1981,5(4):315-322

Population mobility at different scales is important in the analysis of contagious diseases. The role of local mobility among young children is related to spatial and temporal patterns of measles in part of rural Kenya. The dynamics of measles are examined over a period of 3 1/2 years, during which there were two epidemics; the path of the second of these epidemics is studied, using a spatial averaging technique (the centroid). Limited local mobility of children under the age of five, related to low levels of susceptibility provides a probable explanation of the slow diffusion of measles epidemics.This paper is reprinted from Rural Africana, Spring 1981, with the kind permission of the Editors.  相似文献   

Short gravity waves on steady non-uniform currents obtained through up-/downwelling     

Johan N. Hartnack 《Ocean Engineering》2000,27(4):831

Small amplitude water waves propagating in a medium with a steady non-uniform current are investigated. The non-uniform current is obtained by up- or downwelling through the horizontal bed. A new locally valid velocity potential correct to the second order is derived describing the combined wave–current motion. From this solution expressions for the local evolution of the wave amplitude and the wave number are extracted. These expressions are compared with the results found using the principle of wave action conservation and the linear dispersion relation, and good agreement is found at small distances compared to the wavelength. Unlike earlier works there is no restriction to deep water. The results valid for deep water are found as a special case of the general solution and agree with the solution found by Longuet-Higgins, M.S. and Stewart, R.W. (1961) The changes in amplitude of short gravity waves on steady non-uniform currents. Journal of Fluid Mechanics, 10(4), 529–549. Furthermore, it is shown that the principle of wave action conservation in fact holds for waves propagating in a medium with a steady non-uniform current maintained by up-/downwelling also on finite depth.  相似文献