Stratigraphy of the tectonically imbricated lithological succession of the Neves Corvo mine area,Iberian Pyrite Belt,Portugal     

J.?Tomás?Oliveira  Zélia?PereiraEmail author  Pedro?Carvalho  Nelson?Pacheco  Dieter?Korn 《Mineralium Deposita》2004,39(4):422-436

Biostratigraphic research, based on palynomorphs and ammonoids, of the tectonically imbricated lithological succession of the Neves Corvo mine, in the Portuguese part of the Iberian Pyrite Belt, has yielded ages for all formerly recognised lithostratigraphic units. These can be assembled in three main lithological sequences: (1) detrital sandy/shale substrate (Phyllite-Quartzite Formation) of late Famennian age; (2) Volcano-Sedimentary Complex, divided into a lower and an upper suite, in which one basic, three dolerite sills and four felsic volcanic units and a mineralised package of massive sulphides are identified with ages which range from the late Famennian to the late Visean; (3) flysch succession (Mértola Formation) composed of shale and greywacke dated as late Visean to early Serpukhovian. Precise biostratigraphic dating of the sedimentrary host rocks of massive sulphide mineralisation constrains the age of the latter to the late Strunian (354.8–354.0 Ma). Three stratigraphic hiatuses, corresponding to early/middle Strunian, Tournaisian and early Visean respectively and a south-westward progressive unconformity were also recognised. Sequences 1 and 2 are related to extensional episodes while sequence 3 marks the beginning of compressive tectonic inversion which gave rise to south-westward flysch progradation in close relation to a foreland basin development. Our results lead to the reinterpretation of the tectonic structure of the Neves Corvo mine, with implications for the interpretation of the regional basin dynamics and metal exploration.Editorial handling: F. Tornos  相似文献   

Saturn A ring surface mass densities from spiral density wave dispersion behavior     

Linda J. Spilker  Stuart Pilorz  Robert M. Nelson  Christopher T. Russell 《Icarus》2004,171(2):372-390

We have undertaken an analysis of the Voyager photopolarimeter (PPS) stellar occultation data of Saturn's A ring. The Voyager PPS observed the bright star δ Scorpii as it was occulted by Saturn's main rings during the spacecraft flyby of the Saturn system in 1981. The occultation measurement produced a ring profile with radial resolution of approximately 100 m, and radial structure is evident in the profile down to the resolution limit. We have applied an autoregressive technique to the data for estimating the power spectrum as a function of radius, which has allowed us to identify 40 spiral density waves in Saturn's A ring, associated with the strongest torques due to forcing from the moons. The majority of the detected waves are observed to disperse linearly in regions beginning a few kilometers from the resonance location. We have used the dispersion behavior for those waves to calculate local surface mass densities in the vicinity of each wave. We find that the inner three-quarters of the A ring (up to the beginning of the Encke gap) has an average surface mass density of , while the outer region has an average surface mass density of . The two regions have different mean surface mass densities with a significance of approximately 0.999993, as estimated with a T-statistic, which corresponds to about 4.5σ. While the mean optical depth of the A ring increases slightly with increasing distance from Saturn, we find that it is not significantly correlated with the surface mass density; the two quantities having a linear Pearson's correlation coefficient of r_corr≈−0.03. The variation of mass density, independent of PPS optical depth, is consistent with previous conjectures that the particle size distribution and composition are not constant across the entire A ring, particularly in the very outer portion. We estimate the mass of Saturn's A ring from our surface mass density estimates as 4.9×10²¹ gm, or 8.61×10⁻⁹ of the mass of Saturn, roughly equivalent to the mass of a 110-km diameter icy satellite. This mass is almost 25% smaller than estimates from previous studies, but is well within the expected errors of the derived mass densities. We also identified three previously unstudied features which exhibit linear dispersion. The strongest of these features is tentatively identified as the Janus 13:11 density wave. The other two features do not fall near any known satellite resonances and may represent density waves created by previously undetected satellites.  相似文献   

Peak displacement demand of small to moderate magnitude earthquakes in stable continental regions     

Nelson Lam  Adrian Chandler 《地震工程与结构动力学》2005,34(9):1047-1072

A theoretical fault‐slip model has been developed for predicting the notional peak displacement demand (PDD) of earthquakes based on a limiting natural period of 5 s, for application in stable continental regions (SCRs). The developed theoretical expression is simple and robust. Importantly, it envelops predictions arising from a number of existing empirical and seismological (stochastic) models included in the comparison. The notional PDD prediction has been made initially for hard rock crustal conditions and at a reference source–site distance of 30 km. Factors have accordingly been introduced to correct for different distances and geological conditions in completing the PDD prediction model. Assuming displacement‐controlled behaviour, the predicted notional PDD may be compared with the displacement capacity of a structure, or component, for purposes of seismic stability assessment. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

Cusp geometry in MHD simulations   总被引：2，自引：0，他引：2  

George Siscoe  Nancy Crooker  Keith Siebert  Nelson Maynard  Daniel Weimer  Willard White 《Surveys in Geophysics》2005,26(1-3):387-407

The MHD simulations described here show that the latitude of the high-altitude cusp decreases as the IMF swings from North to South, that there is a pronounced dawn–dusk asymmetry at high-altitude associated with a dawn–dusk component of the IMF, and that at the same time there is also a pronounced dawn–dusk asymmetry at low-altitude. The simulations generate a feature that represents what has been called the cleft. It appears as a tail (when the IMF has a B_y component) attached to the cusp, extending either toward the dawn flank or the dusk flank depending on the dawn–dusk orientation of the IMF. This one-sided cleft connects the cusp to the magnetospheric sash. We compare cusp geometry predicted by MHD simulations against published observations based on Hawkeye and DMSP data. Regarding the high-altitude predictions, the comparisons are not definitive, mainly because the observations are incomplete or mutually inconsistent. Regarding the low-altitude prediction of a strong dawn–dusk asymmetry, the observations are unambiguous and are in good qualitative agreement with the prediction.  相似文献   

Low temperature Phanerozoic history of the Northern Yilgarn Craton, Western Australia   总被引：1，自引：0，他引：1  

U.D. Weber  B.P. Kohn  A.J.W. Gleadow  D.R. Nelson 《Tectonophysics》2005,400(1-4):127-151

The Phanerozoic cooling history of the Western Australian Shield has been investigated using apatite fission track (AFT) thermochronology. AFT ages from the northern part of the Archaean Yilgarn Craton, Western Australia, primarily range between 200 and 280 Ma, with mean confined horizontal track lengths varying between 11.5 and 14.3 μm. Time–temperature modelling of the AFT data together with geological information suggest the onset of a regional cooling episode in the Late Carboniferous/Early Permian, which continued into Late Jurassic/Early Cretaceous time. Present-day heat flow measurements on the Western Australian Shield fall in the range of 40–50 mW m⁻². If the present day geothermal gradient of  18 ± 2 °C km⁻¹ is representative of average Phanerozoic gradients, then this implies a minimum of  50 °C of Late Palaeozoic to Mesozoic cooling. Assuming that cooling resulted from denudation, the data suggest the removal of at least 3 km of rock section from the northern Yilgarn Craton over this interval. The Perth Basin, located west of the Yilgarn Craton, contains up to 15 km of mostly Permian to Lower Cretaceous clastic sediment. However, published U–Pb data of detrital zircons from Permian and Lower Triassic basin strata show relatively few or no grains of Archaean age. This suggests that the recorded cooling can probably be attributed to the removal of a sedimentary cover rather than by denudation of material from the underlying craton itself. The onset of cooling is linked to tectonism related to either the waning stages of the Alice Springs Orogeny or to the early stages of Gondwana breakup.  相似文献   

Estimating non‐linear site response by single period approximation     

Hing‐Ho Tsang  Adrian M. Chandler  Nelson T. K. Lam 《地震工程与结构动力学》2006,35(9):1053-1076

Site effects characterize the filtering mechanisms within the soil sedimentary layers overlying bedrock. In regions of high seismicity such as California where strong motion records are relatively abundant, site coefficients can be developed by regression of recorded ground shaking parameters. In regions of low‐to‐moderate seismicity or of high seismicity but with a paucity of recorded strong motion data, such empirical models cannot be obtained in the same way. This study describes the theoretical development of a simple, rational manual procedure to calculate site coefficients, based on a single period approximation (SPA), and to construct displacement response spectra (RSD) for soil sites. The proposed simplified model, which takes into account the non‐linear behaviour of soil that is dependent on the level of shaking, impedance contrast at the soil–bedrock interface and the plasticity of soil material, has been verified by comparison with results obtained from non‐linear shear wave analyses and data recorded during the 1994 Northridge earthquake. The proposed model is believed to be a convenient tool for calculating non‐linear site responses and constructing site‐specific response spectra, which has the potential of being incorporated into code provisions. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

International Study on Artemia LVIII. Morphologic differentiation and reproductive isolation of Artemia populations from South America     

Gonzalo Gajardo  Nelson Colihueque  Margarita Parraguez  Patrick Sorgeloos 《International Journal of Salt Lake Research》1998,7(2):133-151

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Sediment diffusive fluxes of Fe,Mn, and P in a eutrophic lake: Contribution from lateral vs bottom sediments   总被引：3，自引：0，他引：3  

Nelson Belzile  Jaime Pizarro  Montserrat Filella  Jacques Buffle 《Aquatic Sciences - Research Across Boundaries》1996,58(4):327-354

Water column data and porewater profiles are used to study the chemical evolution with time and with depth of a eutrophic lake. By using different approaches, diffusion fluxes for dissolved iron, manganese and phosphate are calculated and used to describe the processes occurring at the sediment-water interface as well as in the hypolimnion of the lake. These data are used in the elaboration of a qualitative model to describe the chemical behaviour of the sedimentary interface of an anoxic lake with emphasis on the Fe/P/S system. Acorona model is proposed to explain the evolution with time of the diffusion process by estimating the relative contribution of bottom and lateral sediment surfaces to the total fluxes of dissolved elements diffusing from the sediment to the overlying water. As the hypolimnion becomes more anoxic, it has been observed that lateral sediment surfaces (16 to 10 meters in depth) represents a larger supplier of diffusing dissolved components than the bottom sediment portion (bottom to 18 meters).  相似文献   

Non-tropical carbonate deposits on the modern New Zealand shelf     

Campbell S. Nelson  Sandra L. Keane  Philip S. Head 《Sedimentary Geology》1988,60(1-4):71-94

Extensive (ca. 50,000 km²) shallow-marine platforms (< 250 m) off northern (34°S) and southern (48°S) New Zealand, and more local areas of shelf between, are blanketed by skeletal carbonate sediments > 70% CaCO₃), despite proximity to a tectonically active plate margin. In these regions the terrigenous sediment supply is presently low, and growth of epibenthos is fostered by firm substrates (rock, gravels, shells, seaweeds) and the generally energetic nature and high nutrient levels of open-shelf waters. Rapid transition into adjacent terrigenous-dominated facies is characteristic. Irrespective of water depth, the carbonates are coarse-grained and fragmental; carbonate mud is rare. Calcite dominates over aragonite. High-Mg calcite, widespread off northern New Zealand, is rare in the south. Skeletal material is dominated by bryozoans and bivalve molluscs, with significant local contributions from foraminifers, barnacles, calcareous red algae and echinoderms. The name bryomol is suggested for this distinctive temperate-region skeletal carbonate facies, which can be usefully subdivided based on dominant zoarial growth forms of the bryozoan component, known to be habitat-related. Bioerosion is an important mechanism of skeletal fragmentation and degradation. Many grains, especially aragontic bivalves, are infested by endolithic borers and have low preservation potential. Ages of skeletal material in the surficial deposits range from more than 20,000 years B.P. to modern, which is consistent with both low rates of carbonate production and sediment accumulation, and the wide range in preservation state of grains. Some data suggest that the skeletal carbonates are dispersed and mixed mainly during infrequent movement of sand ribbons, sand waves and sand sheets driven by storm-assisted tidal flows. Tracts of modern, palimpsest and relict carbonates can occur in juxtaposition.

The facies characteristics of the New Zealand shelf carbonate deposits contrast significantly with those of the classical Bahaman-type carbonate model. However, they are similar to those reported from many other mid- to high-latitude carbonate shelves, and afford good analogues for most onland occurrences of New Zealand Cenozoic limestones.  相似文献   

Dissimilatory Nitrate Reduction in a Waste-Water Contaminated Aquifer   总被引：1，自引：0，他引：1  

Paul R. Bulger  Alan E. Kehew  Richard A. Nelson 《Ground water》1989,27(5):664-671

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