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731.
Itziar Aretxaga Angeles I. Díaz Roberto Terlevich Elena Terlevich 《Astrophysics and Space Science》1990,171(1-2):61-64
In the starburst-warmers scenario, we have used a supernova (SN) explosion model to construct theoretical light curves for active galactic nuclei (AGN). The ionizing flux, in this model, should show variability according to the SN rate in the galaxy.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain. 相似文献
732.
The stable continental margin of northeastern Brazil is unusually narrow, probably because of the small size and tropical character of the drainage basins of the hinterland, and correspondingly low rates of land erosion and marine sedimentation. The continental shelf, which is mainly a marine erosion surface, is also remarkably shallow, either because of upwarping or, more probably, because of the ineffectiveness of Pleistocene marine erosional processes on steeply sloping continental margins. Sediment accumulation is confined to the Sāo Francisco delta, seaward of which are fossil (?) lagoonal deposits, and to a poorly developed nearshore sand prism.The margin formed by seaward progradation of sediment on a subsiding basement, but the present morphology of the continental slope reflects chiefly Pleistocene canyon cutting and mass gravitational movements of sediment, which have exposed older strata in the upper slope. Beneath the continental slope is a magnetic anomaly (like the slope anomaly off the eastern U.S.A.), probably caused by a deeply buried dike of oceanic basalt, and apparently associated with a buried ridge which may have formed the seaward margin of the Sergipe—Alagoas Basin during the early history of the South Atlantic. Similar structures may be typical of the narrow easternmost part of the Brazilian margin. 相似文献
733.
This paper presents the structural response of a fast and relatively small, composite materials catamaran to slamming loads. Finite element method is used to provide valuable information in order to optimise the design of the catamaran. The analysis is carried out using ANSYS 6.0 finite element software.The response of the structure to quasi-static slamming loads according to Det Norske Veritas High Speed and Light Craft crest landing and hollow landing rules [DNV, 1999] has been implemented and studied. An optimisation study for the structural response is carried out by changing the ply orientation in the vessel and suitable recommendations are made. 相似文献
734.
Benthic Community and Sediment Types: A Structural Analysis 总被引:2,自引:0,他引:2
Eugenio Fresi Maria Cristina Gambi Silvano Focardi Roberto Bargagli Franco Baldi Lucia Falciai 《Marine Ecology》1983,4(2):101-121
Abstract. The distribution of the macrobenthic community, in relation to various physical and chemical characteristics of the sediment, was studied along the coast of Southern Tuscany (Central Tyrrhenian) in an area under the influence of the Ombrone River outflow.
The zonation of the benthos was found to be primarily under the control of a gradient of water movement, as is reflected by variations in the sediment structure from the coast to the open sea. This correlation is somehow influenced by secondary gradients such as the degree of mixing of the sediment and the deposition of terrigenous materials along the river plume. 相似文献
The zonation of the benthos was found to be primarily under the control of a gradient of water movement, as is reflected by variations in the sediment structure from the coast to the open sea. This correlation is somehow influenced by secondary gradients such as the degree of mixing of the sediment and the deposition of terrigenous materials along the river plume. 相似文献
735.
736.
737.
738.
Giuseppe Raspa Massimiliano Moscatelli Francesco Stigliano Antonio Patera Fabrizio Marconi Daiane Folle Roberto Vallone Marco Mancini Gian Paolo Cavinato Salvatore Milli Joo Felipe Coimbra Leite Costa 《Engineering Geology》2008,101(3-4):251-268
We are presenting an attempt to evaluate the spatial variability of geotechnical parameters in the upper Pleistocene–Holocene alluvial deposits of Roma (Italy) by means of multivariate geostatistics.The upper Pleistocene–Holocene alluvial deposits of Roma are sensitive to high levels of geohazard. They occupy a sizable and significant part of the city, being the foundation for many monuments, historical neighborhoods, and archaeological areas, and the main host of the present and future subway lines. We have stored information from more than 2000 geotechnical boreholes crossing the alluvial deposits into a relational database. For the present study, only the boreholes with lithologic/textural interpretation and geotechnical information were selected. The set includes 283 boreholes and 719 samples, which have a set of geotechnical information comprising physical properties and mechanical parameters.Techniques of multivariate statistics and geostatistics were combined and compared to evaluate the estimation methods of the mechanical parameters, with special reference to the drained friction angle from direct shear test (φ′). Principal Component Analysis was applied to the dataset to highlight the relationships between the geotechnical parameters. Through cross-validation analysis, multiple linear regression, kriging, and cokriging were tested as estimators of φ′. Cross-validation demonstrates that the cokriging with granulometries as auxiliary variables is the most suitable method to estimate φ′. In addition to proving that cokriging is a good estimator of φ′, cross-validation demonstrates that input data are coherent and this allows us to use them for estimation of geotechnical parameters, although they come from different laboratories and different vintages.Nevertheless, to get the same good results of cross-validation in estimation, it is necessary for granulometries to be available at grid points. Since this information being not available at all grid points, it is expected that, in the future, textural information can be derived in an indirect way, i.e., from lithologic/textural spatial reconstructions. 相似文献
739.
Lena V. S. Monteiro Roberto P. Xavier Emerson R. de Carvalho Murray W. Hitzman Craig A. Johnson Carlos Roberto de Souza Filho Ignácio Torresi 《Mineralium Deposita》2008,43(2):129-159
The Sossego iron oxide–copper–gold deposit (245 Mt @ 1.1% Cu, 0.28 g/t Au) in the Carajás Mineral Province of Brazil consists
of two major groups of orebodies (Pista–Sequeirinho–Baiano and Sossego–Curral) with distinct alteration assemblages that are
separated from each other by a major high angle fault. The deposit is located along a regional WNW–ESE-striking shear zone
that defines the contact between metavolcano–sedimentary units of the ∼2.76 Ga Itacaiúnas Supergroup and tonalitic to trondhjemitic
gneisses and migmatites of the ∼2.8 Ga Xingu Complex. The deposit is hosted by granite, granophyric granite, gabbro, and felsic
metavolcanic rocks. The Pista–Sequeirinho–Baiano orebodies have undergone regional sodic (albite–hematite) alteration and
later sodic–calcic (actinolite-rich) alteration associated with the formation of massive magnetite–(apatite) bodies. Both
these alteration assemblages display ductile to ductile–brittle fabrics. They are cut by spatially restricted zones of potassic
(biotite and potassium feldspar) alteration that grades outward to chlorite-rich assemblages. The Sossego–Curral orebodies
contain weakly developed early albitic alteration and very poorly developed subsequent calcic–sodic alteration. These orebodies
contain well-developed potassic alteration assemblages that were formed during brittle deformation that resulted in the formation
of breccia bodies. Breccia matrix commonly displays coarse mineral infill suggestive of growth into open space. Sulfides in
both groups of deposits were precipitated first with potassic alteration and more importantly with a later assemblage of calcite–quartz–epidote–chlorite.
In the Sequeirinho orebodies, sulfides range from undeformed to deformed; sulfides in the Sossego–Curral orebodies are undeformed.
Very late, weakly mineralized hydrolytic alteration is present in the Sossego/Currral orebodies. The sulfide assemblage is
dominated by chalcopyrite with subsidiary siegenite, and millerite. Pyrrhotite and pyrite are minor constituents of ore in
the Sequerinho orebodies while pyrite is relatively abundant in the Sossego–Curral bodies. Oxygen isotope partitioning between
mineral pairs constrains temperatures in the deposit spatially and through time. In the Sequeirinho orebody, the early sodic–calcic
alteration stage was characterized by temperatures exceeding 500°C and values for the alteration fluid of 6.9 ± 0.9‰. Temperature declines outward and upward from the zone of most intense alteration.
Paragenetically later copper–gold mineralization displays markedly lower temperatures (<300°C) and was characterized by the
introduction of 18O-depleted hydrothermal fluids −1.8 ± 3.4‰. The calculated δDH2O and values suggest that the fluids that formed the early calcic–sodic alteration assemblage were of formational/metamorphic or
magmatic origin. The decrease of values through time may reflect influx of surficially derived waters during later alteration and mineralization events. Influx
of such fluids could be related to episodic fluid overpressure, resulting in dilution and cooling of the metalliferous fluid,
causing deposition of metals transported as metal chloride complexes. 相似文献
740.
Ana M. Dreher Roberto P. Xavier Bruce E. Taylor Sérgio L. Martini 《Mineralium Deposita》2008,43(2):161-184
The Igarapé Bahia Cu–Au deposit in the Carajás Province, Brazil, is hosted by steeply dipping metavolcano-sedimentary rocks
of the Igarapé Bahia Group. This group consists of a low greenschist grade unit of the Archean (∼2,750 Ma) Itacaiúnas Supergroup,
in which other important Cu–Au and iron ore deposits of the Carajás region are also hosted. The orebody at Igarapé Bahia is
a fragmental rock unit situated between chloritized basalt, with associated hyaloclastite, banded iron formation (BIF), and
chert in the footwall and mainly coarse- to fine-grained turbidites in the hanging wall. The fragmental rock unit is a nearly
concordant, 2 km long and 30–250 m thick orebody made up of heterolithic, usually matrix-supported rocks composed mainly of
coarse basalt, BIF, and chert clasts derived from the footwall unit. Mineralization is confined to the fine-grained matrix
and comprises disseminated to massive chalcopyrite accompanied by magnetite, gold, U- and light rare earth element (LREE)-minerals,
and minor other sulfides like bornite, molybdenite, cobaltite, digenite, and pyrite. Gangue minerals include siderite, chlorite,
amphibole, tourmaline, quartz, stilpnomelane, epidote, and apatite. A less important mineralization style at Igarapé Bahia
is represented by late quartz–chalcopyrite–calcite veins that crosscut all rocks in the deposit area. Fluid inclusions trapped
in a quartz cavity in the ore unit indicate that saline aqueous fluids (5 to 45 wt% NaCl + CaCl2 equiv), together with carbonic (CO2 ± CH4) and low-salinity aqueous carbonic (6 wt% NaCl equiv) fluids, were involved in the mineralization process. Carbonates from
the fragmental layer have δ13C values from −6.7 to −13.4 per mil that indicate their origin from organic and possibly also from magmatic carbon. The δ34S values for chalcopyrite range from −1.1 to 5.6 per mil with an outlier at −10.8 per mil, implying that most sulfur is magmatic
or leached from magmatic rocks, whereas a limited contribution of reduced and oxydized sulfur is also evident. Oxygen isotopic
ratios in magnetite, quartz, and siderite yield calculated temperatures of ∼400°C and δ18O-enriched compositions (5 to 16.5 per mil) for the ore-forming fluids that suggest a magmatic input and/or an interaction
with 18O-rich, probably sedimentary rocks. The late veins of the Igarapé Bahia deposit area were formed from saline aqueous fluids
(2 to 60 wt% NaCl + CaCl2 equiv) with δ18Ofluid compositions around 0 per mil that indicate contribution from meteoric fluids. With respect to geological features, Igarapé
Bahia bears similarity with syngenetic, volcanic-hosted massive sulfide (VHMS)-type deposits, as indicated by the volcano-sedimentary
geological context, stratabound character, and association with submarine volcanic flows, hyaloclastite, and exhalative beds
such as BIF and chert. On the other hand, the highly saline ore fluids and the mineral assemblage, dominated by magnetite
and chalcopyrite, with associated gold, U- and LREE-minerals and scarce pyrite, indicate that Igarapé Bahia belongs to the
Fe oxide Cu–Au (IOCG) group of deposits. The available geochronologic data used to attest syngenetic or epigenetic origins
for the mineralization are either imprecise or may not represent the main mineralization episode but a later, superimposed
event. The C, S, and O isotopic results obtained in this study do not clearly discriminate between fluid sources. However,
recent B isotope data obtained on tourmaline from the matrix of the fragmental rock ore unit (Xavier, Wiedenbeck, Dreher,
Rhede, Monteiro, Araújo, Chemical and boron isotopic composition of tourmaline from Archean and Paleoproterozoic Cu–Au deposits in the Carajás Mineral
Province, 1° Simpósio Brasileiro de Metalogenia, Gramado, Brazil, extended abstracts, CD-ROM, 2005) provide strong evidence of the
involvement of a marine evaporitic source in the hydrothermal system of Igarapé Bahia. Evaporite-derived fluids may explain
the high salinities and the low reduced sulfur mineral paragenesis observed in the deposit. Evaporite-derived fluids also
exclude a significant participation of magmatic or mantle-derived fluids, reinforcing the role of nonmagmatic brines in the
genesis of Igarapé Bahia. Considering this aspect and the geological features, the possibility that the deposit was generated
by a hydrothermal submarine system whose elevated salinity was acquired by leaching of ancient evaporite beds should be evaluated. 相似文献