Deep conductivity structure beneath the northern Brasília belt,central Brazil: Evidence for a Neoproterozoic arc-continent collision     

Antonio L. Padilha  Icaro Vitorello  Marcelo B. Pádua 《Gondwana Research》2013,23(2):748-758

The Brasília belt, in central Brazil, experienced a broad range of tectonic events, including several periods of compressional and extensional deformations during the Proterozoic. Magnetotelluric data were collected at 27 sites along a profile crossing the northern part of the belt. Strike analysis and distortion decomposition indicated that most of the data are two-dimensional (2D) and fit a regional geoelectric strike angle of N20E, parallel to the tectonic grain. 2D conductivity model is constrained by available gravity and seismic refraction data and reveals the cumulative effects of the multiple tectonomagmatic episodes that affected the area in the past on current crustal and upper mantle structure. The results show that the resistive lithospheric root of the São Francisco cratonic plate is restricted to the easternmost part of the profile and does not extend westward beneath the Brasília belt. The external zone of the fold belt, composed of slightly metamorphosed rocks at the surface, presents crustal conductors that can be tentatively interpreted as regions with fossil fluid flows at mid-crust depth beneath a gold mineralization zone and as a very shallow mineralized zone within the metamorphic rocks. The external zone is separated to the west from the Goiás massif by a strong conductivity contrast, indicative of a suture zone along the Rio Maranhão fault system. Crustal conductivity structure under the Niquelândia complex is distinct from what is observed in other parts of the Goiás massif, in agreement with the proposal of the complex being an allochthonous block. In spite of the large tectonothermal events that affected the Goiás magmatic arc there are no significant conductive anomalies at crustal depths in its eastern side. The most prominent feature detected in the geoelectric model is a huge high conductivity anomaly below the entire Goiás massif in the central part of the belt, mainly concentrated at the uppermost mantle but also affecting mid to lower crustal depths. It can be interpreted as associated with thermal events either related to Mesoproterozoic and/or Neoproterozoic continental rifting processes or to delamination and detachment processes at the end of the evolution of the belt, in the Neoproterozoic. Effects related to oceanic lithosphere subduction during the collision and accretion of the Goiás massif and Goiás magmatic arc to the western passive margin of the São Francisco cratonic plate in the Neoproterozoic may alternatively explain the anomaly.  相似文献   

Sediment mobility and bed armoring in the St Clair River: insights from hydrodynamic modeling     

Xiaofeng Liu  Gary Parker  Jonathan A. Czuba  Kevin Oberg  Jose M. Mier  James L. Best  Daniel R. Parsons  Peter Ashmore  Bommanna G. Krishnappan  Marcelo H. Garcia 《地球表面变化过程与地形》2012,37(9):957-970

The lake levels in Lake Michigan‐Huron have recently fallen to near historical lows, as has the elevation difference between Lake Michigan‐Huron compared to Lake Erie. This decline in lake levels has the potential to cause detrimental impacts on the lake ecosystems, together with social and economic impacts on communities in the entire Great Lakes region. Results from past work suggest that morphological changes in the St Clair River, which is the only natural outlet for Lake Michigan‐Huron, could be an appreciable factor in the recent trends of lake level decline. A key research question is whether bed erosion within the river has caused an increase in water conveyance, therefore, contributed to the falling lake level. In this paper, a numerical modeling approach with field data is used to investigate the possibility of sediment movement in the St Clair River and assess the likelihood of morphological change under the current flow regime. A two‐dimensional numerical model was used to study flow structure, bed shear stress, and sediment mobility/armoring over a range of flow discharges. Boundary conditions for the numerical model were provided by detailed field measurements that included high‐resolution bathymetry and three‐dimensional flow velocities. The results indicate that, without considering other effects, under the current range of flow conditions, the shear stresses produced by the river flow are too low to transport most of the coarse bed sediment within the reach and are too low to cause substantial bed erosion or bed scour. However, the detailed maps of the bed show mobile bedforms in the upper St Clair River that are indicative of sediment transport. Relatively high shear stresses near a constriction at the upstream end of the river and at channel bends could cause local scour and deposition. Ship‐induced propeller wake erosion also is a likely cause of sediment movement in the entire reach. Other factors that may promote sediment movement, such as ice cover and dredging in the lower river, require further investigation. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

A parsimonious crop-water productivity index: an application to Brazil     

Marco P Maneta  Purnendu N Singh  Marcelo Torres  Wesley W Wallender  Stephen A Vosti  Lineu N Rodrigues  Luis H Bassoi  Julie A Young 《Area》2009,41(1):94-106

Reducing poverty in rural areas of developing countries requires sustained and sustainable increases in agricultural water productivity. However, aside from traditional measures of precipitation, little is known about water available to farmers or how productively they use it. We present a crop-water productivity index (a ratio of the value of annual crop production to a dimensionless potential water availability index) for large water basins using readily available low-resolution data. The index is transferable, permits direct inter-basin comparisons, and is simple to calculate. We calculate the index for each municipality in the São Francisco river basin in Brazil. No clear patterns linking water availability and value of agricultural output are evident, even though clusters of municípios with high- and low-crop-water productivity emerge, and the former may be useful in guiding policies aimed at increasing water productivity. Finally, analyses of the effects of information uncertainty on the crop-water productivity index suggest that the returns to agricultural investments in certain places in the São Francisco river basin are more risky than others. Improvements in data quality and quantity can help refine estimates of the index and reduce their uncertainty.  相似文献   

Spatial and temporal variability of seawater properties,current velocity and SPM concentration off Cassino Beach-Rio Grande-Southern Brazil     

Josefa V. Guerra  Marcelo M. Azevedo  Luciana S. Esteves  Susana B. Vinzon  Nelson Violante-Carvalho  Carlos Augusto F. Schettini  Roberto F. Oliveira  Aleksandro R. Zaleski 《Continental Shelf Research》2009

Water column profiles and near-bed time series of pressure, current velocity, suspended-particulate matter (SPM) concentration and seawater temperature and salinity were collected during three short cruises carried out in May 2005 in the shoreface and inner shelf area adjacent to Cassino Beach, southern Brazil. The measurements were part of the Cassino Experiment, a project conducted at an open, sandy coastal area known for the occurrence of patches of fairly large amounts of muddy sediments that are sporadically fluidized, transported onshore and eventually stranded on the beach. The study area is close to the Patos Lagoon mouth, being influenced by its water and suspended-sediment discharge. The presence of the Patos Lagoon outflow on the inner shelf was detected in one of the cruises (May 13) through measurements of near-surface salinity: while close to shore salinity was 29.4, a minimum value of 13.8 was measured at ∼10 km from the coast. Four days later, no trace of the plume was detected in the area. Regarding seawater temperature, no large temporal or spatial variability was documented with measured values ranging from 19.3 to 20 °C. Water column currents were prominently to N and NE, except at the outermost station, located ∼42 km from the coast, where NW-directed flows were observed at surface and mid-depth. Maximum near-bed current velocity oscillated between 18 and 42 cm s⁻¹ in the east–west direction and between 14 and 42 cm s⁻¹ in the north–south direction. Near-surface concentration of SPM oscillated between 11 and 99 mg L⁻¹, in general one order of magnitude lower than near-bed values. However, near-bed concentration of SPM showed large spatial variability: the highest value (2200 mg L⁻¹) was yielded by a water sample collected at ∼8 m water depth, at a station located ∼2 km away from the shoreline; two water samples collected 500 m, apart from this station, yielded SPM concentrations of 148 and 205 mg L⁻¹, one order of magnitude lower. Spectral analyses of near-bed current speed and SPM concentration indicate the relevance of oscillations in the low-frequency (<0.05 Hz) range. Detailed sampling of bottom sediment indicated that in May 2005 the mud patch was centered at ∼8.5 m water depth.  相似文献   

Subantarctic macrotidal flats, cheniers and beaches in San Sebastian Bay, Tierra Del Fuego, Argentina   总被引：6，自引：0，他引：6  

Federico Vilas  Alfredo Arche  Marcelo Ferrero  Federico Isla 《Marine Geology》1999,160(3-4):301-326

San Sebastian Bay is a large, semicircular coastal embayment situated on the Atlantic coast of Tierra del Fuego, Argentina. It is a high-energy, subantarctic environment with a tidal range of 10.4 m, influenced by large waves of Atlantic and local origin and swept by constant, strong westerly winds. A 17 km long gravel spit protects the Northern part of the Bay giving rise to a gradient in sedimentary processes. From south to north, are seven distinct sedimentary environments. Coastal sedimentation started at least 5200 years before present (BP) and a rapid progradation related to a relative sea-level drop has infilled about 6 kilometres of the Bay with a sedimentary sequence 10–11 m thick. 14-C dating of unabraded shells in the Chenier ridges of the relict part of the complex allows for a precise reconstruction of the stages of the progradation, that has slowed from 2.35 m/year at 5000 years BP to 0.6 m/year at present. The possible causes of the sea-level drop are discussed.  相似文献   

A Tentative Model for U(1) GAUGE VORTEX     

Marcelo de Oliveira Souza  A.A. Morégula  M.M. Som 《Astrophysics and Space Science》1997,249(2):295-301

Some observations on the flat space-time with topological defects are made. An example of a model source that can be approximated by a localU(1) gauge string with Bogomol'nyi parameter equal to unity and winding number equal to 1 is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

A comprehensive study of reported high-metallicity giant H ii regions – I. Detailed abundance analysis     

Marcelo Castellanos  Angeles I. Díaz  Elena Terlevich † 《Monthly notices of the Royal Astronomical Society》2002,329(2):315-335

We present long-slit observations in the optical and near-infrared of 14 H  ii regions in the spiral galaxies NGC 628, 925, 1232 and 1637, all of them reported to have solar or oversolar abundances according to empirical calibrations. For seven of the observed regions, ion-weighted temperatures from optical forbidden auroral to nebular line ratios are obtained and, for six of them, the oxygen abundances derived by standard methods turn out to be significantly lower than solar. The other one, named CDT1 in NGC 1232, shows an oxygen abundance of     , and constitutes, to the best of our knowledge, the first high-metallicity H  ii region for which accurate line temperatures, and hence elemental abundances, have been derived.
For the rest of the regions no line temperature measurements could be made, and the metallicity has been determined by means of both detailed photoionization modelling and the sulphur abundance parameter S ₂₃. Only one of these regions shows values of O ₂₃ and S ₂₃ implying a solar or oversolar metallicity.
According to our analysis, only two of the observed regions can therefore be considered as of high metallicity. These two fit the trends previously found in other high-metallicity H  ii regions, i.e., N/O and S/O abundance ratios seem to be higher and lower than solar respectively.  相似文献   

On unstable ship motions resulting from strong non-linear coupling   总被引：1，自引：0，他引：1  

Marcelo A.S. Neves  Claudio A. Rodríguez 《Ocean Engineering》2006,33(14-15):1853-1883

In this paper, the modelling of strong parametric resonance in head seas is investigated. Non-linear equations of ship motions in waves describing the couplings between heave, roll and pitch are contemplated. A third-order mathematical model is introduced, aimed at describing strong parametric excitation associated with cyclic changes of the ship restoring characteristics. A derivative model is employed to describe the coupled restoring actions up to third order. Non-linear coupling coefficients are analytically derived in terms of hull form characteristics.The main theoretical aspects of the new model are discussed. Numerical simulations obtained from the derived third-order non-linear mathematical model are compared to experimental results, corresponding to excessive motions of the model of a transom stern fishing vessel in head seas. It is shown that this enhanced model gives very realistic results and a much better comparison with the experiments than a second-order model.  相似文献   

Population Dynamics and Secondary Production of Emerita brasiliensis (Crustacea: Hippidae) at Prainha Beach, Brazil   总被引：1，自引：0，他引：1  

Marcelo Petracco  Valeria Gomes Veloso  & Ricardo Silva Cardoso 《Marine Ecology》2003,24(3):231-245

Abstract. The secondary production and population dynamics of the mole crab Emerita brasiliensis Schmitt, 1935 (Decapoda: Hippidae) were studied by taking monthly samples from June 1993 to May 1995 at each of three intertidal transects at Prainha beach, Brazil. The lifespan was ca. 8 months for males and females, but females showed higher growth, mortality, secondary production, and turnover rate. The higher production in spring versus autumn and winter was related to intense recruitment during that period. The population production was estimated at between 39.86 and 46.88 g (AFDW) · m^?2 · a^?1 for the first year (June 93–May 94) and between 150.95 and 156.07 g (AFDW) · m^?2 · a^?1 for the second year (June 94–May 95); the mean annual biomass was 4.91 and 23.09 g (AFDW) · m^?2, respectively. High P/B rates, between ca. 6 and 9 · a^?1, reflected the fast growth, high mortality, and low lifespan of the population, characterized by a high percentage of recently recruited individuals.  相似文献   

Similar potential ATP-P production and enzymatic activities in the microplankton community off Concepción (Chile) under oxic and suboxic conditions     

Rodrigo R. González  Marcelo H. Gutiérrez 《Progress in Oceanography》2007,75(3):550-560

The effects of the oxygen minimum zone on the metabolism of the heterotrophic microplankton community (0.22-100 μm) in the Humboldt Current System, as well as the factors controlling its biomass production, remain unknown. Here we compare the effect of four sources of dissolved organic carbon (glucose, oxaloacetate, glycine, leucine) on microbial biomass production (such as ATP-P) and the potential enzymatic activities involved in catabolic pathways under oxic and suboxic conditions. Our results show significant differences (p < 0.05) in the ATP-P production when induced by the different substrates that are used as dissolved organic carbon herein. The induction of ATP-P production is enhanced from glucose < oxaloacetate < glycine < leucine. Nevertheless, for individual substrates, no significant differences were found between incubation under oxic and suboxic conditions except in the case of leucine. For this amino acid, the induction of ATP-P synthesis was higher under suboxic than oxic conditions. The data sets of all the substrates used showed greater potential ATP-P production under suboxic than oxic conditions. The results of the potential enzymatic activities suggest that malate dehydrogenase has the highest signal of NADH oxidization activity in the microbial assemblage. Furthermore, for all experiments, the malate dehydrogenase activity data set had a significant relationship with ATP-P production. These findings suggest that the microbial community inhabiting the oxygen minimum zone has the same or greater potential growth than the community inhabiting more oxygenated strata of the water column and that malate dehydrogenase is the activity that best represents the metabolic potential of the community.  相似文献