Evidence for coupled reverse and normal active faulting in W Iberia: The Vidigueira–Moura and Alqueva faults (SE Portugal)     

A. Brum da Silveira  J. Cabral  H. Perea  A. Ribeiro 《Tectonophysics》2009,474(1-2):184

The Vidigueira–Moura fault (VMF) is a 65 km long, E–W trending, N dipping reverse left-lateral late Variscan structure located in SE Portugal (W Iberia), which has been reactivated during the Cenozoic with reverse right-lateral slip. It is intersected by, and interferes with the NE–SW trending Alentejo–Plasencia fault. East of this intersection, for a length of 40 km the VMF borders an intracratonic tectonic basin on its northern side, thrusting Paleozoic schists, meta-volcanics and granites, on the north, over Cenozoic continental sediments preserved in the basin, on the south. West of the faults intersection, evidence of Cenozoic reactivation is scarce. In the eastern sector, Plio-Quaternary VMF reactivation is indicated by geomorphologic, stratigraphic, and structural data, showing reverse movement with a right-lateral strike-slip component, in response to a NW–SE trending compressive stress. An average vertical displacement rate of 0.06 to 0.08 mm/yr since late Pliocene (roughly the last 2.5 Ma) is estimated. The Alqueva fault (AF) is a subparallel, northward dipping, 7.5 km long anastomosing fault zone that affects Palaeozoic basement rocks, and is located 2.5 km north and on the hanging block of the VMF. The AF is also a reverse left-lateral late Variscan structure, which has been reactivated during the Tertiary with reverse right-lateral slip; however, Plio-Quaternary reactivation was normal left-lateral, as shown by abundant kinematical criteria (slickensides) and geomorphic evidence. It shows an average displacement rate of 0.02 mm/yr for the vertical component of movement in the approximately last 2.5 Ma. It is proposed that the normal displacements on the AF result from tangential longitudinal strain on the upthrown block of the VMF above a convex ramp of this main reverse structure. According to this model of faults interaction, the AF is interpreted to work as a bending-moment fault sited above the VMF thrust ramp. Consequently, it is expected that the displacements on the AF increase towards the topographic surface with the increase in the imposed extension, declining downwards until they vanish above or at the VMF ramp. In order to constrain the proposed scheme, numerical modeling was performed, aiming at the reproduction of the present topography across the faults using different geodynamic models and fault geometries and displacements.  相似文献   

Mechanics of thick-skinned Variscan overprinting of Cadomian basement (Iberian Variscides)     

António Ribeiro  José Munhá  António Mateus  Paulo Fonseca  Eurico Pereira  Fernando Noronha  José Romão  José Rodrigues  Paulo Castro  Carlos Meireles  Narciso Ferreira 《Comptes Rendus Geoscience》2009,341(2-3):127-139

Remnants of the Cadomian basement can be found in the Iberian Variscides (IBVA) in several key sectors of its autochthonous units (composed of Neoproterozoic to Lower Palaeozoic metasedimentary sequences) and within the Continental Allochthonous Terrane (CAT). Comprehensive characterization of these critical exposures shows that the prevailing features are related to major geological events dated within the age range of 620–540 Ma. Indeed, near the Cambrian–Ordovician boundary, the IBVA Internal Zones experienced pervasive basement thinning and cover thickening, reflecting diffusive displacement of intracratonic rifting that continued until Lower Devonian times. In the thick-skinned Internal Zones, Helvetic/Penninic style nappes were generated, whereas flower upright axial structures developed along transpressive, intraplate shear zones. These features contrast with those preserved in the thin-skinned IBVA External Zones, dominated by décollements above (un-)deformed Palaeozoic and Cadomian basement. The inferred attenuation of rheological contrast between Cadomian basement and Palaeozoic cover can be explained by inherited fabrics due to thermal softening operated during the Cambrian–Lower Devonian extensional regime. Deeper décollements (and subsequent strain partitioning) are also expected to develop at the upper-lower crust (and at the Moho?) transition, as imaged by the available seismic profiling and MT surveys. The whole data implies a significant discontinuity between Cadomian and Variscan Cycles that should have constrained subsequent lithospheric evolution.  相似文献   

Observations of the spring–neap modulation of the gravitational circulation in a partially mixed estuary     

C. H. A. Ribeiro  J. J. Waniek  J. Sharples 《Ocean Dynamics》2004,54(3-4):299-306

The long-term variability of the non-tidal circulation in Southampton Water, a partially mixed estuary, was investigated using 71-day acoustic Doppler current profiler (ADCP) time series. The data show evidence that the spring–neap tidal variability of the turbulent mixing modulates the strength of the non-tidal residual circulation, with subtidal neap tide surface flows reaching 0.12 m s^–1 compared to <0.05 m s^–1 at spring tides. The amplitude of the neap-tide events in this non-tidal circulation is shown to be related to a critical value of the tidal currents, illustrating the strong dependence on tidal mixing. The results suggest that the dominant mechanism for generating these neap-tide circulation events is the baroclinic forcing of the horizontal density gradient, rather than barotropic forcing associated with ebb-induced periodic stratification. While tidal turbulence is thought to be the dominant control on this gravitational circulation, there is evidence of the additional effect of wind-driven mixing, including the effects of wind fetch and possibly wave development with along-estuary winds being more efficient at mixing the estuary than across-estuary winds. Rapid changes in atmospheric pressure also coincided with fluctuations in the gravitational circulation. The observed subtidal flows are shown to be capable of rapidly flushing buoyant material out of the estuary and into the coastal sea at neap tides.Responsible Editor: Iris Grabemann  相似文献   

The Azambuja fault: An active structure located in an intraplate basin with significant seismicity (Lower Tagus Valley, Portugal)   总被引：2，自引：0，他引：2  

J. Cabral  P. Ribeiro  P. Figueiredo  N. Pimentel  A. Martins 《Journal of Seismology》2004,8(3):347-362

The Azambuja fault is a NNE trending structure located 50 km NE of Lisbon, in an area of important historical seismicity. It is sited in the Lower Tagus Basin, a compressive foredeep basin related to tectonic inversion of the Mesozoic Lusitanian Basin in the Miocene. The fault is evident in commercial seismic reflection data, where it shows steep thrust geometry downthrowing the Cenozoic sediments to the east. It has also a clear morphological signature, presenting a NNE-SSW trending, east facing, 15 km long scarp, reaching a maximum height of 80 m. The fault scarp is the geomorphic appearance of a flexure expressed as a zone of distributed deformation, where Miocene and Pliocene sediments are tilted eastwards and are cut by steeply dipping meso-scale faults presenting reverse and normal offsets, with a net downthrow to the east. This pattern at the surface is compatible with a steep fault in the basement that tilts and branches through the overlying Cenozoic sedimentary cover. In order to constrain the neotectonic activity of this structure, detailed geological studies were conducted. Morphotectonics was studied through aerial photo interpretation, analysis of topographic maps and digital mapping. Those studies indicate Quaternary slip on the fault in the ranges of 0.05–0.06 mm per year. Seismogenic behaviour was assumed for the Azambuja fault based on the evidence of Quaternary tectonic activity and its location in an area of significant historical seismicity. M _w 6.4–6.7 maximum earthquakes, with recurrence intervals of 10000–25000 years, were estimated based upon the displaced morphological references, cumulative offsets and fault length.  相似文献   

Corrigendum to “Vegetation dynamics during the late Pleistocene in the Barreirinhas region, Maranhão State, northeastern Brazil, based on carbon isotopes in soil organic matter” [Quaternary Research 62 (2004) 183-193]     

Luiz Carlos Ruiz Pessenda  Adauto de Souza Ribeiro  Ramon Aravena  José Albertino Bendassolli 《Quaternary Research》2005,64(1):112

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Freshwater-saltwater interface and estuarine sediment in situ assays based on post-exposure feeding of chironomids and polychaetes     

Sara Soares  Susana Moreira  Sofia Andrade  Lúcia Guilhermino  Rui Ribeiro 《Estuaries and Coasts》2005,28(2):314-319

An integrated approach for ecological and ecotoxicological sediment assessments along the freshwater saltwater interface (FSI) and estuarine areas in general was developed, through the adaptation of recently proposed methods for the midge larvaChironomus riparius (an in situ assay based on post-exposure feeding for sediment toxicity assessments in freshwater systems) and the development of new techniques for the polychaeteHediste (Nereis) diversicolor (an in situ post-exposure feeding assay). Based on the previously proposed methods, improvements were made to the feeding quantification ofC. riparius fourth instar larvae. A method to quantify the post-exposure feeding ofH. diversicolor was developed and optimized under laboratory conditions using newly hatchedArtemia franciscana nauplii as a food source. Assay chambers and procedures fromC. riparius assays were easily adapted forH. diversicolor in situ exposures. Sediment assays using this integrated approach were successfully performed at an uncontaminated river-estuarine system. They consisted of a 48-h exposure followed by a 1-h post-exposure feeding for chironomids and a 2-h post-exposure feeding for polychaetes. Good recovery rates were obtained after the 48-h field exposure. Fluctuations in various physicochemical variables at the FSI, due to the tidal regime, did not significantly influenceC. riparius larval post-exposure feeding rates, suggesting that uncontaminated freshwater sites located upstream of FSI areas can be used as reference sites forC. riparius in situ evaluations in estuaries. Adapted assay chambers and procedures were suitable forH. diversicolor in situ exposures.  相似文献   

Uberabasuchus terrificus sp. nov., a New Crocodylomorpha from the Bauru Basin (Upper Cretaceous), Brazil     

Ismar de Souza Carvalho  Luiz Carlos Borges Ribeiro  Leonardo dos Santos Avilla   《Gondwana Research》2004,7(4):975-1002

A new fairly complete and articulated skull of a Peirosauridae crocodylomorph from Bauru Basin (Late Cretaceous), Brazil, is described. The fossil is from a level of clayish sandstone within Serra do Veadinho sequence, Peirópolis, Uberaba County, Minas Gerais State. The sedimentary strata of Serra do Veadinho belong to the Marília Formation (Serra da Galga Member), Bauru Group, considered to be Campanian-Maastrichtian in age. The species -Uberabasuchus terrificus sp. nov. - is a peirosaurid with moderately narrow snout, large round orbits protected by supraorbital bones of triangular shape and an antorbital fenestra bounded posteriorly by a deep groove. This fossil resembles Peirosaurus tormini Price, 1955 in the size pattern of premaxillary teeth and by showing a similar wedge-like maxillary process in the premaxilla. It also shares some morphological features with the other species of the Peirosauridae, namely the crocodylomorph Lomasuchus palpebrosus Gasparini, Chiappe and Fernandez, 1991 from Argentina. Their common features comprise a moderately narrow snout and the deep lateral groove at the premaxilla and maxilla articulation for the reception of a large mandibular tooth. However, the nasal participates in the external nares and does not divide the nasal aperture, producing a “beak-like” structure at the extremity of the snout which is unique among peirosaurids. The stratigraphic setting suggests that the specimen was buried when a flash flood overflowed the shallow channels of a braided fluvial system.

Parsimony analysis of 183 morphological characters is performed for 23 crocodylomorphs. Analysis of the morphological data matrix resulted in three most parsimonious trees (374 steps, CI = 0.679; RI = 0.826). The new species is closely related to Mahajangasuchus and both, in addition to Peirosaurus and Lomasuchus, compose the Peirosauridae.  相似文献   

Color centers in topaz: comparison between neutron and gamma irradiation     

K. Krambrock  L. G. M. Ribeiro  M. V. B. Pinheiro  A. S. Leal  M. Â. de B. C. Menezes  J. -M. Spaeth 《Physics and Chemistry of Minerals》2007,34(7):437-444

Neutron- and gamma irradiation-induced paramagnetic centers in natural colorless topaz from four different Brazilian localities were studied by electron paramagnetic resonance (EPR) and optical absorption as a function of irradiation dose and thermal treatment. Gamma irradiation doses were applied up to 1,000 kGy with a dose rate of 15 kGy h⁻¹. For the neutron irradiation experiments, a neutron flux of 4 × 10¹² cm⁻² s⁻¹ was used with an integrated flux of up to 1 × 10¹⁸ cm⁻². From the experiments, it is concluded that brownish colors are induced by gamma-rays and may be associated with a single broad isotropic EPR line with g = 2.015(2). Both the EPR line and the related optical absorption band at 460 nm (2.7 eV) are lost during thermal treatments between 150 and 200°C. Fast neutrons create the paramagnetic peroxyl radicals O₂ ⁻ and the paramagnetic O⁻ hole centers. The O⁻ centers have the same thermal stability as the optical absorption band at 620 nm (2.0 eV). It is confirmed that the absorption due to the O⁻ center is responsible for the blue color in topaz. Both color centers and their absorption bands are discussed in the context of O⁻ bound small polarons.  相似文献   

Tebuthiuron leaching in three Brazilian soils as affected by soil pH     

Autieres Teixeira Faria  Matheus Freitas Souza  Ana Beatriz Rocha de Jesus Passos  Antonio Alberto da Silva  Daniel Valadão Silva  José Cola Zanuncio  Paulo Roberto Ribeiro Rocha 《Environmental Earth Sciences》2018,77(5):214

Underground contamination water by herbicides depends on the interactions between their molecules with physical and chemical soil characteristics and climatic conditions. Studies with columns can estimate the leaching potential of herbicides in soils. This study aimed to determine the effect of soil pH on tebuthiuron leaching, and capacity of bioindicators to detect tebuthiuron residues in three Brazilian soils. Cucumber plants (Cucumis sativus) were more negatively affected when grown in soils with lower amounts of organic matter and clay, and in these soils, the tebuthiuron levels reached greater depths in the column. There was a positive correlation between tebuthiuron concentration and cucumber intoxication, and a negative correlation between tebuthiuron concentration and dry matter cucumber in all soils. The tebuthiuron leached up to 50 cm depth even in soils with higher organic matter and clay content. The increasing of soil pH can affect the leaching of nonionic herbicides, and liming practice may elevate the environmental contamination risk by tebuthiuron. The bioindicator method using Cucumis sativus is viable and can be recommended to detect tebuthiuron concentrations above 0.2 mg kg^?1.  相似文献   

Vulnerability to contamination by phosphorus in a zero-order basin with a high density of pigs and a history of slurry addition: extrapolation of an index     

Rafael da Rosa Couto  Luiz Carloz Pittol Martini  Luciano Colpo Gatiboni  Paulo Belli Filho  Sérgio Roberto Martins  Cleiton Junior Ribeiro Lazzari  Vilmar Müller Júnior  Jucinei José Comin  Paul John Anthony Withers  Ricardo Bergamo Schenato  Gustavo Brunetto 《Environmental Earth Sciences》2018,77(4):144

The application of phosphorus (P)-index methodology to assess the risk of water pollution from agriculture in river basins usually takes time and expends a great amount of resources. This study adapted the P-index methodology using digital mapping of key basin risk criteria for P loss in reference areas to evaluate the wider vulnerability to P loss in a zero-order basin with an intense concentration of pigs and with a history of pig slurry additions to the soil. The P content of eleven reference areas, where ten areas have received various applications of pig slurry and one area has no history of addition, was extrapolated to a zero-order basin using principal component analysis and analysis of hierarchical groupings. Estimated loss of soil and the distance between the P source and the watercourse were mapped using satellite images and in situ evaluations. The methodology of extrapolating was well correlated with a survey of P concentrations in land runoff and provides the potential to improve land management of those areas that are most vulnerable. Principal component analysis and hierarchical clustering analysis proved to be useful tools for the separation of areas with different slurry application times. Our results suggest that the P loss risk could be reduced through the adoption of soil conservation techniques such as reduced-tillage incorporating slurry and crop residue management to provide adequate soil protection, reduce the impact of raindrops and reduce erosion risk and transport of chemical pollutants to water bodies.  相似文献