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41.
Javier Fernández-Suárez Fernando Corfu Ricardo Arenas Alberto Marcos José R. Martínez Catalán Florentino García Jacobo Abati Francisco J. Fernández 《Contributions to Mineralogy and Petrology》2002,143(2):236-253
. A isotope dilution thermal ionisation mass spectrometry U-Pb geochronological study was carried out on the high-pressure and high-temperature units (HP-HT units) overlying the oceanic suture in the Allochthonous Complexes of the NW Iberian Variscan Belt. The rocks investigated are seven granulite- to eclogite-facies paragneisses and one leucosome within mafic high-pressure granulites in the Ordenes and Cabo Ortegal Complexes of NW Spain. U-Pb dating of zircon, monazite, titanite and rutile reveal the presence of a pervasive Early Ordovician metamorphic event at ca. 500-480 Ma and a later Early Devonian event at ca. 400-380 Ma. The U-Pb ages, in conjunction with petrological and structural data, indicate that the high-pressure event recorded by these rocks is Early Ordovician in age. Monazite ages in the paragneisses suggest that peak metamorphic conditions were reached at ca. 500-485 Ma. Subsequently, the rock ensemble underwent exhumation accompanied by partial melting and zircon growth at ca. 485-470 Ma. Melting of mafic granulites was coeval with this latter episode as indicated by zircon crystallisation age in the leucosomes dated at ca. 486 Ma. Based on these data and on the general features of magmatism and metamorphic evolution, it is proposed that this process took place at a convergent plate boundary within a peri-Gondwanan oceanic domain. Monazite, titanite and rutile data in some of the samples studied show evidence of a second metamorphic episode that took place between ca. 400 and 380 Ma (with a peak at ca. 390-385 Ma). This Early Devonian event, at variance with the previous one, was not pervasive, but, rather, was localised in areas of intense Variscan tectonothermal reworking. It is claimed that this later metamorphic event was recorded by the U-Pb system in areas where monazite and titanite growth was enhanced by fluid circulation in highly strained rocks (Variscan shear zones). According to previous structural studies and Ar-Ar dating of fabrics, this Early Devonian episode took place as the HP-HT units were deformed and thrusted upon the ophiolitic units in the early stages of the Variscan collision. 相似文献
42.
Christoph Gaedicke Hans-Ulrich Schlüter Hans Albert Roeser Alexander Prexl Bernd Schreckenberger Heinrich Meyer Christian Reichert Peter Clift Shahid Amjad 《Tectonophysics》2002,355(1-4)
The nature and origin of the sediments and crust of the Murray Ridge System and northern Indus Fan are discussed. The uppermost unit consists of Middle Miocene to recent channel–levee complexes typical of submarine fans. This unit is underlain by a second unit composed of hemipelagic to pelagic sediments deposited during the drift phase after the break-up of India–Seychelles–Africa. A predrift sequence of assumed Mesozoic age occurring only as observed above basement ridges is composed of highly consolidated rocks. Different types of the acoustic basement were detected, which reflection seismic pattern, magnetic anomalies and gravity field modeling indicate to be of continental character. The continental crust is extremely thinned in the northern Indus Fan, lacking a typical block-faulted structure. The Indian continent–ocean transition is marked on single MCS profiles by sequences of seaward-dipping reflectors (SDR). In the northwestern Arabian Sea, the Indian plate margin is characterized by several phases of volcanism and deformation revealed from interpretation of multichannel seismic profiles and magnetic anomalies. From this study, thinned continental crust spreads between the northern Murray Ridge System and India underneath the northern Indus Fan. 相似文献
43.
Pilar Drake Alberto M. Arias Francisco Baldó José A. Cuesta Antonio Rodríguez Alfonso Silva-Garcia Ignacio Sobrino Diego García-González Carlos Fernández-Delgado 《Estuaries and Coasts》2002,25(3):451-468
The aquatic macrofauna of the Guadalquivir estuary were sampled (1 mm mesh persiana net) at 5 sampling sites located along the entire (except the tidal freshwater region) estuarine gradient of salinity (outer 50 km). A total of 134 fish and macroinvertebrate species was collected but only 62 were considered common or regularly present in the estuary. Univariate measures of the community structure showed statistically significant differences among sampling sites: species richness, abundance, and biomass decreased in the upstream direction, being positively correlated with the salinity. Temporal differences of these three variables were also statistically significant. While a clear seasonal pattern (minimum densities in winter and maximum in spring-summer) was observed for abundance and biomass, no such pattern existed for the number of species. Mysids was the most dominant group throughout the estuary (96% to 99% of abundance; 49% to 85% of biomass), although fish biomass was also important at the outer estuary (36% to 38%). Multivariate analyses indicated highly significant spatial variation in the macrofaunal communities observed along the salinity gradient. These analyses suggest that the underlying structure was a continuum with more or less overlapping distributions of the species dependent on their ability to tolerate different physicochemical conditions. There were also significant temporal (intermonthly + interannual) variation of the estuarine community; the relative multivariate dispersion indicated that monthly variation was more considerable (relative multivariate dispersion >1) at the outer part of the estuary during the wet year (last 20 km) and was higher in the inner stations during the dry year (32 to 50 km from the river mouth). Since a clear negative exponential relationship was observed between the freshwater input (from a dam located 110 km upstream) and water salinity at all sampling stations, it is concluded that the human freshwater management is probably affecting the studied estuarine communities. While the higher seasonal (long-term) stability of the salinity gradient, due to the human control of the freshwater input, may facilitate the recruitment of marine species juveniles during the meteorologically unstable early-spring, the additional (short-term) salinity fluctuations during the warm period may negatively affect species that complete their lifecycle within the estuary. 相似文献
44.
Mohamed M. Hantush Rao. S. Govindaraju Miguel A. Marioand Zhonglong Zhang 《Journal of Hydrology》2002,260(1-4):58-74
This paper develops mass fraction models for transport and fate of agricultural pollutants in structured two-region soils. Mass fraction index models, based on a semi-infinite domain solution, are derived that describe leaching at depth, vapor losses through soil surface, absorption, and degradation in the dynamic- and stagnant-water soil regions. The models predict that leaching is the result of the combined effect of the upward vapor-phase transport relative to downward advection, residence time relative to half-life, dispersion, and lateral diffusive mass transfer. Simulations show that leached fraction of volatile compounds does not always decrease monotonically with increased residence time relative to the pollutant half-life, as a result of complex interactions among the different physical and biochemical processes. The results show that leaching, volatilization, and degradation losses can be affected significantly by lateral diffusive mass transfer into immobile-water regions and advection relative to dispersion (i.e. Peclet number) in the mobile-water regions. It is shown that solute diffusion into the immobile phase and subsequent biochemical decay reduces leaching and vapor losses through soil surface. Potential use of the modified leaching index for the screening of selected pesticides is illustrated for different soil textures and infiltration rates. The analysis may be useful to the management of pesticides and the design of landfills. 相似文献
45.
The same mathematical theory can be used to describe physical phenomena of different nature. For instance, the wave equation and the related mathematical developments can be used to describe elastic and electromagnetic wave propagation, and it is also extensively used in quantum mechanics. Fresnel's equations are a classical example of the analogy between shear waves and light waves. George Green in the nineteenth century, used analogies to obtain the reflection coefficients for sound waves and light waves, before the advent of the electromagnetic theory of light.In this work, we investigate the mathematical analogy between elastic waves and electromagentic waves. We obtain a complete parallelism for the reflection and refraction problem, considering the most general situation, that is, the presence of anisotropy and attenuation—viscosity in the elastic case and conductivity in the electromagnetic case. The analogy is illustrated with Fresnel's equations, the Brewster and critical angles, the concept of reflectivity and transmissivity, and the corresponding duals fields. The analysis of the elastic-solid theory of reflection applied by Green to light waves, and a brief historical review of wave propagation through the ether, further illustrate the analogy. 相似文献
46.
47.
Jean-Paul Vicat Hubert Mvondo Luc Willems André Pouclet 《Comptes Rendus Geoscience》2002,334(8):545-550
Numerous karstic features have been recognised in the non-carbonaceous micaschists and gneisses of the Yaoundé Pan-African nappe, south of Cameroon. It is shown that their formation was controlled by the structural features of the rocks. The wells and the pipes in the bedrock outcrops point out a current karstification process, resulting from the plagioclase dissolution by the acid rain waters. Hill wall alveoli and caves, of pre-Miocene age, are exhumed features that were done by dissolution in the aquiferous underground. To cite this article: J.-P. Vicat et al., C. R. Geoscience 334 (2002) 545–550. 相似文献
48.
49.
The Portil lagoon is a natural freshwater reservoir located at the southwest of Spain, near the coast. In its surroundings an important tourist complex has been developed since the earlier 1970s. This has resulted in increased loads of nutrients, specially during summer months. In order to evaluate the impact from humans on the lagoon caused by anthropogenic activities, we have determined vertical profiles of excess 210Pb and 137Cs in a sediment core taken from its bottom. Vertical profile of excess 210Pb seems to indicate that the sediment core was affected by large-scale mixing processes. Nevertheless this possibility was ruled out based on the shape of the 137Cs vertical distribution after modelling the 137Cs profile considering large-scale mixing. The chronology developed from 210Pb and 137Cs has allowed us to calculate the temporal evolution of the mass sedimentation rate during the last century. This mass sedimentation rate increased in 1973: from 0.08(2) g cm-2 a-1 in the period 1900-1973 to 0.17(5) g cm-2 a-1 in the period 1973-1995. This increase could be related to land-movements in the drainage area and to permitted sewage inputs into the lagoon, both with its origin in the growing/running of the surrounding tourist complex. 相似文献
50.
Daily geomagnetic variations on the Iberian Peninsula 总被引:2,自引:0,他引:2