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411.
A. Castro C. Fernández H. El-Hmidi M. El-Biad M. Díaz J. de la Rosa F. Stuart 《International Journal of Earth Sciences》1999,88(1):26-37
The Aracena metamorphic belt (AMB), southwest Iberian peninsula, is characterized by the following geological elements: (a)
a high-temperature/low-pressure (HT/LP) metamorphic belt a few kilometres wide and more than 200 km long; (b) a linear belt
of oceanic amphibolites with a low-pressure inverted metamorphic gradient; (c) crustal-scale ductile shear zones; and (d)
mafic, noritic intrusions of high-Mg andesite (boninite) composition. The relationships between these elements led to the
proposal of a model of ridge subduction for this sector of the Hercynian belt of Europe. This interpretation is supported
by the age relationships displayed between the main rock units considered representative of the main tectonic and petrological
processes responsible for the geological elements mentioned previously. The results of a geochronological study (Ar–Ar, Rb–Sr
and Sm–Nd) clearly support a Late Paleozoic tectonic evolution at an active continental margin. The time evolution of the
metamorphism in the oceanic domain, ranging from 342.6±0.6 Ma in the west to 328.4±1.2 Ma in the east, over a distance of
70 km along the metamorphic belt, support a tectonic model of triple-junction migration responsible for the creation at depth
of a slab-free window with decisive consequences for the thermal evolution of the region. The origin of the linear metamorphic
belt of HT/LP regime may be explained by the migration along a continental margin of a punctual thermal anomaly induced by
the creation of a triple-junction at the continental margin.
Received: 9 March 1998 / Accepted: 9 December 1998 相似文献
412.
Three types of base metal deposits occur in the Devonian of the Central Pyrenees accompanying and post-dating the Hercynian
orogeny. Since spatial and temporal relationships between mineralization can be recognized in the Yenefrito area, they have
been incorporated into a metallogenetic model. Sedex-like ore, composed of sphalerite with minor galena, pyrite, arsenopyrite
and magnetite, is enclosed within a carbonate-siltstone succession containing volcanic sills. Crust-type mineralization is
linked to an unconformity landscape and made up by quartz and minor sphalerite, pyrite and galena. Finally, vein deposit cross-cutting
these deposit types, contains galena, sphalerite and pyrite. Sedex-like and crust-type deposits are linked to Hercynian extensional
tectonics which favoured the development of an unstable depositional environment mainly controlled by synsedimentary faults,
being sedex mineralizations developed prior to crust-type. Both mineralization types suffered diagenesis and deformation during
the Hercynian. The vein-type was generated by extensional movements related to early Alpine rifting.
Received: 18 June 1998 / Accepted: 30 June 1998 相似文献
413.
Influence of DEM resolution on modelling hydrological connectivity in a complex agricultural catchment with woody crops 总被引:2,自引:0,他引:2
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Digital elevation model (DEM) resolution influences hydrological simulation. However, its influence when modelling hydrological connectivity (HC) in woody crops remains to be seen. We assessed surface topography, microtopography and HC in an agricultural sub‐catchment (27.4 ha) using six photogrammetry‐derived DEMs with 0.03, 0.05, 0.1, 0.2, 0.5 and 1 m cell sizes. Land uses included vineyards, olive groves, cereal fields, and forestry patches. We ran an updated version of Borselli's HC index (IC) using the D‐Infinity approach. We assessed HC in woody crops at high spatial resolution for the first time. After analysing the differences in the contributing area, the flow width, the soil roughness, the convergence index and the IC (normalised and non‐normalised) at different scales (hillslope, land uses and compartments, ephemeral gullies, depositional areas and the sub‐catchment outlet) and accounting for the field vertical components, we propose an optimum DEM resolution (0.2 m) to improve modelling of structural HC in woody crops. The modelled hydrological features at this threshold resolution matched well with the geomorphic features associated with the short‐ and medium‐term patterns of soil redistribution. Higher DEM resolutions, especially at 0.03 and 0.05 m, introduced bias in the input data and the IC computations. Finally, we observed good agreement between the outputs at the lowest resolution, 1 × 1 m, and the long‐term soil redistribution patterns (functional connectivity). Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
414.
Evaluation of nitrate levels in groundwater under agricultural fields in two pilot areas in central Chile: A hydrogeological and geochemical approach 总被引:2,自引:0,他引:2
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The aim of this study is to evaluate the impact of the application of industrial fertilizers and liquid swine manure in groundwater in two pilot agricultural areas, San Pedro and Pichidegua, which have been under long‐term historic use of fertilizers. A comprehensive hydrogeological investigation was carried out to define the geology and the groundwater flow system. Chemical and isotopic tools were used to evaluate the distribution and behavior of the nitrate in the groundwater. The isotopic tools included δ18O, δ2H, and 3H, which provide information about the origin and residence time of the groundwater; δ15N‐NO3? and δ18O‐NO3?, which provide information about nitrate sources and processes that can affect nitrate along the groundwater flow system. The application rate of liquid manure and other fertilizers all together with land uses was also evaluated. The hydrogeological investigation identified the presence of a confined aquifer underneath a thick low‐permeability aquitard, whose extension covers most of the two study areas. The nitrate concentration data, excepting a few points in zones located near recharge areas in the upper part of the basins and lower areas at the valley outlets (San Pedro), showed nitrate concentration below 10 mgN/L at the regional scale. The isotope data for nitrate showed no influence of the liquid swine manure in the groundwater at the regional scale, except for the high part of the basins and the outlet of the San Pedro valley, which are areas fertilized by manure. This data showed that the regional aquifer on both pilot study areas is protected by the thick low‐permeability aquitard, which is playing an important role on nitrate attenuation. Evidence of denitrification was also found on both shallow and deep groundwater in the Pichidegua site. This study showed that a comprehensive hydrogeological characterization complemented by chemical and isotope data is key for understanding nitrate distribution and concentration in aquifers from areas with intensive agriculture activities. 相似文献
415.
416.
Rubén?Fernández-Casal Sergio?Castillo-Páez Mario?Francisco-FernándezEmail authorView authors OrcID profile 《Stochastic Environmental Research and Risk Assessment (SERRA)》2018,32(3):675-684
In this work, a fully nonparametric geostatistical approach to estimate threshold exceeding probabilities is proposed. To estimate the large-scale variability (spatial trend) of the process, the nonparametric local linear regression estimator, with the bandwidth selected by a method that takes the spatial dependence into account, is used. A bias-corrected nonparametric estimator of the variogram, obtained from the nonparametric residuals, is proposed to estimate the small-scale variability. Finally, a bootstrap algorithm is designed to estimate the unconditional probabilities of exceeding a threshold value at any location. The behavior of this approach is evaluated through simulation and with an application to a real data set. 相似文献
417.
418.
Linear geophysical inversion via the discrete cosine pseudo‐inverse: application to potential fields
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J.L. Fernández‐Martínez Z. Fernández‐Muñiz J.L.G. Pallero S. Bonvalot 《Geophysical Prospecting》2017,65(Z1):94-111
In this paper, we present a methodology to perform geophysical inversion of large‐scale linear systems via a covariance‐free orthogonal transformation: the discrete cosine transform. The methodology consists of compressing the matrix of the linear system as a digital image and using the interesting properties of orthogonal transformations to define an approximation of the Moore–Penrose pseudo‐inverse. This methodology is also highly scalable since the model reduction achieved by these techniques increases with the number of parameters of the linear system involved due to the high correlation needed for these parameters to accomplish very detailed forward predictions and allows for a very fast computation of the inverse problem solution. We show the application of this methodology to a simple synthetic two‐dimensional gravimetric problem for different dimensionalities and different levels of white Gaussian noise and to a synthetic linear system whose system matrix has been generated via geostatistical simulation to produce a random field with a given spatial correlation. The numerical results show that the discrete cosine transform pseudo‐inverse outperforms the classical least‐squares techniques, mainly in the presence of noise, since the solutions that are obtained are more stable and fit the observed data with the lowest root‐mean‐square error. Besides, we show that model reduction is a very effective way of parameter regularisation when the conditioning of the reduced discrete cosine transform matrix is taken into account. We finally show its application to the inversion of a real gravity profile in the Atacama Desert (north Chile) obtaining very successful results in this non‐linear inverse problem. The methodology presented here has a general character and can be applied to solve any linear and non‐linear inverse problems (through linearisation) arising in technology and, particularly, in geophysics, independently of the geophysical model discretisation and dimensionality. Nevertheless, the results shown in this paper are better in the case of ill‐conditioned inverse problems for which the matrix compression is more efficient. In that sense, a natural extension of this methodology would be its application to the set of normal equations. 相似文献
419.
Miguel Cervantes-Solano Lisa Kapper Avto Goguitchaichvili Vicente Carlos Ruiz-Martínez José Rosas-Elguera Juan Morales Rafael Maciel-Peña Rubén Cejudo-Ruiz 《Studia Geophysica et Geodaetica》2017,61(2):233-248
We present a detailed rock-magnetic and paleomagnetic survey from Autlan volcanic succession in western Mexico. The principal aim of this study is to extend paleomagnetic data from Autlan lavas in order to confirm vertical-axis rotation observed in reconnaissance study and to evaluate long-term variation of the geomagnetic field strength based on existing and global data. The mean inclination (44.7°) is in agreement with the expected inclination for 60 and 70 Ma, as derived from available reference poles for the North American craton. The declination (333.6°), however, is significantly different from those expected, which suggests a statistically significant counterclockwise tectonic rotation ranging between 10° ± 6° and 14° ± 7°. As a measure of paleosecular variation (PSV), we obtained a geomagnetic field dispersion of 9.6° (upper and lower limits: 7.2°–11.9°) in perfect agreement with the previously published PSV compilation of selected Cretaceous data from lavas. The mean virtual dipole moments available for Autlan lavas are about 65% of the present geomagnetic axial dipole but are in reasonably good agreement with other comparable quality determinations between 5 and 90 Ma. This reinforces the hypothesis that low geomagnetic field strengths persisted for the entire Jurassic extending into the Upper Cretaceous. 相似文献
420.
Sergio Zlotnik Manel Fernández Pedro Díez Jaume Vergés 《Pure and Applied Geophysics》2008,165(8):1491-1510
A non-standard new code to solve multiphase viscous thermo–mechanical problems applied to geophysics is presented. Two numerical
methodologies employed in the code are described: A level set technique to track the position of the materials and an enrichment
of the solution to allow the strain rate to be discontinuous across the interface. These techniques have low computational
cost and can be used in standard desktop PCs. Examples of phase tracking with level set are presented in two and three dimensions
to study slab detachment in subduction processes and Rayleigh–Taylor instabilities, respectively. The modelling of slab detachment
processes includes realistic rheology with viscosity depending on temperature, pressure and strain rate; shear and adiabatic
heating mechanisms; density including mineral phase changes and varying thermal conductivity. Detachment models show a first
prolonged period of thermal diffusion until a fast necking of the subducting slab results in the break–off. The influence
of several numerical and physical parameters on the detachment process is analyzed: The shear heating exerts a major influence
accelerating the detachment process, reducing the onset time to one half and lubricating the sinking of the detached slab.
The adiabatic heating term acts as a thermal stabilizer. If the mantle temperature follows an adiabatic gradient, neglecting
this heating term must be included, otherwise all temperature contrasts are overestimated. As expected, the phase change at
410 km depth (olivine–spinel transition) facilitates the detachment process due to the increase in negative buoyancy. Finally,
simple plume simulations are used to show how the presented numerical methodologies can be extended to three dimensions. 相似文献