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861.
Synthesis of regime observations of groundwater quality at deposits under operation and thermodynamic modeling of the geochemical interaction processes that govern the groundwater chemistry were used to recognize the geochemical causes and processes that bring about an increase in the groundwater fluorine content during water withdrawal accompanied by groundwater level drop and partial drying of the aquifer. The causes of the invalidity of previous forecasts are established. 相似文献
862.
Axel K Schmitt Jan M Lindsay Shan de Silva Robert B Trumbull 《Journal of Volcanology and Geothermal Research》2003,120(1-2):43-53
High spatial resolution U–Pb dates of zircons from two consanguineous ignimbrites of contrasting composition, the high-silica rhyolitic Toconao and the overlying dacitic Atana ignimbrites, erupted from La Pacana caldera, north Chile, are presented in this study. Zircons from Atana and Toconao pumice clasts yield apparent 238U/206Pb ages of 4.11±0.20 Ma and 4.65±0.13 Ma (2σ), respectively. These data combined with previously published geochemical and stratigraphic data, reveal that the two ignimbrites were erupted from a stratified magma chamber. The Atana zircon U–Pb ages closely agree with the eruption age of Atana previously determined by K–Ar dating (4.0±0.1 Ma) and do not support long (>1 Ma) residence times. Xenocrystic zircons were found only in the Toconao bulk ignimbrite, which were probably entrained during eruption and transport. Apparent 238U/206Pb zircon ages of 13 Ma in these xenocrysts provide the first evidence that the onset of felsic magmatism within the Altiplano–Puna ignimbrite province occurred approximately 3 Myr earlier than previously documented. 相似文献
863.
L. A. Morgan W. C. Shanks III D. A. Lovalvo S. Y. Johnson W. J. Stephenson K. L. Pierce S. S. Harlan C. A. Finn G. Lee M. Webring B. Schulze J. Dühn R. Sweeney L. Balistrieri 《Journal of Volcanology and Geothermal Research》2003,122(3-4):221-242
‘No portion of the American continent is perhaps so rich in wonders as the Yellow Stone’ (F.V. Hayden, September 2, 1874)Discoveries from multi-beam sonar mapping and seismic reflection surveys of the northern, central, and West Thumb basins of Yellowstone Lake provide new insight into the extent of post-collapse volcanism and active hydrothermal processes occurring in a large lake environment above a large magma chamber. Yellowstone Lake has an irregular bottom covered with dozens of features directly related to hydrothermal, tectonic, volcanic, and sedimentary processes. Detailed bathymetric, seismic reflection, and magnetic evidence reveals that rhyolitic lava flows underlie much of Yellowstone Lake and exert fundamental control on lake bathymetry and localization of hydrothermal activity. Many previously unknown features have been identified and include over 250 hydrothermal vents, several very large (>500 m diameter) hydrothermal explosion craters, many small hydrothermal vent craters (1–200 m diameter), domed lacustrine sediments related to hydrothermal activity, elongate fissures cutting post-glacial sediments, siliceous hydrothermal spire structures, sublacustrine landslide deposits, submerged former shorelines, and a recently active graben. Sampling and observations with a submersible remotely operated vehicle confirm and extend our understanding of the identified features. Faults, fissures, hydrothermally inflated domal structures, hydrothermal explosion craters, and sublacustrine landslides constitute potentially significant geologic hazards. Toxic elements derived from hydrothermal processes also may significantly affect the Yellowstone ecosystem. 相似文献
864.
Statistical analysis of the magnetic fabric of samples from several successive lava flows emplaced under similar conditions can allow determination of the mean flow direction when magnetic fabric data from individual flows do not lead to reliable results. A difference between the obtained flow direction and the present dip direction indicates that the flows were tilted after emplacement. For 2 successive series of flows on the Jeanne d’Arc Peninsula presently NNW dipping, this method shows lava emplacement along a SSW–NNE direction. This indicates a gentle tilting acquired during a period of weak deformation in the whole archipelago. Additionally, the magnetic fabric data allow the reconstruction of the different conditions of emplacement of these two series of lava flows and of formation of the local thick conglomerate interbedded between these series. 相似文献
865.
Buried Quaternary valleys in Denmark are complex structures filled with various deposits consisting primarily of glacio-lacustrine clay, till and meltwater sand, and gravel. The valleys are important geophysical targets, because they often contain significant volumes of groundwater used for public water supply. About 700 km of buried valley structures have been imaged in the western part of Denmark by the transient electromagnetic (TEM) method. The ability to map the valleys depends primarily on valley geometry, infill architecture and the resistivity of the fill sediments as well as the substratum. One-dimensional (1-D) inversion models of the TEM soundings have been used to construct contour maps of 20 m average resistivities and depth to a good conductor, which provide images for geological interpretation. Images of buried valley morphology, fill properties, infill architecture, such as cut-and-fill structures, valley distribution and valley generations, are characterized for case studies from Hornsyld, Holstebro and the Vonsild/Agtrup areas of Denmark. 相似文献
866.
Failure Mode and Spatial Distribution of Damage in Rothbach Sandstone in the Brittle-ductile Transition 总被引:4,自引:0,他引:4
— To elucidate the spatial complexity of damage and evolution of localized failure in the transitional regime from brittle faulting to cataclastic ductile flow in a porous sandstone, we performed a series of triaxial compression experiments on Rothbach sandstone (20% porosity). Quantitative microstructural analysis and X-ray computed tomography (CT) imaging were conducted on deformed samples. Localized failure was observed in samples at effective pressures ranging from 5 MPa to 130 MPa. In the brittle faulting regime, dilating shear bands were observed. The CT images and stereological measurements reveal the geometric complexity and spatial heterogeneity of damage in the failed samples. In the transitional regime (at effective pressures between 45 MPa and 130 MPa), compacting shear bands at high angles and compaction bands perpendicular to the maximum compression direction were observed. The laboratory results suggest that these complex localized features can be pervasive in sandstone formations, not just limited to the very porous aeolian sandstone in which they were first documented. The microstructural observations are in qualitative agreement with theoretical predictions of bifurcation analyses, except for the occurrence of compaction bands in the sample deformed at effective pressure of 130 MPa. The bifurcation analysis with the constitutive model used in this paper is nonadequate to predict compaction band formation, may be due to the neglect of bedding anisotropy of the rock and multiple yield mechanisms in the constitutive model. 相似文献
867.
868.
F. Marra C. Freda P. Scarlato J. Taddeucci D. B. Karner P. R. Renne M. Gaeta D. M. Palladino R. Trigila G. Cavarretta 《Bulletin of Volcanology》2003,65(4):227-247
We present 24 40Ar/39Ar ages for the youngest volcanic products from the Alban Hills volcanic district (Rome). Combined with petrological data
on these products, we have attempted to define the chronology of the most recent phase of activity and to investigate the
magma evolution of this volcanic district. The early, mainly explosive activity of the Alban Hills spanned the interval from
561±1 to 351±3 ka. After approximately 50-kyr of dormancy, a mainly effusive phase of activity took place, accompanied by
the strombolian activity of a small central edifice (Monte delle Faete). This second phase of activity spanned the interval
308±2 to 250±1 ka. After another dormancy period of approximately 50-kyr, a new, hydromagmatic phase of activity started at
200 ka at several centers located to the southwest of the Monte delle Faete edifice. After an initial recurrence period of
approximately 50-kyr, which also characterized this new phase of activity, the longest dormancy period (approximately 80-kyr)
in the history of the volcanic district preceded the start of the activity of the Albano and Giuturna centers at 70±1 ka.
Results of our study suggest a quasi-continuous magmatic activity feeding hydromagmatic centers with a new acme of volcanism
since around 70 ka. Based on data presented in this paper, we argue that the Alban Hills should not be considered an extinct
volcanic district and a detailed re-assessment of the volcanic hazard for the area of Rome is in order.
Published online: 4 April 2003
Editorial responsibility: J. Donnelly-Nolan 相似文献
869.
870.
Numerical viscoelastic modelling by the spectral Laguerre method 总被引:1,自引:0,他引:1
Seismic wave propagation in a viscoelastic media can be described by a system of integro-differential equations. The solution of such equations requires special methods when using finite-difference techniques in the time domain. In the frequency domain, the integral terms are represented by complex elastic parameters. This paper presents an efficient algorithm for viscoelastic modelling based on the integral Laguerre transform for the approximation of temporal derivatives and for the calculation of convolution integrals. For the calculation of spatial derivatives, it is possible to use various methods: finite-difference and finite-element techniques, spectral and pseudo-spectral methods. We then obtain a system of algebraic equations with a matrix independent of the parameter m , i.e. the degree of the Laguerre polynomials. In this case, only the right-hand side of the system has recurrent dependence on the parameter m , which is an analogue of the temporal frequency in the frequency domain. The obtained system with a large number of right-hand sides can be solved using fast methods, where the matrix is transformed only once, as opposed to the frequency-domain approach, when the matrix is transformed for each temporal frequency. 相似文献