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991.
W. G. Ernst 《Contributions to Mineralogy and Petrology》1997,128(1):30-44
Thin mafic dikes, possibly correlative with the Independence dike swarm of SE California, transect uppermost Proterozoic–Cambrian
metasedimentary strata in the White-Inyo Range. Textures and bulk-rock chemistry indicate that the protoliths were diabases
and microdiorites, accompanied by Ca + Mg + Fe +Ni + Cr-rich hornblende (± minor augite) cumulates. Analytical data suggest
crystal settling and fractionation at shallow depths. Most of the dikes lie in the mapped aureoles of – and were metamorphosed
by – voluminous Late Jurassic granitoid plutons; however, a few metadikes cut these plutons and must have been recrystallized
during the emplacement of Cretaceous granitic stocks. The mafic metadikes thus include members of two or more temporally distinct
suites, pre-Late Jurassic, and latest Jurassic–Cretaceous. Neoblastic mineral assemblages and element partitioning within
these nonfoliated mafic metadikes reflect lower-to-upper greenschist facies overprints; metamorphic parageneses, coincident
with those developed in the metasedimentary wallrocks, are defined by the production of chlorite, biotite, white mica, epidote,
and actinolite, and by albitization of the igneous plagioclase. Based on analytical and mineralogic data obtained in this
study, the following conclusions regarding subsolidus recrystallization of the mafic metadikes are advanced: (1) Newly grown
minerals and phase assemblages are systematic in their areal distributions. (2) Metamorphic grade increases chiefly toward
the north and east, toward the Late Jurassic granitoids. (3) Element fractionation among coexisting neoblastic phases is regular,
and compatible with a close approach to chemical equilibrium. (4) Assemblages 3–5 km from the granitic intrusive contacts
reflect lowermost greenschist facies physical conditions. (5) Investigated mafic dikes exhibit mineral parageneses isofacial
with the regional/contact metamorphic assemblages previously documented for the enclosing pre-Mesozoic clastic country rocks.
Clearly, mafic dikes of several ages of injection and recrystallization are present in the central White-Inyo Range, making
correlation with the Independence dike swarm problematic. In any case, the dikes record localized contact metamorphism that
took place sporadically over portions of an approximately 100 million year interval.
Received: 13 March 1996 / Accepted: 24 December 1996 相似文献
992.
The Olmedo bauxite deposit occurs in the Nurra district of northwest Sardinia. It forms a stratiform horizon in Cretaceous
limestone and marl. Uplift in mid-Cretaceous had exposed recently deposited limestone to karst weathering, and a layer of
argillaceous debris accumulated on its surface and was partly converted to bauxite. Intermediate products were desiccated
marl, bauxitic argillite and argillaceous bauxite. Subsidence followed, and the bauxite was preserved by the deposition of
late Cretaceous limestone and other sediments. Uplift in Oligocene-Miocene time, with ensuing erosion, exposed the bauxite
horizon to its present configuration. Concentrations of normative minerals illustrate chemical processes and the build-up
of Al in the bauxite horizon. Plots of chemical data and correlation coefficients show that Al, Ti, Zr, Nb, Th, Cr and V were
immobile during the bauxitization process. Mass changes point to large net removal of Si, Mg and K from the system, although
some of this material and slightly mobile Al were reprecipitated in the underlying argillite and altered marl. Immobile element
ratios trace the source of the bauxite to the underlying argillaceous limestone. Al in the bauxite was accumulated from the
degradation of 25 to 50 m of the argillaceous limestone.
Received: 10 June 1996 / Accepted: 15 April 1997 相似文献
993.
Estimation of groundwater evaporation and salt flux from Owens Lake, California, USA 总被引:3,自引:0,他引:3
S. W. Tyler S. Kranz M. B. Parlange J. Albertson G. G. Katul G. F. Cochran B. A. Lyles G. Holder 《Journal of Hydrology》1997,200(1-4):110-135
Groundwater evaporation and subsequent precipitation of soluble salts at Owens Lake in eastern California have created one of the single largest sources of airborne dust in the USA, yet the evaporation and salt flux have not been fully quantified. In this study, we compare eddy correlation, microlysimeters and solute profiling methods to determine their validity and sensitivity in playa environments. These techniques are often used to estimate evaporative losses, yet have not been critically compared at one field site to judge their relative effectiveness and accuracy. Results suggest that eddy correlation methods are the most widely applicable for the variety of conditions found on large playa lakes. Chloride profiling is shown to be highly sensitive to thermal and density-driven fluxes in the near surface and, as a result, appears to underestimate yearly groundwater evaporation. Yearly mean groundwater evaporation from the playa surface estimated from the three study areas was found to range from 88 to 104 mm year−1, whereas mean evaporation from the brine-covered areas was 872 mm year−1. Uncertainties on these mean rates were estimated to be ±25%, based on comparisons between eddy correlation and lysimeter estimates. On a yearly basis, evaporation accounts for approximately 47 × 106 m3 of water loss from the playa surface and open-water areas of the lake. Over the playa area, as much as 7.5 × 108 kg (7.5 × 105 t) of salt are annually concentrated by evaporation at or near the playa surface, much of which appears to be lost during dust storms in area. 相似文献
994.
P. G. Ditmar 《Pure and Applied Geophysics》1997,150(2):269-284
—This numerical study is devoted to distortions of local anomalies which are revealed by 2-D and 3-D cross-borehole seismic tomography based on first arrival travel times. The fact that prolonged subvertical anomalies may disappear in the final velocity model is well known. But distortions are also inherent to images of local inhomogeneities. These distortions are accompanied with false anomalies of the opposite sign located just above and below true ones. I offer a semi-empirical explanation of their existence, proving that they are an inherent consequence of observation geometry incompleteness. To improve the quality of the model obtained with cross-borehole tomography, a 3-D acquisition geometry may be used when additional boreholes are positioned around the target object. Results of 3-D tomographic experiments however may be considerably distorted for another reason nonaccounted bending of seismic rays.¶To generate the travel times, a ray-tracing procedure making use of the graph theory was applied. Tomographic inversion was performed by an algorithm based on the assumption of the unknown function smoothness. 相似文献
995.
—The clustering structure of the Vesuvian earthquakes occurring is investigated by means of statistical tools: the inter-event time distribution, the running mean and the multifractal analysis. The first cannot clearly distinguish between a Poissonian process and a clustered one due to the difficulties of clearly distinguishing between an exponential distribution and a power law one. The running mean test reveals the clustering of the earthquakes, but looses information about the structure of the distribution at global scales. The multifractal approach can enlighten the clustering at small scales, while the global behaviour remains Poissonian. Subsequently the clustering of the events is interpreted in terms of diffusive processes of the stress in the earth crust. 相似文献
996.
HanBin Xue René Gächter Laura Sigg 《Aquatic Sciences - Research Across Boundaries》1997,59(2):176-189
Data on the cycling of Cu and Zn in two eutrophic lakes are presented: Lake Greifen that becomes seasonally anoxic in its
hypolimnion and Lake Sempach that is aerated during winter and oxygenated during summer. They suggest that hypolimnetic oxygenation
1) enhances the release of copper from the sediment but 2) also accelerates the entrapment and deposition of Cu and Zn by
freshly formed Mn- and Fe-oxides. 相似文献
997.
Measurements of the activation energy of electrical conductance and desiccation rates on subtidal marine algae from Florida
were compared to similar data from the Bay of Fundy, Nova Scotia, on intertidal marine algae frequently subjected to long
periods of exposure to air. We have developed a method for calculating the reaction rate constant of desiccation of fully
hydrated marine algae. Values of activation energies and desiccation rate constants are consistent with the requirements for
survival of these algae under widely different environmental conditions. 相似文献
998.
A. Grafe P. A. Bespalov V. Y. Trakhtengerts A. G. Demekhov 《Annales Geophysicae》1997,15(12):1537-1547
For four geomagnetic storms of middle intensity the relationship between the low-latitude magnetic field asymmetry using ASY indices and the intensity of the auroral eastward and westward electro-jet was considered. It was asked whether there exists a connection between ASY and the eastward electrojet. To answer this question equivalent current systems were estimated in mid-latitudes. It was found that the observations obviously show no correlative relationship between the low-latitude magnetic-field asymmetry and the eastward electrojet, whereas one exists between ASY and the westward electrojet. To explain the generally accepted common three-dimensional current system between the partial ring current and the eastward electrojet, a condensor model of the three-dimensional current system was developed. It could be shown that the short periodic variations of the partial ring current are shielded by the condensor and cannot influence the eastward-electrojet current. 相似文献
999.
The orbits of two geodetic satellites, Starlette and Stella, have been analysed in order to determine ocean-tide parameters. The orbit of Starlette has been determined over a three-year period and Stella over a one-year period. Long-period analysis techniques have been used to determine the evolutions of the orbital inclination, eccentricity and right ascension of the ascending node for each satellite due to ocean tides. The ocean-tide parameters have been determined in a simultaneous fitting of the theoretical orbital variations to the observed variations. The results are compared with ocean-tide models. 相似文献
1000.
—A regional dial-up PT telemetric network of eight digital short-period seismic stations has been in full operation since the beginning of 1995 in Greece. During the first year of full operation, three destructive earthquake sequences struck different regions of central and northern Greece. The dial-up network managed to successfully record more than 85% of these seismic events including foreshocks, main shocks and aftershocks, and 45 of these events were selected in order to determine their seismic source parameters by spectral analysis. The results show that seismic moment correlates very well with earthquake magnitude, fault radius and fault displacement. The determined scaling relations are in general agreement with those obtained by other studies for Greek earthquakes that used different methodologies. Since digital seismic instrumentation is now expanding in Greece, these first results from spectral analysis of digital short-period data can be considered useful for future seismic hazard studies. 相似文献