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871.
An updated analysis of geothermal data from the highland area of eastern Brazil has been carried out and the characteristics of regional variations in geothermal gradients and heat flow examined. The database employed includes results of geothermal measurements at 45 localities. The results indicate that the Salvador craton and the adjacent metamorphic fold belts northeastern parts of the study area are characterized by geothermal gradients in the range of 6–17°C/km. The estimated heat flow values fall in the range of 28–53 mW/m2, with low values in the cratonic area relative to the fold belts. On the other hand, the São Francisco craton and the intracratonic São Francisco sedimentary basin in the southwestern parts are characterized by relatively higher gradient values, in the range of 14–42°C/km, with the corresponding heat flow values falling in the range of 36–89 mW/m2. Maps of regional variations indicate that high heat flow anomaly in the São Francisco craton is limited to areas of sedimentary cover, to the west of the Espinhaço mountain belt. Crustal thermal models have been developed to examine the implications of the observed intracratonic variations in heat flow. The thermal models take into consideration variation of thermal conductivity with temperature as well as change of radiogenic heat generation with depth. Vertical distributions of seismic velocities were used in obtaining estimates of radiogenic heat production in crustal layers. Crustal temperatures are calculated based on a procedure that makes simultaneous use of the Kirchoff and Generalized Integral Transforms, providing thereby analytical solutions in 2D and 3D geometry. The results point to temperature variations of up to 300°C at the Moho depth, between the northern Salvador and southern São Francisco cratons. There are indications that differences in rheological properties, related to thermal field, are responsible for the contrasting styles of deformation patterns in the adjacent metamorphic fold belts. 相似文献
872.
873.
In the ultra-high pressure Metamorphic Kimi Complex widespread tonalitic–trondhjemitic dykes, with an intrusion age ca. 65–63 Ma,
cross-cut boudins and layers of amphibolitized eclogites. Geochemical investigation proclaims the tied genetic relationship
of the amphibolitized eclogites and the associated tonalitic–trondhjemitic dykes. The major and trace element contents and
rare earth element patterns of the amphibolitized eclogites indicate formation of their protoliths by fractional crystallization
of tholeiitic magmas in a back-arc environment. The tonalites and trondhjemites are characterized by moderate to high Sr contents
(>130 ppm), and low Y (<8.2 ppm) and heavy rare earth element contents (Yb content of 0.19–0.88 ppm). The chemical composition
of the tonalitic and trondhjemitic dykes are best explained by partial melting of a tholeiitic source like the amphibolitized
eclogites with residual garnet and amphibole, at the base of a thickened crust during Early Tertiary subduction/accretion
at the southern margins of the European continent. 相似文献
874.
Akinobu Miyakoshi Takeshi Hayashi Atsunao Marui Yasuo Sakura Shinichi Kawashima Masafumi Kawai 《International Journal of Earth Sciences》2008,97(2):401-411
Information on the distribution of subsurface temperature and hydraulic heads at 24 observation wells in and around the Tokyo
Lowland, the eastern part of the Tokyo Metropolitan area, were examined to make clear the relationship between groundwater
and the subsurface thermal environment in the urban area. Minimums in temperature–depth logs due to subsurface temperature
increasing at shallow parts were recognized in 21 wells. This fact shows subsurface temperature is affected by ground surface
warming in almost all of this area. Deeper than minimums, where the effects of surface warming became relatively small, regional
variation is observed as follows: high temperatures are shown in the central part to the southern part, and low temperatures
shown in the inland to eastern part. The high temperature area corresponds to an area where the lower boundary of groundwater
flow is relatively shallow. This area corresponds also to an area with severe land subsidence resulting from excessive groundwater
pumping. It is considered that this high temperature area is formed by the effects of upward groundwater flow affected by
hydrogeological conditions and pumping. On the other hand, a comparison between past data (1956–1967) and present data (2001–2003)
revealed widespread decreasing temperature in the inland area. This is explained by downward groundwater flow based on an
analysis of temperature–depth logs. This fact suggests that subsurface temperature is not only increasing from the effects
of surface warming but also decreasing from the effects of groundwater environment change due to pumping. 相似文献
875.
Hydrogeologic assessment of escalating groundwater exploitation in the Indus Basin, Pakistan 总被引:1,自引:1,他引:0
Groundwater development has contributed significantly to food security and reduction in poverty in Pakistan. Due to rapid population growth there has been a dramatic increase in the intensity of groundwater exploitation leading to declining water tables and deteriorating groundwater quality. In such prevailing conditions, the hydrogeological appraisal of escalating groundwater exploitation has become of paramount importance. Keeping this in view, a surface water–groundwater quantity and quality model was developed to assess future groundwater trends in the Rechna Doab (RD), a sub-catchment of the Indus River Basin. Scenario analysis shows that if dry conditions persist, there will be an overall decline in groundwater levels of around 10 m for the whole of RD during the next 25 years. The lower parts of RD with limited surface water supplies will undergo the highest decline in groundwater levels (10 to 20 m), which will make groundwater pumping very expensive for farmers. There is a high risk of groundwater salinization due to vertical upconing and lateral movement of highly saline groundwater into the fresh shallow aquifers in the upper parts of RD. If groundwater pumping is allowed to increase at the current rate, there will be an overall decline in groundwater salinity for the lower and middle parts of RD because of enhanced river leakage. 相似文献
876.
Mauro Cacace Ulf Bayer Anna Maria Marotta 《International Journal of Earth Sciences》2008,97(5):899-913
The large-scale crustal deformations observed in the Central European Basin System (CEBS) are the result of the interplay
between several controlling factors, among which lateral rheological heterogeneities play a key role. We present a finite-element
integral thin sheet model of stress and strain distribution within the CEBS. Unlike many previous models, this study is based
on thermo-mechanical data to quantify the impact of lateral contrasts on the tectonic deformation. Elasto-plastic material
behaviour is used for both the mantle and the crust, and the effects of the sedimentary fill are also investigated. The consistency
of model results is ensured through comparisons with observed data. The results resemble the present-day dynamics and kinematics
when: (1) a weak granite-like lower crust below the Elbe Fault System is modelled in contrast to a stronger lower crust in
the area extending north of the Elbe Line throughout the Baltic region; and (2) a transition domain in the upper mantle is
considered between the shallow mantle of the Variscan domain and the deep mantle beneath the East European Craton (EEC), extending
from the Elbe Line in the south till the Tornquist Zone. The strain localizations observed along these structural contrasts
strongly enhance the dominant role played by large structural domains in stiffening the propagation of tectonic deformation
and in controlling the basin formation and the evolution in the CEBS. 相似文献
877.
H. Zöllner K. Reicherter P. Schikowsky 《International Journal of Earth Sciences》2008,97(5):1013-1027
The pre-Alpine structural and geological evolution in the northern part of the North German Basin have been revealed on the
basis of a very dense reflection seismic profile grid. The study area is situated in the coastal Mecklenburg Bay (Germany),
part of the southwestern Baltic Sea. From the central part of the North German Basin to the northern basin margin in the Grimmen
High area a series of high-resolution maps show the evolution from the base Zechstein to the Lower Jurassic. We present a
map of basement faults affecting the pre-Zechstein. The pre-Alpine structural evolution of the region has been determined
from digital mapping of post-Permian key horizons traced on the processed seismic time sections. The geological evolution
of the North German Basin can be separated into four distinct periods in the Rerik study area. During Late Permian and Early
Triassic evaporites and clastics were deposited. Salt movement was initiated after the deposition of the Middle Triassic Muschelkalk.
Salt pillows, which were previously unmapped in the study area, are responsible for the creation of smaller subsidence centers
and angular unconformities in the Late Triassic Keuper, especially in the vicinity of the fault-bounded Grimmen High. In this
area, partly Lower Jurassic sediments overlie the Keuper unconformably. The change from extension to compression in the regional
stress field remobilized the salt, leading to a major unconformity marked at the base of the Late Cretaceous. 相似文献
878.
U. Altenberger R. Schmid R. Oberhänsli 《International Journal of Earth Sciences》2008,97(6):1301-1314
As part of the Yangtze plate, segments of the Dabie Shan terrane of Central China underwent ultra-high pressure metamorphism
during Triassic subduction. We studied the geochemistry of the abundant eclogites to evaluate the nature of the protoliths
and their geodynamic setting. Although some previous geochemical work exists, the analyses and interpretation herein are based
on a new subdivision of the ultra-high pressure sequence into basement and cover units (Changpu and Ganghe Unit), revealing
new and important results. In addition, eclogites of the so-called HP Unit south of the UHP units were studied. Whereas the
large ion lithophile elements indicate postmagmatic, metasomatic changes of some samples, the high-field strength elements
and the rare earth elements display original magmatic trends. The geochemical characteristics of the eclogites of the ultra-high
pressure areas display a strong dependence on their “structural” and geographic position. The eclogites of the basement and
the Changpu Unit indicate melt intrusion and extrusion in a continental rift system, i.e. during extensional tectonics. In
contrast, the Ganghe Unit is characterized by a pronounced chemical homogeneity. The composition of the eclogites indicates
generation from a mantle source highly influenced by slab-derived fluids. Those of the HP Unit show similar characteristics.
Magmatism of the Ganghe and HP Unit probably occurred in a continental arc setting. A similar age for both units, geographically
and/or tectonically separated, is possible. The geodynamic interpretation based on the geochemistry of the four units points
to a Neoproterozoic scenario in which the protoliths of the HP and the cover units could have been of similar age and deposited
in one evolving geological system. A rift-related larger-scale basin might have formed, e.g. a continental back-arc basin
behind a magmatic arc after or simultaneous to sedimentation and magmatism in the magmatic arc. Alternatively, magmatism occurred
in independent geodynamic settings, distinct in time and space. The units were juxtaposed during exhumation, after subduction
to varying depths. 相似文献
879.
Irina N. Plotnikova 《International Journal of Earth Sciences》2008,97(5):1131-1142
The study area is the South Tatarstan Arch located in the Volgo-Ural Region, which is an enigmatic crustal segment occupying
one third of the East European Platform. Monitoring studies have shown that fluid discharge processes are still active and
time-dependent. This paper presents an integrated review of the geological, geophysical, hydrochemical and geochemical studies
of the crystalline basement of Tatarstan. These studies are based on the stratigraphic and compositional schemes within the
crystalline basement, the drilling of deep wells, the geodynamic activity of the fractured zones of the crystalline basement
and the presence of fluids therein. Furthermore, the changes in the chemical composition of the basement waters are taken
into account. 相似文献
880.
Autumn Oczkowski Scott Nixon Kelly Henry Peter DiMilla Michael Pilson Stephen Granger Betty Buckley Carol Thornber Richard McKinney Joaquin Chaves 《Estuaries and Coasts》2008,31(1):53-69
Narragansett Bay has been heavily influenced by human activities for more than 200 years. In recent decades, it has been one
of the more intensively fertilized estuaries in the USA, with most of the anthropogenic nutrient load originating from sewage
treatment plants (STP). This will soon change as tertiary treatment upgrades reduce nitrogen (N) loads by about one third
or more during the summer. Before these reductions take place, we sought to characterize the sewage N signature in primary
(macroalgae) and secondary (the hard clam, Mercenaria mercenaria) producers in the bay using stable isotopes of N (δ15N) and carbon (δ13C). The δ15N signatures of the macroalgae show a clear gradient of approximately 4‰ from north to south, i.e., high to low point source
loading. There is also evidence of a west to east gradient of heavy to light values of δ15N in the bay consistent with circulation patterns and residual flows. The Providence River Estuary, just north of Narragansett
Bay proper, receives 85% of STP inputs to Narragansett Bay, and lower δ15N values in macroalgae there reflected preferential uptake of 14N in this heavily fertilized area. Differences in pH from N stimulated photosynthesis and related shifts in predominance of
dissolved C species may control the observed δ13C signatures. Unlike the macroalgae, the clams were remarkably uniform in both δ15N (13.2 ± 0.54‰ SD) and δ13C (−16.76 ± 0.61‰ SD) throughout the bay, and the δ15N values were 2–5‰ heavier than in clams collected outside the bay. We suggest that this remarkable uniformity reflects a
food source of anthropogenically heavy phytoplankton formed in the upper bay and supported by sewage derived N. We estimate
that approximately half of the N in the clams throughout Narragansett Bay may be from anthropogenic sources. 相似文献