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891.
G. A. F. d’Almeida 《Geotectonics》2010,44(3):271-282
The Red Sea Rift has been an object of comprehensive studies by several generations of geologists and geophysicists. Many
publications and open-file reports provide insights into the geological history of this rift. Paleogene and Cretaceous rocks,
which are considered to be prerift, are locally exposed at the margins of the Red Sea Rift. At the same time, some evidence
indicates that at least some of these rocks are related to the early stage of the evolution of the Red Sea Rift. The available
geological data suggest that the Red Sea region started its active evolution in the Cretaceous. As follows from lithostratigraphic
data, the Cretaceous-Paleogene trough that predated the Oligocene-Quaternary rift covered this region completely or partially.
The pre-Oligocene magmatism and geological evidence show that the Cretaceous-Paleogene trough was of the rift type. The Cretaceous-Eocene
and Oligocene-Quaternary phases of rifting were separated by an epoch of uplifting and denudation documented by the erosion
surface and unconformity. 相似文献
892.
Younger and older zircons from rocks of the oceanic lithosphere in the Central Atlantic and their geotectonic implications 总被引:1,自引:0,他引:1
Local U-Pb dating of zircons separated from various rocks in the crest zone of the Mid-Atlantic Ridge (MAR) and Carter Seamount
(Sierra Leone Rise) is performed. Younger zircons formed in situ in combination with older xenogenic zircons are revealed
in enriched basalts, alkaline volcanic rocks, gabbroic rocks, and plagiogranites. Only older zircons are found in depleted
basalts and peridotites. Older zircons are ubiquitous in the young oceanic lithosphere of the Central Atlantic. The age of
the younger zircons from the crest zone of the MAR ranges from 0.38 to 11.26 Ma and progressively increases receding from
the axial zone of the ridge. This fact provides additional evidence for spreading of the oceanic floor. The rate of half-spreading
calculated from the age of the studied zircons is close to the rate of half-spreading estimated from magnetic anomalies. The
age of the younger zircons from Carter Seamount (58 Ma) corresponds to the age of the volcanic edifice. Older zircons make
up an age series from 53 to 3200 Ma. Clusters of zircons differing in age reveal quasiperiodicity of about 200 Ma, which approximately
corresponds to the global tectonic epochs in the geological evolution of the Earth. Several age groups of older zircons combine
grains close in morphology and geochemistry: (1) Neoproterozoic and Phanerozoic (53–700 Ma) prismatic grains with slightly
resorbed faces, well-preserved or translucent oscillatory zoning, and geochemical features inherent to magmatic zircons; (2)
prismatic grains dated at 1811 Ma with resorbed faces and edges, fragmentary or translucent zoning, and geochemical features
inherent to magmatic zircons; (3) ovoid and highly resorbed prismatic grains with chaotic internal structure and metamorphic
geochemical parameters; the peak of their ages is 1880 Ma. The performed study indicates that older xenogenic zircons from
young rocks in the crest zone of the MAR were captured by melt or incorporated into refractory restite probably in the sublithospheric
mantle at the level of magma generation in the asthenosphere. It is suggested that zircons could have crystallized from the
melts repeatedly migrating through the asthenosphere during geological history or were entrapped by the asthenosphere together
with blocks of disintegrated and delaminated continental lithosphere in the process of breakup of the continents older than
Gondwana. The variability in the age of older zircons even within individual samples may be regarded as evidence for active
stirring of matter as a result of periodically arising and destroyed within-asthenospheric convective flows varying in orientation
and scale. 相似文献
893.
V. V. Emel’yanenko 《Solar System Research》2010,44(4):281-289
One of the main particular features of the structure of the Kuiper Belt is that it contains clusters of objects of small orbital
eccentricity and inclination (“cold population”). In order to solve the problem of the origin of the objects, we considered
the process of the gravitational interaction of a comparatively small-mass planet with a planetesimal disk. We found that
one particular property of the process is that the planet changes its direction of migration. The interaction with the planet
results in the transportation of a considerable portion of planetesimals from the inner zone out to the Kuiper Belt. After
such a transition of the objects, the planet returns to the inner regions of the planetesimal disk. Numerical simulations
show that the reversible migration of a planet of a mass similar to that of the Earth can explain the main properties of the
Kuiper Belt’s cold population orbit distribution. 相似文献
894.
G. Ya. Smol’kov V. P. Maksimov D. V. Prosovetskii A. M. Uralov I. A. Bakunina 《Bulletin of the Crimean Astrophysical Observatory》2010,106(1):31-33
An experience of radioheliographic prediction of powerful solar flares using active region microwave emission intensity and
circular polarization distribution is briefly described. 相似文献
895.
Long-term visual observations of the Lyrid meteoroid shower have been analyzed to determine the mass distribution of Lyrid
meteoroids. The value of the parameter S has been confirmed to be less than 1.8, which is normally assumed for meteoroid streams. The inclination of the descending
and ascending branches of the S curve, depending on the longitude of the Sun, does not seem to exceed 3°. Observations carried out from 1987 until 2007 reveal
that the minimum value of S corresponding to the longitude of the Sun 32.19 ± 0.04° is equal to 1.54 ± 0.02 (2000.0). The analysis of the S parameter derived from visual observations did not discover any particularities in the mass distribution of the meteoroids
in the stream connected with the assumed 12-year enhancement period in the activity of the Lyrids. 相似文献
896.
D. S. Rodionov G. Klingelhoefer E. N. Evlanov M. Blumers B. Bernhardt J. Gironés J. Maul I. Fleischer O. F. Prilutskii A. F. Shlyk V. M. Linkin C. d’Uston 《Solar System Research》2010,44(5):362-370
Möessbauer spectroscopy is a powerful tool for the mineralogical analysis of Fe-bearing materials. The miniaturized Möessbauer spectrometer MIMOS II has already been working on the surface of Mars for 6 years as part of the NASA Mars Exploration Rovers mission. The improved version of the instrument is a component of the scientific payload of the Phobos-Grunt mission. The scientific objectives of the instrument are the following: to identify the iron-bearing phases, to determine the quantitative distribution of iron among these phases, and to determine the distribution of iron among its oxidation states. 相似文献
897.
M. Yu. Piotrovich Yu. N. Gnedin T. M. Natsvlishvili N. A. Silant’ev 《Astronomy Letters》2010,36(6):389-395
We analyze the spectropolarimetric observations of 12 candidates for quasars from the spectroscopic database of the SDSS Catalog.
The magnetic fields of these objects are estimated in the context of a theory that includes the Faraday rotation of the polarization
plane on the mean free path of a photon in the outflow from an accretion disk. As a result, we have determined the column
density in the outflow, N
H ∼ 6 × 1023 cm−2, and the radial, B ∼ 1 G, and toroidal, B ∼ 600 G, magnetic fields. 相似文献
898.
Using the method of two-dimensional spectroscopy, we have investigated the kinematics and distribution of the gas and stars
at the center of the early-type spiral galaxy NGC 7177 with a mediumscale bar as well as the change in the mean age of the
stellar population along the radius. A classical picture of radial gas inflow to the galactic center along the shock fronts
delineated by dust concentration at the leading edges of the bar has been revealed. The gas inflow is observed down to a radius
R = 1″.5−2″, where the gas flows at the inner Lindblad resonance concentrate in an azimuthally highly inhomogeneous nuclear
star formation ring. The bar in NGC 7177 is shown to be thick in z coordinate—basically, it has already turned into a pseudo-bulge as a result of secular dynamical evolution. The mean stellar
age inside the star formation ring, in the galactic nucleus, is old, ∼10 Gyr.Outside, at a distance R = 6″−8″ from the nucleus, the mean age of the stellar population is ∼2 Gyr. If we agree that the bar in NGC 7177 is old,
then, obviously, the star formation ring has migrated radially inward in the last 1–2 Gyr, in accordance with the predictions
of some dynamical models. 相似文献
899.
The structure of the AGN object 1803+784 has been investigated at a wavelength of 7 mm with a limiting angular resolution
reaching 20 μas. The ejector nozzle surrounded by a ring structure, an accretion disk, has been identified. The nozzle size
is ∼0.1 pc, the diameter of the ring structure is ∼1.4 pc, and its width is ∼0.25 pc. The reaction of the plasma flow produces
a multimode precession responsible for the conical helical structure of the jet with a variable step and a curved axis. The
viewing angle of the flow ejection is ∼40°. The central part of the ejected flow moving along the axis accelerates to a relativistic
velocity. The apparent velocity reaches 12 s at a distance of ∼1 mas or ∼6 pc from the ejector. The outer part of the flow
moves along a helix around a high-velocity component whose step is a factor of 4 smaller, because the longitudinal velocity
is relatively low. The plasma is ejected almost toward the observer, as confirmed by its high brightness temperature T
b ≈ 8 × 1013 K and highly beamed emission. The polarized emission from the nozzle is axisymmetric. The orientation of the polarization
of the flow along the whole length is aligned with the direction of its motion, suggesting the excitation of a ring magnetic
field around it and self-focusing. 相似文献
900.