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941.
ABSTRACT Fanning structures radiating from a central perturbation are known in various geological environments, where different processes have produced similar geometry. The present contribution describes and analyses fanning clastic dykes in the Dead Sea Rift, a new example of diapir-related deformation. The dykes are opening-mode fractures exposed in lacustrine varved marl of the Lisan Formation, deposited 70–15 ka. They are arranged mainly in a radial and tangential geometry. The radial traces converge at the 'Black Hill' structural dome. The geometry of the fractures is consistent with stresses exerted by the rise of a salt diapir located underneath the Black Hill. The estimated extension of the radial fractures is in good agreement with the present topographic elevation of the hill. The absence of fractures in the overlying Holocene alluvium probably indicates that either the rise of the Black Hill salt diapir paused or is associated now with a different style of deformation. 相似文献
942.
Giacomo Corti Marco Bonini Francesco Mazzarini Mario Boccaletti Fabrizio Innocenti Piero Manetti Genene Mulugeta Dimitrios Sokoutis 《Tectonophysics》2002,348(4)
Scaled centrifuge experiments have been used to investigate the dynamic relations between deformation and magma distribution in rift-related transfer zones. The physical models were built using suitable analogue materials, such as sand to represent the brittle upper crust, various kinds of silicone mixtures to simulate the lower crust and upper mantle and glycerol to reproduce magma. Models simulated the development of transfer zones across pre-existing glycerol reservoirs placed at the base of the analogue continental crust. In plan view, different geometries, dimensions and positions of subcrustal reservoirs were reproduced in three different sets of experiments; to compare results, models were also performed without magma-simulating glycerol.Set 1 experiments, incorporating a narrow rectangular glycerol reservoir, show that the low-viscosity material is able to localise deformation into the overlying crust, giving rise to discrete transfer zones. This concentrated surface deformation corresponds at depth to major magma accumulation. Set 2 experiments, with an initial wide squared glycerol reservoir, show instead that deformation is distributed across the whole model surface, corresponding at depth to relatively minor magma accumulation. Set 3 experiments explored various positions of a small squared reservoir that invariably localised faulting in the overlying analogue brittle crust at the onset of model deformation.The overall model behaviour suggests that magma distribution at depth can effectively control the strain distribution in the overlying crust and the deformative pattern of transfer zones. Strain distribution, in turn, may control magma emplacement as localized deformation would favour major accumulation of magma at transfer zones. Coupled to a strong thermal weakening of the country rocks, this process may ultimately lead to a positive feedback interaction between magma and deformation. 相似文献
943.
Bruno Capaccioni Laura Valentini Marco B. L. Rocchi Giovanni Nappi Damiano Sarocchi 《Bulletin of Volcanology》1997,58(7):501-514
Computer-assisted image analysis can be successfully used to derive quantitative textural data on pyroclastic rock samples.
This method provides a large number of different measurements such as grain size, particle shape and 2D orientation of particle
main axes (directional- or shape-fabric) automatically and in a relatively short time. Orientation data reduction requires
specific statistical tests, mainly devoted to defining the kind of particle distribution pattern, the possible occurrence
of preferred particle orientation, the confidence interval of the mean direction and the degree of randomness with respect
to pre-assigned theoretical frequency distributions. Data obtained from image analysis of seven lithified ignimbrite samples
from the Vulsini Volcanic District (Central Italy) are used to test different statistics and to provide insight about directional
fabrics. First, the possible occurrence of a significant deviation from a theoretical circular uniform distribution was evaluated
by using the Rayleigh and Tukey χ
2 tests. Then, the Kuiper test was performed to evaluate whether or not the observation fits with a unimodal, Von Mises-like
theoretical frequency distribution. Finally, the confidence interval of mean direction was calculated. With the exception
of one sample (FPD10), which showed a well-developed bimodality, all the analysed samples display significant anisotropic
and unimodal distributions. The minimum number of measurements necessary to obtain reasonable variabilities of the calculated
statistics and mean directions was evaluated by repeating random collections of the measured particles at increments of 100
particles for each sample. Although the observed variabilities depend largely on the pattern of distribution and an absolute
minimum number cannot be stated, approximately 1500–2000 measurements are required in order to get meaningful mean directions
for the analysed samples.
Received: 9 April 1996 / Accepted: 26 December 1996 相似文献
944.
The Chisone Valley is located in the internal NW Alps, in the Pinerolese District, an area characterized by present low to medium seismicity. Fine-grained sediments (sand, silt and clay with interbedded gravel) crop out in the lower Chisone Valley: they were first interpreted as glaciolacustrine deposits, and then as a lacustrine infilling of the valley floor probably due to differential uplifting of the valley mouth. Review of this data, together with new field and palynological observations, lead us to refer the lacustrine deposits to approximately the Lower Pleistocene (Villafranchian). In many outcrops, the lacustrine deposits show strong soft-sediment deformation such as convolute laminations, water-escape structures and disrupted beds, some of them associated with folds and faults (cm to dm in size); only two sites show metric to decametric folds and faults trending E-W and N-S. Detailed structural analysis conducted along a recently exposed section (Rio Gran Dubbione site) shows several soft-sediment deformation features on the limbs of mesoscale folds. Because of their intimate structural association, the origin of these minor structures seems to be connected to synsedimentary activity on reverse and normal faults (m to dm in size) affecting the lacustrine deposits in the same locality. Soft-sediment deformation features can be interpreted as possible paleoseismites. If so, the present seismicity of the Pinerolese District, which is the major area of such activity in NW Italy, cannot be considered an isolated episode in the geological evolution of the region; even if there is no supporting evidence for continuous seismicity, the deformations in the lacustrine sediments of the Chisone Valley testify to Early Pleistocene seismic activity, probably related to the recent tectonic evolution of the internal side of the NW Alps. 相似文献
945.
946.
Marco Landolfi 《Solar physics》1987,109(2):287-306
We present a diagnostic method for detecting magnetic field gradients and velocity gradients in sunspots through the analysis of Stokes parameters profiles in magnetoactive lines. On one hand, the method is based on the concept of response functions for the Stokes profiles introduced by Landi Degl'Innocenti and Landi Degl'Innocenti (1977); on the other hand, it takes advantage of the diagnostic content of the residuals between observed Stokes profiles and best-fit Unno-profiles. The analysis of synthesis profiles obtained for the line 6302.502 FeI forming in the Sunspot Sunspot Model (Avrett, 1981) suggests that the method could actually be promising, at least if the dependence of the magnetic field vector on optical depth is sufficiently simple. 相似文献
947.
Marco Missana 《Astrophysics and Space Science》1979,61(2):277-286
The analysis of the Skylab measurements on the ultraviolet limb spectra confirm the presence of a generalized Compton effect in the solar spectrum which can be explained by Thomson scattering theory. The present measurements on the Orion spectrum and interstellar line towards -Arae give a large generalized Compton effect which could be explained by a resonance scattering theory. These numerical results cannot be due to random errors in the measurements, as follows from the statistical discussion. The need for further measurements is pointed out. 相似文献
948.
949.
Toms Vega Fernndez Marco Milazzo Fabio Badalamenti Giovanni D'Anna 《Estuarine, Coastal and Shelf Science》2005,65(4):645-653
An extensive Posidonia oceanica meadow was partially destroyed by excavation, resulting in areas of seagrass habitat of equal complexity (shoot density) but different heterogeneity (degree of fragmentation). The fish assemblages associated with differently fragmented beds were compared from a landscape perspective. Differences in the fish assemblages were detected, with several species showing different patterns: (1) species that increased their abundance along with the degree of bed fragmentation, (2) species that were more abundant in fragmented beds, but did not show differences between more or less fragmented beds, and (3) species that were mostly abundant in large seagrass patches or in the continuous meadow. The fish assemblages were also affected by depth, but further research is needed to determine properly this effect. Some ecological mechanisms are suggested to operate in the interaction between P. oceanica and the held fish assemblage. 相似文献
950.
Near-infrared, adaptive optics observations of AGN achieve a spatial resolution comparable to that of the Hubble Space Telescope
at visible and ultraviolet wavelengths. Using the ESO adaptive optics system, we have mapped the nuclear region of NGC 1068
in the near-infrared at 2.2, 3.5 and 4.8 μm. These data show the presence of strong near-infrared emission within the central
100 pc around the nucleus. At the K, L and M bands, the dominant emission peak is unresolved at resolutions of 35 pc (K),
15 pc (L) and 23 pc (M). We find that the emission peaks at 2.2, 3.5 and 4.8 μm are offset by 0.3±0.1" south of the HST optical
continuum peak. The infrared peaks are coincident with the HST imaging polarimetry center thought to mark the hidden nucleus.
We conclude that the infrared emission peaks are also directly related with the hidden nucleus and that they outline the dusty
torus around the central engine, as expected from current AGN models. From our first estimate of the fluxes in L and M, we
derive a dust temperature of 750 K for the unresolved component.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献