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31.
32.
John L. Rich introduced the revolutionary concept that many folds in the Appalachian Mountains can be explained as superficial structures formed by passive translation of thrust blocks over ramps in detachment surfaces. The amount of layer-parallel shortening can be negligible in the formation of these folds. Rich primarily was concerned with an explanation for the Powell Valley anticline, in the southern Appalachians, but the essential kinematic features of his model of folding have been verified in other folds in the Appalachians, in the Canadian Rockies, in the Idaho-Wyoming thrust belt, and in the Pyrenees. In this paper we solve the boundary-value problem for an idealized thrust block moving over a detachment surface and ramp with zero drag, and produce theoretical fold forms in the thrust block that closely resemble those in Rich's idealized model. The anticline is narrow and rounded if the translation is small, and broad and flat-topped if the translation is large. The limbs of the anticline are symmetric. We also incorporate drag along the ramp part of the detachment surface in order to derive a possible explanation for the asymmetry of dips of the two limbs of the Powell Valley anticline. We show that drag can explain the asymmetry, particularly if drag between relatively competent rocks in opposition at the ramp caused an initial anticline to form as the thrust block began to move, and then drag reduced markedly as relatively soft shales at the base of the block were thrust over competent rocks in the ramp. The existence of the initial anticline should be reflected in asymmetry of the two limbs and in a bulge at the distal edge of the broad anticline. 相似文献
33.
The West Cycladic Detachment System (WCDS) has been mapped from the western Cycladic islands to Lavrion (mainland Greece), where several top‐to‐SSW low‐angle normal faults at different structural levels are observed. Near the detachment horizon, hydrothermal fluid flow originating from Miocene intrusions lead to high‐temperature carbonate strata‐bound Pb–Ag–Zn ore deposits. Zircon (U–Th)/He cooling ages from the mineralized footwall are 7.1 ± 0.6 to 7.9 ± 0.6 Ma. Carbon‐ and O‐isotope analyses of the carbonate host indicate high water‐to‐rock ratios during brittle deformation‐induced metasomatism, and that this interaction, unrelated to proximity of the intrusions, plays the dominant role in the mineralization. The Late Miocene geology of Lavrion is strikingly similar to Serifos island on the SE termination of the WCDS, both characterized by strong localization of detachment faulting and the interaction of brittle deformation with the injection of fluids related to granitoid intrusions. 相似文献
34.
We study dynamics of quiescent prominences using several data sets taken with the Solar Optical Telescope (SOT) on Hinode. We find a number of processes occurring at different stages of prominence evolution that are common for all of our chosen cases and, having universal character, can be related to fundamental plasma instabilities. We combine the observational evidence and theory to identify these instabilities. Here we discuss three examples: i) prominence cavity formation and its evolution, associated with a screw-pinch instability; ii) development of a regular series of plumes and spikes typical to the Rayleigh?–?Taylor (RT) instability; and iii) the appearance of growing ripples at the prominence/corona interface, often followed by a sudden collimated mass upflow, attributed to the Kelvin?–?Helmholtz (KH) instability. The conditions for transition from a linear (rippling mode) to nonlinear stage of the KH instability, known to have an explosive character, are specified. Given excellent Hinode data, all three aspects of prominence dynamics allow quantitative analysis. 相似文献
35.
Guido Schreurs Jörg Giese Alfons Berger Edwin Gnos 《International Journal of Earth Sciences》2010,99(8):1827-1847
The Ranotsara shear zone in Madagascar has been considered in previous studies to be a >350-km-long, intracrustal strike-slip
shear zone of Precambrian/Cambrian age. Because of its oblique strike to the east and west coast of Madagascar, the Ranotsara
shear zone has been correlated with shear zones in southern India and eastern Africa in Gondwana reconstructions. Our assessment
using remote sensing data and field-based investigations, however, reveals that what previously has been interpreted as the
Ranotsara shear zone is in fact a composite structure with a ductile deflection zone confined to its central segment and prominent
NW–SE trending brittle faulting along most of its length. We therefore prefer the more neutral term “Ranotsara Zone”. Lithologies,
tectonic foliations, and axial trace trajectories of major folds can be followed from south to north across most of the Ranotsara
Zone and show only a marked deflection along its central segment. The ductile deflection zone is interpreted as a result of
E–W indentation of the Antananarivo Block into the less rigid, predominantly metasedimentary rocks of the Southwestern Madagascar
Block during a late phase of the Neoproterozoic/Cambrian East African Orogeny (c. 550–520 Ma). The Ranotsara Zone shows significant
NW–SE striking brittle faulting that reactivates part of the NW–SE striking ductile structures in the flexure zone, but also
extends along strike toward the NW and toward the SE. Brittle reactivation of ductile structures along the central segment
of the Ranotsara Zone, confirmed by apatite-fission track results, may have led to the formation of a shallow Neogene basin
underlying the Ranotsara plain. The present-day drainage pattern suggests on-going normal fault activity along the central
segment. The Ranotsara Zone is not a megascale intracrustal strike-slip shear zone that crosscuts the entire basement of southern
Madagascar. It can therefore not be used as a piercing point in Gondwana reconstructions. 相似文献
36.
Philippe Besson Jefferson Degboe Benjamin Berge Valérie Chavagnac Sébastien Fabre Gilles Berger 《Geostandards and Geoanalytical Research》2014,38(3):355-362
We report a measurement procedure to determine simultaneously the major cation concentrations (Na, Ca, K and Mg) of seawater‐derived solutions by inductively coupled plasma‐atomic emission spectrometry. The best results were obtained when the IAPSO (‘standard’) seawater reference material was diluted by thirty times with Milli‐Q® water. We obtained an average reference value rK (the ratio of the mass fraction of potassium to that of chlorine, i.e., (g kg?1)/(g kg?1)) for IAPSO seawater of 0.0205 ± 0.0006 (2.9% RSD), not significantly different from 0.0206 ± 0.0005 (2.4% RSD) for seawater composition reported in the literature. The measured Na, Ca and Mg concentrations correspond to rNa, rCa and rMg values of 0.5406 ± 0.0026 (0.5% RSD), 0.02192 ± 0.00048 (2.2% RSD) and 0.06830 ± 0.00047 (0.7% RSD), respectively, in line with previous values measured by wet‐chemistry and atomic absorption spectrophotometry or wet‐chemical titration. Our measurement procedure was used successfully on synthetic seawater solutions and high‐temperature hydrothermal fluids. 相似文献
37.
We derive the magnetic helicity for configurations formed by flux tubes contained fully or only partially in the spatial domain
considered (called closed and open configurations, respectively). In both cases, magnetic helicity is computed as the sum
of mutual helicity over all possible pairs of magnetic flux tubes weighted by their magnetic fluxes. We emphasize that these
mutual helicities have properties which are not those of mutual inductances in classical circuit theory. For closed configurations, the mutual helicity of two closed flux
tubes is their relative winding around each other (known as the Gauss linkage number). For open configurations, the magnetic
helicity is derived directly from the geometry of the interlaced flux tubes so it can be computed without reference to a ground
state (such as a potential field). We derive the explicit expression in the case of a planar and spherical boundary. The magnetic
helicity has two parts. The first one is given only by the relative positions of the flux tubes on the boundary. It is the
only part if all flux tubes are arch-shaped. The second part counts the integer number of turns each pair of flux tubes wind
about each other. This provides a general method to compute the magnetic helicity with discrete or continuous distributions
of magnetic field. The method sets closed and open configurations on an equal level within the same theoretical framework. 相似文献
38.
Arnaud M. Aubourg E. Bareyre P. Br';ehin S. Caridroit R. de Kat J. Dispau G. Djidi K. Gros M. Lachièze-Rey M. Laigneau Y. Laurent B. Lesquoy E. Lavocat Ph. Magneville C. Mazeau B. Milsztajn A. Moscoso L. Pasquaud J. Paul B. Perrin P. Petibon J. Piret Y. Queinnec F. Rich J. Spiro M. de Trogoff J. Vigroux L. Zylberajch S. Ansari R. Cavalier F. Moniez M. Beaulieu J. P. Ferlet R. Grison Ph. Vidal-Madjar A. Adrianzyk G. Berger J. P. Burnage R. Delclite J. C. Kohler D. Magnan R. Richaud A. Guibert J. Moreau O. Tajahmady F. Baranne A. Maurice E. Prévôt L. Gry C. 《Experimental Astronomy》1994,4(3-4):279-296
Conclusion To summarize, the readout and the control system of the CCD mosaic camera are running since December 1991 at the La Silla Observatory (ESO). The overall performance of the camera has been good. About 12000 pictures (data and flat-fields) have been successfully registered up to now. We will report in the near future preliminary scientific results of the EROS experiment. 相似文献
39.
Blue continuum images of active regions at ∼ 60° from the center of the solar disk obtained with the new Swedish 1-m Solar
Telescope reveal heretofore unreported structure of the magnetized solar atmosphere. Perhaps the most striking aspect of these
images is that, at an angular resolution of 0.12″, they show clearly the three-dimensional structure of the photosphere. In
particular, the Wilson depression of the dark floors of pores is readily apparent. Conversely, the segmented structure of
light bridges running through sunspots and pores reveal that light bridges are raised above the dark surroundings. The geometry
of light bridges permits estimates of the height of their central (slightly darker) ridge: typically in the range 200–450 km.
These images also clearly show that facular brightenings outside of sunspots and pores occur on the disk-center side of those
granules just limbward of intergranular lanes that presumably harbor the associated plage magnetic flux. In many cases the
brightening extends 0.5″ or more over those granules. Furthermore, a very thin, darker lane is often found just centerward
of the facular brightening. We speculate that this feature is the signature of cool down flows that surround flux tubes in
dynamical models. These newly recognized observational aspects of photospheric magnetic fields should provide valuable constraints
for MHD models of the magnetized photosphere, and examination of those models as viewed from oblique angles is encouraged. 相似文献
40.
The source of coronal magnetic energy and helicity lies below the surface of the Sun, probably in the convective zone dynamo. Measurements of magnetic and velocity fields can capture the fluxes of both magnetic energy and helicity crossing the photosphere. We point out the ambiguities which can occur when observations are used to compute these fluxes. In particular, we show that these fluxes should be computed only from the horizontal motions deduced by tracking the photospheric cut of magnetic flux tubes. These horizontal motions include the effect of both the emergence and the shearing motions whatever the magnetic configuration complexity is. We finally analyze the observational difficulties involved in deriving such fluxes, in particular the limitations of the correlation tracking methods. 相似文献