Uplift by thermal expansion of the lithosphere     

Jean-Claude Mareschal 《Geophysical Journal International》1981,66(3):535-552

Summary. If one can measure the anomalous horizontal magnetic field associated with a (locally bounded) two-dimensional conductivity anomaly, the transfer function which results from correlating the vertical with the anomalous horizontal magnetic field directly indicates the depth to an equivalent line-current. A. numerical model can be used to illustrate this. If three-dimensional effects (including current channelling) produce the current anomaly, interpretation in terms of conductive structure would be less clear. It has been claimed (Babour & Mosnier etc.) that such three-dimensional effects are experimentally observed in the highly coherent transfer functions determined from differential geomagnetic sounding experiments. These effects are, specifically, the 'linear polarization' of the anomalous fields, and the invariance of the phase of the measured anomalous field across the anomaly. It is suggested in this present paper that both these effects can be explained in terms of simple local induction models.
If the embedded two-dimensional anomaly is sufficiently close to the Earth's surface, the transfer function (between the vertical and the total horizontal field) contains more information than is usually interpreted. With this in mind, the magnetovariational data collected by Rooney & Hutton in the Kenyan Rift is re-examined.  相似文献   

GEOGRAPHIC SERIATION REVISITED*     

Jean-Claude Muller 《The Professional geographer》1983,35(2):196-202

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Deep structure of the West African continental margin (Congo, Zaïre, Angola), between 5°S and 8°S, from reflection/refraction seismics and gravity data     

Isabelle Contrucci  Luis Matias  Maryline Moulin  Louis Géli  Frauke Klingelhofer  Hervé Nouzé  Daniel Aslanian  Jean-Louis Olivet  Jean-Pierre Réhault  Jean-Claude Sibuet 《Geophysical Journal International》2004,158(2):529-553

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How was Taiwan created?   总被引：4，自引：0，他引：4  

Jean-Claude Sibuet  Shu-Kun Hsu   《Tectonophysics》2004,379(1-4):159-181

Since the beginning of formation of proto-Taiwan during late Miocene (9 Ma), the subducting Philippine (PH) Sea plate moved continuously through time in the N307° direction at a 5.6 cm/year velocity with respect to Eurasia (EU), tearing the Eurasian plate. Strain states within the EU crust are different on each side of the western PH Sea plate boundary (extensional in the Okinawa Trough and northeastern Taiwan versus contractional for the rest of Taiwan Island). The B feature corresponds to the boundary between the continental and oceanic parts of the subducting Eurasian plate and lies in the prolongation of the ocean–continent boundary of the northern South China Sea. Strain rates in the Philippines to northern Taiwan accretionary prism are similar on each side of B (contractional), though with different strain directions, perhaps in relation with the change of nature of the EU slab across B. Consequently, in the process of Taiwan mountain building, the deformation style was probably not changing continuously from the Manila to the Ryukyu subduction zones. The Luzon intra-oceanic arc only formed south of B, above the subducting Eurasian oceanic lithosphere. North of B, the Luzon arc collided with EU simultaneously with the eastward subduction of a portion of EU continental lithosphere beneath the Luzon arc. In its northern portion, the lower part of the Luzon arc was subducting beneath Eurasia while the upper part accreted against the Ryukyu forearc. Among the consequences of such a simple geodynamic model: (i) The notion of continuum from subduction to collision might be questioned. (ii) Traces of the Miocene volcanic arc were never found in the southwestern Ryukyu arc. We suggest that the portion of EU continental lithosphere, which has subducted beneath the Coastal Range, might include the Miocene Ryukyu arc volcanoes formed west of 126°E longitude and which are missing today. (iii) The 150-km-wide oceanic domain located south of B between the Luzon arc and the Manila trench, above the subducting oceanic EU plate (South China Sea) was progressively incorporated into the EU plate north of B.  相似文献   

Radiogenic lead mobility in the Shea Creek unconformity-related uranium deposit (Saskatchewan,Canada): migration pathways and Pb loss quantification     

Philippe Kister  Michel Cuney  Viacheslav N. Golubev  Jean-Jacques Royer  Christian Le Carlier De Veslud  Jean-Claude Rippert 《Comptes Rendus Geoscience》2004,336(3):205-215

The average Pb/U ratio of the Shea Creek unconformity-type uranium deposit has been estimated at 0.071±0.015. The calculation was performed on a volume enclosing the orebody to take into account the possible radiogenic lead migration within the ore zone. Despite this precaution, this ratio is significantly lower than the expected ratio (0.211) assuming a main U deposition around 1315 Ma, as suggested by previous UPb isotopic dating. Although part of the radiogenic lead can be trapped as galena within the orebody, about 60% of Pb have migrated more than 700 m away from the orebody, preferentially along the unconformity. To cite this article: P. Kister et al., C. R. Geoscience 336 (2004).  相似文献   

Fundamental Structures for the Design of Machine Vision Systems     

Jeffrey Johnson  Jean-Claude Simon 《Mathematical Geology》2001,33(3):331-352

The chicken-and-egg problem of machine vision is that (a) abstracting objects requires images to be segmented, (b) segmentation requires greyscale statistics, (c) greyscale statistics are defined by segmented regions, and this leads back to (a). New image structures called gradient runs and gradient polygons are presented. These lead to new types of three-dimensional histograms.The histogram of a whole image mixes histograms from different regions within the image. These regions and their histrograms may need separating. Global histrograms can be used for preliminary segmentations. These then generate new historagms of the statistics for more coherent locally structured regions. These ideas have used to binarise documents for automated document reading systems, and they may be developed in the desingn of machine vision systems for other areas of application.  相似文献   

The influence of local and non-local forcing effects on the subtidal circulation of Patos Lagoon     

Osmar O. Möller  Patrice Castaing  Jean-Claude Salomon  Pascal Lazure 《Estuaries and Coasts》2001,24(2):297-311

Some basic features concerning the subtidal circulation of Patos Lagoon were studied through time series analysis of wind, freshwater discharge, and water level records, as well as by means of experiments carried out with a 3D numerical model. The results indicate that during low to moderate river discharge the wind is the main forcing mechanism in time scales associated with meteorological fronts. The two types of wind action, local and non-local effects, are distinguished and their relative importance is evaluated. Salt water enters the system due to a combination of both remote and local wind effects that favors the development of a pressure gradient towards the lagoon during southwesterly winds. This situation is reversed when northeasterly winds dominate. In the inner parts of the lagoon, local wind plays the major role by inducing set up/set down oscillations. An upwind return flow is then developed under these conditions. During high flood periods, normally observed in late winter, the circulation is driven by freshwater discharge.  相似文献   

Morphosedimentary evolution of the macrotidal Seine estuary subjected to human impact     

Sandric Lesourd  Patrick Lesueur  Jean-Claude Brun-Cottan  Jean-Paul Auffret  Nicole Poupinet  Benoît Laignel 《Estuaries and Coasts》2001,24(6):940-949

Many estuaries in the world have been subjected to significant human impact since the 19th century. Natural infilling and human activities, including building of embankments, dikes and jetties have modified both the morphology of these estuaries and the distribution of sedimentary facies. The Seine, a macrotidal estuary, provides a good opportunity to study such modifications, as the latest morphosedimentary observations could be compared to old sediment maps as well as to geotechnical drilling data. In the mouth of the Seine, originally a sand/gravel system sediment distribution, has now been transformed into a muddy system. The result is a regressive sequence several meters thick. It is typically fining-up and corresponds to a shift from a distal term (pebbles, gravel, and coarse sands) at its base to a proximal term (mud and fine sands) at its top. Civil engineering works have reduced the available amount of space within the estuary, leading to an increase in the natural downstream shift of the depocenter of mud brought by winter river floods. The deposition area of the mud is today in the open marine zone, where waves and tidal currents render the balance of this process precarious.  相似文献   

Modern and fossil Potamids (Gastropoda) in saline lakes     

Jean-Claude Plaziat 《Journal of Paleolimnology》1993,8(2):163-169

Potamids generally live in estuaries, lagoons, and along protected sea shores. However, their ability to colonize intracontinental lakes has been underestimated due to non- or misidentifications. Potamides conicus, the only present lacustrine Potamid currently found in North Africa, belongs to a lagoon-type (thalassoid) malacofauna, dominated by the Cardium Cerastoderma glaucum: this fauna occurs in association with Foraminifera (especially Ammonia beccarii) and Ostracoda (Cyprideis gr. Torosa). Potamids have been found 250 to 900 km from the nearest sea shore: passive dispersal (probably through birds) is the primary dispersal mechanism. Shell morphology and ornamentation can be used to characterize life environment (lacustrine versus marine) but not distance from sea shore. The presence of Potamid-Foraminifera fauna in the fossil record has been interpreted as providing evidence for a sea connection (e.g., gulfs, lagoons). However, more recent paleoecological studies have demonstrated that such assemblages span an ecological gradient from freshwater to hypersaline lacustrine environments. Thus, the occurrence of modern Potamids in lacustrine (in addition to maritime) environments has important implications for Quaternary and Tertiary paleogeographic investigations. Lacustrine Potamids have been recorded from the European Tertiary (France, Spain) and from the Sahara Holocene. More research is required for the better understanding of the distribution and ecology of Potamids in modern and fossil lakes of other continents.This publication is the fifth of a series of papers presented at the Conference on Sedimentary and Paleolimnological Records of Saline Lakes. This Conference was held August 13–16, 1991 at the University of Saskatchewan, Saskatoon, Canada. Dr Evans is serving as Guest Editor.  相似文献   

A TEM investigation of shock metamorphism in quartz from the Vredefort dome, South Africa     

Hugues Leroux  Wolf Uwe Reimold  Jean-Claude Doukhan 《Tectonophysics》1994,230(3-4):223-239

The origin of the Vredefort structure in South Africa is still debated. Several causes have been discussed, namely asteroid impact, internal gas explosion or tectonic processes. Evidence of dynamic rock deformation is pervasive in the form of planar features in quartz grains, shatter cones, veins of pseudotachylite and occurrence of coesite and stishovite (high-pressure quartz polymorphs). A number of these characteristics is widely believed to support an impact origin. However, the planar features in quartz, which are generally considered as one of the strongest indicators of impact, are in the Vredefort case considered as anomalous when compared with those from accepted impact structures.

We have investigated by optical and transmission electron microscopy (TEM) the defect microstructures in quartz grains from different lithologies sampled at various places at the Vredefort structure. Whatever the locality, only thin mechanical Brazil twin lamellae in the basal plane are observed by TEM. So far, such defects have only been found in quartz from impact sites, but always associated with sets of thin glass lamellae in rhombohedral planes 10−1n with n = 1, 2, 3, and 4. At the scale of the optical microscope, Brazil twins in (0001) are easily detected in Vredefort quartz grains because of the numerous tiny fluid inclusions which decorate them. Similar alignments of tiny fluid inclusions parallel to other planes are also detected optically, but at the TEM scale no specific shock defects are detected along their traces. If these inclusion alignments initially were shock features, they are now so severely weathered that they can no longer be recognized as unambiguous shock lamellae. Fine-grained coesite was detected in the vicinity of narrow pseudotachylite veinlets in a quartzite specimen, but stishovite was not found, even in areas where its occurrence was previously reported. Finally, definite evidence of high-temperature annealing was observed in all the samples. These observations lead us to the conclusion that our findings regarding microdeformation in quartz are consistent with an impact origin for the Vredefort structure. Most of the original shock defects are now overprinted by an intense post-shock annealing episode. Only the thin mechanical twin lamellae in the basal plane have survived.  相似文献