New Early Permian paleomagnetic results from the Brive basin (French Massif Central) and their implications for Late Variscan tectonics     

Yan Chen  Bernard Henry  Michel Faure  Jean-François Becq-Giraudon  Jean-Yves Talbot  Lucien Daly  Maxime Le Goff 《International Journal of Earth Sciences》2006,95(2):306-317

In order to assess the structural evolution of the Brive basin and the Paleozoic activity of surrounding major faults in the French Massif Central, we carried out a paleomagnetic study on Early Permian rocks from this basin. Positive-fold tests and solely reversed polarities indicate that the characteristic remanent magnetization is likely to be primary. Early Permian tilt-corrected site mean declinations vary from 207°–167° indicating that the Brive basin experienced internal vertical-axis rotations. On the contrary, Late Permian paleomagnetic site means exhibit a circular Fisherian distribution showing no relative rotations. Detailed analyses of Permian paleomagnetic data from five contemporaneous basins of the French Massif Central reveal that these basins share the same equatorial paleolatitude with stable Europe throughout the Permian. However, in Early Permian, three of the five basins experienced differential rotations. The Saint-Affrique basin not only suffered internal deformation during the Early Permian, but the basin as a whole underwent a full-scale counterclockwise vertical-axis block rotation with respect to stable Europe. As a consequence, paleomagnetic data from similar late orogenic basins have to be thus carefully considered for establishment of Apparent Polar Wander paths.  相似文献   

Seismicity, deformation and seismic hazard in the western rift of Corinth: New insights from the Corinth Rift Laboratory (CRL)   总被引：1，自引：2，他引：1  

P. Bernard  H. Lyon-Caen  P. Briole  A. Deschamps  F. Boudin  K. Makropoulos  P. Papadimitriou  F. Lemeille  G. Patau  H. Billiris  D. Paradissis  K. Papazissi  H. Castarde  O. Charade  A. Nercessian  A. Avallone  F. Pacchiani  J. Zahradnik  S. Sacks  A. Linde 《Tectonophysics》2006,426(1-2):7

This paper presents the main recent results obtained by the seismological and geophysical monitoring arrays in operation in the rift of Corinth, Greece. The Corinth Rift Laboratory (CRL) is set up near the western end of the rift, where instrumental seismicity and strain rate is highest. The seismicity is clustered between 5 and 10 km, defining an active layer, gently dipping north, on which the main normal faults, mostly dipping north, are rooting. It may be interpreted as a detachment zone, possibly related to the Phyllade thrust nappe. Young, active normal faults connecting the Aigion to the Psathopyrgos faults seem to control the spatial distribution of the microseismicity. This seismic activity is interpreted as a seismic creep from GPS measurements, which shows evidence for fast continuous slip on the deepest part on the detachment zone. Offshore, either the shallowest part of the faults is creeping, or the strain is relaxed in the shallow sediments, as inferred from the large NS strain gradient reported by GPS. The predicted subsidence of the central part of the rift is well fitted by the new continuous GPS measurements. The location of shallow earthquakes (between 5 and 3.5 km in depth) recorded on the on-shore Helike and Aigion faults are compatible with 50° and 60° mean dip angles, respectively. The offshore faults also show indirect evidence for high dip angles. This strongly differs from the low dip values reported for active faults more to the east of the rift, suggesting a significant structural or rheological change, possibly related to the hypothetical presence of the Phyllade nappe. Large seismic swarms, lasting weeks to months, seem to activate recent synrift as well as pre-rift faults. Most of the faults of the investigated area are in their latest part of cycle, so that the probability of at least one moderate to large earthquake (M = 6 to 6.7) is very high within a few decades. Furthermore, the region west to Aigion is likely to be in an accelerated state of extension, possibly 2 to 3 times its mean interseismic value. High resolution strain measurement, with a borehole dilatometer and long base hydrostatic tiltmeters, started end of 2002. A transient strain has been recorded by the dilatometer, lasting one hour, coincident with a local magnitude 3.7 earthquake. It is most probably associated with a slow slip event of magnitude around 5 ± 0.5. The pore pressure data from the 1 km deep AIG10 borehole, crossing the Aigion fault at depth, shows a 1 MPa overpressure and a large sensitivity to crustal strain changes.  相似文献   

Structure and tectonics of the central segment of the Eastern Cordillera of Colombia     

Martín Corts  Bernard Colletta  Jacques Angelier 《Journal of South American Earth Sciences》2006,21(4):437

In the Eastern Cordillera of Colombia, a new structural model constrained by field data, paleontologic determinations, and interpretations of seismic reflection profiles is proposed. The model implies 70 km of shortening, including reactivation of basement structures as inverse faults in both flanks of the chain. These faults propagated within the lower Cretaceous strata, inducing passively rooted and transported thrust sheets as the successive basement faults were reactivated. Two structural styles are identified in the western flank: (1) positive flower structures in a transpressive regime, which affected rocks older than upper Paleocene and were unconformably covered by post–late Paleocene sediments, and (2) compressive structures during the late Miocene–Recent Andean phase. Presently, WNW-ESE compression reactivates Late Paleocene structures, which locally affect Andean trends. In the western margin of the Eastern Cordillera, the Cambao thrust takes up most displacement, whereas the Bituima fault takes only a minor part. To the south, this relationship reverses, suggesting complementary behavior by the Bituima and Cambao faults, as well as a transfer zone. This suggestion explains the southward termination of the Guaduas syncline as a structure related to the Cambao fault, whereas the Bituima fault increases its displacement southward, generating the Girardot foldbelt that takes over the structural position of the Guaduas syncline.  相似文献   

Special issue “Multiscale Approaches to Geomaterials”     

Cristian Dascalu  Bernard Cambou 《Acta Geotechnica》2008,3(1):1-1

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Determination of porosity and storage capacity of a calcareous aquifer (France) by correlation and spectral analyses of time series     

Stephane Bernard  Frederick Delay 《Hydrogeology Journal》2008,16(7):1299-1309

Time-series analyses were used to investigate the relationships between barometric pressure changes, earth tides, and water-level fluctuations in a confined aquifer. The method was applied to data from the fractured aquifer at the Hydrogeological Experimental Site in Poitiers (France) and used to yield estimates of the aquifer’s storage capacity, porosity and barometric efficiency. The aim is to address relevance of these analyses for an aquifer showing both fracture draining and confined karstic flow in thin strata. Cross-correlation and spectral analysis are used to compare water-level head and atmospheric-pressure fluctuations. Porosity and storage capacity are calculated using this method and compared to results from petrophysical measurements and hydraulic pumping tests, respectively. The storage coefficients calculated by the time-series analyses are in agreement with those obtained by interpretation of the interference pumping tests. Conversely, porosity values calculated by time-series analyses are underestimated compared to those obtained by other methods.  相似文献   

Anomalous behavior of tellurium abundances   总被引：1，自引：0，他引：1  

Bernard L. Cohen 《Geochimica et cosmochimica acta》1984,48(1):203-205

The cosmic abundance of Te is larger than for any element with atomic number greater than 40, but it is one of the least abundant elements in the earth's lithosphere and it is one of the five elements never reported in sea water. On the other hand, it is the fourth most abundant trace element in the human body (after Fe, Zn, and Rb), and is unusually abundant in human food. It is shown that the high abundance in human food combined with the low abundance in soil requires that it be picked up by plant roots very much more efficiently than any other trace element.  相似文献   

Geodynamic evolution of the Taiwan-Luzon-Mindoro belt since the late eocene   总被引：1，自引：0，他引：1  

Jean Franois Stephan  Ren Blanchet  Claude Rangin  Bernard Pelletier  Jean Letouzey  Carla Muller 《Tectonophysics》1986,125(1-3)

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Lead-lead age of komatiitic lavas and limitations on the structure and evolution of the Precambrian mantle     

Olivier Brvart  Bernard Dupr  Claude J. Allgre 《Earth and Planetary Science Letters》1986,77(3-4)

Using lead isotopes, we have studied komatiites and associated sulfides from three complexes. At Barberton, the lead-lead isochron gives an age of 3.46 ± 0.07Gy in good agreement with the Sm—Nd age. At Munro Township, three distinct komatiitic flows yield significantly different ages: Pyke Hill is 2.72 ± 0.02Gy, Theo's Flow is 2.47 ± 0.13Gy and Fred's Flow is 2.58 ± 0.02Gy old. 2.72 Gy is the emplacement age while the alignment obtained on Theo's Flow and Fred's Flow clearly points out the effect of a metamorphism posterior to 2.7 Gy. At Cape Smith, the age obtained by the lead-lead method is 1.6 ± 0.13Gy. This age is younger than that obtained by other methods. In contrast the isochron obtained on sulfide gives an age of 1.88 ± 0.17Gy in agreement with Sm/Nd methods. These results together with others from the literature are used to describe the evolution of Pb in the mantle. The important result inferred is that the mantle 2.70 Gy ago was very different from the primitive mantle according to both Th/U and U/Pb values. Indeed, the latter are lower than the planetary values. This evolution is the result of the formation of the continental crust.  相似文献   

Epidote phenocrysts in dacitic dikes,Boulder County,Colorado   总被引：1，自引：0，他引：1  

Bernard W. Evans  Joseph A. Vance 《Contributions to Mineralogy and Petrology》1987,96(2):178-185

Epidote (Ps21%) crystallized early as elongate phenocrysts in Late Cretaceous rhyodacitic dikes in the vicinity of Ward, Boulder County, Colorado. Other unusual phenocryst phases are garnet (Gr17–24%) and muscovite. In a xenolith containing kyanite, corundum, biotite, and plagioclase, magmatic garnet grew as a rim around xenocrystic pyrope-rich (Py37%) garnet. The xenolith was derived from a granulite-facies zone, not represented at the present-day erosion surface which is composed of upper amphibolite-facies cordierite and sillimanite-bearing gneisses. The dike magmas were fed not from an immediately underlying batholith but from a magma chamber at a depth corresponding to a pressure of 8–13 kilobars. Phenocrysts cystallized in the temperature range 800 to 700° C, under H₂O and O₂ activities greater than normal for silicic magmas. This occurrence shows convincingly not only that epidote can be magmatic but that it is a possible early-crystallization phase in silicic magmas.  相似文献   

Interprétation quantitative de l'anisotropie de susceptibilité magnétique     

Bernard Henry 《Tectonophysics》1983,91(1-2)

We propose a model in which the magnetic susceptibility anisotropy is considered as resulting from the distinct properties of two rock components. This model suggests that differences in the susceptibility ellipsoid can be principally related to mineralogical variations. Thus, some difficulties in the quantitative interpretation of anisotropy data may be explained. This model is consistent with a detailed analysis carried on Dalradian rocks from Scotland.A new methodology of anisotropy analysis is derived. It is based on the fact that, in specimens from a same sample, the susceptibility ellipsoid often varies, but relations nevertheless exist between values of the principal and mean susceptibilities. For each sample, measurements are made on 6–12 specimens. The study of the relations between principal and mean susceptibilities permits separation of the anisotropies of the rock components, the anisotropy determination appearing the best for the high susceptibility component.

Résumé

L'examen d'un modèle de roche, où l'anisotropie est considérée comme résultant des propriétés de deux constituants, montre que l'ellipsoïde de susceptibilité dépend fortement de la composition minéralogique. Ainsi s'expliquent certaines difficultés de l'interprétation quantitative des résultats d'anisotropie. Ce modèle est testé avec succès sur une série du Dalradien d'Écosse.Une nouvelle méthodologie d'analyse de l'anisotropie en découle. Elle est basée sur la constatation que l'ellipsoïde de susceptibilité présente souvent des fluctuations dans les différents spécimens taillés dans un même échantillon, et que des relations simples existent cependant entre les différentes valeurs des susceptibilités principales et moyenne. Pour chaque échantillon, les mesures sont faites sur 6–12 spécimens. L'étude des relations entre susceptibilités principales et moyenne permet une séparation des anisotropies des constituants de la roche, la détermination de l'anisotropie semblant d'autant meilleure que la susceptibilité est plus forte.  相似文献