B. Henry
Centre national de la recherche scientifique
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Featured researches published by B. Henry.
Geophysical Journal International | 2012
P. F. Silva; B. Henry; Fernando O. Marques; A. Hildenbrand; Pedro Madureira; C. A. Mériaux; Zuzana Kratinová
We present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1.3 Myr on S. Jorge Island (Azores ArchipelagoNorth Atlantic Ocean). The sections sampled in the southeastern and central/western parts of the island record reversed and normal polarities, respectively. They indicate a mean palaeomagnetic pole (81.3 degrees N, 160.7 degrees E, K= 33 and A95= 3.4 degrees) with a latitude shallower than that expected from Geocentric Axial Dipole assumption, suggesting an effect of non-dipolar components of the Earth magnetic field. Virtual Geomagnetic Poles of eight flows and two dykes closely follow the contemporaneous records of the Cobb Mountain Subchron (ODP/DSDP programs) and constrain the age transition from reversed to normal polarity at ca. 1.207 +/- 0.017 Ma. Volcano flank instabilities, probably related to dyke emplacement along an NNWSSE direction, led to southwestward tilting of the lava pile towards the sea. Two spatially and temporally distinct dyke systems have been recognized on the island. The eastern is dominated by NNWSSE trending dykes emplaced before the end of the Matuyama Chron, whereas in the central/western parts the eruptive fissures oriented WNWESE controlled the westward growth of the S. Jorge Island during the Brunhes Chron. Both directions are consistent with the present-day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre-existing fractures. The distinct timing and location of each dyke system likely results from a slight shift of the magmatic source.
Tectonophysics | 2001
Mohamed El-Messaoud Derder; Brigitte Smith; B. Henry; A.K. Yelles; B. Bayou; H. Djellit; R. Ait ouali; H. Gandriche
Abstract A paleomagnetic study was carried out on the Middle Carboniferous sediments of the eastern margin of the Reggane Basin of Algeria. Seven sites (108 samples) in the Lower Serpukhovian and 11 sites (129 samples) in the Upper Serpukhovian, Bashkirian and Lower Moscovian levels were investigated. Besides a common, but generally limited, viscous remanent magnetization (component A) and a recent chemical remanent magnetization of reversed polarity (A′), two main components were identified: one of these (component B), is characterized by a negative fold test and has been identified as a Lower Jurassic remagnetization. The associated paleomagnetic pole obtained in the seven zones by combining characteristic remanent magnetization directions (ChRM) and great circles (λ=71.1°N, ϕ=251.4°E, A95=3.8°, K=254) lies in the vicinity of the NW African poles of similar ages. The second (component C) displays both normal and reversed polarities. Also determined by the combination of ChRM or stable end points and remagnetization circles, it yields a positive fold test which constrains the magnetization acquisition time and a positive reversal test which argues in favor of a “non-composite” nature of the component C. The normal polarities observed in the Lower Serpukhovian levels represent the latest normal event observed in Africa before the Kiaman superchron. The paleomagnetic South pole calculated from 10 sites (n=64 data) gathered in four large areas (λ=26.5°S, ϕ=44.7°E, A95=4.7°, K=383) is the first African Carboniferous pole founded on both positive reversal and fold tests. It lies only slightly apart from other Middle Carboniferous poles previously published for the northern part of Africa where no intraformational test were available to constrain the magnetization age.
Journal of African Earth Sciences | 2001
M.E.M. Derder; B. Henry; Nacer-Eddine Merabet; M. Amenna; S. Bourouis
A palaeomagnetic study was conducted at different stratigraphical positions in rocks of Upper Carboniferous age (Bashkirian) in the Illizi Basin (Algeria). Three distinct remanence directions were isolated in these rocks. The first direction is associated with a Cenozoic magnetic overprint. The second direction yields a palaeomagnetic pole (40.3°S, 64.3°E) that is similar to previously published African Permian poles and is interpreted as a Permian remagnetisation. The third direction gives a new Late Carboniferous (Bashkirian) African palaeomagnetic pole (28.2°S, 55.5°E, K = 207, A95 = 3.4°). This pole is in a good agreement with previously published Saharan poles for the neighbouring periods. The combination of the new given datum with the previous Carboniferous poles of Gondwana shows that the reconstruction of Gondwana should be based on the parameters of Ricou. (1990) rather than on those of the classical du Toit (1937) model.
Studia Geophysica Et Geodaetica | 1999
N. Merabet; B. Henry; H. Bouabdallah; Said Maouche
The paleomagnetic study of the Namurian of Reouina (28.9°N, 08.0°W) revealed the existence of two magnetization components, either juxtaposed or superimposed, besides a viscous component. The high blocking temperature component, carried by hematite, has a mean direction defined by D = 126.9° and I = 10.8°. It provides a Namurian paleomagnetic pole located at 28.4°S and 56.9°E (K = 642, A95=1.7°). The second component is carried at least in part, by grains with blocking temperatures lower than 550°C. Though well defined, it consists of two superimposed components, the high unblocking temperature component with a likely Permian overprint.
Tectonophysics | 1998
Nacer-Eddine Merabet; H Bouabdallah; B. Henry
Abstract One of the reference Permian paleomagnetic poles for the African Apparent Polar Wander Path (APWP) was determined by (Morel et al., 1981, Earth Planet. Sci. Lett. 55, 65–74) in the upper unit of the 2000 m thick Abadla redbeds formation (31.2°N, 2.7°W). Unfortunately, this upper unit remains undated, although its age was presumed to be Autunian, whereas microflora fossils characteristic of the Autunian were discovered in the lower unit of the same formation. The paleomagnetic data (N=11, D=130.1°, I=13.0°, k=138, α95=3.6° and pole: 29.1°S, 57.8°E, A95=2.0°) obtained from the better dated Autunian levels must take the place of the previous Abadla pole for the Autunian part of the African APWP. This new pole is very close to the Morel et al.s pole determined from the unfossiliferous upper unit of the Abadla formation, giving an Autunian age for this upper unit. Consequently, the subsidence of the Abadla basin and the deposition of the whole succession occurred during a period not longer than 20–25 Myr.
Tectonophysics | 2001
P. F. Silva; Fernando O. Marques; J. M. Miranda; B. Henry; Antonio F Mateus
Abstract In this paper, we present an anisotropy of magnetic susceptibility (AMS) and structural study of high-grade metamorphic and ophiolitic rocks from the Braganca Massif, trying to contribute to the debate concerning the identification of transport directions during the closure of the Variscan Ocean. The study area comprises high-grade metamorphic rocks of the Continental Allochthonous Terrane (CAT) and the Northern Ophiolite Terrane (NOT) in the Braganca Massif. Mineral parageneses and fabrics were characterized through conventional petrographic and microstructural studies, electron microprobe, and high field and thermomagnetic analysis. With very few exceptions, the magnetic lineations and foliations were coincident, within limited error, with the observed mesoscopic mylonitic and metamorphic foliations, and with the mineral and stretching lineations. In places of the studied major tectonic contacts where the mineral and stretching lineations were obliterated by post-kinematic recrystallization, AMS data revealed a magnetic lineation. AMS results confirmed the NNW–SSE to N–S amphibolite facies lineation common to CAT and NOT, and revealed an E–W greenschist facies lineation in NOT. AMS results combined with detailed structural and metamorphic data from previous work showed that there are two E–W lineations separated in time: (1) an E–W mesoscopic and magnetic lineation in competent granulites that is older than the N–S magnetic lineation in more ductile gneisses of a major shear zone within the CAT; (2) an E–W magnetic lineation in incompetent greenschists of the NOT, younger than the NNW–SSE to N–S mesoscopic and magnetic lineations found in more competent amphibolites of CAT and NOT. The NNW–SSE to N–S lineations can be interpreted as the result of the uppermost allochthonous terranes transport to the NNW over the Iberian Terrane, dated with 40 Ar/ 39 Ar at ca. 390 Ma.
Physics and Chemistry of The Earth | 1997
B. Henry
Abstract The intersection of magnetic foliation represents the zone axis of the anisotropy carriers. It is a useful complementary direction for the knowledge of the structure, sometimes (particularly in sites with a strongly oblate fabric) difficult, to determine at the origin of the magnetic lineation.
Arabian Journal of Geosciences | 2017
M. Amenna; M. E. M. Derder; B. Henry; S. Maouche; B. Bayou; R. Bestandji; H. Bouabdallah; A. Ouabadi; M. Ayache; M. Beddiaf
In Ordovician and Silurian sedimentary formations of the Murzuq basin (Saharan platform, Algeria), different remagnetization processes have been highlighted. These magnetic overprints totally replaced the primary magnetization. They are mainly due to chemical phenomena. Even in a site affected by contact metamorphism during Devonian, chemical changes, associated to the acquisition of the thermo-remanent overprint, were important, affecting the characteristics of the magnetite grains. In the remaining sites, remagnetizations of Cenozoic age have also a chemical origin and are carried by magnetite as well as by hematite. Contrary to what is generally deemed, these remagnetizations processes appeared limited to very short duration of acquisition, and to very local geographical extension.
Geophysical Journal International | 2012
P. F. Silva; B. Henry; Fernando O. Marques; Anthony Hildenbrand; Pedro Madureira; C. A. Meriaux; Z. Kratinova
We present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1.3 Myr on S. Jorge Island (Azores ArchipelagoNorth Atlantic Ocean). The sections sampled in the southeastern and central/western parts of the island record reversed and normal polarities, respectively. They indicate a mean palaeomagnetic pole (81.3 degrees N, 160.7 degrees E, K= 33 and A95= 3.4 degrees) with a latitude shallower than that expected from Geocentric Axial Dipole assumption, suggesting an effect of non-dipolar components of the Earth magnetic field. Virtual Geomagnetic Poles of eight flows and two dykes closely follow the contemporaneous records of the Cobb Mountain Subchron (ODP/DSDP programs) and constrain the age transition from reversed to normal polarity at ca. 1.207 +/- 0.017 Ma. Volcano flank instabilities, probably related to dyke emplacement along an NNWSSE direction, led to southwestward tilting of the lava pile towards the sea. Two spatially and temporally distinct dyke systems have been recognized on the island. The eastern is dominated by NNWSSE trending dykes emplaced before the end of the Matuyama Chron, whereas in the central/western parts the eruptive fissures oriented WNWESE controlled the westward growth of the S. Jorge Island during the Brunhes Chron. Both directions are consistent with the present-day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre-existing fractures. The distinct timing and location of each dyke system likely results from a slight shift of the magmatic source.
Geophysical Journal International | 2012
P. F. Silva; B. Henry; Fernando O. Marques; A. Hildenbrand; Pedro Madureira; C. A. Mériaux; Zuzana Kratinová
We present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1.3 Myr on S. Jorge Island (Azores ArchipelagoNorth Atlantic Ocean). The sections sampled in the southeastern and central/western parts of the island record reversed and normal polarities, respectively. They indicate a mean palaeomagnetic pole (81.3 degrees N, 160.7 degrees E, K= 33 and A95= 3.4 degrees) with a latitude shallower than that expected from Geocentric Axial Dipole assumption, suggesting an effect of non-dipolar components of the Earth magnetic field. Virtual Geomagnetic Poles of eight flows and two dykes closely follow the contemporaneous records of the Cobb Mountain Subchron (ODP/DSDP programs) and constrain the age transition from reversed to normal polarity at ca. 1.207 +/- 0.017 Ma. Volcano flank instabilities, probably related to dyke emplacement along an NNWSSE direction, led to southwestward tilting of the lava pile towards the sea. Two spatially and temporally distinct dyke systems have been recognized on the island. The eastern is dominated by NNWSSE trending dykes emplaced before the end of the Matuyama Chron, whereas in the central/western parts the eruptive fissures oriented WNWESE controlled the westward growth of the S. Jorge Island during the Brunhes Chron. Both directions are consistent with the present-day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre-existing fractures. The distinct timing and location of each dyke system likely results from a slight shift of the magmatic source.