Éditeur(s) : HAL CCSD American Geophysical Union (AGU) American Geophysical Union
Résumé : International audience
Environmental magnetism of speleothems is still in its early stage of development. Here we report on our investigation of the environmental and paleomagnetic information that has been recorded in speleothems, and what are the factors that control its preservation and reliability. To address these issues, we used a multidisciplinary approach, including rock magnetism, petrography, scanning electron microscopy, stable carbon and oxygen isotope compositions, and major and trace element concentrations. We applied this to a set of samples from different stages of speleothem evolution: present-day dripwater (glass plates), a weathered stalactite, a fresh stalagmite, cave sediments, and terra rossa soils. These samples come from the Penico and Excentricas caves, located in two distinct aquifers of the Algarve region, South Portugal. Our results show that the main magnetic carriers of the speleothems under study are primary (detrital) and consist of maghemite (and magnetite?). Similarities in coercivity and temperature dependence of the studied set of samples suggest that iron oxides are inherited from the terra rossa soils that cap the cave and were transported to the speleothems by dripwater. Hence, they represent a regional environmental signature. Interestingly, a stable and probably detrital remanent magnetization could be isolated in the fresh stalagmite, whereas the weathered stalactite yielded chaotic magnetic directions and very low remanent intensities. We propose that these low intensities can be the result from (i) different remanence acquisition mechanisms between stalagmite and stalactite and/or (ii) iron dissolution by fungal activity. We also suggest that magnetic properties and color and the content in detrital elements in the fresh speleothem inform about environmental processes acting on the interface of rock (soil)-atmosphere, while oxygen isotope composition and alkaline-earth element concentrations inform about calcite-water interaction processes. These results provide a better understanding of how environmental information is recorded in speleothems and what the factors are that control the reliability of the paleomagnetic and paleo-environmental signal.
ISSN: 0148-0227

hal-01114953
https://hal.archives-ouvertes.fr/hal-01114953
DOI : 10.1002/2014JB011381

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
The Tian Shan Mountains constitute central Asia's longest and highest mountain range. Understanding their Cenozoic uplift history thus bears on mountain building processes in general, and on how deformation has occurred under the influence of the India-Asia collision in particular. In order to help decipher the uplift history of the Tian Shan, we collected 970 samples for magnetostratigraphic analysis along a 4571-m-thick section at the Jingou River (Xinjiang Province, China). Stepwise alternating field and thermal demagnetization isolate a linear magnetization component that is interpreted as primary. From this component, a magnetostratigraphic column composed of 67 polarity chrons are correlated with the reference geomagnetic polarity timescale between ~1 Ma and ~23.6 Ma, with some uncertainty below ~21 Ma. This correlation places precise temporal control on the Neogene stratigraphy of the southern Junggar Basin and provides evidence for two significant stepwise increases in sediment accumulation rate at ~16–15 Ma and ~11–10 Ma. Rock magnetic parameters also undergo important changes at ~16–15 Ma and ~11–10 Ma that correlate with changes in sedimentary depositional environments. Together with previous work, we conclude that growth history of the modern Tian Shan Mountains includes two pulses of uplift and erosion at ~16–15 Ma and ~11–10 Ma. Middle to upper Tertiary rocks around the Tian Shan record very young (<~5 Ma) counterclockwise paleomagnetic rotations, on the order of 15° to 20°, which are interpreted as because of strain partitioning with a component of sinistral shear that localized rotations in the piedmont.
ISSN: 0278-7407

insu-00376345
https://hal-insu.archives-ouvertes.fr/insu-00376345
https://hal-insu.archives-ouvertes.fr/insu-00376345/document
https://hal-insu.archives-ouvertes.fr/insu-00376345/file/2007TC002137.pdf
DOI : 10.1029/2007TC002137

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Imaging deep active volcanic areas remains a challenge in our understanding of their activity and evolution, especially in subduction zones. Study of magnetic anomalies is appropriate to access such dynamics in depth. The magnetic anomaly pattern of the Lesser Antilles Arc (LAA) subduction is studied through Curie Point Depth (CPD), interpreted as the depth of the 580 °C isotherm, and developed to better assess the deep thermal structure of the arc. The depth of the estimated CPD exhibits a complex topography. Keeping in mind the overall uncertainty associated with this method, a main doming is evidenced below the Guadeloupe archipelago. Its apex is shifted towards the ancient arc, suggesting a very hot state of the fore-arc/arc domain. To better understand the LAA thermal state, we perform 2D thermo-mechanical simulations of the subduction zone. Recalling that magnetite is a serpentinization by-product, we simulate water transfer triggered by slab dehydration to test the assumption of fore-arc serpentinization suggested by the positive magnetic anomaly in the vicinity of the Guadeloupe archipelago. In this area, the subduction-induced arc lithosphere hydration and related weakening trigger a fast heating of the upper plate by basal convective removal. This process of fast arc lithosphere thinning may apply where simultaneously the volcanic arc is split in two and normal convergence is high enough. As serpentinization strongly decreases P-wave velocity, we propose a new interpretation of a published seismic profile below Guadeloupe. The seismic layer previously interpreted as the arc lower crust may rather be a layer of serpentinized mantle, as supported by spatial correlations between gravimetric and magnetic anomalies. Consequently, at the scale of Guadeloupe Island, the fore-arc Moho would be shallower than initially assumed, with a dome shape more consistent with both the extensive deformation active since the Oligocene in the inner fore-arc and the CPD doming.
ISSN: 0040-1951

Droits : http://creativecommons.org/licenses/by-nc-nd/
insu-01505300
https://hal-insu.archives-ouvertes.fr/insu-01505300
https://hal-insu.archives-ouvertes.fr/insu-01505300/document
https://hal-insu.archives-ouvertes.fr/insu-01505300/file/Gailler-2017.pdf
DOI : 10.1016/j.tecto.2017.03.014

Éditeur(s) : HAL CCSD Mineralogical Society
Résumé : The diagenetic evolution of the magnetic minerals during burial in sedimentary basins has been recently proposed. In this study, we provide new data from the Grès d’Annot basin, SE France. We analyze fine-grained clastic rocks that suffered a burial temperature from ~60 to >250 °C, i.e., covering oil and gas windows. Low temperature magnetic measurements (10–300 K), coupled with vitrinite reflectance data, aim at defining the magnetic mineral evolution through the burial history. Magnetite is documented throughout the entire studied transect. Goethite, probably occurring as nanoparticles, is found for a burial temperature <80 °C. Micron-sized pyrrhotite is highlighted for a burial temperature >200 °C below the Alpine nappes and the Penninic Front. A model of the evolution of the magnetic assemblage from 60 to >250 °C is proposed for clastic rocks, containing iron sulfides (pyrite) and organic matter. This work provides the grounds for a better understanding of the magnetic properties of petroleum plays.
ISSN: 0009-8558

hal-01122812
https://hal.archives-ouvertes.fr/hal-01122812
DOI : 10.3390/min4030667

Éditeur(s) : HAL CCSD Springer/Terra Scientific Publishing Company
Résumé : International audience
We carried out a detailed paleomagnetic, rock-magnetic, and Thellier paleointensity study of all similar to 3.6 My Pliocene lava flow succession in southern Georgia. An earlier study (Camps et al., 1996) revealed that several consecutive lava flows record an intermediate polarity direction at the base of the section followed by a thick reverse polarity zone. The transitional field was interpreted as an excursion within chron 2Ar or all upper Cochiti-Gilbert reversal. New paleomagnetic data reported here have been obtained from nearby lava successions. In total, about 170 standard paleomagnetic cores belonging to 22 lava flows were collected during the 2005 sample collection campaign. Rock-magnetic experiments showed that the remanence is carried by Ti-poor titanomagnetite in most of the samples. The fraction of grains with multidomain magnetic structure does not seem to be important. Characteristic remanent magnetization was successfully determined oil all samples. The direct correlation with the original (Thoki) sequence, previous preliminary measurements of natural remanent magnetization (Sologashvili, 1986), and field observations allowed us to establish a new magnetic stratigraphy. The lower part of section is characterized by intermediate magnetic polarity followed by thick reversely magnetized lavas. The upper sequence, represented by 18 consecutive flows yielded normal magnetic polarity. The mean paleointensity of the intermediate field is drastically reduced with respect to the post-transitional field strength. Based on all available radiometric ages and new paleomagnetic data, it may be speculated that Gilbert-Gauss (R-N) reversal was recorded at the upper part of sequence. Lower intermediate polarity flows possibly represent a form of precursor of this reversal that is similar to the Matuyama-Brunhes geomagnetic transition.
ISSN: 1343-8832

hal-00413099
https://hal.archives-ouvertes.fr/hal-00413099

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The N-S oriented Raghane shear zone (8 degrees 30') delineates the western boundary of the Saharan metacraton and is, with the 4 degrees 50' shear zone, the most important shear zone in the Tuareg shield. It can be followed on 1000 km in the basement from southern Air, Niger to NE Hoggar, Algeria. Large subhorizontal movements have occurred during the Pan-African orogeny and several groups of granitoids intruded during the Neoproterozoic. We report U-Pb zircon datings (laser ICP-MS) showing that three magmatic suites of granitoids emplaced close to the Raghane shear zone at c. 790 Ma, c. 590 and c. 550 Ma. A comprehensive and detailed (158 sites, more than 1000 cores) magnetic fabric study was performed on 8 plutons belonging to the three magmatic suites and distributed on 200 km along the Raghane shear zone. The main minerals in all the target plutons do not show visible preferential magmatic orientation except in narrow shear zones. The AMS study shows that all plutons have a magnetic lineation and foliation compatible with the deformed zones that are zones deformed lately in post-solidus conditions. These structures are related to the nearby mega-shear zones, the Raghane shear zone for most of them. The old c. 793 Ma Touffok granite preserved locally its original structures. The magnetic structures of the c. 593 Ma Ohergehem pluton, intruded in the Aouzegueur terrane, are related to thrust structures generated by the Raghane shear zone while it is not the case of the contemporaneous plutons in the Assode-Issalane terrane whose structures are only related to the subvertical shear zones. Finally, the c. 550 Ma granite group has magnetic structure related to the N-S oriented Raghane shear zone and its associated NNE-SSW structures when close to them, but NW-SE oriented when further. These NW-SE oriented structures appear to be characteristic of the late Neoproterozoic evolution of the Saharan metacraton and are in relation to the convergence with the Murzuq craton. This evolution reflects the rheological contrast existing along the Raghane shear zone marking the western boundary of the Saharan metacraton.
ISSN: 0040-1951

hal-00429354
https://hal.archives-ouvertes.fr/hal-00429354
DOI : 10.1016/j.tecto.2009.04.022

Éditeur(s) : HAL CCSD
Résumé : Guadeloupe Island (West French Indies) is one of the twenty islands that compose the Lesser Antilles Arc, which results from the subduction of the Atlantic Ocean plate beneath the Caribbean one. The island lies in a complex volcano-tectonic system and the need to understand its geological context has led to numerous on- and offshore geophysical investigations. This work presents the compilation and processing of available, on-land, airborne and marine, gravity and magnetic data acquired during the last 40 years on Guadeloupe Islands and at the scale of the Lesser Antilles Arc. The overall dataset provides new Bouguer and reduced to the pole magnetic anomaly maps at the highest achievable resolution. Regionally, the main central negative gravity trend of the arc allows defining two subsident areas. The first one is parallel to the arc direction (~N160°E) to the north, whereas the second unexpected southern one is oriented parallel to oceanic ridges (N130°E). Along the Outer Arc, the long wavelength positive anomaly is interpreted, at least along the Karukera Spur, as an up-rise of the volcanic basement in agreement with the seismic studies. To the NE of Guadeloupe, the detailed analysis of the geophysical anomalies outlines a series of structural discontinuities consistent with the main bathymetric morphologies, and in continuity of the main fault systems already reported in this area. Based on geophysical evidences, this large scale deformation and faulting of the Outer Arc presumably primarily affects the Atlantic subducting plate and secondarily deforms the upper Caribbean plate and the accretion prism. At the scale of Guadeloupe Island, joined gravity and magnetic modeling has been initiated based on existing interpretation of old seismic refraction profiles, with a general structure in three main layers. According to our geophysical anomalies, additional local structures are also modeled in agreement with geological observations: i) the gravity and magnetic signals confirm an up-rise of the volcanic basement below the limestone platforms outcropping on Grande-Terre Island ; ii) the ancient volcanic complexes of Basse-Terre Island are modeled with high density and reverse magnetized formations; iii) the recent volcanic centre is associated with formations consistent with the low measured density and the underlying hydrothermal system. The EW models coherently image a NNW-SSE depression structure in half-graben beneath Basse-Terre Island, its western scarp following the arc direction in agreement with bathymetric and seismic studies to the north of the island. The so-defined depressed area, and particularly its opening in half-graben toward the SW, is interpreted as the present-day front of deformation of the upper plate associated with the recent volcanic activity on and around Guadeloupe. Based on this regional deformation model, perspectives are given for further integrated investigation of key targets to address the internal structure and evolution of the Lesser Antilles Arc and Guadeloupe volcanic system.
Colloque 2012 du Comité National Français de Géodésie et Géophysique (CNFGG)
Clermont-Ferrand, France

hal-00742653
https://hal-brgm.archives-ouvertes.fr/hal-00742653
https://hal-brgm.archives-ouvertes.fr/hal-00742653/document
https://hal-brgm.archives-ouvertes.fr/hal-00742653/file/CNFGG_Caraibes.pdf

Éditeur(s) : HAL CCSD Springer Verlag (Germany) Springer Verlag
Résumé : International audience
La Cabaña peridotite is part of a dismembered ophiolite complex located within the metamorphic basement of the Coastal Cordillera of south-central Chile, and is the only location in Chile were Cr-spinels have been described so far. The La Cabaña peridotite is part of the Western Series unit, which comprises meta-sedimentary rocks, metabasites, and serpentinized ultramafic rocks. This unit has been affected by greenschist-facies metamorphism with reported peak PT conditions of 7.0–9.3 kbar and 380°–420 °C. Within La Cabaña peridotite Cr-spinels are present in two localities: Lavanderos and Centinela Bajo. In Lavanderos, Cr-spinel occurs in small chromitite pods and as accessory/disseminated grains with a porous or spongy texture in serpentinite, whereas in Centinela Bajo Cr-spinel is present as accessory zoned grains in partly serpentinized dunites, and in chromitite blocks. All Cr-spinels display variable degrees of alteration to Fe2+-rich chromite with a variation trend of major elements from chromite to Fe2+-rich chromite similar to those observed in other locations, i.e., an increase in Fe2O3 and FeO, a decrease in Al2O3 and MgO. Cr2O3 content increases from chromite to Fe2+-rich chromite in chromitite pods from Lavanderos and chromitite blocks from Centinela Bajo, but decreases in ferrian chromite zones in accessory grains from Centinela Bajo. The minor element (Ti, V, Zn, Ni) content is mostly low and does not exceed 0.4 wt.%, with the exception of MnO (<0.9 wt.%), which shows a correspondence with increasing degree of alteration. Cr# (Cr/Cr + Al) versus Mg# (Mg/Mg + Fe2+) and Fe3+/Fe3++Fe2+ versus Mg# plots are used to illustrate the Cr-spinel alteration process. Overall, the Cr-spinels from Lavanderos (chromitite pods and disseminated grains) exhibit Cr# values ranging from 0.6 to 1.0, Mg# (Mg/Mg + Fe2+) below 0.5, and (Fe3+/Fe3++Fe2+) <0.4. Cr-spinels from chromitites in Centinela Bajo have Cr# and Mg# values that range from 0.65 to 1.0, and 0.7-0.3, respectively, and (Fe3+/Fe3++Fe2+) < 0.4. Accessory Cr-spinels from Centinela Bajo have Cr# and Mg# values that range from 0.55 to 1.0, and 0.6- < 0.1, respectively. The (Fe3+/Fe3++Fe2+) ratio is less than 0.4 in chromite cores and Fe2+-rich chromite, and >0.5 in ferrian chromite and Cr-magnetite. Interpretation of the data obtained and Cr-spinel textures indicate that the alteration of Cr-spinel is a progressive process that involves in its initial stages the reaction of chromite with olivine under water-saturated conditions to produce clinochlore and Fe2+-rich chromite. During this stage the chromite can also incorporate Ni, Mn, and/or Zn from the serpentinization fluids. As alteration progresses, Fe2+-rich chromite loses mass resulting in the development of a spongy texture. In a later stage and under more oxidizing conditions Fe3+ is incorporated in chromite/Fe2+-rich chromite shifting its composition to an Fe3+-rich chromite (i.e., ferrian chromite). Depending on the fluid/rock and Cr-spinel/silicate ratios, Cr-magnetite can also form over Fe2+-rich chromite and/or ferrian chromite as a secondary overgrowth. The compositional changes observed in Cr-spinels from La Cabaña reflect the initial stages of alteration under serpentinization conditions. Results from this study show that the alteration of Cr-spinels is dependent on temperature. The degree and extent of alteration (formation of Fe2+-rich and/or ferrian chromite) are controlled by the redox nature of the fluids, the Cr-spinel/silicate and the fluid/rock ratios.
ISSN: 0930-0708

hal-01172266
https://hal.archives-ouvertes.fr/hal-01172266
DOI : 10.1007/s00710-014-0335-5

Éditeur(s) : The American Association of Petroleum Geologlists
Résumé : The west African continental margin between Abidjan (Ivory Coast) and Walvis Boy (Southwcst Africa) was surveyed in 1971 by the R/V Jean Charcot. Fiftyeight seismic-reflection (flexotir source), bathymetric (3.5 and 12 kc), gravimetric, and magnetic profiles were obtained. The seaward limit of an evaporitic zone outlined during the survey is at the boundary between continental slope and continental rise. Variation of depth of this limit as a function of latitude shows the presence of a large offset at 11°S which seems to be related to an east-east-southeast line of magnetic seamounts cutting into the continental slope. The Annobon-Cameroun volcanic axis separates the salt zones of the Nigerian basin and Congo-Angola basin. The northern limit of Salt deposition in the Congo-Angola basin is marked near 1°N by a strong southwest-northeast magnetic trend. On the south it extends to necr 14°s. The Walvis Ridge, located farther south, does not seem to have an effect on evaporite deposition. [NOT CONTROLLED OCR]
The American Association of Petroleum Geologlists Bulletin (The American Association of Petroleum Geologlists), 1973 , Vol. 57 , N. 9 , P. 1658-1671

Droits : The American Association of Petroleum Geologlists
http://archimer.ifremer.fr/doc/1973/publication-5123.pdf
http://archimer.ifremer.fr/doc/00000/5123/

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Three kilns and a collection of baked bricks from Italian archaeological sites have been studied for archaeointensity determination using the Thellier method as modified by Coe. All sites are dated based on archaeological information and their ages range from 500 to 800 AD and 1500 to 1700 AD. Rock magnetic studies identify magnetite, Ti- magnetite and hematite as the main magnetic minerals, and magnetic susceptibility versus temperature shows a good thermal stability of the samples. The intensity results have been corrected for anisotropy of the thermoremanent magnetization and cooling rate effects. The new data together with 136 previously published results are used to estimate the variation of the Earth's magnetic field over the past three millennia. The time distribution of the Italian absolute intensity data is irregular with the majority of determinations concentrated during the last four centuries, while older periods are very poorly covered. Most of the data come from volcanic rocks and show significant discrepancies. All data have been compared with archaeointensity results from Europe and regional and global models predictions. This work indicates the need to obtain more high quality archaeointensity results, in particular for the periods older than 200 BC and between 200 and 1000 AD, in order to determine a robust Italian intensity secular variation curve that, in combination with directional data, could be used for archaeomagnetic dating.
ISSN: 0956-540X

hal-00452451
https://hal.archives-ouvertes.fr/hal-00452451
DOI : 10.1111/j.1365-246X.2009.04455.x

Éditeur(s) : HAL CCSD
Résumé : International audience
In this work, a series of zinc partially substituted magnetite nanoparticles (Fe3-xZnxO4, 0 ≤ x ≤ 0.4), were synthesised through a facile precipitation-oxidation method. The partial substitution of zinc into the magnetite (Fe3O4) structure was confirmed by the collective findings of nitrogen sorption, XRD and TG-DTG analysis. It was found that the partial substitution of zinc slightly changed the textural properties of the resultant Fe3-xZnxO4 nanoparticles. From the XRD analysis, there was no visible formation of secondary phase or impurity peaks in the nanoparticles. These findings indicated the partial substitution of zinc into the Fe3O4 crystal structure with a good dispersion within the Fe3O4 matrix. © 2016 Author(s).
1st International Conference on Advanced Science,
Penang, Malaysia

hal-01683217
https://hal.univ-antilles.fr/hal-01683217

Éditeur(s) : American Geophysical Union
Résumé : We present four companion digital models of the age, age uncertainty, spreading rates, and spreading asymmetries of the world's ocean basins as geographic and Mercator grids with 2 arc min resolution. The grids include data from all the major ocean basins as well as detailed reconstructions of back-arc basins. The age, spreading rate, and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments. The age uncertainties for grid cells coinciding with marine magnetic anomaly identifications, observed or rotated to their conjugate ridge flanks, are based on the difference between gridded age and observed age. The uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities. Asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges, resulting in ridge jumps toward hot spots. We also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age-depth relationships. The new set of grids helps to investigate prominent negative depth anomalies, which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow. A combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems.
Geochemistry Geophysics Geosystems - G3 (1525-2027) (American Geophysical Union), 2008-04 , Vol. 9 , P. NIL_18-NIL_36

Droits : 2008 American Geophysical Union
http://archimer.ifremer.fr/doc/2008/publication-3900.pdf
DOI:10.1029/2007GC001743
http://archimer.ifremer.fr/doc/00000/3900/

Éditeur(s) : HAL CCSD Société géologique de France
Résumé : International audience
Guadeloupe island (West French Indies) is one of the twenty islands that compose the Lesser Antilles arc, which results from the subduction of the Atlantic ocean plate beneath the Caribbean one. The island lies in a complex volcano-tectonic system and the need to understand its geological context has led to numerous on- and offshore geophysical investigations. This work presents a compilation and the processing of available, on-land, airborne and marine, gravity and magnetic data acquired during the last 40 years on Guadeloupe islands and at the scale of the Lesser Antilles arc. The overall dataset provides new Bouguer and reduced to the pole magnetic anomaly maps at the highest achievable resolution. Regionally, the main central negative gravity trend of the arc allows defining two subsident areas. The first one is parallel to the arc direction (~N160°E) to the north, whereas the second unexpected southern one is oriented parallel to oceanic ridges (N130°E). Along the Outer arc, the long wavelength positive anomaly is interpreted, at least along the Karukera spur, as an up-rise of the volcanic basement in agreement with the seismic studies. To the NE of Guadeloupe, the detailed analysis of the geophysical anomalies outlines a series of structural discontinuities consistent with the main bathymetric morphologies, and in continuity of the main fault systems already reported in this area. Based on geophysical evidences, this large scale deformation and faulting of the Outer arc presumably primarily affects the Atlantic subducting plate and secondarily deforms the upper Caribbean plate and the accretion prism, as evidenced in bathymetry as well as on the islands. At the scale of Guadeloupe island, combined gravity and magnetic modeling has been initiated based on existing interpretation of old seismic refraction profiles, with a general structure in three main layers. According to our geophysical anomalies, additional local structures are also modeled in agreement with geological observations: i) the gravity and magnetic signals confirm an up-rise of the volcanic basement below the limestone platforms outcropping on Grande-Terre island ; ii) the ancient volcanic complexes of Basse-Terre island are modeled with high density and reverse magnetized formations; iii) the recent volcanic centre is associated with formations consistent with the low measured density and the underlying hydrothermal system. The E-W models coherently image a NNW-SSE depression structure in half-graben beneath Basse-Terre island, its western scarp following the arc direction in agreement with bathymetric and seismic studies to the north of the island. The so-defined depressed area, and particularly its opening in half-graben toward the SW, is interpreted as the present-day front of deformation of the upper plate, associated with the recent volcanic activity on and around Guadeloupe. Based on this regional deformation model, perspectives are given for further integrated investigation of key targets to address the internal structure and evolution of the Lesser Antilles arc and Guadeloupe volcanic system.
EISSN: 1777-5817

hal-00853352
https://hal.archives-ouvertes.fr/hal-00853352
DOI : 10.2113/gssgfbull.184.1-2.77

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Serpentinization of olivine-rich troctolite from core 227, Integrated Ocean Drilling Program (IODP) Hole U1309D ranges from 10 to 90. Two episodes of serpentinization are recognized. The first, dominant in weakly serpentinized samples, is an approximately isochemical (except for water) replacement of olivine (Fo(8485)) by a mixture of serpentine (antigorite, Mg-number 92) and brucite (amakinite-rich; Mg-number 65). The compositions of the minerals in type 1 veins are a reflection of FeMg exchange between olivine and the brucite serpentine formed during early serpentinization. The early serpentinite veins (type 1) are thin ( 005 mm), irregular, and exploit pre-existing cracks in olivine. The presence of antigorite suggests that early serpentinization occurred at T 300C. Type 1 veins reflect rock-dominated serpentinization, became isolated early in their history, and persist as relicts in all but the most altered samples. The main episode of serpentinization is manifested by through-going lizardite (average Mg-number 96)magnetite veins (type 2). Type 2 veins define an anastomosing foliation, may be several millimeters in width and appear to exploit pre-existing, favorably oriented type 1 veins. Type 2 veins reflect open-system serpentinization. Magnetite in these veins formed by oxidation of the Fe in brucite and serpentine, whereas addition of silica to the system converted the Mg-component of the brucite to serpentine. Magnetite forms one or more distinct bands in the interior of the vein and is never in direct contact with relict olivine. A bruciteserpentine mixture, similar to that found in type 1 veins, but with lizardite instead of antigorite, is commonly present at the margins of type 2 veins (i.e. where they are in reaction contact with relict olivine). We interpret type 2 veins as a steady-state system where brucite continually forms at the olivinevein contact and then reacts out in the interior of the vein. This continual formation and destruction of brucite imposes an exceptionally low a(SiO2) on the system. Magnetite and olivine are never in contact in type 2 veins (or anywhere else) because the olivine-out reaction yields ferroan brucite and not magnetite. The desilication of serpentine in the type 2 veins is a reflection of the inherent instability of Fe-rich serpentine with respect to magnetite at low silica activity. Thus, the composition of serpentine in equilibrium with magnetite in serpentinites is a function of serpentinemagnetite and not serpentineolivine equilbria.
ISSN: 0022-3530

hal-00420931
https://hal.archives-ouvertes.fr/hal-00420931
DOI : 10.1093/petrology/egp004

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Espinho Branco anatexite, located within a transcurrent, high-temperature shear zone in NE Brazil, was the subject of a comprehensive petrostructural study (Anisotropy of Magnetic Susceptibility - AMS, Anisotropy of Anhysteretic Remanence - AAR, Electron Backscatter Diffraction - EBSD) to evaluate the compatibility of different fabrics with the kinematics of melt deformation. Magnetite dominates susceptibilities larger than 1 mSI and biotite displays [001] lattice directions consistent with AMS k3 axes. In contrast, migmatites with a susceptibility lower than 0.5 mSI and no visible mesoscopic foliation provide crystallographic fabrics distinct from AMS and AAR. However, AAR remains consistent with the regional strain field. These results suggest that the correlation of field, AMS and crystallographic fabrics is not always straightforward despite the relatively simple organisation of the magnetic fabric in the anatexite. We conclude that AMS recorded the final stages of the strain field in the migmatite irrespective of its complex mesoscale structures and contrasting crystallographic fabrics.
ISSN: 0191-8141

hal-00823056
https://hal.archives-ouvertes.fr/hal-00823056
DOI : 10.1016/j.jsg.2012.12.008

Éditeur(s) : HAL CCSD Elsevier
Résumé : Reliable measurements of the intensity of the palaeomagnetic field are notoriously difficult to obtain. The approach generally taken is to produce multiple estimates per rock unit and assume that those which meet certain minimum standards of technical quality will combine to produce an accurate mean. However, here, using results produced from a package of 20th century basaltic lava flows from Mount Etna in Sicily, we demonstrate that this approach can fail. In this case, the application of typical sets of selection criteria actually introduces bias into the mean determination that we measure. We demonstrate that this is caused by two types of non-ideal behaviour acting in combination. The first is a result of the multidomain grains that the samples contain and the second is caused by differences in the natural and laboratory cooling rates. We discuss means of avoiding these sources of error in future palaeointensity studies performed on ancient rocks. We also develop a new, more general reliability criterion which is effective here and which we argue should be applied wherever possible in future palaeointensity studies in conjunction with standard criteria. It requires two distinct types of materials which both produce some good-quality palaeointensity measurements and uses their range of overlap to constrain the true palaeointensity. Both the application of this criterion and general reliability considerations require that future palaeointensity studies should measure many samples per cooling unit and that these should be as diverse in terms of their rock magnetic properties and cooling histories as possible.
ISSN: 0012-821X

hal-00407943
https://hal.archives-ouvertes.fr/hal-00407943
DOI : 10.1016/j.epsl.2007.03.017

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
A central target in Earth sciences is the study of deformation at various depth levels within the Earth. Seismology has offered a remarkable tool for doing this via seismic anisotropy. It is however not always clear how to interpret those observations. A question of interest is to understand the relation between the deformation of the mantle and the crust, and in studying the relation between the two. Mantle deformation is expressed in seismic anisotropy. In this paper we seek an objective way of extracting information about crustal fabric as well, to be able to compare with seismic anisotropy. The magnetization of crustal rocks offers an attractive possibility for doing this. We thus explore the use of magnetic data, and we compare magnetic crustal fabric orientation with mantle fabric observations from seismic anisotropy. We apply our technique to the North American craton for which we have an excellent magnetic dataset, and we show that there is a clear relation between crustal and mantle fabric for the cratonic region. This has important implications for crustal formation, and for interpreting seismic anisotropy observations.
ISSN: 0012-821X

hal-00424536
https://hal.archives-ouvertes.fr/hal-00424536
DOI : 10.1016/j.epsl.2008.10.032

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
IODP Expedition 352 cored igneous rocks from the Izu-Bonin-Mariana forearc crust. Cores from Sites U1440 and U1441 recovered Eocene basalts and related rocks and cores from Sites U1439 and U1442 recovered Eocene boninites and related rocks. We selected samples from Holes U1439C, U1440B and U1442A for palaeointensity measurements. Hysteresis measurements and high and low-temperature magnetization curves show that samples from Hole U1440B undergo magneto-chemical changes when heated and are mostly composed of single-domain (SD) or pseudo-single-domain (PSD) titanomaghemite. In contrast, the same measurements show that most selected samples from Holes U1439C and U1442A are thermally stable and are composed of either SD or PSD titanomagnetite with very little titanium content, or SD ferromagnetic grains with a large paramagnetic contribution. Thellier–Thellier palaeointensity experiments carried out on U1439C and U1442A samples give a good success rate of 25/60 and Virtual Dipole Moment (VDM) values between 1.3 and 3.5 × 1022 Am2. Multispecimen palaeointensity experiments with the domain-state corrected method carried out on 55 samples from Hole U1440B (divided into four groups) and 20 from Hole U1439C gave poor quality results, but indicated a VDM around 4–6 × 1022 Am2 in Hole U1440B forearc basalts. These results are in agreement with the few, low VDM values previously measured on Eocene rocks. However, they do not support an inverse relationship between field intensity and reversal rate for this period of time, since the Eocene reversal rate was low.
ISSN: 0956-540X

hal-01622775
https://hal.archives-ouvertes.fr/hal-01622775
https://hal.archives-ouvertes.fr/hal-01622775/document
https://hal.archives-ouvertes.fr/hal-01622775/file/carvalloGJI2017.pdf
DOI : 10.1093/gji/ggx208

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
IODP Expedition 324 cored igneous rocks from Shatsky Rise, an oceanic plateau in the northwest Pacific Ocean that formed mainly during late Jurassic and Early Cretaceous times. We selected 60 samples from 3 different holes for Thellier-Thellier palaeointensity determinations. Induced and remanent magnetization curves measured at low- and high-temperature suggest a diverse and complex magnetic mineralogy, with large variations in Ti content and oxidation state. Hysteresis and FORC measurements show that most samples contain singledomain magnetic grains. After carrying out the palaeointensity determinations, only 9 samples satisfied all reliability criteria. These gave palaeointensity values between 16.5 and 21.5 μT, which correspond to average VDM values of (4.9 ± 0.2) × 1022 Am2 for an estimated age of 140-142 Ma. This value is lower than that for the recent field, which agrees with the hypothesis of a Mesozoic Dipole Low.
ISSN: 0956-540X

hal-00795491
https://hal.archives-ouvertes.fr/hal-00795491
DOI : 10.1093/gji/ggs100

Éditeur(s) : Gauthier-villars
Résumé : During the Kanaut cruise of the submersible Nautile and its mothership Le-Nadir (15 November-16 December 1992), one dive was devoted to the detailed study of a specific area of the neovolcanic ridge of the MAR around 29-degrees-06'N and 43-degrees-11'W. This study emphasizes the importance of the juxtaposition and the stacking of hundreds of small volcanoes in the construction of a neovolcanic ridge and confirms the general observation made from multibeam and side-scan sonar data. The very large magnetic anomaly associated with these massifs favours an extreme freshness.
Lors de la campagne Kanaut (15 novembre-16 décembre 1992) du Nautile et de son navire support Le-Nadir, une plongée a été consacrée B l'étude détaille d'un secteur de la ride néovalcanique de l'axe de la dorsale médio-Atlantique autour de 29"06" et 43"ll'W. Cette étude permet de souligner le rôle prépondérant que jouent les centaines de petits édifices volcaniques dans la construction de la ride néovolcanique, par leur juxtaposition et leur superposition. Ceci confirme et affine les observations faites B partir des données des sondeurs multifaisceaux et les sonars latéraux. La très forte anomalie magnétique associé B ces édifices penche en faveur de leur extrème fraicheur.
Comptes Rendus de l'Academie des Sciences Serie II (1251-8069) (Gauthier-villars), 1993-05 , Vol. 316 , N. 10 , P. 1415-1422

Droits : Académie des Sciences
http://archimer.ifremer.fr/doc/00183/29475/27887.pdf
http://archimer.ifremer.fr/doc/00183/29475/