Éditeur(s) : HAL CCSD European Geosciences Union
Résumé : International audience
During the GWADASEIS cruise (Lesser Antilles volcanic arc, February-March 2009) a very high resolution (VHR) seismic-reflection survey was performed in order to constrain Late Quaternary to Present faulting. The profiles we obtained evidence frequent "ponding" of reworked sediments in the deepest areas, similar to the deposition of Mediterranean "homogenites". These bodies are acoustically transparent (few ms t.w.t. thick) and are often deposited on the hanging walls of dominantly normal faults, at the base of scarps. Their thickness appears sufficient to compensate (i.e. bury) co-seismic scarps between successive earthquakes, resulting in a flat and horizontal sea floor through time. In a selected area (offshore Montserrat and Nevis islands), piston coring (4 to 7 m long) was dedicated to a sedimentological analysis of the most recent of these particular layers. It corresponds to non-stratified homogenous calcareous silty sand (reworked calcareous plankton and minor volcanoclastics). This layer can be up to 2 m thick, and overlies finegrained hemipelagites. The upper centimeters of the latter represent the normal RedOx water/sediment interface. 210Pb and 137Cs activities lack in the massive sands, while a normal profile of unsupported 210Pb decrease is observed in the hemipelagite below, together with a 137Cs peak correspond- ing to the Atmospheric Nuclear Experiments (1962). The RedOx level was thus capped by a recent instantaneous major sedimentary event considered as post-1970 AD; candidate seismic events to explain this sedimentary deposits are either the 16 March 1985 earthquake or the 8 October 1974 one (Mw=6.3 and Mw=7.4, respectively). This leads to consider that the syntectonic sedimentation in this area is not continuous but results from accumulation of thick homogenites deposited after the earthquakes (as observed in the following weeks after Haiti January 2010 event, McHugh et al., 2011). The existence of such deposits suggests that, in the area of study, vertical throw likely results from cumulated effects of separated earthquakes rather than from aseismic creep. Examination of VHR profiles shows that all major co-seismic offsets are recorded in the fault growth sequence and that co-seismic offsets can be precisely estimated. By using a sedimentation rate deduced from 210Pb decrease curve (0.5 mm yr−1) and taking into account minor reworking events detected in cores, we show that the Redonda system may have been responsible for five >M6 events during the last 34 000 yr. The approach presented in this work differs from fault activity analyses using displaced sets of isochronous surfaces and postulating co-seismic offsets. Combining VHR seismic imagery and coring we can decipher co-seismic vs. slow continuous displacement, and thus actually estimate the amplitude and the time distribution of major co-seismic offsets.
ISSN: 1561-8633

insu-00844581
https://hal-insu.archives-ouvertes.fr/insu-00844581
https://hal-insu.archives-ouvertes.fr/insu-00844581/document
https://hal-insu.archives-ouvertes.fr/insu-00844581/file/nhess-12-1755-2012.pdf
DOI : 10.5194/nhess-12-1755-2012

Éditeur(s) : HAL CCSD AMIFRAM
Résumé : International audience
L'auteur ébauche ici un certain nombre de pistes didactiques afin d'améliorer l'évaluation en langues: pour une meilleure prise en compte de la validité de construit des épreuves, pour éviter les biais, pour améliorer la transparence des processus...Un plébiscite pour une démarche qualité en éducation.
ISSN: 2007-8986

Droits : info:eu-repo/semantics/OpenAccess
halshs-01400379
https://halshs.archives-ouvertes.fr/halshs-01400379
https://halshs.archives-ouvertes.fr/halshs-01400379/document
https://halshs.archives-ouvertes.fr/halshs-01400379/file/quelques%20r%C3%A9flexions%20sur%20l%27%C3%A9valuation%20en%20langues%2C%20comment%20faire%20mieux.pdf

Éditeur(s) : Elsevier
Résumé : The French Guiana transform margin and Demerara abyssal plain have been recently surveyed in the framework of the EXTRAPLAC French Program of extension of the continental shelf (Guyaplac survey, Ifremer-IFP-SHOM-IPEV). Based on the interpretation of some of the data collected during the Guyaplac survey (Simrad-EM12 multibeam bathymetric data, backscatter imagery, and 3.5 kHz profiles), the area can be divided into three morphostructural domains. (1) The western Guiana margin, including a part of the Demerara plateau, an important bathymetric relief prolonging the continental platform off Guiana and Surinam. This domain is bounded by (1a) the NWSE trending northern border of the Demerara plateau which appears quite steep and corresponds to a transform segment of the margin, (1b) the N-S eastern border of the Demerara plateau which corresponds to a divergent segment of the margin. The Demerara plateau shows a segmented morphology, low slope gradients, and a very rough surface (ripples perpendicular to the slope direction). NNW-SSE structural steps seem to correspond to collapses of 100 km long blocs towards the east. Slumps initiate along these directions. The observed rough bathymetry seems to be related to creeping processes. At a greater scale (seismic data), this part of the margin has been totally destabilized (numerous imbricate transparent masses rooted at about 0.5 s.t.w.t.t. below seafloor). The NW-SE trending northern border of the Demerara plateau corresponds to a cliff-like continental slope, probably slightly smoother than other transform margins (Ghana/cote d'Ivoire margin). The N-S eastern border of Demerara plateau is characterized by numerous small-scale imbricate slumps. Some of these failures seem to be emplaced in the prolongation of the NNW-SSE structural steps identified on the Demerara plateau. (2) The eastern Guiana margin corresponds to a NW-SE oriented gullied transform margin segment. The associated continental slope is very steep and characterized by numerous imbricate slumps and related debris flows. Some undulated masses, probably corresponding to creeping sediments or to older mass-wasting events are still imprinted on bathymetry. This transform margin segment is nearly entirely destabilized and eroded. (3) The Demerara abyssal plain. This domain is characterized eastwards by channels belonging to the Amazon turbidite system and westwards, at the foot of Demerara continental slope, by sediment waves probably created by contour currents. To conclude, it seems that there is a strong relationship between the structure (transform and divergent segments) and the emplacement of recurrent slope instabilities. These are probably related to the steepness of the slopes but also to subsidence histories generating in some cases huge deep-seated collapses of the whole margin. Fluid ascents are common everywhere in the area, probably enhancing slope instability. Their origin is not constrained but the black shales or Cretaceous organic-rich layers could be good candidates.
Marine and Petroleum Geology (0264-8172) (Elsevier), 2009-05 , Vol. 26 , N. 5 , P. 711-723

Droits : 2008 Elsevier Ltd All rights reserved.
http://archimer.ifremer.fr/doc/2009/publication-6856.pdf
DOI:10.1016/j.marpetgeo.2008.02.010
http://archimer.ifremer.fr/doc/00000/6856/

Éditeur(s) : HAL CCSD Elsevier
Résumé : The current interpretation of the Messinian Salinity Crisis (MSC) involves the deposition of the so-called “peripheral or marginal” evaporites in onshore basins, as well as the erosion of the margins and the formation of thick evaporites in the deep parts of the basins. The present study focuses on intermediate depth basins, i.e. located between the onshore outcrops and the deep basins. Indeed, the Balearic Promontory shows small stepped basins filled with MSC deposits between its western extremity on the Alicante shelf and its eastern end around the Menorca block. New and already available seismic reflection profiles and onshore data allow us to investigate the nature and geometrical relationships of these deposits in the area between the islands of Ibiza and Mallorca.Our observations suggest the existence of three MSC-related units in this area. The lowermost transparent seismic unit can be interpreted as a 100 to 200-m-thick salt layer deposited in the deepest part of the Central Mallorca Depression. These evaporites pass laterally and upward into a thin-bedded seismic unit, which could be lithologically equivalent to the Upper Evaporites of the deep basins. All around the borders of the Central Mallorca Depression, this bedded seismic unit onlaps a Slope Unit which shows bedded to chaotic and rafted facies. The Messinian continental shelf is eroded and incised by a valley extending from the Palma onshore area to the Mallorca downslope domain.Our results suggest the presence offshore of at least two generations of diachronous MSC deposits. The observation of salt on a continental high and the widespread MSC deposits on the margins of the Central Mallorca Depression call into question the location of halite preferentially in the deep abyssal basins and the significance of the deep basin MSC units that onlap onto the Margin Erosional Surface (MES), as observed all around the passive margins of the Northwest-Mediterranean Basin. The dual terminology used for peripheral/deep MSC evaporites cannot be applied to any of the MSC units found in the study area.
ISSN: 0025-3227

hal-01115656
https://hal.archives-ouvertes.fr/hal-01115656
DOI : 10.1016/j.margeo.2014.10.001

Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
1. The use of targets to provide measurable objectives and benchmarks for management, conservation, andrestoration of ecosystems is commonplace. In the marine and coastal realms, targets have been successful insetting sustainable limits to fisheries harvests, thresholds for pollutants, and recommended amounts ofrepresentative habitat included in marine protected area (MPA) networks. Quantifiable targets can dissuadegovernments from making dubious claims about investments in ocean protection that sound impressive butcannot be verified. Examples are presented where protection targets have been used successfully for marinemanagement, and instances where measurable and meaningful benchmarks serve to allow tracking of true progress.2. However, the setting of targets can also be a double-edged sword. In some cases, targets have proven useful,but in many instances, interventions made to fulfil targets not only give a false illusion of progress or even success,they present opportunity costs that impede further conservation.3. Some of these issues were raised in the 2003 article ‘Dangerous Targets?: Unresolved issues and ideologicalclashes around marine protected areas’ that appeared in Aquatic Conservation: Marine and FreshwaterEcosystems. Since its publication, the article’s warnings about how targets can sometimes be dangerous andcounter-productive have led to intense debate among scientists and policy-makers alike, and the paper has beencited in more than 500 publications. Yet today, more than a dozen years after the first ‘Dangerous Targets’publication, new targets are driving more MPA designations and conservation strategies than ever before, andthe ‘dangerous’ aspects of target setting have been largely ignored.4. This paper discusses old ‘dangers’ in the context of new developments in marine conservation, including thelingering problem of having simplistic metrics drive marine policies, and the unintended result that can often occurwhen outputs (percentage of area under MPA designation) do not align with true outcomes of effectivemanagement and conservation. Newly emerging ‘dangers’ in letting areal targets (percentage of area underMPA designation) drive MPA designations are also discussed, including how the rush to fulfil obligations toprotect a certain proportion of area is taking place in planning, separate from broader level, and potentiallymore holistic, marine spatial planning (MSP).5. The paper suggests five recommendations that would allow policy-makers to use targets more effectively,including: (1) increase transparency in planning, especially around specific goals and objectives of MPAestablishment; (2) use time-based areal targets when representativity is a goal of the protected area strategy; (3)
ISSN: 1052-7613

hal-01380963
https://hal-univ-perp.archives-ouvertes.fr/hal-01380963
DOI : 10.1002/aqc.2675

Éditeur(s) : HAL CCSD Copernicus Publications on behalf of the European Geosciences Union
Résumé : International audience
During the GWADASEIS cruise (Lesser Antilles volcanic arc, February–March 2009) a very high resolution (VHR) seismic-reflection survey was performed in order to constrain Late Quaternary to Present faulting. The profiles we obtained evidence frequent " ponding " of reworked sediments in the deepest areas, similar to the deposition of Mediterranean " homogenites ". These bodies are acoustically transparent (few ms t.w.t. thick) and are often deposited on the hanging walls of dominantly normal faults, at the base of scarps. Their thickness appears sufficient to compensate (i.e. bury) co-seismic scarps between successive earthquakes, resulting in a flat and horizontal sea floor through time. In a selected area (offshore Montserrat and Nevis islands), piston coring (4 to 7 m long) was dedicated to a sedimento-logical analysis of the most recent of these particular layers. It corresponds to non-stratified homogenous calcareous silty sand (reworked calcareous plankton and minor volcanoclas-tics). This layer can be up to 2 m thick, and overlies fine-grained hemipelagites. The upper centimeters of the latter represent the normal RedOx water/sediment interface. 210 Pb and 137 Cs activities lack in the massive sands, while a normal profile of unsupported 210 Pb decrease is observed in the hemipelagite below, together with a 137 Cs peak corresponding to the Atmospheric Nuclear Experiments (1962). The RedOx level was thus capped by a recent instantaneous major sedimentary event considered as post-1970 AD; candidate seismic events to explain this sedimentary deposits are either the 16 March 1985 earthquake or the 8 October 1974 one (M w = 6.3 and M w = 7.4, respectively). This leads to consider that the syntectonic sedimentation in this area is not continuous but results from accumulation of thick homogen-ites deposited after the earthquakes (as observed in the following weeks after Haiti January 2010 event, McHugh et al., 2011). The existence of such deposits suggests that, in the area of study, vertical throw likely results from cu-mulated effects of separated earthquakes rather than from aseismic creep. Examination of VHR profiles shows that all major co-seismic offsets are recorded in the fault growth sequence and that co-seismic offsets can be precisely estimated. By using a sedimentation rate deduced from 210 Pb decrease curve (0.5 mm yr −1) and taking into account minor reworking events detected in cores, we show that the Redonda system may have been responsible for five > M 6 events during the last 34 000 yr. The approach presented in this work differs from fault activity analyses using displaced sets of isochronous surfaces and postulating co-seismic offsets. Combining VHR seismic imagery and coring we can decipher co-seismic vs. slow continuous displacement, and thus actually estimate the amplitude and the time distribution of major co-seismic offsets.
Natural Hazards and Earth System Science

insu-01585969
https://hal-insu.archives-ouvertes.fr/insu-01585969
https://hal-insu.archives-ouvertes.fr/insu-01585969/document
https://hal-insu.archives-ouvertes.fr/insu-01585969/file/nhess-12-1755-2012-beck.pdf
DOI : 10.5194/nhess-12-1755-2012

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Secondary migration is defined as the movement of hydrocarbons through relatively permeable rocks from source to trap: a two-phase flow within a porous medium. Depending on the geometry and capillary pressure distributions of carrier beds, secondary migration has both vertical and lateral components. The present paper focuses on that part of the migration where the movement is mainly vertical. Its objective is to propose a description of the dynamics governing the vertical part of secondary migration based on the main physical aspects of two-phase flow in a homogeneous porous medium. The study is illustrated by laboratory observations performed in a vertical, 2-D Hele-Shaw cell filled with a transparent porous medium where the flow of dyed oil invading a wetting fluid is visually observed. These observations help us to understand the effect of buoyant, capillary and viscous forces on the resulting flow, the relative importance of which is characterized by non-dimensional numbers. Extrapolating these observations to natural media, it is proposed that vertical secondary migration can be described as a percolation of disconnected and vertically-elongated stringers. These stringers do not move continuously but as a succession of snap-off and re-feeding events which result in a jerky upward movement. Using parameters characterizing the physical properties of the fluid and of the porous medium, the geometry and the dynamic behavior of the stringers are estimated. The width of stringers occurring during secondary migration in geological media is centimetric and their vertical size ranges from several centimeters to a few meters. An upper limit of the mean upward velocity of stringers is proposed, as well as an estimate of their spatial density and of the minimum, average horizontal distance (decametric) between two stringers. The stringers are sparsely distributed, resulting in a low average oil loss and a high efficiency of the vertical migration process.
ISSN: 0264-8172

hal-00831370
https://hal.archives-ouvertes.fr/hal-00831370
https://hal.archives-ouvertes.fr/hal-00831370/document
https://hal.archives-ouvertes.fr/hal-00831370/file/pekins-V-edited.pdf
DOI : 10.1016/j.marpetgeo.2013.04.020

Éditeur(s) : HAL CCSD
Résumé : L'introduction des restitutions organisées de connaissances (ROC) dans les épreuves du baccalauréat, à partir de 1995, est une réponse de l'institution à la volonté de rendre plus efficace l'enseignement en cycle terminal. Cet article s'appuie sur l'analyse de la ROC du sujet Antilles Guyane session 2006 par une approche anthropologique en proposant la construction de son « site mathématique ». Au travers de cet exemple sont interrogées certaines caractéristiques du concept ROC comme sa cohérence (notamment institutionnelle) et sa transparence de cette évaluation.
https://hal.archives-ouvertes.fr/hal-00319519

Droits : info:eu-repo/semantics/OpenAccess
hal-00319519
https://hal.archives-ouvertes.fr/hal-00319519
https://hal.archives-ouvertes.fr/hal-00319519/document
https://hal.archives-ouvertes.fr/hal-00319519/file/PropositionArticleVersionHAL1.pdf