Éditeur(s) : HAL CCSD Springer
Résumé : International audience
We present here a synthesis of the evolution of rifted continental margin systems in the Gulf of Aden. These margins are volcanic to the west of the Gulf of Aden, where they are influenced by the Afar hotspot, and non-volcanic east of longitude 46° E. The combined use of magnetics, gravity, seismic reflection, field observations (tectonic, stratigraphic and sedimentological) and oil well data allowed us to obtain better constraints on the timing of continental rifting and seafloor spreading. From the Permo-Triassic to the Oligocene, the Arabian-African plate was subject to distributed extension, probably due, at least from the Cretaceous, to tensile stresses related to the subduction of the Tethysian slab in the north. In Late Eocene-Early Oligocene, 34-33 Ma ago, rifting started to localise along the future area of continental breakup. Initially guided by the inherited basins, continental rifting then occurred synchronously over the entire gulf before becoming localised on the northern and southern borders of the inherited grabens, in the direction of the Afar hotspot. In the areas with non-volcanic margins (in the east), the faults marking the end of rifting trend parallel to the inherited grabens. Only the transfer faults cross-cut the inherited grabens, and some of these faults later developed into transform faults. The most important of these transform faults follow a Precambrian trend. Volcanic margins were formed in the west of the Gulf, up to the Guban graben in the southeast and as far as the southern boundary of the Bahlaf graben in the northeast. Seaward dipping reflectors can be observed on many oil industry seismic profiles. The influence of the hotspot during rifting was concentrated on the western part of the gulf. Therefore, it seems that the western domain was uplifted and eroded at the onset of rifting, while the eastern domain was characterised by more continuous sedimentation. The phase of distributed deformation was followed by a phase of strain localisation during the final rifting stage, just before formation of the Ocean-Continent Transition (OCT), in the most distal graben (DIM graben). About 20 Ma ago, at the time of the continental break-up, the emplacement of the OCT started in the east with exhumation of the subcontinental mantle. Farther west, the system was heated up by the strong influence of the Afar hotspot, which led to breakup with much less extension. In the Gulf of Aden (s.str), up to the Shukra El Sheik fracture zone, oceanic spreading started 17.6 Ma ago. West of this fracture zone, oceanic accretion started 10 Ma ago, and 2 Ma ago in the Gulf of Tadjoura. Post-rift deformation of the eastern margins of the Gulf of Aden can be seen in the distal and proximal domains. Indeed, the substantial post-rift uplift of these margins could be associated with either the continental break-up, or activity of the Afar hotspot and related volcanic/magmatic activity. Uplift of the northern proximal margin was still active (e.g. stepped beach rocks exposed at 60 m of 2 Ma; 30 m of 35,200 years; 10 and 2 m) and active volcanoes can be inferred at depths of between 70 and 200 km beneath the margin (at 5-10 km distance from the coast). On the distal margin, heat flow measurements show a high value that is associated with post-rift volcanic activity and the development of a volcano (with flows and sills) shortly after the formation of the OCT. The Afar hotspot is therefore important for several reasons. It allows the localisation of deformation along the Red Sea/Aden system and the rapid opening of the Gulf after the continental break-up; its influence also seems to persist during the post-rift period.
ISSN: 1866-7411

hal-01053089
https://hal.archives-ouvertes.fr/hal-01053089
DOI : 10.1007/s12517-011-0475-4

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
We image the lithospheric and upper asthenospheric structure of western continental Yemen with 24 broadband stations to evaluate the role of the Afar plume on the evolution of the continental margin and its extent eastwards along the Gulf of Aden. We use teleseismic tomography to compute relative P-wave velocity variations in southwestern Yemen down to 300 km depth. Published receiver function analysis suggest a dramatic and localized thinning of the crust in the vicinity of the Red Sea and the Gulf of Aden, consistent with the velocity structure that we retrieve in our model. The mantle part of the model is dominated by the presence of a low velocity anomaly in which we infer partial melting just below thick Oligocene flood basalts and recent off-axis volcanic events (from 15 Ma to present). This low velocity anomaly could correspond to an abnormally hot mantle and could be responsible for dynamic topography and recent magmatism in western Yemen. Our new P-wave velocity model beneath western Yemen suggests the young rift flank volcanoes beneath margins and on the flanks of the Red Sea rift are caused by focused small-scale diapiric upwelling from a broad region of hot mantle beneath the area. Our work shows that relatively hot mantle, along with partial melting of the mantle, can persist beneath rifted margins after breakup has occurred.
ISSN: 1525-2027

hal-01015496
https://hal.archives-ouvertes.fr/hal-01015496
https://hal.archives-ouvertes.fr/hal-01015496/document
https://hal.archives-ouvertes.fr/hal-01015496/file/Korostelev_etal_GGG_2014.pdf
DOI : 10.1002/2014GC005316

Éditeur(s) : HAL CCSD Linnean Society of London
Résumé : International audience
Although migratory pelagic fishes generally exhibit little geographic differentiation across oceans, as expected from their life-history (broadcast spawning, pelagic larval life, swimming ability of adults) and the assumed homogeneity of the pelagic habitat, exceptions to the rule deserve scrutiny. One such exception is the narrow-barred Spanish mackerel (Scomberomorus commerson), where strong genetic heterogeneity at the regional scale has been previously reported. We investigated the genetic composition of S. commerson across the Indo-West Pacific range using control-region sequences (including previously published datasets), cytochrome-b gene partial sequences, and eight microsatellite loci, to further explore its phylogeographic structure. All haplotypes sampled from the Indo-Malay-Papua archipelago (IMPA) and the southwestern Pacific coalesced into a clade (Clade II) that was deeply separated (14.5% nucleotide divergence) from a clade grouping all haplotypes from the Persian Gulf and Oman Sea (Clade I). Such a high level of genetic divergence suggested the occurrence of two sister-species. Further phylogeographic partition was evident between the western IMPA and the regions sampled east and south of it, i.e. northern Australia, West Papua, and the Coral Sea. Strong allele-frequency differences were found between local populations in the southwestern Pacific, both at the mitochondrial locus (ΦST=0.282-0.609) and at microsatellite loci (^θ=0.202-0.313). Clade II consisted of four deeply divergent subclades (9.0-11.8% nucleotide divergence for the control region; 0.3-2.5% divergence at the cytochrome b locus). Mitochondrial sub-clades within Clade II generally had narrow geographic distribution, demonstrating further genetic isolation. However, one particular haplogroup within Clade II was present throughout the central Indo-West Pacific; that haplogroup was found to be sister-group to an haplogroup restricted to West Papua and the Coral Sea, yielding evidence of recent secondary westward colonization. Such a complex structure is in sharp contrast with the generally weak phylogeographic patterns uncovered to date in other widely distributed, large pelagic fishes with pelagic eggs and larvae. We hypothesize that in S. commerson and possibly other Scomberomorus species, philopatric migration may play a role in maintaining the geographic isolation of populations by annihilating the potential consequences of passive dispersal.
Biological Journal of the Linnean Society

ird-00759711
http://hal.ird.fr/ird-00759711
http://hal.ird.fr/ird-00759711/document

Éditeur(s) : HAL CCSD Linnean Society of London
Résumé : International audience
Although migratory pelagic fishes generally exhibit little geographic differentiation across oceans, as expected from their life-history (broadcast spawning, pelagic larval life, swimming ability of adults) and the assumed homogeneity of the pelagic habitat, exceptions to the rule deserve scrutiny. One such exception is the narrow-barred Spanish mackerel (Scomberomorus commerson), where strong genetic heterogeneity at the regional scale has been previously reported. We investigated the genetic composition of S. commerson across the Indo-West Pacific range using control-region sequences (including previously published datasets), cytochrome-b gene partial sequences, and eight microsatellite loci, to further explore its phylogeographic structure. All haplotypes sampled from the Indo-Malay-Papua archipelago (IMPA) and the southwestern Pacific coalesced into a clade (Clade II) that was deeply separated (14.5% nucleotide divergence) from a clade grouping all haplotypes from the Persian Gulf and Oman Sea (Clade I). Such a high level of genetic divergence suggested the occurrence of two sister-species. Further phylogeographic partition was evident between the western IMPA and the regions sampled east and south of it, i.e. northern Australia, West Papua, and the Coral Sea. Strong allele-frequency differences were found between local populations in the southwestern Pacific, both at the mitochondrial locus (ΦST=0.282-0.609) and at microsatellite loci (^θ=0.202-0.313). Clade II consisted of four deeply divergent subclades (9.0-11.8% nucleotide divergence for the control region; 0.3-2.5% divergence at the cytochrome b locus). Mitochondrial sub-clades within Clade II generally had narrow geographic distribution, demonstrating further genetic isolation. However, one particular haplogroup within Clade II was present throughout the central Indo-West Pacific; that haplogroup was found to be sister-group to an haplogroup restricted to West Papua and the Coral Sea, yielding evidence of recent secondary westward colonization. Such a complex structure is in sharp contrast with the generally weak phylogeographic patterns uncovered to date in other widely distributed, large pelagic fishes with pelagic eggs and larvae. We hypothesize that in S. commerson and possibly other Scomberomorus species, philopatric migration may play a role in maintaining the geographic isolation of populations by annihilating the potential consequences of passive dispersal.
ISSN: 0024-4066

ird-00759711
http://hal.ird.fr/ird-00759711
http://hal.ird.fr/ird-00759711v2/document
http://hal.ird.fr/ird-00759711/file/FauvelotBorsa_Scomberomorus%20commerson%20p%20HAL.pdf

Éditeur(s) : HAL CCSD Blackwell Publishing
Résumé : International audience
In the Causses platform (south-east France), Late Hettangian to Sinemurian deposits were interpreted previously as shallow-water carbonate ramp deposits. A new look at these deposits has shown a fault-controlled mosaic carbonate platform that is different from the carbonate ramp models. Within the platform mosaic, 15 lithofacies have been recognized, which are organized in four facies associations, including peritidal, restricted shallow sub-tidal, sand dunes and sub-tidal shelf facies associations. The rapid lateral and vertical facies changes, and the lack of consistent landward or seaward direction indicated by the pattern of facies changes, question the existence of a shoreline suggested by the traditional models for this region. Instead, the facies organization and cycle stacking pattern suggest deposition in a mosaic of intertidal islands between which sub-tidal restricted or open conditions could coexist in very close proximity. Such a platform mosaic would have been defined by tectonic activities along normal faults which segmented the shallow-water Causses platform. The facies and facies associations are arranged into metre-scale, peritidal and sub-tidal cycles that are also variable. Certain cycles show the same stacking pattern in all the sections and seem to be traceable over tens of kilometres. On the contrary, other cycles cannot be correlated; they are present only in specific sections and have a maximum lateral extension of 1 or 2 km. These metre-scale cycles stack to form four medium-scale cycles bounded by surfaces that display sub-aerial exposure features. Medium-scale cycles stack into two larger-scale cycles (tens of metres thick) and are bounded by well-defined karstic surfaces. Based on their lateral continuity and their stacking pattern, the metre-scale cycles are controlled probably by high frequency eustatic variations overprinting the topographic irregularities formed by differential subsidence of fault-bounded blocks. Episodic fault activities may reorganize the topography so that, even if eustatic changes may still be the major control of cycles, the expression and number of cycles could be different. Cycles of medium and large-scale are interpreted as being allogenic, controlled by changes in eustasy and/or subsidence rates as evidenced by their lateral continuity and the correlations of the large-scale cycles with third-order depositional sequences.
ISSN: 1469-3496

hal-00411940
https://hal.archives-ouvertes.fr/hal-00411940
DOI : 10.1111/j.1365-3091.2007.00896.x

Éditeur(s) : Montpellier SupAgro
Résumé : One of the main challenges in ecology is to understand how global changes affect biodiversity and what are the consequences on ecosystem functioning. In this perspective, the functional diversity of communities is a cornerstone since it allows linking environment, community structure and ecosystem properties. The aim of this thesis is thus to improve the understanding of functional diversity dynamic (i) in relation to natural variability of environmental conditions and (ii) under anthropogenic disturbances. As a first step we have developed a new methodological framework allowing to (1) describe fish functional niches based on functional traits, and (2) to quantify functional diversity within (α) and among (β) sites thanks to new indices. In a second step we have studied the spatiotemporal dynamic of the functional structure of fish and nektonic communities from the Terminos lagoon (Mexico). This tropical estuarine ecosystem is an appropriate ecological model for our problematic as it is characterized by a strong environmental variability, a high biological diversity and is under a strong human pressure. We have put in light the stability of the functional and trophic structures of communities along environmental gradients, despite a very strong species turnover. This stability is determined by the dominance of a couple of functional groups inside which species are replacing each others according to their environmental preferences. However, at a long-term scale, we have demonstrated a functional diversity loss in a part of the lagoon despite an increase of species richness. This paradox has to be related to the decrease of species associated to seagrass and the increase of more estuarine species.
Un des enjeux majeurs de l'écologie est de comprendre comment les changements globaux affectent la biodiversité et quelles en sont les conséquences sur le fonctionnement des écosystèmes. Dans cette perspective, la diversité fonctionnelle des communautés est un outil clé permettant de lier l'environnement, la structure des communautés et les propriétés écosystémiques. L'objectif de cette thèse est donc d'améliorer la compréhension de la dynamique de la diversité fonctionnelle (i) face à des conditions environnementales naturellement variables et (ii) face à des perturbations d'origine anthropique. Dans un premier temps nous avons mis en place un socle méthodologie nous permettant de (1) définir la niche fonctionnelle des poissons à partir de traits fonctionnels et (2) de quantifier les diversités fonctionnelles intra (α) et inter (β) échantillons avec de nouveaux indices. Dans un second temps nous avons étudié la dynamique spatio-temporelle de la structure fonctionnelle des communautés ichtyologiques et nectoniques peuplant la lagune de Terminos (Mexique). Cet écosystème estuarien tropical est un modèle d'étude adapté à notre problématique car il présente une forte variabilité environnementale, une forte diversité biologique et est sous forte pression anthropique. Nous avons mis en évidence une stabilité de la structure fonctionnelle et trophique des communautés face aux gradients environnementaux très marqués, et donc malgré un fort taux de remplacement des espèces entre les communautés. Cette stabilité est due à la dominance de quelques groupes fonctionnels à l'intérieur desquels les espèces se remplacent suivant leurs preferendums environnementaux. Néanmoins, à plus long terme, nous avons démontré qu'une portion de la lagune avait subi une perte de diversité fonctionnelle et ce malgré une augmentation de la richesse spécifique. Ce paradoxe est à relier au remplacement des espèces inféodées aux herbiers de phanérogames par des espèces plus estuariennes.

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/2008/these-6178.pdf
http://archimer.ifremer.fr/doc/00000/6178/

Résumé : Fishing crayfish on the gulf's mudflats is a traditional business practised by 450 boats of 12 to 18 metres in length and employs 2,500 sailors. In the north of the gulf, this activity is exclusive, in the south, immature hake are also caught, because the crayfish areas coincide, in part, with the nurseries of the northern stock of this species. Management of these fisheries, called mixed, of crayfish and hake is one of the greatest problems of the Gulf of Gascony. All assessments tend to show the responsibility of the crayfish boats in the future of the hake stock. The solution is likely to pass through a generalised increase of the 80 mm meshing that would lead, eventually, the hake stock yield to its maximum, but would eliminate a great many trades that are only practicable for the moment by using small meshing: crayfish boats, shrimp boats, etc. It was shown moreover that the times of maximum abundance of hake and crayfish do not necessarily coincide. Indeed, hake does not swim along the bottom but rather moves in the channel of water. There are therefore definitely crayfish areas and hake areas, but overlapping such that a ship passing from one to the other during a single pass cannot judge in advance the immediate composition of its catch. The method used for describing these mixed fisheries and providing a solution to their problems of management relies on simulations aided by a bio-economic model known as "compartmental". This structure was adopted to be able to take into account all possible scenarios of the fleets' strategies. The fishery is first divided into geographic elements corresponding to the specific sedimentary structures where the stocks of exploited species are concentrated, the major constraint being the limits of these elements corresponding to those usually employed in fishing statistics, since it is based on these statistics that we can determine the fleets' profiles and their target species. The ratios between the fleet and the species are measured by calculating the fishing power.
La pêche de la langoustine sur les vasières du golfe est une activité traditionnelle pratiquée par 450 bateaux de 12 à 18 mètres et occupe 2500 marins . Dans le nord du golfe, cette activité est exclusive, vers le sud, elle s'accompagne de la capture de merlus immatures, car les zones à langoustine coïncident, pour partie, avec les nourriceries du stock nord de cette espèce. La gestion de ces pêcheries, dites mixtes, de langoustine et de merlu est un des problèmes les plus importants du golfe de Gascogne. Toutes les évaluations tendent à montrer la responsabilité des langoustiniers dans le devenir du stock de merlu. La solution passerait par une augmentation généralisée du maillage à 80 mm qui conduirait, à terme, le rendement du stock de merlu à son maximum, mais ferait disparaître quantité de métiers qui ne sont praticables pour le moment que par l'emploi de petits maillages : langoustiniers, crevettiers etc.. Il a été montré par ailleurs que les abondances maximales de merlu et de langoustine ne coïncident pas obligatoirement. En effet, le merlu n'est pas posé sur le fond mais se déplace dans la tranche d'eau. Il existe donc bien des zones à langoustine et des zones à merlu, mais imbriquées de telle sorte qu'un navire passant de l'une à l'autre au cours d'un même trait ne peut préjuger de la composition immédiate de sa capture. La méthode retenue pour décrire ces pêcheries mixtes et fournir une solution à leurs problèmes de gestion repose sur des simulations à l'aide d'un modèle bio-économique dit " à compartiments". Cette structure a été adoptée pour permettre la prise en compte de tous les cas de figures possibles de stratégies des flottilles. La pêcherie est d'abord divisée en éléments géographiques correspondant à des structures sédimentaires particulières où se concentrent les stocks d'espèces exploitées, la contrainte majeure étant les limites de ces éléments correspondant à celles habituellement employées dans les statistiques de pêche, puisque c'est à partir de ces statistiques que l'on peut déterminer le profil des flottilles et de leurs espèces cîbles. Les rapports entre la flottille et l'espèce sont mesurés par le calcul des puissances de pêche. Chaque compartiment est, sur une base trimestrielle, le résultat de la superposition d'un élément géographique, d'une flottille, d'une espèce ou d'un groupe d'espèces-cibles. L'association espèce-flottille définit le métier. Les stocks sont dits mobiles quand ils recouvrent par le biais des migrations la totalité de la pêcherie, immobiles quand il n'existe pas d'échanges entre éléments géographiques. Dans le deuxième cas, la mortalité par pêche est ventilée au prorata des captures sur chaque stock élémentaire. A l'intérieur de chaque composante un navire a le choix entre poissons démersaux, poissons de fond ou langoustine. L'inflexion de comportement dans les simulations est gouvernée par deux paramètres qui leur permettent d'orienter leur effort vers d'autres cibles pour optimiser leurs bénéfices. On connaît pour chaque flottille les éléments de coûts de production dérivés des comptes d'exploitation de l'année en cours. Pour chaque espèce, le chiffre d'affaire est le produit du prix prédit par l'équation des prix et des quantités débarquées. Les bénéfices escomptés sont déduits de ces deux valeurs. Les simulations ont porté sur diverses hypothèses : - maillage de 50 mm pour la langoustine et 65 mm pour le poisson ; - l'augmentation de l'effort ; - stratégies visant au profit maximal avec ou sans quota sur le merlu. Dans tous les cas, à terme, les schémas classiques des conséquences des procédures de gestion sont vérifiés : les brusques augmentations de maillage créent des situations transitoires catastrophiques mais permettent d'espérer au bout de 15 ans des soldes tout à fait attirants. Les conséquences d'une augmentation de l'effort agissent dans le sens inverse : augmentation immédiate du solde global, pertes à long terme. Enfin quand on laisse les flottilles suivre une stratégie "libre" orientée vers des bénéfices optimaux, les soldes sont importants car il existe toujours un report très marqué vers le merlu, dans les limites d'un quota quand on l'impose. Les phénomènes biologiques sont aisément prévisibles et théoriquement contrôlables. La stratégie des flottilles l'est beaucoup moins et dépend du contexte politico-économique. Le modèle présenté ici peut donner des réponses au gestionnaire et à l'économiste.

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/1988/rapport-2569.pdf
http://archimer.ifremer.fr/doc/00000/2569/