Building of the Amsterdam-Saint Paul plateau: A 10 Myr history of a ridge-hot spot interaction and variations in the strength of the hot spot source

International audienceThe Amsterdam-Saint Paul plateau results from a 10 Myr interaction between the South East Indian Ridge and the Amsterdam-Saint Paul hot spot. During this period of time, the structure of the plateau changed as a consequence of changes in both the ridge-hot spot relative distance and in the strength of the hot spot source. The joint analysis of gravity-derived crust thickness and bathymetry reveals that the plateau started to form at ~10 Ma by an increase of the crustal production at the ridge axis, due to the nearby hot spot. This phase, which lasted 3-4 Myr, corresponds to a period of a strong hot spot source, maybe due to a high temperature or material flux, and decreasing ridge-hot spot distance. A second phase, between ~6 and ~3 Ma, corresponds to a decrease in the ridge crustal production. During this period, the hot spot center was close to the ridge axis and this reduced magmatic activity suggests a weak hot spot source. At ~3 Ma, the ridge was located approximately above the hot spot center. An increase in the hot spot source strength then resulted in the building of the shallower part of the plateau. The variations of the melt production at the ridge axis through time resulted in variations in crustal thickness but also in changes in the ridge morphology. The two periods of increased melt production correspond to smooth ridge morphology, characterized by axial highs, while the intermediate period corresponds to a rougher, rift-valley morphology. These variations reveal changes in axial thermal structure due to higher melting production rates and temperatures

O Complexo Geoidal Leste Brasileiro: características geológicas e geofísicas

The Eastern Brazilian continental margin is a typical passive continental margin derived from a rifting process that has been modified by a set of features, here named Abrolhos Magmatic Province (AMP). Abrolhos continental shelf, Royal Charlotte bank, Bahia seamounts, and Vitória/Trindade ridge, along with isolated seamounts, and igneous bodies interbedded within the sedimentary sequences are the main components of such Province. This study is based on satellite altimetry derived bathymetric data as well as gravity and multichannel seismic data in order to investigate the spatial and temporal relationship between AMP and a group of gravity anomalies formerly designated as East Brazilian Geoidal Complex. Among these anomalies is the Geoid High of Abrolhos. The interpretation of seismic profiles and bathymetric, gravity, and isopach maps allowed to unravel distinct characteristics of the oceanic basement and sedimentary sequences so as related gravity anomalies along the different sectors of the Abrolhos Magmatic Province. Gravity anomalies could be primarily classified as shallow (crustal) and deeper ones. Deeper anomalies can be separated into a northern and a southern sector. Northern sector strikes NW-SE with a smaller geoid high, can be directly related to Bahia seamounts and extends southeasterly up to Bragation fracture zone. Contrarily southern sector is a prominent WSWENE striking geoid high, which is not related to any important topographic feature. So apparently these two anomalies are related to distinct mantle sources differing in age and in depth as suggested by amplitude and strike, respectively. Nevertheless it can not be discarded the hypothesis that these two anomalies come from different parts of a single plume that have been actives in different times; 80 Ma. for the northern sector while the southern sector can be considered still in activity nowadays. It has to be added that there existed yet weakness zones striking NW-SE and W-E in northern and southern regions, respectively. Evidences that the whole study area has been constantly influenced by magmatic events since the Tertiary suggest that it is of fundamental importance to take them into account while establishing geological models and in the application of such models either in academic research or in oil and gas industry no matter the origin of such events to be related to a single or multiple sources. Knowledge of the evolution of these thermal anomalies is important since it is considered their influence in the process of oil and gas origin and accumulation (structural changes) in the sedimentary basins of the region. It became evident in this work the need of continuing the study of these features in order to allow the development of geological and geophysical models capable to estimate the nature, origin, and effects of these thermal anomalies in depositional environments of the eastern Brazilian continental margin.Agência Nacional do PetróleoA margem continental leste brasileira apresenta uma fisiografia típica de margens continentais do tipo passiva derivada de processo de rifteamento, que foi posteriormente modificada por um conjunto de feições, aqui denominado Província Magmática de Abrolhos (PMA), em que se destacam a plataforma de Abrolhos, o banco Royal Charlotte, os montes submarinos da Bahia e a Cadeia Vitória/Trindade, além de montes submarinos isolados e corpos magmáticos intercalados aos sedimentos. O presente estudo concentrou-se na investigação da correlação espacial e temporal entre PMA e anomalias gravimétricas, anteriormente nomeadas Complexo Geoidal Leste Brasileiro (CGLB), entre elas o Alto Geoidal de Abrolhos (AGA). Para tanto foram utilizados dados de batimetria derivada da altimetria de satélites, gravimetria e sísmica multicanal. A partir da interpretação dos perfis de sísmica multicanal e dos mapas e blocos diagramas elaborados, pôde-se observar as distintas características do embasamento e coluna sedimentar, bem como das anomalias gravimétricas correspondentes ao longo de diferentes setores da Província Magmática de Abrolhos. As anomalias gravimétricas puderam ser primariamente diferenciadas em rasas (crustais) e outras mais profundas.. As mais profundas podem ser separadas em dois setores, norte e sul. O setor norte de alinhamento NW-SE apresenta um menor valor de altura do geóide, está diretamente relacionado com os montes submarinos da Bahia e se estende até a Z.F. Bragation. Diferentemente, o setor sul é constituído de um proeminente alto geoidal alinhado na direção WSW-ENE e não está justaposto a nenhuma feição topográfica proeminente. Estas anomalias gravimétricas aparentemente estão relacionadas a duas diferentes fontes mantélicas de idades e profundidades diferentes, o que é sugerido pelas diferenças de amplitude e alinhamentos, respectivamente. Entretanto, não se pode eliminar por completo a hipótese de serem estas anomalias de diferentes setores de uma única pluma, e que tiveram suas fases de atividade em diferentes épocas, sendo 80 Ma para o setor norte, enquanto o setor sul pode ser considerado ainda em atividade. A isso associa-se ainda a existência pretérita de zonas de fraquezas com direções de alinhamento distintos, NW-SE e W-E para as regiões norte e sul respectivamente. As evidências de que toda a região de estudos vêm sendo constantemente influenciada por eventos magmáticos desde o Terciário sugerem que, independente da gênese destes eventos estar relacionada a apenas uma ou a várias fontes, é fundamental levá-las em consideração no estabelecimento de modelos geológicos e na aplicação destes seja em pesquisa acadêmica, seja na indústria de Óleo e Gás. Haja vista a importância do conhecimento da evolução destas anomalias térmicas, considerando-se a influência destas no processo de geração (maturação da matéria orgânica) e acumulação (modificações estruturais) nas bacias sedimentares da região. Evidenciou-se neste trabalho a necessidade de continuidade do estudo destas feições, de forma a permitir o desenvolvimento de modelos geofísicos e geofísicos capazes de estimar a natureza, origem e os efeitos destas anomalias térmicas nos ambientes deposicionais da margem continental leste brasileira

Évolution Temporelle et Spatiale du Système d'Interaction entre le point chaud de Tristan da Cunha et la Dorsale de l'Atlantique Sud.

In this work we have studied the space and time evolution of the Tristan hotspot and south Mid-Atlantic Ridge interaction system. The Tristan hotspot and its associated bathymetrical features (i.e. Walvis Ridge in the African plate and Rio Grande Rise in the South American plate) are ones of the most significant features of the South Atlantic Ocean. However, this system is not as well studied as the systems in the North Atlantic (e.g. Azores and Iceland), and their origin and evolution is an open subject. Data compilation of the available surveys from the National Geophysical Data Center (NGDC) and satellite-derived bathymetry were treated to generate a Digital Elevation Model (DEM) for the entire South Atlantic Ocean. Satellite-derived free-air anomaly data and sediment thickness data were used to calculate the mantle Bouguer anomaly (MBA) and digital isochrons of the ocean floor were used to calculate the residual mantle Bouguer anomaly (RMBA), residual bathymetry and new poles for the South America and Africa relative plate motions. Satellite-derived geoid data and the Earth Gravitational Model - EGM2008 were used for the separation of the different components in the geoid data with the calculation of filters using different levels and degree of spherical harmonics. From kinematic reconstructions based on new rotation poles calculated in this work and using the most accepted hypothesis for the current position of the hotspot, as the volcanic island of Tristan da Cunha, the chronology of the emplacement of the volcanic structures could be detailed. This highlighted the existence of several axes jumps and periodic variations in the magma supply to the axis resulting in a periodic alternation between periods of construction and axial ruptures of the axial plateau. The detailed comparison of the estimated position for the Tristan da Cunha hotspot and the location of the volcanic structures of the Walvis Ridge whose ages are known, underlined that the formation of the Walvis Ridge cannot be explained in its entirety by the evolution model involving an interaction between the South Atlantic Mid Ocean Ridge and a hotspot whose current position is in the island of Tristan da Cunha. Next, we evaluated the temporal evolution of the influence of the hotspot to the South Atlantic Mid-Ocean Ridge axis with respect to changes in the residual bathymetry (RB) and the residual mantle Bouguer anomaly (RMBA) with the analysis of variations in the spreading rate and the hotspot-ridge distance. For the early stages of the South Atlantic opening, the RMBA and BR anomalies are stronger for the African plate then for the South American plate. This suggests that the position of the hotspot was beneath the African plate since the beginning of the opening of the South Atlantic Ocean. In summary, there are three different periods for the Tristan da Cunha hotspot and South Atlantic Mid-Ocean Ridge interaction system. A period of rapprochement of the ridge to the hotspot between 115 Ma and 100 Ma, in which it is observed the influence of the hotspot on the ridge axis for at least the distance of 150 km. Then, between 95 Ma and 60 Ma, we observed that the ridge is directly above or very close (<50 km) of the hot spot. Then, since 60 Ma, it is observed a gradual and progressive separation of the ridge from the hotspot position. Have been shown that in periods of rapprochement and ridge directly above of the hotspot the observed periodic variations of the magmatic contribution are related to changes in the flux of the plume. From the analysis of the evolution of volcanic structures, together with the free-air anomaly and geochemistry data, the volcanic structures have been divided into two groups. A group formed by the NW border of the Walvis Ridge was called first phase, consistent with the evolution of structures from a context of the mid-ocean ridge directly above the hotspot to the emplacement of the intraplate situation, in agreement with the three periods, rapprochement, directly above and displacement, as described above. The other group, formed by the SE border of the Walvis Ridge was called second phase and shows a gravity signature that is characteristic of intraplate setting up even for structures with ages in which the position of the mid-ocean ridge was directly above the hotspot. The two groups have different geochemical signatures, Zr/Nd ratio and Ti abundance. Then, from the comparative analysis of different degrees of spherical harmonics of the geoid and the new results available in the literature for the seismic tomography, we have shown the existence of an deep association between the plume responsible for the volcanic structures of the Rio Grande Rise and Walvis ridge with the African superplume. The identification of three mantle anomalies close to the islands of Gough and RSA seem a better alternative for the hotspot actual position than the location of the island of Tristan da Cunha accepted today.Dans ce travail nous avons abordé l'évolution spatiale et temporelle du système d'interaction entre le point chaud de Tristan da Cunha et la dorsale de l'Atlantique Sud. Le point chaud de Tristan da Cunha et les structures associées du fond océanique (à savoir la chaîne volcanique de Walvis dans la plaque africaine et l'élévation du Rio Grande dans la plaque sud-américaine) sont parmi les plus importantes de l'océan l'Atlantique Sud. Cependant, ce système est moins étudié que les systèmes présents dans l'Atlantique Nord (par exemple Açores et Islande), et leur origine et évolution restent sujets à débat. La compilation des données de sondage bathymétrique hébergées auprès du Centre National de Données Géophysique (National Geophysical Data Center - NGDC), et les données de bathymétrie dérivées de l'altimétrie de satellites furent utilisées pour l'élaboration d'un modèle numérique de terrain (MNT) de l'ensemble de l'Atlantique Sud. Les données d'anomalie à l'air libre dérivées de l'altimétrie de satellites et les données d'épaisseurs sédimentaires furent utilisées pour le calcul de l'anomalie de Bouguer réduite au manteau (Mantle Bouguer Anomaly - MBA) et avec les données des isochrones du fond océanique furent calculées l'anomalie résiduelle de Bouguer réduite au manteau (Residual Mantle Bouguer Anomaly - RMBA), la bathymétrie résiduelle et des nouveaux pôles du mouvement relatif entre les plaques de l'Amérique du Sud et Afrique. Les données de géoïde dérivées de l'altimétrie de satellites et le Modèle Gravitationnel de la Terre (Earth Gravitational Model - EGM2008) furent utilisés pour la séparation des différentes composantes dans le signal du géoïde, grâce à des filtres retenant différents ordres et degrés d'harmoniques sphériques. À partir des reconstructions cinématiques basées sur les nouveaux pôles calculés dans ce travail, et en se basant surl'hypothèse communément admise que l'île volcanique de Tristan da Cunha se situe à l'aplomb dela position actuelle du point chaud, la chronologie des phases de mise en place des structures volcaniques a pu être précisée. Ceci a mis en évidence l'existence de plusieurs sauts d'axes et des variations périodiques de l'apport magmatique à l'axe entrainant une alternance entre périodes de construction et de rupture du plateau axial. La comparaison détaillée de la position estimée pour le point chaud de Tristan da Cunha et de la localisation des structures volcaniques de la chaine Walvis dont les âges sont connus a souligné que la formation de la chaîne Walvis ne peut pas être expliquée dans sa totalité par un modèle d'évolution faisant appel à une interaction entre la dorsale de l'Atlantique Sud et un point chaud dont la position actuelle serait l'île de Tristan da Cunha. Ensuite, nous avons évalué l'évolution temporelle de l'influence du point chaud à l'axe de la dorsale, sur aux variations de la bathymétrie résiduelle (BR) et de l'anomalie résiduelle de Bouguer réduite au manteau (RMBA), et avec l'analyse des variations du taux d'ouverture et de la distance point chaud-dorsale. Les valeurs des anomalies de RMBA et BR, lors des premières étapes d'ouverture de l'Atlantique Sud, sont plus fortes sur la plaque africaine que sur la plaque sud-américaine. Cela suggère que la position du point chaud a été sous la plaque africaine depuis le début de l'ouverture. En synthèse, on observe trois différentes périodes dans le système d'interaction entre le point chaud de Tristan da Cunha et la dorsale de l'Atlantique Sud. Une période de rapprochement de la dorsale vers le point chaud entre 115 Ma et 100 Ma, lors de laquelle on remarque l'influence du point chaud sur la dorsale sur une distance d'au moins 150 km. Ensuite, entre 95 Ma et 60 Ma, on observe que la dorsale est à l'aplomb ou très proche (< 50 km) du point chaud. Et depuis 60 Ma, un éloignement graduel et progressif de la dorsale par rapport à la position du point chaud est observé. On a pu montrer que lors des périodes de rapprochement et de proximité de la dorsale au point chaud, les variations périodiques d'apport magmatique observées sont liées aux variations du flux de panache. A partir de l'analyse de l'évolution des structures volcaniques, en association avec les données d'anomalie à l'air libre et de géochimie, les structures volcaniques ont pu être divisées en deux groupes. Un groupe formé par la bordure nord-ouest de la chaîne Walvis et appelé première phase, cohérent avec l'évolution des structures dans un contexte de transition d'une mise en place à l'axe vers une situation intraplaque, en accord avec les trois étapes (rapprochement, fixation à l'aplomb et éloignement) décrites ci-dessus. L'autre groupe, formé par la bordure sud-est de la chaîne Walvis et appelé deuxième phase, montre une signature gravimétrique caractéristique d'une mise en place intraplaque, y compris pour les structures à l'âge desquelles la dorsale est à l'aplomb du point chaud. Les deux groupes possèdent signatures géochimiques différentes en termes de rapports Zr/Nb et d'abondance en Ti. Ensuite, à partir de l'analyse comparative des différents degrés d'harmoniques sphériques du géoïde et des nouveaux résultats de tomographie sismique disponibles dans la littérature, nous avons pu montrer l'existence d'une association en profondeur entre les panaches responsables des structures volcaniques de l'élévation du Rio Grande et de la chaîne Walvis et le superpanache africain. L'identification de trois anomalies mantéliques proches des îles de Gough et RSA semblent en faire une meilleure alternative d'emplacement que l'île de Tristan da Cunha, qui est l'hypothèse communément acceptée aujourd'hui

Évolution temporelle et spatiale du sytème d interaction entre le point chaud de Tristan de Cunha et la Dorsale de l Atlantique Sud

Dans ce travail nous avons abordé l évolution temporelle spatiale du système d interaction entre le point chaud de Tristan da Cunha et la dorsale de l'Atlantique Sud. Le point chaud de Tristan da Cunha et les structures associées du fond océanique (i.e. la chaîne volcanique de Walvis dans la plaque africaine et l élévation du Rio Grande dans la plaque sud-américaine) sont parmi les plus importantes de l océan l Atlantique Sud. Cependant, ce système est moins étudié que les systèmes présents dans l Atlantique Nord (e.g. Açores et Islande), et leur origine et évolution reste sujet de grand débat. La compilation des données de sondage bathymétrique hébergé auprès du Centre National de Données Géophysique (National Geophysical Data Center - NGDC), et les données de bathymétrie dérivées de l altimétrie de satellites furent utilisées pour l élaboration d un modèle numérique de terrain (MNT) de l ensemble de l Atlantique Sud. Les données d anomalie à l air libre dérivées de l altimétrie d satellites et les données d épaisseurs sédimentaires furent utilisés pour le calcul de l anomalie de Bouguer réduite au manteau (Mantle Bouguer Anomaly - MBA) et avec les données des isochrones du fond océanique furent calculé l anomalie résiduelle de Bouguer réduite au manteau (Residual Mantle Bouguer Anomaly - RMBA), la bathymétrie résiduelle et des nouveaux pôles du mouvement relatif entre le plaques de l Amérique du Sud et Afrique.In this work we have studied the space and time evolution of the Tristan hotspot and south Mid-Atlantic Ridge interaction system. The Tristan hotspot and its associated bathymetrical features (i.e. Walvis Ridge in the African plate and Rio Grande Rise in the South American plate) are one of the most significant features of the South Atlantic Ocean. However, this system is not as well studied as the systems in the North Atlantic (e.g. Azores and Iceland), and their origin and evolution is an open subject. Data compilation of the available surveys from the National Geophysical Data Center (NGDC) and satellite-derived bathymetry were treated to generate a Digital Elevation Model (DEM) for the entire South Atlantic Ocean. Satellite-derived free-air anomaly data and sediment thickness data were used to calculate the mantle Bouguer anomaly (MBA) and digital isochrons of the ocean floor were used to calculate the residual mantle Bouguer anomaly (RMBA), residual bathymetry and new poles for the South America and Africa relative plate motions. Satellite-derived geoid data and the Earth Gravitational Model - EGM2008 were used for the separation of the different components in the geoid data with the calculation of filters using different levels and degree of spherical harmonics. From kinematics reconstructions based on new rotation poles calculated in this work and using the most accepted hypothesis for the current position of the hotspot, as the volcanic island of Tristan da Cunha, the chronology of the emplacement of the volcanic structures could be detailed.BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

Extreme mantle uplift and exhumation along a transpressive transform fault

International audienc

In Vitro Anti-Toxoplasma gondii and Antimicrobial Activity of Amides Derived from Cinnamic Acid

Most cinnamic acids, their esters, amides, aldehydes, and alcohols present several therapeutic actions through anti-inflammatory, antitumor, and inhibitory activity against a great variety of microorganisms. In this work, eight amines derived from cinnamic acid were synthesized and tested against host cells infected with Toxoplasma gondii and the bacteria Escherichia coli, Pseudomonas aeruginosa, Staphylococcus epidermidis, and three strains of Staphylococcus aureus. Compounds 3 and 4 showed the best result against intracellular T. gondii, presenting antiparasitic activity at low concentrations (0.38 and 0.77 mM). The antibacterial activity of these compounds was also evaluated by the agar microdilution method, and amides 2 and 5 had a minimum inhibitory concentration of 250 µg mL−1 against two strains of S. aureus (ATCC 25923 and bovine strain LSA 88). These also showed synergistic action along with a variety of antibiotics, demonstrating that amines derived from cinnamic acid have potential as pharmacological agents

Antihypertensive Activity of the Alkaloid Aspidocarpine in Normotensive Wistar Rats

The alkaloid Aspidocarpine was isolated from the bark of Aspidosperma desmanthum. Its structure was elucidated by the spectral data of 1H and 13C-NMR (1D and 2D) and high-resolution mass spectrometry (HRESIMS). The antihypertensive activity was investigated by intravenous infusion in Wistar rats. This alkaloid significantly reduced (p < 0.05) the systolic, median, and diastolic blood pressures of rodents, without causing motor incoordination and imbalance in the rotarod test. The results indicate that the alkaloid Aspidocarpine exerts its antihypertensive activity without causing sedation or the impairment of motor functions