L'inversion conjointe géophysique est la méthode la plus efficace pour imager l'intérieur de la Terre. En intégrant plusieurs techniques géophysiques elle permet de réduire les incertitudes inhérentes à chacune et ainsi améliorer la compréhension de la Terre. Dans cette étude, nous utilisons les techniques des fonctions récepteur (RF) en sismologie, de la magnétotellurique (MT) et de la gravimétrie qui permettent de caractériser respectivement la vitesse des ondes S, la résistivité électrique et la densité du sous-sol.Le but de ce travail de recherche se divise en deux parties: une première, méthodologique, sur le développement d'une nouvelle approche d'inversion conjointe en 3D et une deuxième avec l'application de ces techniques (en approche jointe ou séparée) sur la Divergence Nord Tanzanienne pour mieux comprendre le phénomène de rupture continentale. Pour la partie méthodologie deux approches ont été développées : une entre les données de MT et de gravimétrie avec un calcul original de l'effet gravimétrique de la topographie qui permet de réduire le nombre de mailles tout en gardant une résolution satisfaisante ; et une deuxième méthode entre les données de MT et de RF par une nouvelle approche d'extrapolation des modèles 1D de vitesse en pseudo modèle 3D de vitesse. L'application de ces techniques sur la Tanzanie a permis de mettre en évidence un certain nombre de structures lithosphériques dont deux zones majeures à faible vitesse dans la croûte inférieure et dans le manteau supérieur. Cette dernière semble refléter des interactions entre des structures héritées d'âge protérozoïque et le panache mantellique Africain. / Geophysical joint inversion attempts to reproduce as best as possible the interior of the Earth. By integrating several geophysical techniques the joint inversion reduces the uncertainties of each methods and improves our understanding of the Earth structure. In this study we use the receiver functions (RF), the magnetotelluric (MT) and the gravity methods which enable to charaterize the Swave velocity, the electrical resistivity and the density, respectively. The objective of this research work is divided in two parts; first with the development of a new 3D joint inversion approach and then with the application of these methods (on a joint or separate approach) on the North Tanzanian Divergence to better understand the continental breakup.For the methodologic part two approaches have been developed; one between the MT and gravity data with an original computation of the topographic effect which decreases the number of cells while keeping a satisfaying resolution. And a second method between the MT and RF data where pseudo 3D velocity model are created and combined with the MT models to better takes into account the physical properties of the receiver function. The application of these methods on the Tanzania highlighted several lithospheric structures and particularly two low-velocity areas in the lower crust and the upper mantle. This latter suggests interactions with Proterozoic inherited structures and the African plume material.

Dans cette thèse nous quantifions les interactions entre phénomènes tectoniques et magmatiques au niveau d'un rift proche de la rupture continentale: celui du Dabbahu / Manda Hararo en Afar (Ethiopie), qui a été le siège d'un épisode de rifting majeur en 2005. Nous apportons ici des contraintes temporelles sur ces interactions à l'échelle du millier à la dizaine de millier d'années, afin de mieux comprendre l'évolution des processus responsables de l'acquisition de la topographie. Pour cela nous avons produits des âges d'expositions de surfaces de coulées de laves et d'escarpement de failles par la technique des isotopes cosmogéniques. Ces datations ont été combinées à des travaux de cartographie et de géochimie (éléments majeurs et traces). Nous montrons que nord du segment est sous l'influence de la présence d'un volcan terminal qui impacte le développent de la topographie. Le milieu du segment permet d'évaluer l'impact des variations spatiales et temporelles de la localisation des réservoirs sur la topographie. Le long du segment, il apparaît que la topographie se développe quand les réservoirs magmatiques locaux ont une activité modérée à faible, alors que la topographie est inhibée lors des phases de magmatisme intense. Nos observations ont permis de monter que l'épisode de rifting de 2005 n'est pas représentatif de l'évolution globale de la morphologie du rift, et que ce type d'intrusion ne représente sans doute qu'un dixième des injections totales. Enfin, l'accrétion magmatique est soutenue à l'axe par différents réservoirs magmatiques qui semblent se relayer dans le temps, et dont la position peut varier de 15 kilomètres de part et d'autre de l'axe / In this thesis we quantify the interactions between tectonic and magmatic processes in the active Dabbahu rift segment, which experienced in 2005 a major rifting event. Here, we provide accurate constraints at this time scale by producing cosmogenic exposure ages of lavas surfaces and faults scarps, in order to decipher the morphological evolution of a rift depression, and to identify the processes that are responsible for the topography acquisition. The study is based on samples taken on two separate areas of the rift. The field areas were systematically mapped, and all lavas were chemically analysed in major and trace elements. The first studied focused the northern portion of the rift segment, where the lavas of the segment terminating volcano meets the neo-volcanic zone. This area allowed the influence of the presence of such volcanic edifice to be assessed. The second studied area is located at the middle of the segment, and covers the rift western margin up to the rift axis. The topography in this area is sensitive to the evolution in space and time of the location of the different axial magma reservoirs. Our results show that along the rift segment, topography develops during phases of low magmatic activity, and is inhibited during phases of lower magmatic activity. The 2005 appears to be an unusual event that represents about a tenth of the total intrusions necessary to build the depression. The maintaining of magmatic accretion at the axis seems to be the result of a relay between different magma reservoirs. The position of these reservoirs can vary between 0 and 15 km from both sides of the rift axis

Este trabalho tem como objetivo a identificação de feições que permitam (1) a construção do arcabouço crustal profundo e da porção superior do manto em parte da Costa de Angola, (2) a comparação deste arcabouço com o embasamento aflorante e (3) a tentativa de adequar estes resultados aos modelos de ruptura continental já publicados. Para alcançar estes objetivos foi feita a interpretação de cinco linhas sísmicas de reflexão profundas (25 Km de profundidade) na costa de Angola, nas Bacias de Kwanza e Baixo Congo, adquiridas pela ION-GXT. As feições identificadas na sísmica de reflexão auxiliaram na determinação dos limites da crosta continental superior e inferior, no reconhecimento das camadas que compõem a crosta oceânica e na identificação da Descontinuidade de Mohorovicic (que marca o limite entre crosta e manto). A interpretação sísmica associada a dados da literatura (que proporcionaram valores de densidade para os pacotes identificados na interpretação sísmica) permitiram a realização de uma modelagem gravimétrica que foi comparada ao dado gravimétrico adquirido durante a aquisição sísmica. A modelagem gravimétrica serviu para validar a interpretação sísmica, atuando como um controle de qualidade para a interpretação. Caso a anomalia gravimétrica gerada pela modelagem não estivesse de acordo com a anomalia medida, a interpretação sísmica era revista na tentativa de um melhor ajuste entre o resultado modelado e o medido. Este ajuste, no entanto, sempre foi feito honrando os refletores que estavam bem marcados na sísmica. Somado a isto, ainda foi utilizado o dado magnético adquirido no campo, no auxilio da interpretação. O arcabouço crustal obtido com a utilização deste método permitiu a comparação dos resultados da interpretação com os modelos de evolução de margens passivas existentes na literatura, mostrando muitos pontos em comum aos modelos que defendem a possibilidade de ocorrência de manto exumado em margens passivas pobres em magmatismo. A interpretação final destes dados mostrou um domínio proximal marcado por uma crosta continental espessa porém pouco afinada em contato com um domínio distal marcado por uma crosta continental muito afinada (crosta hiper-estirada) e, em direção ao centro do oceano, uma região em que ocorre a exumação do manto. A passagem do domínio proximal para o distal ocorre de forma rápida em uma região denominada Zona de Estrangulamento. À oeste do manto exumado é possível identificar a crosta oceânica. O cruzamento dos resultados obtidos neste trabalho com dados do embasamento aflorante no continente africano sugerem um controle do deste nos valores finais de afinamento da crosta continental sob a bacia e nas regiões de manto exumado. Trabalhos recentes realizados na costa de Angola e do Brasil mostram feições semelhantes às identificadas nesta dissertação. / The main objectives of this study are to identify features on seismic data that allow (1) the building of a deep crustal framework and of the upper portion of the mantle, in part of the Angolan margin; (2) to compare this framework with the adjacent outcropping basement of the African continent and; (3) to try to fit these results to the published continental breakup models. In order to achieve these objectives, five deep reflection seismic lines (25 km of depth) situated in the in Kwanza and Lower Congo Basins on the passive margin of Angola were interpreted. The features identified on seismic were useful to recognize the tripartite division that caracterize the oceanic crust and in defining the Mohorovicic Discontinuity (that represents the limit between crust and mantle). The seismic interpretation associated with the data obtained from the scientific literature (that provided density values for the packages identified on seismic interpretation) allowed the establishment of a gravity modeling that was compared to the gravity data acquired during the seismic acquisition. The gravity model was useful to validate the seismic interpretation, acting as a quality control of the latter. In case of the gravity anomaly generated by the modeling not being in accordance with the measured anomaly, the seismic interpretation was revised in order to obtain a better adjustment between the modeled and the measured result. This adjustment, however, was always done honoring the reflectors that were clearly positioned on seismic. In addition, the magnetic data acquired on the field was used to help on interpretation. The crustal framework obtained by the methodology described above was compared with the passive margin evolution models found on scientific literature, showing some points in common with the models that postulate the occurrence of exhumed mantle in magma-poor passive margins. The final interpretation of these data showed the existence of a proximal domain characterized by a thick continental crust slightly thinned in contact with a distal domain marked by a hyper-extended continental crust. Oceanwards there is a region where the exhumation of the mantle took place. The passage of the proximal to the distal domain is abrupt, here termed as a Necking Zone. Oceanic crust is identified to the west of exhumed mantle. The comparison of the results obtained in this study with data from the outcropping basement on the African continent suggests a basement control on the vales of continental crust thinning attained under the basins and on the regions of exhumed mantle. Recent works done on the Angolan and Brazilian margins show features similar to the ones identified on this dissertation.

Gangschwärme nehmen eine bedeutende Stellung im Verständnis zur kontinentalen Fragmentierung ein. Einerseits markieren sie das Paläo-Spannungsfeld und helfen bei der Rekonstruktion der strukturellen Entwicklung der gedehnten Lithosphäre, andererseits gibt ihre petrologische Beschaffenheit Aufschluß über die Entstehung des Magmas, Aufstieg und Platznahme und schließlich erlaubt ihre Altersbestimmung die Rekonstruktion einer chronologischen Reihenfolge magmatischer und struktureller Ereignisse.<br><br> Das Arbeitsgebiet im namibianischen Henties Bay-Outjo Dike swarm (HOD) war zur Zeit der Unterkreide einem Rifting mit intensiver Platznahme von überwiegend mafischen Gängen unterworfen. Geochemische Signaturen weisen die Gänge als erodierte Förderkanäle der Etendeka Plateaubasalte aus. Durch den Einsatz von hochauflösenden Aeromagnetik- und Satellitendaten war es möglich, die Geometrie des Gangschwarmes erstmals detailliert synoptisch zu erfassen. Viele zu den Schichten des Grundgebirges foliationsparallel verlaufende magnetische Anomalien können unaufgeschlossenen kretazischen Intrusionen zugeordnet werden.<br><br> Bei der nach Norden propagierenden Südatlantiköffnung spielte die unterschiedliche strukturelle Vorzeichnung durch die neoproterozoischen Faltengürtel sowie Lithologie und Spannungsfeld des Angola Kratons eine bedeutende Rolle. Im küstennahen zentralen Bereich war dank der Vorzeichnung des Nordost streichenden Damara-Faltengürtels ein Rifting in Nordwest-Südost-Richtung dominierend, bis das Angola Kraton ein weiteres Fortscheiten nach Nordosten hemmte und die Vorzeichnung des Nordwest streichenden Kaoko-Faltengürtels an der Westgrenze den weiteren Riftverlauf und die letztendlich erfolgreiche Öffnung des Südatlantiks bestimmte. Aus diesem Grund kann das Gebiet des HOD als ein failed rift betrachtet werden.<br><br> Die Entwicklung des Spannungsfeldes im HOD kann folgendermaßen skizziert werden:<br><br> 1. Platznahme von Gängen bei gleichzeitig hoher Dehnungsrate und hohem Magmenfluß.<br> 2. Platznahme von Zentralvulkanen entlang reaktivierter paläozoischer Lineamente bei Abnahme der Dehnungsrate und fortbestehendem hohen Magmenfluß.<br> 3. Abnahme/Versiegen des Magmenflusses und neotektonische Bewegungen führen zur Bildung von Halbgräben. / Dike swarms play a fundamental role in understanding continental breakup. On the one hand they represent strain markers of the paleo-deformation field and help to reconstruct the structural evolution of the rifted lithosphere. On the other hand their magmatic infill contains information about the conditions of magma generation, ascent and emplacement. Finally, dating of dikes allows reconstructing a chronological order of magmatic and structural events. The study area of the Namibian Henties Bay-Outjo Dike swarm (HOD) underwent tectonic extension in the Lower Cretaceous associated with the widespread emplacement of predominantly mafic dikes and intrusive ring complexes representing the remnants of volcanic centres. Geochemical signatures of the dikes prove them to be the feeder structures of the Etendeka Plateau Basalts. The application of recent high resolution aeromagnetic surveys and satellite imaging revealed the dike swarm's extent and geometry for the first time. The distribution and geometry of the dikes shown in the aeromagnetics reflect the propagation of the South Atlantic opening from south to north by their relative-ages.<br><br> Northwest-southeast-directed rifting was dominant in the central coastal area, due to the structural control of the northeast striking basement structures until further propagation was hampered by the Angola Craton. Subsequently the structural control of the coast-parallel Kaoko Belt became dominant and determined the successful opening of the South Atlantic. Hence, the area of the HOD can be considered as a failed rift.<br><br> The stress field evolution within the HOD can be outlined as follows:<br><br> 1. Intrusion of dikes when extension rates as well as magma supply were high.<br> 2. Intrusion of volcanic ring complexes along reactivated Panafrican lineaments when extension rates decreased and magma supply remained high.<br> 3. Neotectonic movements create half-grabens after the termination of magmatism.

Este trabalho tem como objetivo a identificação de feições que permitam (1) a construção do arcabouço crustal profundo e da porção superior do manto em parte da Costa de Angola, (2) a comparação deste arcabouço com o embasamento aflorante e (3) a tentativa de adequar estes resultados aos modelos de ruptura continental já publicados. Para alcançar estes objetivos foi feita a interpretação de cinco linhas sísmicas de reflexão profundas (25 Km de profundidade) na costa de Angola, nas Bacias de Kwanza e Baixo Congo, adquiridas pela ION-GXT. As feições identificadas na sísmica de reflexão auxiliaram na determinação dos limites da crosta continental superior e inferior, no reconhecimento das camadas que compõem a crosta oceânica e na identificação da Descontinuidade de Mohorovicic (que marca o limite entre crosta e manto). A interpretação sísmica associada a dados da literatura (que proporcionaram valores de densidade para os pacotes identificados na interpretação sísmica) permitiram a realização de uma modelagem gravimétrica que foi comparada ao dado gravimétrico adquirido durante a aquisição sísmica. A modelagem gravimétrica serviu para validar a interpretação sísmica, atuando como um controle de qualidade para a interpretação. Caso a anomalia gravimétrica gerada pela modelagem não estivesse de acordo com a anomalia medida, a interpretação sísmica era revista na tentativa de um melhor ajuste entre o resultado modelado e o medido. Este ajuste, no entanto, sempre foi feito honrando os refletores que estavam bem marcados na sísmica. Somado a isto, ainda foi utilizado o dado magnético adquirido no campo, no auxilio da interpretação. O arcabouço crustal obtido com a utilização deste método permitiu a comparação dos resultados da interpretação com os modelos de evolução de margens passivas existentes na literatura, mostrando muitos pontos em comum aos modelos que defendem a possibilidade de ocorrência de manto exumado em margens passivas pobres em magmatismo. A interpretação final destes dados mostrou um domínio proximal marcado por uma crosta continental espessa porém pouco afinada em contato com um domínio distal marcado por uma crosta continental muito afinada (crosta hiper-estirada) e, em direção ao centro do oceano, uma região em que ocorre a exumação do manto. A passagem do domínio proximal para o distal ocorre de forma rápida em uma região denominada Zona de Estrangulamento. À oeste do manto exumado é possível identificar a crosta oceânica. O cruzamento dos resultados obtidos neste trabalho com dados do embasamento aflorante no continente africano sugerem um controle do deste nos valores finais de afinamento da crosta continental sob a bacia e nas regiões de manto exumado. Trabalhos recentes realizados na costa de Angola e do Brasil mostram feições semelhantes às identificadas nesta dissertação. / The main objectives of this study are to identify features on seismic data that allow (1) the building of a deep crustal framework and of the upper portion of the mantle, in part of the Angolan margin; (2) to compare this framework with the adjacent outcropping basement of the African continent and; (3) to try to fit these results to the published continental breakup models. In order to achieve these objectives, five deep reflection seismic lines (25 km of depth) situated in the in Kwanza and Lower Congo Basins on the passive margin of Angola were interpreted. The features identified on seismic were useful to recognize the tripartite division that caracterize the oceanic crust and in defining the Mohorovicic Discontinuity (that represents the limit between crust and mantle). The seismic interpretation associated with the data obtained from the scientific literature (that provided density values for the packages identified on seismic interpretation) allowed the establishment of a gravity modeling that was compared to the gravity data acquired during the seismic acquisition. The gravity model was useful to validate the seismic interpretation, acting as a quality control of the latter. In case of the gravity anomaly generated by the modeling not being in accordance with the measured anomaly, the seismic interpretation was revised in order to obtain a better adjustment between the modeled and the measured result. This adjustment, however, was always done honoring the reflectors that were clearly positioned on seismic. In addition, the magnetic data acquired on the field was used to help on interpretation. The crustal framework obtained by the methodology described above was compared with the passive margin evolution models found on scientific literature, showing some points in common with the models that postulate the occurrence of exhumed mantle in magma-poor passive margins. The final interpretation of these data showed the existence of a proximal domain characterized by a thick continental crust slightly thinned in contact with a distal domain marked by a hyper-extended continental crust. Oceanwards there is a region where the exhumation of the mantle took place. The passage of the proximal to the distal domain is abrupt, here termed as a Necking Zone. Oceanic crust is identified to the west of exhumed mantle. The comparison of the results obtained in this study with data from the outcropping basement on the African continent suggests a basement control on the vales of continental crust thinning attained under the basins and on the regions of exhumed mantle. Recent works done on the Angolan and Brazilian margins show features similar to the ones identified on this dissertation.