An early phase continental rift is an emerging plate boundary where tectonic forces stretch and thin the continental lithosphere, shaping the Earth's surface. Continental breakup and its progression are typically driven by the interplay between repeated magmatic and tectonic activities, which have been explored through both tectonic and magma-assisted rifting models. Understanding volcano-tectonic interactions is key for evaluating the role of magmatic fluids in facilitating the initiation of continental breakup during early phase rifting. This study applies the magma-assisted rifting model to the Natron Rift and investigates volcano-tectonic interactions during early phases of continental breakup associated with observed changes in the volcanic plumbing system of the active volcano Ol Doinyo Lengai. The Natron Rift is a magma-rich rift in the southern segment of the Eastern Branch in northern Tanzania providing an ideal setting to explore the interactions between tectonic and magmatic processes in the early stages of rifting. To investigate tectonic and magmatic interactions, we began by characterizing the magmatic plumbing system of Ol Doinyo Lengai using Global Navigational Satellite System (GNSS) data from our TZVOLCANO network and Interferometric Synthetic Aperture Radar (InSAR) observations. We inverted the GNSS and InSAR data independently to identify potential deformation sources using the software dMODELS. We then conducted a joint inversion of both datasets and found results that were consistent with the independent inversions within 2-sigma uncertainty. Our findings suggest that Ol Doinyo Lengai is fed by an offset multi-tiered reservoir system, consisting of a shallow magma reservoir located east of the volcano connected to a deeper reservoir through a network of fractures. This magmatic system likely influences the nature, style, and magnitude of volcanic activity at the edifice. We also assessed temporal and spatial changes in surface motion observed with GNSS stations associated with magmatic activity to help mitigate risks to nearby communities, tourism, and air traffic. Detecting transient deformation is essential for forecasting eruptions since these signals often precede eruptive events. To detect transient signals using GNSS data from the TZVOLCANO network, we employed the Targeted Projection Operator (TPO) program which projects GNSS time-series data onto a target spatial pattern. We analyzed seven years of continuous GNSS data and divided the observations into three-year intervals. The TPO method detected rapid uplift between March 2022 and December 2022 followed by steady-state uplift through August 2023. The method also identified quiescent periods and non-eruptive inflation signals that enhance our understanding of the dynamic magma plumbing system of Ol Doinyo Lengai. When integrated with the TZVOLCANO network, which streams real-time GNSS data, this approach enables continuous monitoring and early detection of potential volcanic hazards. Ongoing monitoring is crucial for assessing volcanic risks and improving emergency response plans. Finally, we examined the role of interactions between tectonic and magmatic processes in the Natron Rift during the early stages of continental breakup, focusing on the evolution of the magma plumbing system beneath Ol Doinyo Lengai. Using the code PyLith, we developed a 3D model of the region. The modeling experiments test both homogeneous and heterogeneous medium, with and without topography to estimate surface deformation and stress changes on the Natron fault due to geodetically constrained magma source inflation and deflation. Our analysis focused on stress transfer from the magma sources to assess the likelihood of fault slip, considering the typical 0.1 MPa threshold for triggering slip in magmatic rift settings. Results indicate that during the inflation period from 2016 to 2023, slip on the Natron fault is inhibited adjacent to the volcano under all scenarios. During the magma source deflation phase that occurred from 2007 to 2008 due to explosive eruptions, slip on the Natron fault was promoted adjacent to the volcano under all scenarios. Shear stress change analyses reveal that during the magma deflation scenario, slip of the Natron fault is consistent with oblique normal fault movement that is dominated by normal faulting and has components of strike-slip motion. Finite numerical modeling results demonstrate that topography considerably influences stress changes caused by dynamic magma sources as compared to material heterogeneity highlighting the importance of incorporating topography in volcano-tectonic settings. This work suggests that the potential ongoing magmatic activity at Ol Doinyo Lengai and its proximity to the Natron Fault influence the development of the youthful Natron Rift during early phase rifting. However, this influence likely inhibits fault slip at present on the adjacent section of the Natron fault due to magma source inflation. / Doctor of Philosophy / Continental rifts in their early phases mark the initial stage of plate boundary formation, characterized by the stretching and thinning of the Earth's outer, rigid shell under tectonic forces. Rifts are a significant agent in shaping the Earth's rigid, outer shell, ultimately leading to the formation of oceanic basins and volcanoes. Rifting occurs when tectonic plates break apart, creating faults and allowing magma that formed deep in the Earth to rise to shallower depths. This process not only contributes to the geological evolution of our planet, but it also poses significant hazards in the form of earthquakes and volcanic eruptions. Understanding the interaction between tectonic activity, like slip on faults, and magmatic processes, like volcanic deformation, is essential for assessing rift behavior, particularly in the early, immature stages of rifting when volcanic and tectonic activities are closely linked. This research focuses on the Natron Rift, a magma-rich segment of the southern part of the Eastern Branch of the East African Rift System located in Northern Tanzania. This region includes the active volcano Ol Doinyo Lengai, which is known for its unique magma composition and a history of explosive eruptions. The Natron Rift is an ideal setting to study the interactions between volcanic and faulting processes since it is still in the early stages of rifting. We examined the volcanic structure beneath the active volcano Ol Doinyo Lengai and its surroundings to assess the sources of magma supplying the volcano. We analyzed the geometry and location of a magma source using Global Navigational Satellite System (GNSS) data from our TZVOLCANO monitoring network and satellite images. We used the software dMODELS to independently model the surface displacements and identify potential magma sources. We also combined both datasets and jointly modeled them to test the independent results, which suggested a shallow, deflating magma source located to the east of Ol Doinyo Lengai. The magma source we found is likely connected to a deeper one through fractures that feed Ol Doinyo Lengai. The magmatic system determined from this study influences the nature and intensity of volcanic activity. We further assessed how the surface of Ol Doinyo Lengai volcano changes over time in response to magmatic activity to better understand and reduce the risks posed by eruptions. Volcanic eruptions at Ol Doinyo Lengai pose a risk to nearby communities, tourism, and air traffic, making it crucial to detect surface changes that could indicate an impending eruption. We developed computer models that identified potential non-eruptive volcanic signals due to magma source changes using seven years of continuous GNSS data from our monitoring network. The detected transient signals include a period of rapid uplift from March 2022 to December 2022 followed by steady uplift through August 2023. When the difference between the observed data and the expected pattern three times larger, this difference indicates transient surface motion that could signal an eruption in the near future. This information provides valuable context for eruption forecasting and serves as an early-warning system for the surrounding communities. Continuous monitoring using real-time data from the GNSS network is essential for the early detection of volcanic hazards and improving emergency response efforts. Finally, we investigate the roles played by the interactions between tectonic and magmatic processes in developing the Natron Rift during early stages of continental breakup. We use advanced modeling software called PyLith to create a 3D model of the region that incorporates known magma sources and the Natron fault. We estimate the surface motions and stress changes on the Natron fault due to changes in the known magma sources (inflation or deflation). Our stress transfer analysis indicates that during magma source inflation from 2016 to 2023 the Natron fault near the volcano section is clamped and prevents fault slip. For the deflating magma source associated with 2007-2008 explosive eruptions, stress changes on the Natron fault adjacent to the volcano section indicated fault slip likely occurred with dominantly normal faulting that includes a small component of strike-slip motion. The incorporation of topography significantly affects the amount of stress transferred on the fault under all scenarios. This study suggests that current magmatic activity at Ol Doinyo Lengai along with its closeness to the Natron Fault affects how the early stage Natron Rift develops. However, this influence likely prevents fault slip currently on the volcanic section of the Natron fault because of magma source inflation inhibiting slip the fault.

Continental rifting ultimately creates a deep accommodation space for sediment. When a major river flows into a late-stage rift, thick deltaic sediment can change the thermal regime and alter the mechanisms of extension and continental breakup. The Salton Trough, the northernmost rift segment of the Gulf of California plate boundary, has experienced the same extension as the rest of the Gulf, but is filled to sea level by sediment from the Colorado River. Unlike the southern Gulf, seafloor spreading has not initiated. Instead, seismicity, high heat flow, and minor volcanoes attest to ongoing rifting of thin, transitional crust. Recently acquired controlled-source seismic refraction and wide-angle reflection data in the Salton Trough provide constraints upon crustal architecture and active rift processes. The crust in the central Salton Trough is only 17-18 km thick, with a strongly layered but relatively one-dimensional structure for ~100 km in the direction of plate motion. The upper crust includes 2-3 km of Colorado River sediment. The basement below the sediment is interpreted to be similar sediment metamorphosed by the high heat flow and geothermal activity. Meta-sedimentary rock extends to at least 7-8 km depth. A 4-5 km thick layer in the middle crust is either additional meta-sedimentary rock or stretched pre-existing continental crust. The lowermost 4-5 km of the crust is rift-related mafic magmatic material underplated from partial melting in the hot upper mantle. North American lithosphere in the Salton Trough has been almost or completely rifted apart. The gap has been filled by ~100 km of new transitional crust created by magmatism from below and sedimentation from above. These processes create strong lithologic, thermal, and rheologic layering. Brittle extension occurs within new meta-sedimentary rock. The lower crust, in comparison, stretches by ductile flow and magmatism is not localized. This seismic interpretation is also supported by 1D thermal and rheological modeling. In this passive rift driven by far-field extensional stresses, rapid sedimentation keeps the crust thick and ductile, which delays final breakup of the crust and the initiation of seafloor spreading. / Ph. D.

La compréhension des processus et des modalités de l’inversion des systèmes extensifs et plus particulièrement les domaines de marges amincies, dans les chaines de collision est un enjeu majeur. La chaîne intracontinentale des Pyrénées constitue un exemple d’inversion de marges passives hyper-amincies, associées à un métamorphisme HT-BP et intégrées dans le prisme orogénique. La première partie de cette étude est centrée sur l’étude de la répartition du métamorphisme HT-BP associé à la phase de rifting et de l’exhumation du manteau lithosphérique. L’apport des données de TRSCM a permis, dans un premier temps, de définir l’enveloppe de la ZIM caractérisée par des températures comprises entre 400 et 630°C à l’échelle de l’ensemble de la chaîne et de montrer qu’il n’existe pas de gradient significatif des températures maximales à cette échelle. Dans un second temps, cette étude a permis de mettre en évidence des sauts de température importants au travers de failles majeures et de distinguer des gradients de températures latéraux à l’échelle des différents bassins constituant la ZIM, en particulier dans l’ouest de la chaîne sur l’exemple de la Nappe de Marbres. Cette partie de l’étude montre également l’importance d’une tectonique salifère antérieure au métamorphisme de HT-BP. La seconde partie de cette étude, concernant la structure de la ZIM met en évidence trois phases de déformation, associées à l’orogenèse pyrénéenne ainsi que le rôle du niveau de décollement des évaporites du Trias supérieur dans l’allochtonie généralisée de la ZIM. De plus, les failles majeures observées dans la ZNP, montrent un mouvement inverse avec une composante décrochante sénestre. L’interprétation de l’ensemble de ces résultats suggère que la ZIM et la ZNP ne forme qu’une seule unité découplée du socle varisque au niveau du Trias supérieur et déplacée par des chevauchements plats issus de l’héritage extensif, lors du début de la convergence. Le mode de déformation est alors de type thin-skinned, puis devient, lors de la collision des deux paléomarges, de type thick-skinned, avec le développement de faille majeures associées à l’exhumation des blocs de socle (Massifs Nord-Pyrénéens) qui ont découpé l’ensemble de la ZIM. / The understanding of the processes and scenarios of the inversion of extensional systems, and more specifically of hyper-extended margins, in collision thrust belts is a major issue. The intracontinental belt of the Pyrenees is an example of inversion of hyper-extended margins, associated with a HT-LP metamorphism and then integrated within the orogenic wedge. The first part of this study is focused on the distribution of the HT-LP metamorphism associated with rifting and the exhumation of lithospheric mantle. A new set of TRSCM data allows the recognition of the geometry of the IMZ, characterized with temperature ranging from 400 to 630°C and shows the absence of a regional gradient at this scale. This study then shows significant temperature gaps across major faults and distinguishes lateral temperature gradients at the scale of the different basins constituting the IMZ, especially in the westernmost part of the belt, in the Nappe des Marbres Basin. This part of the study moreover shows the importance of a salt tectonics prior to the HT-LP metamorphism. The second part shows the existence of three main tectonics phases during the Pyrenean orogeny and the role played by the Late Triassic evaporites as a decollement level in the generalized allochthony of the IMZ. A left-lateral component along the main faults within and along the boundaries of the North Pyrenean Zone (ZNP) is also shown. The interpretation of these observations is that the IMZ and ZNP form a single tectonic unit, decoupled from the Variscan basement by the decollement in the Late Triassic deposits and displaced above shallow-dipping thrust faults inherited from the rifting episode, during the first stages of the convergence. Deformation mode is then thin-skinned and becomes thick-skinned when the two paleomargins collide, with the development of major steeper faults linked with the exhumation of basement blocks (North Pyrenean Massifs) that dissected the IMZ.

Cette thèse présente la première étude Source-to-Sink de la marge Atlantique Equatoriale africaine au Méso-Cénozoïque. Nous avons dans un premier temps produit, à partir d'une nouvelle méthode intégrant les limites d'érosion des dépôts préservés dans les bassins et leur extension initiale minimum, une nouvelle reconstruction paléogéographique et structurale couplant pour la première fois le continent ouest africain et l'Atlantique Equatoriale au cours du Méso-Cénozoïque. Ceci nous permet de suivre l'évolution depuis 200 Ma des domaines en érosion (sources) et en sédimentation (puits) à l'échelle continentale. Nous montrons en particulier qu'au Crétacé supérieur la zone correspondait à un grand bassin intracratonique Saharien qui exportaient ses sédiments à la fois vers la Téthys et vers l'Atlantique Equatoriale. La fragmentation de ce bassin a eu lieu à l'Oligocène par le soulèvement du bouclier du Hoggar qui a isolé les petits dépôt-centres résiduels actuels. Le développement de cette topographie particulière est issu de la superposition de différentes longueurs d'onde de déformation à l'échelle continentale combinant les bourrelets marginaux longeant la marge équatoriale et un bombement de type " point chaud ".Nous avons ensuite caractérisé, à partir de l'interprétation des données sismiques et des puits, la segmentation de la marge continentale équatoriale en segments transformants et divergents et l'architecture stratigraphique post-rift du prisme stratigraphique associée au Crétacé Supérieur. Nous montrons que les parties proximales (dépôts de plateforme et pente continentale) des prismes stratigraphiques du Crétacé Supérieur ne sont préservés que le long des segments divergents de la marge, et pas le long des segments transformants. Nous interprétons cette différence de préservation comme résultant de mouvements verticaux post-rift plus importants dans les domaines proximaux des segments transformants empêchant la préservation des termes proximaux des systèmes sédimentaires. La caractérisation des architectures stratigraphiques post-rifts a ensuite permis la quantification des volumes sédimentaires préservés dans ces bassins de marges passives. En parallèle, de nouvelles données de thermochronologie basse-température (AFTA et (U-Th-Sm/He sur apatite) acquises à l'Université de Glasgow sur les échantillons de trois profils perpendiculaires à la marge équatoriale ont permis de quantifier l'histoire de l'érosion et les volumes dénudés sur le domaine continental au cours du Méso-Cénozoïque. Ces données montrent que le seul événement thermique majeur enregistré correspond au refroidissement lié à la dénudation d'une topographie syn-rift le long de la marge. Le bilan d'érosion et d'accumulation montre que les ordres de grandeur des volumes dénudés et accumulés sont comparables à l'échelle de la marge équatoriale au cours du Méso-Cénozoïque. Certaines périodes (Crétacé Supérieur et depuis le Miocène Supérieur), montrent cependant un excès d'accumulation qui pourrait être associé à la remobilisation de sédiments précédemment stockés dans des bassins intracontinentaux ou sur la plateforme de la marge. / This PhD thesis presents the first source-to-sink study of the African Atlantic Equatorial margin. We established new Meso-Cenozoic paleogeographic and structural reconstructions, integrating the West African sub-continent and the Equatorial Atlantic Ocean, based on a new mapping method defining both erosion limits of preserved deposits and their minimum original extension. We show the evolution over 200 Myrs of the eroding (sources) and accumulating domains (sinks) at continental scale. We demonstrate in particular that during the Cretaceous, a large Saharan intracratonic basin was exporting sediments toward both the Tethys and the future Atlantic Equatorial Ocean. The fragmentation of this basin occurred in the Oligocene, by the growth of the Hoggar swell that isolated the present-day small residual depot-centers. The development of this specific "basin and swell" topography results from the superimposition of various deformation wavelength at continental scale combining a marginal upwarp along the equatorial margin and a hot spot swell. We then characterized, from the interpretation of seismic data and well logs, the segmentation of the Equatorial Atlantic passive margin and the stratigraphic architecture of the post-rift Upper Cretaceous sedimentary wedge. We show that the proximal parts of the Late Cretaceous sedimentary wedge are only preserved along divergent segments of the margin and not along transform segments. We interpret this differential preservation as the result of a greater uplift, during the early post-rift, in the proximal parts of the transform segments preventing the preservation of the proximal terms of the sedimentary systems. The transform segments are associated with narrow necking zone, resulting in greater flexural uplift than divergent segments showing wider necking zones, in particular during the early post-rift. The characterization of the stratigraphic architecture of the post-rift sedimentary wedge then allowed for the quantification of accumulation history in the passive margin basins. New low-temperature thermochronological data (AFTA and Apatite (U-Th-Sm)/He) acquired at the University of Glasgow on the samples of three regional transects perpendicular to the margin allowed for the quantification of the denudation history and eroded volume on the continental domain. These data shows that the major thermal event recorded by those samples is the cooling phase related to the erosion of a rift-related topography along the margin. Erosion and accumulation budgets fall within the same order of magnitude. During some given periods (Late Cretaceous and since the Late Miocene), excess in accumulation may be associated with the reworking of sediments previously stored within intracontinental basins or on the shelf of the margin.

L'architecture structurale de l’avant-pays sud atlasique tunisien est caractérisée par un style tectonique mixte résultant de la réactivation de failles normales connectées avec le socle, de la mise en place de décollements dans la couverture sédimentaire ainsi que d’un diapirisme non négligeable. La géométrie et l’orientation des structures extensives préexistantes, issues du rifting Trias à Turonien, contrôlent la déformation de la couverture sédimentaire au cours des phases compressives d’âge fini-mésozoïque et cénozoïque. En effet, la marge tunisienne a enregistrée une longue période de rifting, de la fin du Permien-Trias jusqu’au Turonien. Une inversion tectonique s’est initiée probablement pendant le Crétacé supérieur. Les compressions tectoniques tertiaires se sont produites au cours de trois périodes: l’Eocène, le Mio-Pliocène et le Plio-Quaternaire. Notre étude montre une variation temporel du champ de contrainte régional, d’un régime tectonique compressif de direction NW-SE d’âge Mio-Pliocène à un régime tectonique compressif de direction N-S à NNE-SSW d’âge Quaternaire à l’actuel. Ce changement de régime tectonique a lieu, soit à la fin du Pliocène, soit au début du Quaternaire. Et une variation spatiale du champ de contrainte, de la compression (Domaine atlasique de la Tunisie) à la transtension (Golfe de Gabès), semble se faire progressivement du Nord vers le Sud-Est. Cette étude souligne le rôle prépondérant des failles profondes héritées et acquises au cours de l'évolution de la marge passive sud téthysienne. Dans ce domaine, la restauration de notre coupe équilibrée montre un raccourcissement modéré en surface de l’ordre de 8.1 km (~7,3%). / The structural architecture of the Tunisian foreland consists in a mixed tectonic style with deep-seated basement faults, shallower décollements within sedimentary cover and salt diapirism. Structural geometry and orientation of the pre-existing Triassic-Turonian extensional structures controlled subsequent contractional deformation within the sedimentary cover. The rifting of the margin started in the late Permian–Triassic and continued up to the Turonian. From the inversion of the successive compressions, the development of ENE-trending thrust-related anticlines such as the Orbata and Chemsi structures are controlled by the reactivation of the inherited Mesozoic faults. Geologic data from this region indicate that the positive tectonic inversion occurred probably during Late Cretaceous period. The Cenozoic tectonic compressions in the southern Atlassic domain occurred during three periods: Late Eocene, Late Miocene and Plio-Quaternary. The Fault kinematic analysis reveals a temporal change in states of stress that occurred during the Late Cenozoic. A paleostress (Miocene-Pliocene) state is characterized by a regional compressional tectonic regime with a mean N134±09°E trending compressional axis (σ1). A modern (Quaternary to present-day) state of stress also corresponds to compressional tectonic regime with a regionally mean N05±10°E trending horizontal σ1. This study underlines the predominant role of inherited basement structures acquired during the evolution of the southern Tethyan margin, and their influence on the geometry of the Atlassic fold-and-thrust belt. At the southern Atlas of Tunisia our restoration shows a surface shortening of ~8.1 km (~7.3%).

O paralelismo entre antigos cinturões colisionais e sistemas de riftes mais jovens é amplamente reconhecido e especialmente observado ao longo do Oceano Atlântico. A relação entre estruturas litosféricas herdadas e a nucleação de sistemas de riftes tem sido o foco de muitos estudos. Entretanto, a relação entre estruturas herdadas e estruturas neo-formadas (i.e. durante a extensão) e a estruturação de margens passivas conjugadas ainda é pouco entendida. Na presente tese, usamos uma versão muito eficiente do código computacional Lagrangiano- Euleriano de elementos finitos FANTOM para modelar fluxos termo-mecânicos acoplados visco-plásticos de forma a entender a geodinâmica do processo de rifteamento considerando os efeitos das estruturas tectônicas herdadas e da estrutura reológica na litosfera continental. Os modelos consideram quantias variadas de extensão e contração prévios para desenvolver a herança tectônico-estrutural, a qual é ou não reativada durante o desenvolvimento da margem passiva. Nossos resultados mostram que: 1) as primeiras reativações da cunha orogênica ocorrem seguindo zona de cisalhamento principal de escala litosférica (antiga sutura); 2) zonas de cisalhamento crustais (antigos cavalgamentos) são parcial ou totalmente reativadas dependendo da quantia de contração anterior; 3) com pouca contração anterior, os cavalgamentos thick-skin são eficientemente reativados e controlam a formação da margem passiva; 4) maiores quantias de contração prévia permitem que zonas de cisalhamento afastadas da sutura não dão reativadas e preservam a estrutura orogênica; 5) a reativação de zonas de cisalhamento compressivas dominam durante os estágios iniciais do rifteamento, enquanto que em estágios finais e de afinamento crustal, dominam estruturas neo-formadas. Nossos modelos ajudaram a explicar algumas características das margens passivas conjugadas do Espírito Santo - Kwanza no Atlântico Central Sul, e da margem passiva norueguesa no Mar do Norte. Nossos modelos também mostraram que diferenças reológicas entre a crosta superior e a crosta inferior causam um desacoplamento durante subducção e posterior educção. Este último processo, associado ao necking termal da astenosfera ascendente, é responsável por deixar lascas de crosta inferior no canal de subducção-educção no manto litosférico, como é evidenciado por antigas zonas de subdução fósseis associadas à margem conjugada Newfoundland-Iberia. Dessa forma, a remoção de crosta inferior em margens passivas conjugadas pode ocorrer muito antes do processo de rifteamento. / The parallelism between older collisional belts and younger rift systems is widely known and particularly portrayed along the Atlantic Ocean. In what follows, the relationship between lithospheric inherited structures and nucleation of rift systems has been focus of many studies. Nevertheless, it is still poorly understood how inherited tectonic and new-formed structures (i.e. during extension) affect the final architecture of rifted conjugate passive margins. In this thesis we use a modified highly efficient version of the Arbitrary Lagrangian-Eulerian finite- element code FANTOM to model thermal-mechanical coupled, plane-strain, viscous-plastic creeping flows to understand the geodynamics of the rifting process considering the effects of tectonic structural inheritance and rheology on the final architecture of rifted conjugate margins. The models consider different amounts of previous extension and contraction to produce the structural inheritance that is reactivated or not during rifting. Our results show that: 1) first reactivations occur along the lithospheric former suture zone; 2) upper crustal thick skinned basement thrusts are partially or fully reactivated depending on the amount of prior contraction and size of the orogen; 3) with a small amount of contraction, thick skinned thrusts are efficiently reactivated in extension and provide the template for rifted margin formation; 4) with larger amounts of precursor contraction, thick skinned thrusts distal to the lithospheric suture zone do not reactivate in extension; 5) reactivation of prior contractional shears dominates during the early stages of rifting, whereas during the final stage of rifted passive margin formation new-formed extensional shears dominate. Models with less precursor extension and more contraction resulted in a rifted conjugate passive margin similar to the Espírito Santo-Kwanza conjugate in the Central South Atlantic, whereas more precursor extension and less contraction, develops into margin showing similar behavior of fault reactivations in the Norwegian margin, North Atlantic. Our models also show that rheological differences between upper and lower continental crusts cause them to decouple both during subduction and subsequent eduction. The latter process, associated to thermal necking of the upwelling asthenosphere, is responsible to leave slivers of previously subducted lower continental crust within the eduction channel in the mantle lithosphere, as it is seismically evidenced under the Newfoundland-Iberia rifted conjugate margins. In what follows, lower continental crust of the former subducting plate can be removed long before depth-dependent extension during magma-poor rifted margin development.

O paralelismo entre antigos cinturões colisionais e sistemas de riftes mais jovens é amplamente reconhecido e especialmente observado ao longo do Oceano Atlântico. A relação entre estruturas litosféricas herdadas e a nucleação de sistemas de riftes tem sido o foco de muitos estudos. Entretanto, a relação entre estruturas herdadas e estruturas neo-formadas (i.e. durante a extensão) e a estruturação de margens passivas conjugadas ainda é pouco entendida. Na presente tese, usamos uma versão muito eficiente do código computacional Lagrangiano- Euleriano de elementos finitos FANTOM para modelar fluxos termo-mecânicos acoplados visco-plásticos de forma a entender a geodinâmica do processo de rifteamento considerando os efeitos das estruturas tectônicas herdadas e da estrutura reológica na litosfera continental. Os modelos consideram quantias variadas de extensão e contração prévios para desenvolver a herança tectônico-estrutural, a qual é ou não reativada durante o desenvolvimento da margem passiva. Nossos resultados mostram que: 1) as primeiras reativações da cunha orogênica ocorrem seguindo zona de cisalhamento principal de escala litosférica (antiga sutura); 2) zonas de cisalhamento crustais (antigos cavalgamentos) são parcial ou totalmente reativadas dependendo da quantia de contração anterior; 3) com pouca contração anterior, os cavalgamentos thick-skin são eficientemente reativados e controlam a formação da margem passiva; 4) maiores quantias de contração prévia permitem que zonas de cisalhamento afastadas da sutura não dão reativadas e preservam a estrutura orogênica; 5) a reativação de zonas de cisalhamento compressivas dominam durante os estágios iniciais do rifteamento, enquanto que em estágios finais e de afinamento crustal, dominam estruturas neo-formadas. Nossos modelos ajudaram a explicar algumas características das margens passivas conjugadas do Espírito Santo - Kwanza no Atlântico Central Sul, e da margem passiva norueguesa no Mar do Norte. Nossos modelos também mostraram que diferenças reológicas entre a crosta superior e a crosta inferior causam um desacoplamento durante subducção e posterior educção. Este último processo, associado ao necking termal da astenosfera ascendente, é responsável por deixar lascas de crosta inferior no canal de subducção-educção no manto litosférico, como é evidenciado por antigas zonas de subdução fósseis associadas à margem conjugada Newfoundland-Iberia. Dessa forma, a remoção de crosta inferior em margens passivas conjugadas pode ocorrer muito antes do processo de rifteamento. / The parallelism between older collisional belts and younger rift systems is widely known and particularly portrayed along the Atlantic Ocean. In what follows, the relationship between lithospheric inherited structures and nucleation of rift systems has been focus of many studies. Nevertheless, it is still poorly understood how inherited tectonic and new-formed structures (i.e. during extension) affect the final architecture of rifted conjugate passive margins. In this thesis we use a modified highly efficient version of the Arbitrary Lagrangian-Eulerian finite- element code FANTOM to model thermal-mechanical coupled, plane-strain, viscous-plastic creeping flows to understand the geodynamics of the rifting process considering the effects of tectonic structural inheritance and rheology on the final architecture of rifted conjugate margins. The models consider different amounts of previous extension and contraction to produce the structural inheritance that is reactivated or not during rifting. Our results show that: 1) first reactivations occur along the lithospheric former suture zone; 2) upper crustal thick skinned basement thrusts are partially or fully reactivated depending on the amount of prior contraction and size of the orogen; 3) with a small amount of contraction, thick skinned thrusts are efficiently reactivated in extension and provide the template for rifted margin formation; 4) with larger amounts of precursor contraction, thick skinned thrusts distal to the lithospheric suture zone do not reactivate in extension; 5) reactivation of prior contractional shears dominates during the early stages of rifting, whereas during the final stage of rifted passive margin formation new-formed extensional shears dominate. Models with less precursor extension and more contraction resulted in a rifted conjugate passive margin similar to the Espírito Santo-Kwanza conjugate in the Central South Atlantic, whereas more precursor extension and less contraction, develops into margin showing similar behavior of fault reactivations in the Norwegian margin, North Atlantic. Our models also show that rheological differences between upper and lower continental crusts cause them to decouple both during subduction and subsequent eduction. The latter process, associated to thermal necking of the upwelling asthenosphere, is responsible to leave slivers of previously subducted lower continental crust within the eduction channel in the mantle lithosphere, as it is seismically evidenced under the Newfoundland-Iberia rifted conjugate margins. In what follows, lower continental crust of the former subducting plate can be removed long before depth-dependent extension during magma-poor rifted margin development.

Les marges passives présentent de nombreux indices de déformations postérieures à l'épisode de subsidence thermique, successif au rifting. Parmi ces déformations, la plus argumentée dans la littérature est la surrection, bien reconnue sur les marges nord-ouest européennes, ouest africaines, sud-ouest brésiliennes et indiennes. Elle est responsable d'une topographie élevée, ainsi que de perturbations importantes des systèmes sédimentaires marins et continentaux (identifiables sur images satellitaires). Des indices d'une tectonique récente en raccourcissement (plissements, failles...) sont également décrits sur ces marges et les données de sismicité, de mouvements de plaques tectoniques (DORIS), et des indications géologiques sont en faveur de contraintes compressives actuelles sur les marges passives péri-atlantiques Les causes suggérées de ces déformations sont diverses. Facteurs climatiques, tectonique régionale, ridge-push, point chaud, etc, sont envisagés dans la littérature. La modélisation nous permet de tester l'influence de certains de ces mécanismes sur la surrection et/ou le style et la localisation de la déformation. Le raccourcissement tectonique et l'influence des point-chauds ont ainsi été testés grâce à la modélisation analogique. Il en découle que les rhéologies des lithosphères océaniques et continentales, et les contrastes de résistance et de densité existant, ont une grande influence sur le style et la localisation des déformations. Un modèle numérique 2D en éléments finis nous permet de connaître l'influence de la thermicité initiale sur la déformation observée après le breakup. Ce modèle démontre notamment qu'une surrection des marges passives peut être induite par leur évolution thermique et que cette surrection est plus importante pour les marges volcaniques que pour les marges non-volcaniques. La rhéologie aura quant à elle une grande influence sur le style et la localisation de la déformation en compression, lorsque la marge est soumise à un raccourcissement horizontal. Il résulte de ces observations et modélisations que la déformation récente des marges passives est liée à des associations différentes de mécanismes selon chaque marge. Cependant certains mécanismes pourraient favoriser ces déformations (ridge-push ou contrainte compressive régionale, points chauds mis en place lors de leur formation ou postérieurement).

Ce travail est consacré à l'étude des structures, de la sédimentation et de la subsidence jurassiques sur une transversale E-W aux Alpes Occidentales, du Massif Cristallin Externe de Belledonne à la frontière italienne, représentant la marge occidentale de la Téthys Ligure (chapitre 1). il est fondé sur des coupes stratigraphiques levées dans les zones dauphinoise, subbriançonnaise, briançonnaise et piémontaise. En vue de déterminer les positions relatives des diverses coupes, le chapitre 2 rappelle les diverses étapes de l'histoire tectonique tertiaire, puis présente une carte des unités structurales alpines à la fln du Crétacé, sur la portion des Alpes étudiée ici. La description de coupes-types (chapitre 3) met en évidence divers faciès sédimentaires et de leur paléo-environnement. Une attention particulière est portée aux brèches synsédimentaires et aux formations olistolitiques, dont la genèse s'explique par plusieurs épisodes de démantèlement. Ce chapitre s'achève sur une tentative d'attribution des faciès sédimentaires, qu'ils soient macroscopiques ou microscopiques, à un milieu de dépôt, donc à une profondeur. Des cartes de faciès sédimentaires (chapitre 4) illustrent les situations respectivement à la fin de l'Hettangien, du Carixien, du Toarcien, de l'Aalénien, du Bathonien et de l'Oxfordien. Des cartes d'isopaques vraies permettent d'apprécier le début de la différenciation des aires à forte et à faible subsidence en domaine dauphinois et subbriançonnais. Les faciès sédimentaires restent uniformes jusqu'au Toarcien, mais des aires plus subsidentes que d'autres apparaissent dès le Sinémurien. De plus, des failles synsédimentaires séparent des aires à fortes et à faibles épaisseurs. Les accidents synsédimentaires résultant des distensions liées au rifting limitent diverses structures (chapitre 5) : des demis-blocs basculés, le plus typique étant celui de Bourg d'Oisans; des grabens symétriques, tel celui de Besse; des horsts de grande dimension, comme le Massif du Pelvoux. Enfin, lorsque des évaporites triasiques sont présentes à la base de la la série sédimentaire, des mouvements diapiriques sont initiés, en relation avec la tectonique extensive. Le meilleur exemple se trouve en zone subbriançonnaise dans le Massif du Perron des Encombres. Ces accidents synsédimentaires, de quelque type qu'ils soient, ont tous eu des jeux multiples, à diverses époques. Pour préciser quantitativement les observations précédentes, la subsidence de diverses séries de la transversale est calculée dans le chapitre 6. Le paramètre le plus contraignant à ce stade est la profondeur d'eau à l'époque des dépôts. Celle-là est déterminée à l'aide de deux méthodes: d'une part, grâce aux faciès synsédimentaires, de l'autre, grâce aux caractères morphologiques des fragments de crinoïdes. La subsidence au cours du Lias et du Dogger a lieu en trois étapes, en trois à-coups, le premier au début, le deuxième vers la fin du Lias et le troisième vers la fin du Dogger. Cette subsidence est toujours positive en domaine dauphinois et piémontais. Par contre, elle devient négative vers la fin du Lias en domaine briançonnais et dans une partie du domaine subbriançonnais, qui émergent alors. L'inversion de subsidence, ainsi que les ordres de grandeur des sauts des courbes de subsidence peuvent s'expliquer par un modèle d'extensions multiples non uniformes dans la lithosphère (chapitre 7). A trois reprises, à la fin du Trias, du Lias et du Dogger, une extension rapide a causé un amincissement de la croûte et du manteau lithosphérique, qui se traduit par une subsidence initiale, suivie d'un refroidissement thermique. Le modèle unidimentionnel bicouche permet d'estimer les valeurs des taux d'amincissement, grâce aux contraintes fournies par le chapitre 6. A partir de ces taux, une image possible de la structure profonde de la marge téthysienne peut être dessinée. Parmi les explications possibles de cette image, l'hypothèse d'une zone de cisaillement inclinée soit vers l'Est soit vers l'Ouest est l'une des plus séduisante. En effet, elle permettrait d'expliquer les caractéristiques du fond océanique du domaine ligure, qui apparait à la fin du rifting.

La partie centrale de la dépression Afar (Sud Mer Rouge) est bordée de blocs sans déformation récente (Danakil et d'Aysha). En raison du climat désertique, la morphologie des structures récentes et/ou actives est très bien conservée. Dans le cadre du programme TECTOSCOPE Afar financé par l'INSU/CNRS et pour la partie sud de la dépression, l'analyse détaillée de 8 couples d'images stéréoscopiques SPOT a permis non seulement de cartographier et d'interpréter le champ de failles mais aussi de mesurer le rejet vertical des failles. Cette interprétation a été réalisée d'une part sur des tirages photographiques Vizir réalisés à l'INSU, d'autre part sur station de travail par images "anaglyphes". Il a ainsi été possible de définir la géométrie et la cinématique du champ de déformation finie post-stratoïde (4Ma). Les caractéristiques du champ de failles ont été déterminées en utilisant des critères structuraux (intersection de failles, segmentation de coulées volcaniques, etc ... ) et l'analyse statistique de la distribution spatiale des orientations, rejets, et densités de la fracturation. Deux domaines de déformations (blocs basculés et horst-grabens) sont caractérisés et des zones de transfert entre ces domaines et les zones non déformées accommodent les variations latérales de la déformation. Cette étude met en évidence une direction/ moyenne d'extension N020° dans le centre de la dépression, s'orientant N040° en bordure du bloc danakil. Cette configuration structurale résulte d'une déformation progressive du domaine central de l'Afar et les hétérogénéités du champ de déformations sont interprétables en termes d'effets de bordure le long des blocs non déformés Dankaly et Aysha. Les implications cinématiques en sont déduites et comparées avec des expériences sur modèles réduits fragilesductiles inspirés du modèle de la "Biellette" Danakil (Sichler 1980). La rotation senestre de 10° du bloc Danakil offre une explication cohérente de la geométrie du champ de failles, des rotations dextres des blocs basculés du Sud de la dépression et de l'extension oblique du golfe de Tadjoura.