The major structure of the Shabani area is a north-south trending syncline, involving "Greenstones" which rest unconformably on various schists and gneisses of pre-Bulawayan ago. These basement gneisses have been divided into: migmatites, banded gneisses, granitic gneisses andgneissic granite, a sub-division that has proved extremely useful in evaluating the mutual relationships within the gneisses and between the gneisses and the later formations. The Basement Rocks; The oldest rocks found within the area are the migmatites and banded gneisses and, where more granitized, the granitic gneisses. Although no lithological distinction may be seen, it is believed that the strongly banded gneisses and the migmatites together consist of at least two structurally distinct generations of rocks; an earlier group having an east-south-east, west-north-west trend and a later group having a predominantly north-south trend. It is suggested that a second period of deformation caused the refolding of the already deformed first series of gneisses. This situation is most clearly demonstrated in the area to the north-west of the "Great Dyke", A gradation into only partially migmatized rocks is found within both of these groups, the original rocks were evidently iron rich sandstones, greenstones and a more calcareous sediment. A west-north-west, east-south-east trending batholith, into which the earlier migmatites grade both laterally and marginally, is responsible for the formation of the earlier migmatites. This granite batholith must have been at least partially fluid during its emplacement, since during the growth of microcline phonocrysts within the rock, small euhedral crystals of plagioclase, sphene and muscovite, attached themselves onto the growing crystal faces of the phenocryst. The Lower Sedimentary and Volcanic Greenstones : Over-lying the basement gneisses (structurally higher), mainly in the south of the area, is a series of sedimentary and volcanic greenstones that have been thermally metamorphosed by the intrusion of later granitic rocks. Near the top of this sedimentary sequence, opposite the Ngesi Antimony Claims, at the Ngesi River bridge on the Shabani-Belingwe road, a silt of subaqueously deposited tuff occurs. Within this deposit, graded bedding, flame structures, ripple marks and accretionary volcanic lapilli may be observed, demonstrating the sedimentary nature of this horizon, and in this locality its slightly overturned attitude. After the deposition of this series, the intrusion of concordant ultrabasic sills occurred, the presence of suchsills seems characteristic of these deposits. The Middle Sedimentary and Volcanic Greenstone Group: Resting unconformably upon the Lower Sedimentary and Volcanic Greenstones, which had already been folded about north-east, south-west trending axial planes, are the Middle Sedimentary and Volcanic Greenstone rocks, composed of volcanic lavas,tuffs and sub-aqueous sediments, This sequence begins as a predominantly sedimentary series of ironstones and conglomerates, volcanic materials increasing in amount at higher stratigraphic levels. The Middle Sedimentary and folded Volcanic rocks overstep the folded Lower Sedimentary and Volcanic Group onto the gneisses of the basement. The Middle Sedimentary and Volcanic Greenstone Group compose the most widespread lithological unit of the greenstone rocks and are preserved in a tight synform, trending north-south and slightly overturned towards the east. Throughout this large structure, graded bedding, ripple marks, flame structures and pillow lavas, all demonstrate that the rocks arc the "right way up", and that the structure is an upward "facing" syncline. The Upper Sedimentary Group i In the south of the area, the synclinal core of the Middle Sedimentary and Volcanic Greenstone Group is occupied by rocks of a distinctly different lithology; conglomerates, grits, siltstones and mudstones. The strike of this younger series is on the whole parallel to that of the Middle Sedimentary and Volcanic Greenstone Group, the dip vortical, but there are many local variations in strike and many minor, internal unconformities. The outcrop pattern of this Upper Sedimentary Group is suggestive of one that has been modified by the slumping of still unconsolidated sediments during the latter stages of synclinal infilling. Later Tectonic Events: Following the deposition of the Upper Sedimentary Group, the major synclinal structure suffered an up doming about a west-north-west, east-south-east axis, which was accompanied in its later stages of development by the intrusion of the Younger Granite. The latest major tectonic event was the emplacement of the Great Dyke, parallel to the north-south axis of the Middle Sedimentary and Volcanic Greenstone syncline. Later, normal both sinistrai faulting both sinistral and dextral wrench faulting, trending west-north-west, east-south-east, has affected the whole area.

This thesis reports on over 27 sills and 213 associated vents. The sills and vents were investigated using 3D seismic data, in a ~1000 km2 area, offshore Norway between the Møre and Vøring Basins (the Edvarda survey). A wide range of sill geometries are observed which are interpreted to be the result of five different processes acting on the sills. Three of these processes relate to how the host deforms. If sill intrusion causes deformation of the seafloor, creating folds, or the sills interact with folds created by neighbouring sills, sills are found to cross bedding (transgress) abruptly. Alternatively, if deformation is interpreted to be local, then continuously increasing Young’s Modulus with depth is interpreted to result in sills which transgress continuously upwards, akin to smooth ‘bowls’. At shallow depths the host is interpreted to fluidise, leading to limited transgression or in some cases multiple bowls. The seismic amplitude responses of shallow sills include flow related features such as channels and lobes. The other two processes interpreted to affect sill propagation stem from structures in the host: abrupt changes in lithology and pre-existing faults. Multiple sills are found to terminate, and in some cases form, at sand rich units in the otherwise mudstone dominated host. Additionally, some sills are interpreted to have intruded into a host with pre-existing polygonal faults, which led to angular sill geometries. Vents are found to occur directly above sills, often along the margins of sills, but in some cases over sill interiors, especially where the sills are locally shallower. Additionally, a cluster of 98, relatively small vents occur above the shallowest sill. Differential compaction and slumping are found to affect some larger vent morphologies. Overall, vent size is found to closely follow a power-law such that smaller vents are significantly more numerous than larger vents.

Dyke injection transports large volumes of magma over great distances, controlling the supply of magma to volcanoes and effectively releasing tensional stress at divergent plate margins. This thesis aims to improve understanding of dyke injection processes on different scales. Dyke shapes measured on the Isle of Rum have been analysed and show a mismatch between the currently accepted theory used to describe their shape, and the measured data. The measured dykes show wider edges than expected, consistent with wedging and cooling of magma in the dyke tips; wedged dykes can act as conduits for longer. Finite difference one- and two-dimensional models for the thermal evolution of the crust due to heat transfer from multiple dyke injection have been developed and applied to the geological setting of the actively spreading Main Ethiopian and Red Sea rifts, where the spreading rates are 5 and 16 mm yr-1 respectively. The model has shown that the spreading rate is the first order control on the temperature build up. Differences in crustal thickness exist between these two regions; the crust has thinned under the Red Sea Rift whilst under the Main Ethiopian Rift there has been no appreciable thinning. This difference has led to the conclusion that the spreading rate, and thus the temperature profile, is the principal cause for the differences in crustal thicknesses. Above the brittle-ductile transition temperature, the crust is likely to undergo pre- dominantly ductile deformation; for slow spreading rates (e.g. 5 mm yr-1), it takes up to 142 ka for the dyke injection site to reach this temperature. The position of the locus of strain at an actively rifting margin migrates with time. For slow spreading rates, the strain locus must remain fixed for at least 142 ka before appreciable crustal heating allows the onset of ductile stretching. Where the spreading rate is faster, the locus of strain must remain fixed for shorter lengths of time. Thus Ethiopia's evolving locus of strain and low spreading rate have likely caused much of the extension to be accommodated by magmatic intrusion rather than by stretching. Comparisons between the thermal model results and geophysical observations from a segment of the Red Sea rift have been made. The mag- netotelluric survey across the rift axis of the actively spreading Red Sea Rift segment has shown two bodies of hot material; one explanation is that the rift axis has jumped. Scaled experimental models have been used to study multiple dyke injection in an extensional tectonic setting. For a fixed overpressure, larger spacings between injections give smaller rotation angles between injections. This is consistent with the rotation angles and injection spacings observed between the recent dyke injections on the Red Sea Rift.

Volcanic earthquakes can provide significant insight into physical processes acting at volcanoes, such as magma accumulation and the mechanisms of deformation of the volcanic edifice. At the same time a statistical analyses of volcanic seismicity prior to an eruption (for example variations in the Gutenberg-Richter b-value – a measure of the proportion of large and small events) are a key component of the practical problem of forecasting eruptions. This thesis aims to tackle two key areas of research that are closely related to these important overall goals, by comparing seismic data obtained from currently-active volcanoes with direct field observation of faulting and fracturing from an exhumed extinct volcano. First I introduce a new approach that improves the accuracy and reliability of calculating spatial and temporal variations of the seismic b-value for frequency-magnitude distributions at active volcanoes, and apply it to several test cases. An extensive literature review highlights a large variability and lack of standardisation of methodology used to analyse frequency-magnitude distributions in the past. Motivated by this, I introduce and test a new workflow to standardise calculating completeness magnitudes of seismic catalogues. The review also highlights the fact that uncertainties in estimating the threshold magnitude of complete reporting have been ignored to date. Here I use synthetic catalogues to quantify this previously unidentified source of error, and provide a template to estimate the total error in b-value. In standard analysis it is also common to sample time windows subjectively, although this can introduce bias. Here I develop a new objective, iterative sampling method that calculates the b-value as a full probability density function which need not have a Gaussian error structure. Application of this method reveals ‘mode-switching’ behaviour for the first time in volcanic seismic catalogues. The results also show b-values often do have a value indistinguishable from that of tectonic seismicity (b=1 within error). Nevertheless there are also several robust examples of real high b-values, as high as 3.3. The second part of the study is based on a field campaign to investigate the fracture zones from an exhumed volcanic setting on the Isle of Rum, NW Scotland. Lithological and structural mapping is used to collect structural data that is then used to quantify and explain complex fracture patterns and the underlying intra-magma chamber processes that occurred there in the geological past. In particular I identify a singular collapse event within the youngest volcanic unit, the Central Intrusion. This is responsible for forming the observed igneous breccias and the lineaments on satellite images that I interpret as contemporaneous faults. Using appropriate scaling relations, I infer the b-value for the Rum lineaments data. This would have been relatively high, at a value of approximately 1.9. The final part of the study compares the fracture data on Rum to earthquake distributions at El Hierro volcano, Canary Islands. Here I show the level of fractal clustering is similar in both an extinct (60 Ma) and a currently active volcano. Both show similar high levels of clustering. However, in both cases there is a difference between the capacity and correlation dimensions (D₀≠D₂), implying the set of rupture sources or mapped fault traces form a multi-fractal set. Broadly, the scaling of fracture sets in an ancient volcano has similar properties to those observed in a modern volcano, except that the Rum data imply a greater absolute degree of spatial clustering of deformation than that for the recent unrest at El Hierro.

The Afar region of Ethiopia is slowly being torn apart by the Red Sea, Gulf of Aden and Main Ethiopian rifts which all meet at this remote, barren corner of Africa. Prior to rifting, volcanism probably started here some 30 million years ago, marked by the arrival of the Afar mantle plume and subsequent eruption of kilometres thick flood basalts. To the north and east the Red Sea and Gulf of Aden rifts have already progressed to become sea-floor spreading centres where new oceanic crust is produced. Active spreading on the Red Sea rift takes a landward step west into Eritrean Afar at approximately 15oN, after which divergence between the Nubian and Arabian tectonic plates is localised into 60 km long, 20 km wide magmatic segments that undergo periodic rifting cycles. This part of Afar is a unique natural laboratory where the process of transition from continental rifting to sea floor spreading can be studied. In September 2005 a dramatic rifting episode began on one such segment of the Red Sea rift in Afar (the Dabbahu magmatic segment), whereby a 60 km long dyke containing an estimated 2.5 km3 magma was intruded in just two weeks, allowing opening of up to 8 m. Since then a further 13 smaller dykes have been intruded, some with fissural eruptions of basaltic lava. Subsidence observed via geodetic observations can only account for a small fraction of the magma supply required to in ate the dykes, suggesting a deep crustal or upper mantle source must exist. The magnetotelluric (MT) method is a passive geophysical technique, used to probe the Earth to reveal subsurface conductivity. The presence of fluids can dramatically increase conductivity by orders of magnitude making the MT method ideally suited to detecting them. MT data collected from 22 sites on profiles near to and crossing the active rift are analysed and interpreted in conjunction with seismic and petrological constraints. They reveal for the first time, the existence of both a mid to lower-crustal magma chamber directly below the rift, and an o -axis zone of partial melt well within the mantle. The volume of melt contained within the crust and upper mantle below the Dabbahu segment is estimated to be at least 350 km3; enough to supply the rift at current spreading rates for almost 30 thousand years, assuming that both melt containing regions supply the rift. Vast amounts of highly conductive material, suggesting the existence of pure melt in places, are also required in the shallow crust close to Dabbahu volcano which lies at the northern end of the segment. Further data collected on the currently inactive Hararo segment which is the next one to the south of Dabbahu, show a smaller zone of partial melt that appears to be pooling at the Moho, inferred seismically to be at about 22 km, but little or no melt is required within the mid-crust. The minimum amount of melt estimated to be contained here is just 21 km3; an order of magnitude less than on the Dabbahu segment, but similar to estimates for melt within the crust found below the rift axis in the continental Main Ethiopian rift. This, along with other morphological evidence, suggests that this rift segment is less mature than the Dabbahu segment to the north, rather than it simply being at a different stage of a rifting cycle. A wide spread layer of highly conductive sediments up to 2 km thick has been imaged at most locations. This was unexpected on the Dabbahu segment where the surface of the Earth is dominated by heavily faulted basalts erupted from fissures, which are seen as a resistive uppermost layer several hundred metres thick. The high conductivity of the sediments is attributed to high heat flow and the presence of brines.

The north-east Atlantic margin (NEAM) is important for hydrocarbon exploration because of the growing evidence of hydrocarbon reserves in the region. However, seismic exploration of the sub-surface is hampered by large deposits of flood basalts, which cover possible hydrocarbon-bearing reservoirs underneath. There are several hypotheses as to why imaging beneath basalt is a problem. These include: the high impedance contrast between the basalt and the layers above; the thin-layering of the basalt due to the many flows which make up a basalt succession; and the rough interfaces on the top-basalt interface caused by weathering and emplacement mechanisms. I perform forward modelling to assess the relative importance of these factors for imaging of sub-basalt reflections. The boundary element method (BEM) is used for the rough-interface modelling. The method was selected because only the interfaces between layers need to be discretized, in contrast to grid methods such as finite difference for which the whole model needs to be discretized, and so should lead to fast generation of shot gathers for models which have only a few homogeneous layers. I have had to develop criteria for accurate modelling with the boundary element method and have considered the following: source near an interface, two interfaces close together, removal of model edge effects and precise modelling of a transparent interface. I have improved efficiency of my code by: resampling the model so that fewer discretization elements are required at low frequencies, and suppressing wrap-around so that the time window length can be reduced. I introduce a new scheme which combines domain decomposition and a far-field approximation to improve the efficiency of the boundary element code further. I compare performance with a standard finite difference code. I show that the BEM is well suited to seismic modelling in an exploration environment when there are only a few layers in the model and when a seismic profile containing many shot gathers for one model is required. For many other cases the finite difference code is still the best option. The input models for the forward modelling are based on real seismic data which were acquired in the Faeroe-Shetland Channel in 2001. The modelling shows that roughness on the surface of the basalt has little effect on the imaging in this particular area of the NEAM. The thin layers in the basalt act as a low-pass filter to the seismic wave. For the real-data acquisition, even the topbasalt reflection is a low frequency event. This is most likely to be due to high attenuation in the layers above the basalt. I show that sea-surface multiple energy is considerable and that it could mask possible sub-basalt events on a seismic shot gather, but any shallow sub-basalt events should still be visible even with the presence of multiple energy. This leaves the possibility that there is only one major stratigraphic unit between the base of the basalt and the crystalline basement. The implication of the forward modelling and real data analysis for acquisition is that the acquisition parameters must emphasize the low frequencies, since the high frequencies are attenuated before they even reach the top-basalt interface. The implication for processing is that multiple removal is of prime importance.

The Neoarchaean Agnew-Wiluna greenstone belt (AWB) of the Kalgoorlie Terrane, within the Eastern Goldfields Superterrane (EGS) of the Yilgarn Craton, Western Australia, contains several world-class, komatiite-hosted, nickel-sulphide ore bodies. These are commonly associated with felsic volcanic successions, many of which are considered to have a tonalite-trondhjemite-dacite (TTD) affinity. The Cosmos greenstone sequence lies on the western edge of the AWB and this previously unstudied mineralised volcanic succession contrasts markedly in age, geochemistry, emplacement mechanisms and probable tectonic setting to that of the majority of the AWB and wider EGS. Detailed subsurface mapping has shown that the footwall to the Cosmos mineralised ultramafic sequence consists of an intricate succession of both fragmental and coherent extrusive lithologies, ranging from basaltic andesites through to rhyolites, plus later-formed felsic and basaltic intrusions. The occurrence of thick sequences of amygdaloidal intermediate lavas intercalated with extensive sequences of dacite lapilli tuff, coupled with the absence of marine sediments or hydrovolcanic products, indicates the succession was formed in a subaerial environment. Chemical composition of the non-ultramafic lithologies is typified by a high-K calc-alkaline to shoshonite signature, indicative of formation in a volcanic arc setting. Assimilation-fractional crystallisation modelling has shown that at least two compositionally distinct sources must be invoked to explain the observed basaltic andesite to rhyolite magma suite. High resolution U-Pb dating of several units within the succession underpins stratigraphic relationships established in the field and indicates that the emplacement of the Cosmos succession took place between ~2736 Ma and ~2653 Ma, making it significantly older and longer-lived than most other greenstone successions within the Kalgoorlie Terrane. Extrusive periodic volcanism spanned ~50 Myrs with three cycles of bimodal intermediate/felsic and ultramafic volcanism occurring between ~2736 Ma and ~2685 Ma. Periodic intrusive activity, related to the local granite plutonism, lasted for a further ~32 Myrs or until ~2653 Ma. The Cosmos succession either represents a separate, older terrane in its own right or it has an autochthonous relationship with the AWB but volcanism initiated much earlier in this region than currently considered. Dating of the Cosmos succession has demonstrated that high-resolution geochronology within individual greenstone successions can be achieved and provides more robust platforms for interpreting the evolution of ancient mineralised volcanic successions. The geochemical affinity of the Cosmos succession indicates a subduction zone was operating in the Kalgoorlie Terrane by ~2736 Ma, much earlier than considered in current regional geodynamic models. The Cosmos volcanic succession provides further evidence that plate tectonics was in operation during the Neoarchaean, contrary to some recently proposed tectonic models.

Afin de parvenir à une meilleure compréhension de la dynamique d’ascension et d’éruption des magmas basaltiques, nous avons réalisé des expériences de décompression à haute pression (200–25 MPa) et haute température (1200°C) spécifiquement orientées pour documenter la nucléation des bulles de gaz ; ce processus, qui constitue la première étape du dégazage magmatique, conditionne l’évolution de la phase gazeuse (force motrice des éruptions explosives) dans le conduit volcanique. Quatre principaux ensembles d’expériences ont été menés afin de mieux comprendre le rôle des volatils majeurs (H2O, CO2, S), ainsi que les effets de la vitesse d’ascension et de la présence de cristaux sur la cinétique de vésiculation (nucléation, croissance, coalescence) des bulles dans les magmas basaltiques. L’objectif est de comprendre les mécanismes qui contrôlent les caractéristiques texturales (nombre, taille, forme des bulles) et chimiques (teneur en volatils dissous, composition des gaz) des produits naturels et de les approcher expérimentalement. Dans ce sens, les verres expérimentaux ont été analysés avant et après décompression sur le plan textural (microtomographie par rayons X, MEB) et chimique (FTIR, microsonde électronique). Nos résultats démontrent une forte influence du CO2 sur les processus ainsi que sur le mode (équilibre vs. déséquilibre) de dégazage des magmas basaltiques, en lien avec des différences de solubilité et de diffusivité entre les espèces volatiles. Nos données, obtenues dans des conditions voisines des conditions naturelles, ont des implications volcanologiques pour l’interprétation des textures de bulles et des mesures de gaz en sortie de conduit, ainsi que, plus spécifiquement, pour la dynamique des éruptions paroxysmales au Stromboli. / For a better understanding of the dynamics of ascent and eruption of basaltic magmas, we have performed high pressure (200–25 MPa) and high temperature (1200°C) decompression experiments specifically oriented to document gas bubble nucleation processes. Bubble nucleation occurs first during magma degassing and, so, it is critical to understand bubble nucleation processes to constrain the evolution of the gas phase (which is the driving force of explosive eruptions) in the volcanic conduit. Four main sets of experiments were conducted to better assess the role of the major volatiles (H2O, CO2, S), as well as the effects of ascent rate and crystals, on bubble vesiculation (nucleation, growth, coalescence) kinetics in basaltic magmas. The aim of the study is to understand the mechanisms which control the textural (number, size, shape of bubbles) and the chemical (dissolved volatile concentrations, gas composition) characteristics of natural products, and also to approach them experimentally. In this way, experimental melts, before and after decompression, were analysed texturally (by X-ray microtomography and MEB) and chemically (by FTIR and electron microprobe). Our results demonstrate a strong influence of CO2 on degassing mode (equilibrium vs. disequilibrium) and mechanisms, which are shown to be controlled by differences in solubility and diffusivity between the main volatile species. Finally, our data, obtained under conditions closely approaching natural eruptions, have volcanological implications for the interpretation of bubble textures and gas measurements, as well as, more specifically, for the dynamics of Strombolian paroxysms.

Le bassin de Levant, localisé à l’extrémité la plus orientale de la Méditerranée, se situe à jonction de trois plaques tectoniques majeures (Afrique, Arabie, Eurasie ainsi que la microplaque Anatolienne). Il est bordé à l’Est par la faille du Levant (frontière Arabie/Afrique), qui représente un système transformant de 1000 km de long, reliant le rift dans la Mer Rouge au sud avec la zone de convergence le long du Taurus au nord (frontière Arabie/Eurasie). Son extrémité nord est marquée par la frontière convergente Afrique/Anatolie soulignée par l’arc de Chypre. Le bassin Levantin a enregistré l’interaction entre ces différentes plaques au cours du Cénozoïque et sa bordure Est a été en particulier déformée par la mise en place de la faille du Levant. Cette limite de plaque majeure est marquée au Liban par un relais compressif qui a été actif depuis la fin du Miocène. Jusque récemment, l’absence de données sismiques dans la partie centrale du bassin levantin (offshore Liban) a constitué un handicap important dans la caractérisation de ce basin. Dans ce secteur, la géométrie, cinématique, l’âge des structures tectoniques ne sont pas connus. Plusieurs questions en découlent. Quel est impact de la frontière transformante du Levant sur la structure du bassin? Le bassin a-t-il enregistré d’autres déformations au cours du Cénozoïque ? Quel est l’effet de la structuration ancienne et profonde de la marge sur la déformation actuelle ? Ce travail s’est appuyé sur l’interprétation des données sismiques 2D et 3D de haute qualité dont deux cubes 3D de 4290 m3 et sept lignes 2D de 830 km de long. Cette étude a permis d’identifier les structures tectoniques affectant le secteur offshore Libanais et de caractériser leurs origines. Plusieurs familles de failles tout au long de la marge Est du bassin ont été identifiées et témoigne d’une histoire tectonique méso-cénozoïque longue et complexe. Les structures reconnues sont tout d’abord (1) des failles chevauchantes NNE-SSW actives depuis le début du Tertiaire jusqu’à la fin Miocène, (2) des anticlinaux NNE-SSW formés durant le Miocène supérieur et se localisant sur des structures préexistantes et (3) des failles décrochantes dextres, héritées des structures mésozoïques et réactivées durant le Miocène supérieur. Seules les failles décrochantes dextres montrent des preuves d’une activité actuelle, liée à la transpression au long de la faille du Levant. Ces structures constituent le prolongement vers l’ouest de la frontière de plaque du Levant sous un régime transpressif et une compression NW-SE. Nous mettons en évidence que cette frontière de plaque montre une évolution au cours du Néogène avec une forte décroissance de la composante de raccourcissement à partir du Pliocène. La mise en évidence de jeux plus anciens témoigne d’une structuration profonde E-W de la marge, vraisemblablement héritée des tectoniques mésozoïques. L’impact de cette structuration a été évalué à travers une modélisation analogique. Les résultats démontrent le rôle considérable de cet héritage sur l’évolution du relais compressif de la faille du Levant au Liban, entre autre en localisant la déformation le long de couloirs E-W et en segmentant les structures transpressives NNE-SSW. Ces résultats nous conduisent à interpréter les structures E-W comme majeures et traduisant la prolongation vers l’ouest du bassin mésozoïque des Palmyrides. Nous mettons ici en évidence le rôle majeure d’une marge sur la structure d’une frontière de plaques transformante. Le développement de failles antithétiques (failles dextres dans une frontière transformante senestre), connus dans d’autres frontières de ce type, est ici clairement associé à une anisotropie profonde forçant la localisation de la déformation. / The Levant Basin is located at the easternmost Mediterranean at the intersection of three major tectonic plates (Africa, Arabia, Eurasia and the smaller Anatolian microplate). The Levant Fracture System (Arabia-Africa plate boundary) borders the basin to its east and represents a 1000 km long left-lateral transform system linking rifting in the Red Sea with plate convergence along the Taurus Mountains (Arabia-Eurasia plate boundary). The Levant Basin is bordered to the north by the Cyprus Arc (Africa-Eurasia plate boundary). The interaction between these tectonic plates had important consequences on the evolution of the Levant Basin whereby its eastern boundary has been affected by deformation along the Levant Fracture System. This major plate boundary is associated with a restraining bend in Lebanon and has been active since the Late Miocene. Until recent days, the absence of seismic data in the central Levant Basin was an obstacle against characterizing the tectonic setting of the basin. In this area, the geometry, kinematics and the age of the tectonic structures are poorly understood. A focal question thus remains on how the Levant Basin was affected by this adjacent plate boundary. Therefore, what is the impact of the deformation along the Levant Fracture System since the Late Miocene on this basin and how can we assess it? Has the latter been affected by other tectonic regimes prior to the onset of transpression? If so, how would the existing structures influence the style of modern deformation? In this study, high quality 2D and 3D seismic reflection data (with two 4290 m3 3D seismic cubes and seven 830 km long 2D seismic lines) were interpreted allowing identification and timing of the structures in the Levant Basin offshore Lebanon. Several fault families, mapped along the margin, are remnants of a lasting and complex tectonic history since Mesozoic times. These include NNE-SSW striking thrust faults active during the early Tertiary and inactive since the Pliocene; NNE-SSW striking anticlines folded during the Late Miocene and overlying pre-existing structuresd; and ENE-WSW striking dextral strike-slip faults inherited from Mesozoic times and reactivated during the Late Miocene. Only the dextral strike-slip faults show evidence of current activity and are interpreted to be linked to transpression along the Levant Fracture System. They constitute the westward extension of the plate boundary, formed under a transpressif regime and a NW-SE compression. We have showed how this plate boundary has evolved through the Neogene with a decrease in the shortening component during the Pliocene.The identification of pre-existing structures along the eastern Levant margin shed the light on the deep structuration affecting this area, inherited from Mesozoic tectonic events. The impact of these structures was tested through analogue modeling. Results indicated a considerable impact of pre-existing structures on the development of the restraining bend, localizing deformation at the onset of transpression and responsible of segmenting the restraining bend along an ENE direction. These ENE-WSW faults are thus major and are most likely associated with the deformation affecting the Palmyra basin since the Mesozoic, which is thus extending westward to Lebanon. This study has shown the important role of a margin on a strike-slip plate boundary. Namely, the development of antithetic faults (local dextral strike-slip faults in a regional sinistral strike-slip plate boundary) known in other similar plate boundaries is associated with a deep crustal anisotropy localizing the subsequent deformation.

Afin de cartographier la structure souterraine des volcans et détecter des ressources géothermiques de haute température, on utilise souvent l’imagerie de résistivité électrique. La résistivité électrique des volcans est affectée par plusieurs facteurs: volume et salinité de l’eau interstitielle, abondance de minéraux conducteurs, température de la roche et présence de magma. Ce travail de thèse tente de contraindre l'interprétation des structures de résistivité électrique autour des volcans actifs, afin de développer des outils innovants pour l'exploration des ressources géothermiques. La contribution des minéraux conducteurs est au cœur de la thèse: conducteurs ioniques solides (minéraux argileux, en particulier la smectite) ou semi-conducteurs électroniques (pyrite, oxydes de fer), mais l’influence de la porosité, de la salinité, de la température et de la présence de magma est aussi étudiée. La thèse utilise le volcan Krafla comme terrain d’étude pour affiner les interprétations des structures de résistivité électriques, du fait de la disponibilité de carottes, de données, de bibliographie et d’infrastructure. La smectite et la pyrite sont formées par altération hydrothermale des roches volcaniques et témoignent ainsi des convections hydrothermales. Les oxydes de fer en revanche sont plutôt formés lors de la cristallisation du magma et sont dissous lors des circulations hydrothermales. La contribution de la smectite à la conductivité électrique de roches volcaniques, saturées en eau à différentes salinités, est d'abord étudiée en laboratoire (à température ambiante) par spectroscopie d’impédance électrique « résistivité complexe ». Des variations non linéaires de la conductivité électrique à 1 kHz avec la salinité sont observées et discutées. La conduction interfoliaire est suggérée comme un mécanisme important par lequel la smectite conduit le courant électrique. L'influence de la pyrite et des oxydes de fer sur les effets de polarisation provoquée est ensuite analysée en utilisant l'angle de phase de l'impédance, qui dépend de la fréquence. Un angle de phase maximal supérieur à 20 mrad est attribué à la pyrite si la roche est conductrice et aux oxydes de fer si la roche est résistive. L'angle de phase maximal augmente d'environ 22 mrad pour chaque pourcent de pyrite ou d'oxyde de fer. Ces résultats de laboratoire en domaine fréquentiel sont appliqués à l’interprétation de tomographies de résistivité complexe sur le terrain en domaine temporel. Smectite, pyrite et oxydes de fer ont pu être identifiés jusqu'à 200 m de profondeur. La température in-situ, plus élevées qu’en laboratoire, semble augmenter la conductivité de la smectite. De manière générale, la tomographie de résistivité complexe est recommandée comme méthode complémentaire aux sondages électromagnétiques pour l'exploration géothermique. / Electromagnetic soundings are widely used to image the underground structure of volcanoes and look for hightemperature geothermal resources. The electrical resistivity of volcanoes is affected by several characteristics of rocks: volume and salinity of pore fluid, abundance of conductive minerals, rock temperature and presence of magma. This thesis aims at improving the interpretation of electrical resistivity structures around active volcanoes, in order to develop innovative tools for the assessment of geothermal resources. I focus on conductive minerals, which can either be solid ionic conductors (clay minerals, in particular smectite) or electronic semi-conductors (pyrite and iron-oxides), but I also investigate the effects of porosity, salinity, temperature and presence of magma. I use Krafla volcano as a laboratory area, where extensive literature, borehole data, core samples, surface soundings and infrastructures are available. Smectite and pyrite are formed upon hydrothermal alteration of volcanic rocks and thus witness hydrothermal convection. On the other hand, iron-oxides are mostly formed during the primary crystallization of magma and dissolved by hydrothermal fluids. The contribution of smectite to the electrical conductivity of volcanic rocks saturated with pore water at different salinity is first investigated in the laboratory (room temperature) by electrical impedance spectroscopy “complex resistivity”. Non-linear variations of the conductivity at 1 kHz with salinity are observed and discussed. Interfoliar conduction is suggested as an important mechanism by which smectite conducts electrical current. The influence of pyrite and iron-oxides on induced polarization effects is then analyzed, using the frequency-dependent phase-angle of the impedance. A maximum phase-angle higher than 20 mrad is attributed to pyrite if the rock is conductive and to ironoxides if the rock is resistive. The maximum phase-angle increases by about 22 mrad for each additional per cent of pyrite or iron-oxide. These laboratory frequency-domain findings are partly upscaled to interpret field time-domain complex resistivity tomography at Krafla: smectite, pyrite and iron-oxides can be identified down to 200 m. The in-situ temperature, higher than in laboratory conditions, appears to significantly increase the conductivity associated to smectite. In general, time-domain complex resistivity measurements are recommended as a complementary method to electromagnetic soundings for geothermal exploration.