No description available.

No description available.

The uniform stretching model successfully accounts for the general features of many extensional sedimentary basins. However, the amount of extension measured across normal faults in the upper crust is often thought to be significantly less than that calculated from subsidence analysis and crustal thinning. At present, more complicated models, which incorporate two-layer stretching, multiple stretching phases and flexural rigidity, are used to explain this extension discrepancy. The principal aim of this dissertation is to show that the extension discrepancy can be resolved in the northern North Sea without abandoning the uniform stretching model. Other observations are also explained by minor changes to the model. Basin evolution is addressed both on a small and on a large scale. A kinematic model for hanging wall deformation, which is assumed to occur by arbitrarily inclined simple shear and by differential compaction, is proposed. Fault geometries can be calculated from sedimentary horizons within hanging walls using an inversion scheme based on this model. Results suggest that hanging wall shear is inclined towards the main fault. This implies that the amount of extension across a fault is considerably greater than the apparent horizontal displacement. Syn-rift footwall uplift is explained by combining the simple domino-style fault model with the uniform stretching model. The 'steer's head' cross-sectional geometry of sedimentary basins is usually explained either by fluctuations in sea-level or by increasing flexural rigidity of the continental lithosphere during post-rift cooling. Here, a two-layer stretching model is proposed, where the lithospheric mantle is stretched over a fractionally wider region than is the crust. This accounts for the observed extent of post-rift stratigraphic onlap in the North Sea and does not alter conclusions concerning the extension discrepancy. The geometrical and thermal consequences of lithospheric simple shear are investigated using a numerical model. Results predict that, as for the uniform stretching model, crustal thinning is symmetrical about the basin. Maximum thinning is also coincident with maximum subsidence. However, the magnitude of post-rift subsidence varies across the basin, allowing the uniform stretching model and the lithospheric simple shear model to be distinguished. The different models described here have been applied to regional seismic reflection profiles and well-log information from the northern North Sea. On the best constrained profile, the extension measured across normal faults agrees well with that calculated by subsidence analysis. The major observations are thus consistent with the predictions of the uniform stretching model.

The Pampean flat slab subduction in west-central Argentina (latitude 30-33S) and the steeply dipping Northern Costa Rica subduction zone (latitude 9-11N) show significant along-trench variations in both the subducting and overriding plates. This dissertation contains the results of three seismological studies using broadband instruments conducted in these subduction zones, with the aim of understanding the structure of the lithosphere and the correlation between the variability observed in the downgoing and the overriding plates. In the Costa Rica region, by analyzing teleseismic receiver functions we investigate the variability in the hydration state of the subducting Cocos Plate and the nature of three distinct crustal terranes within the overriding Caribbean Plate: the Nicoya and Chorotega terranes that display an oceanic character, and the Mesquito Terrane, which is more compatible with continental crust.In the Pampean region of Argentina, we apply a regional-scale double-difference tomography algorithm to earthquake data recorded by the SIEMBRA (2007-2009) and ESP (2008-2010) broadband seismic networks to obtain high-resolution images of the South America lithosphere. We find that most of the upper mantle has seismic properties consistent with a depleted lherzolite or harzburgite, with two anomalous regions above the flat slab: a higher Vp/Vs ratio anomaly consistent with up to 10% hydration of mantle peridotite and a localized lower Vp/Vs ratio anomaly consistent with orthopyroxene enrichment. In addition, we study the geometry and brittle deformation of the subducting Nazca Plate by determining high-quality earthquake locations, slab contours, and focal mechanisms. Our results suggest that the subduction of the incoming Juan Fernandez Ridge controls the slab geometry and that ridge buoyancy and slab pull are key factors in the deformation of the slab. The spatial distribution of the slab seismicity suggests variability in the hydration state of the subducting Nazca Plate and/or in strain due to slab bending. These observations support the hypothesis that the along-trench variability in bathymetric features and hydration state of the incoming plate has profound effects in the subducting slab geometry and the upper plate structure in both flat and steeply dipping subduction zones.

A major change in seismic velocities between Earth’s crust and mantle is known as the Mohorovicic discontinuity (Moho). The depth of the Moho plays an important role in characterizing the overall structure of the crust and can be related to the tectonic setting of a region. Teleseismic P-wave receiver function techniques can provide estimates of the depth of the Moho and therefore crustal thickness under a broadband station. In this research we are interested in the structure of the crust and mantle across the northern Canadian cordillera, described by various tectonic settings. The teleseismic data recorded by broadband three-component seismic stations are used to perform receiver function analysis to determine the lateral variations of Moho depth under northern Canadian cordillera and map out the crustal thickness under the broadband stations. Based on visual inspection of receiver function results in the region, we find evidence of anisotropy or dipping reflectors in the crustal structure of the northern cordillera observed in back-azimuthal variations of transverse component receiver functions. We further provide a quantitative interpretation of receiver function in terms of anisotropy or dipping structure by decomposing the azimuthal variations of depth migrated receiver functions into back-azimuthal harmonics. This technique can be used to map out the orientation of anisotropy that may be related to cracks and/or rock texture caused by deformation. We resolve the Moho at an average depth of ~35 km along the western profile of the study area. Harmonic decomposition along the study area yields crustal anisotropy at depth 5-20 km, which does not extend in the lower crust. This can be the result of complex deformation at a detachment zone like a quasi-rigid displacement of the upper crust over a lower crust. The detected anisotropy over the study area is not coherent as the slow symmetry directions detected by harmonic decomposition are highly variable.

The neotectonic evolution of the eastern Mediterranean is intimately tied to interactions between the underthrusting/subducting slab along the southern margin of Anatolia and the overriding plate. The lateral variations in the subduction zone can be viewed as a temporal analogue of the transition between continuous subduction and subduction termination by continent-continent collision. By investigating the lateral variations along this subduction zone in the overriding plate, we can gain insight into the processes that precede continent collision. This dissertation summarizes the results of three studies that focus on different parts of the subduction margin: 1) In the west, where the development of a slab tear represents the transition between continuous and enigmatic subduction, 2) In the east, where continent-continent collision between the Arabian and Eurasian Plate is leading to the development of the third largest orogenic plateau on earth after complete slab detachment, and 3) In central Anatolia, where the subducting slab is thought to be in the processes of breaking up, which is affecting the flow of mantle material leading to volcanism and uplift along the margin. In the first study, we interpret that variations in the composition of material in the downgoing plate (i.e. a change from the subduction of oceanic material to continental material) may have led to the development of a slab tear in the eastern Aegean. This underthrusting, buoyant continental fragment is controlling overriding plate deformation, separating the highly extensional strains of western Anatolia from the much lower extensional strains of central Anatolia. Based on intermediate depth seismicity, it appears that the oceanic portion of the slab is still attached to this underthrusting continental fragment. In the second study, we interpret that the introduction of continental lithosphere into the north-dipping subduction zone at the Arabian-Eurasian margin led to the rollback and eventual detachment of the downgoing oceanic lithosphere attached to the Arabian Plate. After detachment, high rates of exhumation in the overriding plate are recorded due to the removal of the oceanic lithosphere and accompanying rebound of the Arabian continental lithosphere. In the third study, we image a transitional stage between the complete slab breakoff of the second study and the continuous subduction slab of the first study. We interpret that trench-perpendicular volcanism and ~2 km of uplift of flat-lying carbonate rocks along the southern margin of Turkey can be attributed to the rollback and ongoing segmentation of the downgoing slab as attenuated continental material is introduced into the subduction zone. Combining these three studies allows us to understand the terminal processes of a long-lived subduction zone as continental material is introduced.

The presence of chlorine in the subcratonic lithospheric mantle (SCLM) has been evaluated by compiling the compositional data of fluid inclusions in fibrous diamonds. Chlorine associates with potassium, dissolving in water and forming a KCl-bearing brine with the Cl/(Cl+H2O) molar ratio of 0.05-0.68. To examine the effect of such a KCl-bearing brine on the melting behavior of the SCLM, we conducted experiments in the Mg2SiO4-MgSiO3-H2O and Mg2SiO4-MgSiO3-KCl-H2O systems at 5 GPa and 1100-1700C. In the Mg2SiO4-MgSiO3-H2O system, the solidus temperature of forsterite+enstatite is ~1230C. In the Mg2SiO4-MgSiO3-KCl-H2O systems with molar Cl/(Cl+H2O) ratios of 0.2, 0.4 and 0.6, the solidus temperatures are ~1430C, ~1530C and ~1580C, respectively. The increase in the temperature of the solidus demonstrates that KCl elevates the solidus of the Mg2SiO4-MgSiO3-H2O system. Therefore, KCl in the SCLM can prevent melting at the H2O-saturated solidus, and a KCl-bearing fluid can be a robust agent for mantle metasomatism.

No description available.

Cette étude vise caractériser le manteau lithosphérique du massif du Hoggar (Algérie) et son évolution, grâce à une étude multidisciplinaire (pétrologique, géochimique et pétrophysique) d'enclaves mantelliques échantillonnées par le volcanisme cénozoïque. L'échantillonnage provient de deux districts volcaniques (Tahalagha et Manzaz) situés respectivement en périphérie et au coeur du bombement du Hoggar. Le district de Tahalgha est par ailleurs situé à cheval sur un grand cisaillement pan-africain (le 4°35), séparant deux domaines structuraux majeurs du socle du Hoggar : le Hoggar Central Polycyclique à l'Est (domaine LATEA) et le Hoggar occidental à l'Ouest (bloc d'Iskel). Les xénolites étudiés apportent des informations sur l'évolution du manteau lithosphérique depuis l'orogenèse pan-africaine, au cours de laquelle s'est structuré le socle de cette région (le Bouclier Touareg), jusqu'aux événements cénozoïques responsables du bombement topographique et du volcanisme.L'héritage pan-africain est essentiellement préservé dans les échantillons du district périphérique de Tahalgha, sous la forme de lherzolites équilibrées à basse température (750 - 900°C), à clinopyroxènes appauvris en terres rares légères. Ces échantillons sont considérés comme représentant la lithosphère sous-continentale à l'issue des processus de réjuvénation qui ont marqué les derniers stades de l'orogenèse pan-africaine. Ils montrent des textures de déformation (porphyroclastiques à equigranulaires) bien préservées, attribuées à ces événements et caractérisées par des orientations cristallographiques préférentielles (OPRs) de l'olivine (axiales-[010]) compatibles avec un régime transpressif. Les événements cénozoïques sont marqués par un recuit partiel de ces textures, particulièrement prononcé à Manzaz et dans les échantillons de Tahalgha équilibrés à des températures moyennes à élevées (900-1150°C), et affectés par différents degrés de métasomatisme. Les xénolites de Tahalgha représentent un cas d'étude exemplaire du métasomatisme mantellique, couplant variations texturales, minéralogiques et chimiques le long de gradient locaux de température. Une modification des OPRs d'olivine est observée, qui résulterait à la fois de l'infiltration de liquides métasomatiques et d'une réactivation des accidents pan-africains en cisaillement pur.Des implications importantes de cette étude résident dans l'échelle des variations de premier ordre attribuées aux interactions lithosphère-asthénosphère au Cénozoïque. Celles-ci sont essentiellement à l'échelle du bombement du Hoggar (différences entre Manzaz et Tahalga, c'est-à-dire entre Hoggar central et périphérique) ou à celle de conduits magmatiques et de leurs épontes (variabilité locale des xénolites de Tahalgha). Par contre, les résultats obtenus montrent peu de variations significatives pour les échelles intermédiaires, notamment pour des localités de Tahlagha situées de part et d'autre ou à différentes distances du 4°35. Ceci favorise plutôt, pour l'origine du bombement volcanique du Hoggar, les modèles faisant appel à des structures d'assez grande échelle telle qu'un panache mantellique ou une cellule de convection asthénosphérique de type « Edge Driven Convection », plutôt qu'un processus essentiellement lié à la réactivation des failles lithosphériques pan-africaines. / This study aims to characterize the lithospheric mantle of the Hoggar swell (Algeria) and its evolution through time via a multidisciplinary (petrological, geochemical and petrophysical) study of mantle xenoliths sampled by Cenozoic volcanism. The samples were collected in two volcanic districts (Tahalagha and Manzaz) located in the periphery and in the central part of the Hoggar massif, respectively. The Tahalgha sampling also straddles a mega pan-African shear zone (the 4°35 fault) between two major structural domains of the Tuareg Shield basement: the Central Polycyclic Hoggar to the East (LATEA terranes) and the Western Hoggar domain to the West (Iskel block). The studied xenoliths provide information on the evolution of the lithospheric mantle from the Pan-African orogeny – i.e. the period when the Tuareg Shield was structured – to the Cenozoic events responsible for topographic upwelling and volcanism in the Hoggar swell.The Pan-African heritage is found in xenoliths from the peripheral Tahalgha district. These samples are distinguished by low equilibrium temperatures (750-900°C) and LREE-depleted clinopyroxene compositions. They are considered to represent the sub-continental lithosphere after the rejuvenation process that marked the later stages of the Pan-African orogeny. They show well preserved deformation textures (porphyroclastic to equigranular) assigned to these events and characterized by preferential crystallographic orientations (CPOs) of olivine (axial-[010]) consistent with a transpressional regime. The Cenozoic events are marked by partial annealing of these textures, particularly pronounced in the Manzaz samples, as well as in the Tahalgha xenoliths equilibrated at medium to high temperatures (900-1150°C). These samples were affected by different degrees of metasomatism. The Tahalgha xenoliths represent a rather unique case study of mantle metasomatism, where coupled textural, mineralogical and chemical variations occur along local temperature gradients. The Cenozoic events were also responsible for a change in olivine CPOs, resulting from both infiltration of metasomatic fluids and reactivation of Pan-African accidents in a pure-shear regime.Important implications of this study lie in the scale at which the first-order lithosphere modifications ascribed to the Cenozoic event are observed, i.e. either at the scale of the whole Hoggar swell, as shown by the increasing degree of textural annealing and metasomatism from Tahalgha to Manzaz (i.e. from outer to central Hoggar), or at the small scale of magma conduits and their wall rocks, as shown by the local variability registered by the Tahalgha xenoliths. Conversely, our data show little changes at intermediate scales, as might be expected, for instance, among the Tahalgha localities situated on either sides - or at different distances - from the 4°35. As regards the origin of the Hoggar volcanic swell, this result favours the models involving relatively large-scale structures such as a mantle plume or "Edge Driven Convection", rather than a process involving merely the reactivation of pan-African lithospheric faults.

The oceanic lithosphere forms as a residue of partial melting of the mantle beneath the mid-ocean ridge axis. Subduction of this residual layer has a profound impact on the Earth’s thermal and geochemical cycles as the recycling of this layer facilitates heat loss from the Earth’s interior and induces geochemical heterogeneities in the mantle. The goal of this study is to understand the thermal and geochemical evolution of the oceanic lithospheric mantle from a petrological perspective. An empirical geobarometer is calibrated for ocean island xenoliths in order to understand the thermal structure of the oceanic lithospheric mantle. The results of 0.1 MPa experiments from this study and high-pressure experiments from previous studies are used in the calibration. The uncertainties on pressures derived using the above geobarometer are high and hence could not be tested against thermal models for the oceanic lithosphere. The geochemical evolution of the oceanic lithospheric mantle involves post-melting geochemical modifications such as metasomatism. The geochemical evolution of the uppermost oceanic lithospheric mantle is studied using harzburgites from Hess Deep ODP Site 895, which are depleted in moderately incompatible elements relative to the global suite of abyssal peridotites. A comparison between Yb-abundances in Hess Deep harzburgites iii iv and those of a model depleted MORB mantle (DMM) residue reveals that the harzburgites have undergone up to 25% melting, assuming 0.5% melt porosity. Higher light and middle rare earth elements in the Hess Deep harzburgites than the model DMM melting residue are interpreted as the result of plagioclase crystallisation from melts being extracted by diffuse porous flow through the upper mantle. The effect of plagioclase crystallisation does not affect the chemistry of residual mineral phases as evidenced from the depleted light rare earth element abundances in clinopyroxene relative to the bulk rock. Ocean island xenoliths are studied to understand when and where metasomatism occurs in the deeper portion of the oceanic lithosphere. The median values of measured and reconstructed bulk concentration of Al2O3 in most ocean island xenoliths is lower than in abyssal peridotites, which generally would be interpreted as indicating a higher extent of melting in the former. However, a comparison between Yb- abundances in ocean island xenoliths and abyssal peridotites with a model DMM melting residue suggests that the extents of melting in the suites of rocks are broadly similar. Although fewer in number than ocean island xenoliths, abyssal peridotites from several locations have low concentrations of moderately incompatible elements. Metasomatism is observed in both, ocean island xenoliths and abyssal peridotites in the form of higher bulk rock Ce and Nd concentration than the model DMM melting residue but the extent of metasomatism is higher in ocean island xenoliths. There is no correlation between the concentrations of bulk rock Ce, Nd, Sm and Eu of ocean island xenoliths and age of the oceanic lithosphere from which the xenoliths originate. It is interpreted that metasomatism in the lower oceanic lithospheric mantle occurs near the ridge axis above the wings of the melting column. / Graduate / 0996 / 0372