Dissertations / Theses on the topic 'Crustal anomalies'

Cette étude s’intéresse en premier lieu aux caractéristiques magnétiques d’échantillons de BIFs (Banded Iron Formations) en provenance de Mauritanie, où est observable la plus grande partie de l’Anomalie Magnétique Ouest Africaine (AMOA). On met en évidence des valeurs de susceptibilité magnétique allant jusqu’à 3.4 SI, et d’aimantation rémanente jusqu’à 1350 A/m, dans les BIFs. Des valeurs de rapport de Koenigsberger pour la plupart supérieures à 1 impliquent également que la rémanence devrait être prise en compte pour expliquer l’AMOA. L’effet de la pression sur les propriétés magnétiques de la titanomagnétite a également été étudié, car ce paramètre est peu connu, et la plupart du temps négligé dans les modèles numériques des sources des anomalies magnétiques. On montre ainsi une dépendance à la pression de la température de Curie de l’hématite et de la titanomagnétite, ainsi qu’une augmentation de l’Aimantation Thermo Rémanente (ATR) acquise à des pressions lithosphériques, allant jusqu’à +100% à 675 MPa. Un modèle numérique de la croûte sous l’AMOA est ensuite construit, à l’aide de ces contraintes et de données magnétiques et gravimétriques. On utilise une approche directe pour investiguer la profondeur, l’épaisseur et l’intensité d’aimantation des lithologies crustales. Nos résultats montrent que des couches de BIFs sont les seules sources crustales aimantées nécessaires pour expliquer l’anomalie, et qu’elles pourraient être enfouies à plusieurs kilomètres de profondeur. Les résultats de cette étude fournissent de nouvelles perpectives pour l’étude des sources d’anomalies magnétiques dans d’autres régions cratoniques comportant des affleurements de BIFs
The geological sources of major magnetic field anomalies are still poorly constrained, in terms of nature, geometry and vertical position. A common feature of several anomalies is their spatial correlation with cratonic shields and, for the largest anomalies, with Banded Iron Formations (BIF). This study first unveils the magnetic properties of some BIF samples from Mauritania, where the main part of the West African magnetic anomaly is observed. It shows magnetic susceptibility values up to 3.4 SI and natural remanent magnetization up to 1350 A/m can be reached by BIF rocks. Koenigsberger ratios mostly superior to 1 imply that the remanent magnetization should be taken into account to explain the anomaly. I also investigated the impact of pressure on magnetic properties of titanomagnetite, because it is not well known and most of the time neglected in numerical models of the geological sources of magnetic anomalies. My results show a pressure-dependent Curie temperature increase, as well as an intensity increase for TRM acquired under lithospheric pressure (up to +100% at 675 MPa). A numerical modeling of the crust beneath the West African anomaly is then performed using these constraints and both gravity and magnetic field data. A forward approach is used, investigating the depth, thickness and magnetization intensity of all possible crustal lithologies. Our results show that BIF slices may be the only magnetized lithology needed to explain the anomaly, and that they could be buried several kilometers deep. The results of this study provide a new perspective to address the investigation of magnetic field anomaly sources in other cratonic regions with BIF outcrops

Cratonic basins cover more than 10% of Earth's continental surface area, yet their origin remains enigmatic. In this thesis a suite of new and legacy geophysical and geological data are integrated to constrain the origin of the Parnaíba basin, a cratonic basin in Northeast Brazil. These data include a 1400 km long, deep (20 s two-way travel time) seismic reflection profile, five +/- 110 km offset wide-angle split-spread receiver gathers, gravity anomaly, and well data. In the centre of the basin, the depth to pre-Paleozoic basement is ~ 3.3 km, a zone of midcrustal reflectivity (MCR) can be traced laterally for ~ 250 km at depths between 17-25 km and Moho depth is ~ 42 +/- 2 km. Gravity and P-wave modelling suggests that the MCR represents the upper surface of a high density (2985 kg m³) and V_p (6.7 - 7.0 km s^-1) lower crustal body, likely of magmatic origin. Backstripping of well data shows a concave up decreasing tectonic subsidence, similar in form to that commonly observed in rift-type basins. It is shown, however, that the seismic and gravity data are inconsistent with an extensional origin. It is shown that an intrusive body in the lower crust that has loaded and flexed the surface of the crust, combined with sediment loading, provides a satisfactory fit to the observed gravity anomaly, sediment thickness and basin shape. A buried load model is also consistent with seismic data, which suggest that the Moho is as deep or deeper beneath the basin centre than its flanks and accounts for at least part of the tectonic subsidence through a viscoelastic stress relaxation that occurs in the lithosphere following load emplacement. Comparative analysis of the Michigan and Congo basins shows gravity data from these basins is also consistent with a lower crustal mass excess, while subsidence analysis shows viscoelastic stress relaxation may also contribute to their early subsidence histories. However, unlike Parnaíba, both of these basins appear to have been subjected to secondary tectonic processes that obscure the primary 'cratonic basin' subsidence signals. Parnaíba basin, therefore, offers an excellent record for the investigation of cratonic basin formation.

Maps of relatively strong crustal magnetic field anomalies detected at low altitudes with the magnetometer instrument on Lunar Prospector are presented. On the lunar nearside, relatively strong anomalies are mapped over the Reiner Gamma Formation on western Oceanus Procellarum and over the Rima Sirsalis rille on the southwestern border of Oceanus Procellarum. The main Rima Sirsalis anomaly does not correlate well with the rille itself but is centered over an Imbrian-aged smooth plains unit interpreted as primary or secondary basin ejecta. The stronger Reiner Gamma anomalies correlate with the locations of both the main Reiner Gamma albedo marking and its northeastward extension. Both the Rima Sirsalis and the Reiner Gamma anomalies are extended in directions approximately radial to the center of the Imbrium basin. This alignment suggests that Imbrium basin ejecta materials (lying in many cases beneath the visible mare surface) are the sources of the nearside anomalies. If so, then the albedo markings associated with the stronger Reiner Gamma anomalies may be consistent with a model involving magnetic shielding of freshly exposed mare materials from the solar wind ion bombardment. Two regions of extensive magnetic anomalies are mapped in regions centered on the Ingenii basin on the south central farside and near the crater Gerasimovic on the southeastern farside. These regions are approximately antipodal to the Imbrium and Crisium basins, respectively. The Imbrium antipode anomaly group is the most areally extensive on the Moon, while the largest anomaly in the Crisium antipode group is the strongest detected by the Lunar Prospector magnetometer. A consideration of the expected antipodal effects of basin-forming impacts as well as a combination of sample data and orbital measurements on the nearside leads to the conclusion that the most probable sources of magnetic anomalies in these two regions are ejecta materials from the respective impacts. In both regions the strongest individual anomalies correlate with swirl-like albedo markings of the Reiner Gamma class visible on available orbital photography.

Lunar Prospector (LP) electron reflectometer measurements show that surface fields are generally weak in the large mare basalt filled impact basins on the near side but are stronger over highland terranes, especially those lying antipodal to young large impact basins. Between the Imbrium and Nectaris basins, many anomalies correlate with the Cayley and Descartes Formations. Statistical analyses show that the most strongly magnetic nearside terranes are Cayley-type light plains, terra materials, and pre-Imbrian craters. Light plains and terrae include basin impact ejecta as a major component, suggesting that magnetization effects from basin-forming impacts were involved in their formation. The magnetization of pre-Imbrian craters, however, may be evidence of early thermal remanence. Relatively strong, small-scale magnetic anomalies are present over the Reiner Gamma feature on western Oceanus Procellarum and over the Rima Sirsalis rille on the southwestern border of Procellarum. Both Apollo subsatellite and LP data show that the latter anomaly is nearly aligned with the rille, though LP magnetometer and reflectometer data show that the anomaly peak is actually centered over a light plains unit. This anomaly and the Reiner Gamma anomaly are approximately radially aligned with the center of Imbrium, suggesting an association with ejecta from this basin.

The study deals with the relative movements between Africa and Antarctica in Middle Jurassic to Lower Cretaceous times using new data from four campaigns (2006-2010). Aeromagnetic measurements in the 5W Enderby Basin clearly image the continent-ocean-transition east of the Gunnerus Ridge. Spreading anomalies are absent, pointing to a formation of the oceanic crust during Cretaceous Normal superchron times. Velocity and density modelling of two wide-angle seismic profiles across the Central Mozambique continental margin reveal continental crust, thinning seawards by 50% over a distance of ~130 km. Farther south, oceanic crust is overlain by sediments with high P-wave velocities around 4. 8 km/s. An extensive lower crustal high-velocity-body has been found in both profiles. Identifications of magnetic spreading anomalies reveal that the COT is located closer to the coast than supposed so far. M41 n is interpreted as oldest existing spreading anomaly. New magnetic and gravity data across the Mozambique Ridge and the Northern Natal Valley and their similarity to potential field data on the Mozambique Coastal Plains point to a mainly oceanic nature from the plains south and two southward jumps of the spreading centre. The Astrid Ridge is subdivided by the Astrid Fracture Zone into a northern and a southern part with different magnetic signatures and is interpreted to consist of oceanic crust. The regional results were implemented into a new kinematic model between Africa and Antarctica, postulating a tight continental Gondwana fit, a two-stage breakup and a southward movement of the Grunehogna Craton east of the Mozambique Ridge during the second stage of breakup
L’étude traite des mouvements relatifs de l’Afrique et de l'Antarctique, du Jurassique moyen jusqu’à la fin du Crétacé inférieur, en utilisant les données de quatre campagnes (2006-2010). Des mesures aéromagnétiques dans le bassin Enderby Sud-Ouest montrent la COT à l’est de la dorsale de Gunnerus. Il n’y a pas d’anomalie évidente dans les données, ce qui indique une formation de croûte océanique durant le superchron normal du Crétacé. La modélisation de deux profils sismiques grand-angles à travers la marge continentale du Mozambique central révèle une croûte continentale qui s’amincit ~50% sur une distance de ~130 km vers le large. Plus au sud, de la croûte océanique se trouve sous des sédiments à hautes vitesses d’ondes P autour de 4,8 km/s. Dans la croûte inférieure se retrouve un vaste corps de grande vitesse d’ondes. Des identifications des anomalies (M41n étant interprétée comme la plus vieille) révèlent que la COT est plus proche de la côte qu’on ne le croyait. De nouvelles données magnétiques et gravimétriques à la ride du Mozambique et au bassin nord du Natal et leur similitude avec les données des plaines de la côte du Mozambique indiquent une nature de croûte océanique majoritaire des plaines vers le sud ainsi que deux sauts du centre d’écartement vers le Sud. La ride d’Astrid est scindée en deux parties magnétiquement différentes, interprétées en croûte océanique. Les résultats régionaux ont été implémentés dans un nouveau modèle cinématique, qui postule un ajustement étroit des continents du Gondwana, une fracturation diphasée et un mouvement du Craton Grunehogna vers le sud à l’est de la ride du Mozambique pendant la deuxième phase de la fracturation

Ce travail, realise dans le cadre du programme ecors-crop alpes 1986-1987, a pour but l'etude de la structure profonde sous la chaine des alpes occidentales a l'aide de donnees gravimetriques acquises et incorporees aux leves anterieurs. Deux profils gravimetriques ont ainsi ete modelises a l'aide des contraintes apportees par les donnees sismiques. Ils ont permis la confirmation du caractere cylindrique de la chaine ainsi que le mode probable de raccourcissement qui d'opererait grace a de grands chevauchements intracrustaux. Une carte d'anomalie de bouguer sur l'ensemble de la chaine des alpes occidentales a ete etalie avec une precision superieure a 410##5 m. S##2 grace aux soins apportes a la realisation des corrections de relief. Les donnees topographiques acquises dans ce but ont egalement permis la realisation d'une carte d'anomalie isostatique qui a servi de controle indirect pour etudier le comportement mecanique de la lithosphere europeenne en flexion. Plusieurs modeles sont presentes, utilisant des approches elastiques pour tenter de rendre compte de la geometrie de la lithosphere europeenne. L'un d'entre eux, adoptant une approche en rigidite variable, explique en partie la pente et la rupture du gradient du moho observe, et permet de mieux comprendre le comportement mecanique de la lithosphere lors de collisions continentales. D'autres approches, faisant appel a des modeles mecaniques plus sophistiques, sont envisagees et donnent une idee sur le type d'etudes a realiser dans l'avenir

La crevette d'élevage penaeus indicus a fait l'objet d'une étude caryologique en vue d'induire la tétraploïdie par traitement thermique. En utilisant la technique de coloration au giemsa le nombre diploïde 2n = 88, a été déterminé par comptage de 155 métaphases mitotiques de nauplii, et confirme par le nombre haploide, n = 44 compte sur 50 métaphases méiotiques de spermatocytes i. Les analyses du caryotype ont été effectuées à partir des meilleures métaphases. Les mesures morphométriques ont ete effectuées à l'aide du système d'analyse d'image et l'appariement a été réalisé automatiquement par un programme informatique spécifique. Le caryotype obtenu montre que les paires de chromosomes se répartissent en : 27 à 35 métacentriques (m), 8 à 15 submetacentriques (sm), et de 1 à 4 subtelocentriques (st). La technique de marquage de bandes c appliquée pour la première fois aux crustacés décapodes a mis en évidence une paire de chromosomes telocentriques. En s'appuyant sur ces deux techniques le caryotype le plus représentatif de p. Indicus se composerait de 27-28 m, 11-12 sm, 4 st et 1t. La tétraploïdie a été induite chez cette espèce par traitement thermique des ufs, applique avant la première division mitotique. Les meilleures performances des tétraploïdes induits ont été obtenues lors de chocs a 38c appliques 21 mn après la ponte pendant 2 mn 45 s. Les effets cytologiques induits par ce traitement ont été analyses en suivant le développement des embryons à l'aide de la microscopie confocale à balayage laser couplée aux techniques d'immunofluorescence. Mais les faibles rendements obtenus au cours des traitements n'ont pas permis une mise en élevage larvaire satisfaisante. D'autres expériences sont d'ores et déjà envisagées avec différentes techniques.

Cette etude se propose d'interpreter les anomalies gravimetriques magnetiques du domaine precambrien de la republique centrafricaine et du sud cameroun. La reduction au pole des donnees magnetiques de la r. C. A. Montre que l'anomalie magnetique de bangui est l'addition de deux anomalies majeures qui recouvrent respectivement une unite archeenne et la chaine proterozoique inferieure de bogoin-ouham. La carte reduite a l'equateur suggere l'existence des sources ayant un allongement ew notable et des aimantations essentiellement induites. Les anomalies magnetiques se superposent a deux importantes signatures gravimetriques. Les zones de contacts entre le craton du congo et les zones mobiles panafricaines du sud cameroun et de l'est de la r. C. A. Montrent deux signatures gravimetriques inversees, caracterisees par un gradient majeur qui separe une anomalie positive, d'une anomalie negative de grandes longueurs d'ondes. L'interpretation quantitative montre que: le domaine archeen est traverse par une faille intracrustale ew a pendage nord qui recoupe en profondeur un large corps dense et magnetique. Ces deux structures sont integrees dans une evolution de type granite-greenstone belts. Dans la region de bogoin-ouham, le bloc crustal reactive au proterozoique inferieur, d'orientation submeridienne, se deverse vers l'est le long d'une discontinuite assimilee a une suture de collision. En profondeur, un corps dense et magnetique s'allonge parallelement aux unites de la chaine. La chaine de bogoin-ouham est interpretee comme une chaine de collision de type rift-intracratonique. Au sud cameroun et dans l'est de la r. C. A. , le craton plonge sous le domaine reactive au panafricain le long d'une discontinuite intracrustale. Les structures crustales modelisees presentent des caracteristiques deja reconnues ailleurs, notamment en afrique de l'ouest, qui permettent de les interpreter comme des structures issues d'une compression et d'une collision continentale

Submitted by Cleide Dantas (cleidedantas@ufpa.br) on 2014-05-30T14:40:43Z No. of bitstreams: 2 license_rdf: 23898 bytes, checksum: e363e809996cf46ada20da1accfcd9c7 (MD5) Tese_NovaAbordagemInterpretacao.pdf: 15232651 bytes, checksum: 8838ce5885d35952a159a082516bf81f (MD5)
Rejected by Irvana Coutinho (irvana@ufpa.br), reason: Definir palavras-chave on 2014-08-06T16:32:59Z (GMT)
Submitted by Cleide Dantas (cleidedantas@ufpa.br) on 2014-09-12T13:53:09Z No. of bitstreams: 2 license_rdf: 23898 bytes, checksum: e363e809996cf46ada20da1accfcd9c7 (MD5) Tese_NovaAbordagemInterpretacao.pdf: 15232651 bytes, checksum: 8838ce5885d35952a159a082516bf81f (MD5)
Approved for entry into archive by Irvana Coutinho (irvana@ufpa.br) on 2014-09-15T16:03:50Z (GMT) No. of bitstreams: 2 license_rdf: 23898 bytes, checksum: e363e809996cf46ada20da1accfcd9c7 (MD5) Tese_NovaAbordagemInterpretacao.pdf: 15232651 bytes, checksum: 8838ce5885d35952a159a082516bf81f (MD5)
Made available in DSpace on 2014-09-15T16:03:50Z (GMT). No. of bitstreams: 2 license_rdf: 23898 bytes, checksum: e363e809996cf46ada20da1accfcd9c7 (MD5) Tese_NovaAbordagemInterpretacao.pdf: 15232651 bytes, checksum: 8838ce5885d35952a159a082516bf81f (MD5) Previous issue date: 1989
CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico
CNRS - Centre National de la Recherche Scientifique
FINEP - Financiadora de Estudos e Projetos
A interpretação de anomalias gravimétricas é de grande importância no estudo de feições geológicas que ocorrem na crosta terrestre. Esta interpretação é, no entanto, dificultada pelo fato das anomalias gravimétricas serem resultantes da soma total dos efeitos produzidos por todos os contrastes de densidades de subsuperfície. Desse modo, com o intuito de separar efeitos de feições mais profundas de efeitos de feições mais rasas, bem como a caracterização da geometria desses dois conjuntos de feições, apresentamos um método de separação das componentes regional e residual do campo e a subsequente interpretação de cada componente. A separação regional-residual de dados gravimétricos é efetuada através da aproximação do campo regional por um polinômio ajustado ao campo observado por um método robusto. Este método é iterativo e usa como aproximação inicial a solução obtida através do ajuste polinomial pelo método dos mínimos quadrados. O método empregado minimiza a influência de observações contendo forte contribuição do campo residual no ajuste do campo regional. A componente regional obtida a partir da separação regional-residual é transformada em um mapa de distâncias verticais em relação a um nível de referência. Esta transformação compreende duas etapas. A primeira consiste na obtenção da continuação para baixo da componente regional, que é pressuposta ser causada por uma interface suave separando dois meios homogêneos, representando a interface crosta-manto, cujo contraste de densidade é supostamente conhecido. A segunda consiste na transformação do mapa de continuação para baixo em um mapa de distâncias verticais entre o nível de continuação (tomado como nível de referência) e a interface. Este método apresenta duas dificuldades. A primeira está ligada à instabilidade, havendo portanto a necessidade do emprego de um estabilizador o que acarreta a perda de resolução das feições que se desejam mapear. A segunda, inerente ao método gravimétrico, consiste na impossibilidade da determinação das profundidades absolutas da interface em cada ponto, bastando entretanto o conhecimento da profundidade absoluta em um ponto, através de informação independente, para que todas as outras profundidades absolutas sejam conhecidas. A componente residual obtida a partir da separação regional-residual é transformada em um mapa de contrastes de densidade aparente. Esta transformação consiste no cálculo do contraste de densidade de várias fontes prismáticas através de uma inversão linear pressupondo que as fontes reais estejam das a uma placa horizontal, com contrastes de densidade variando apenas nas direções horizontais. O desempenho do método de separação regional-residual apresentado foi avaliado, através de testes empregando dados sintéticos, fornecendo resultados superiores em relação aos métodos dos mínimos quadrados e da análise espectral. O método de interpretação da componente regional teve seu desempenho avaliado em testes com dados sintéticos onde foram produzidos mapeamentos de interfaces bem próximas das estruturas reais. O limite de resolução das feições que se desejam mapear depende não só do grau do polinômio ajustante, como também da própria limitação inerente ao método gravimétrico. Na interpretação da componente residual é necessário que se postule ou tenha informação a priori sobre a profundidade do topo e espessura da placa onde as fontes estão supostamente confinadas. No entanto, a aplicação do método em dados sintéticos, produziu estimativas razoáveis para os limites laterais das fontes, mesmo na presença de fontes interferentes, e pressupondo-se valores para profundidade do topo e espessura da placa, diferentes dos valores verdadeiros. A ambiguidade envolvendo profundidade do topo, espessura e densidade pode ser visualizada através de gráficos de valores de densidade aparente contra profundidade do topo presumida para a placa para vários valores postulados para a espessura da placa. Estes mesmos gráficos permitem, pelo aspecto das curvas, a elaboração de uma interpretação semi-quantitativa das profundidades das fontes reais. A seqüência dos três métodos desenvolvidos neste trabalho foi aplicada a dados gravimétricos da região norte do Piauí e noroeste do Ceará levando a um modelo de organização crustal que compreende espessamentos e adelgaçamentos crustais associados a um evento compressivo que possibilitou a colocação de rochas densas da base da crosta a profundidades rasas. Este modelo ê compatível com os dados geológicos de superfície. É ainda sugerida a continuidade, por mais 200 km em direção a sudoeste, do Cinturão de Cisalhamento Noroeste do Ceará por sob os sedimentos da Bacia do Parnaíba, com base nas evidências fornecidas pela interpretação da anomalia residual. Embora esta seqüência de métodos tenha sido desenvolvida com vistas ao estudo de feições crustais de porte continental, ela também pode ser aplicada ao estudo de feições mais localizadas como por exemplo no mapeamento do relevo do embasamento de/bacias sedimentares onde os sedimentos são cortados por rochas intrusivas mais densas.
Despite its great importance to the study of global geologic structures, interpreting gravity anomalies is not a trivial task because the observed gravity field is the resultant of every gravity effect produced by every elementary density contrast. Therefore, in order to isolate the effects produced by shallow sources from those produced by deep sources, I present a new method for regional-residual separation and methods for interpreting each isolated component. The regional-residual separation is perfomed by approximating the regional field by a polynomial fitted to the observed field by a robust method. This method is iterative and its starting value is the least-squares fitting. Also, the influence of observations containing substantial contributions of the residual field in the regional field fitting is minimized. The computed regional field is transformed into a map of vertical distances relative to a given datum. This transformation consists of two stages. The first one is the downward continuation of the regional field which is assumed to be produced by a smooth interface separating two homogeneous media: the crust and the mantle. The density contrast between the media is presumably known. The second stage consists in transforming the downward continued field into a map of vertical distances relative to a given datum by means of simple operations. This method presents two difficulties. The first one is related to the instability inherent to the downward continuation operation. The use of a stabilizer is therefore mandatory, leading to an inevitable loss of resolution of the features being mapped. The second difficulty, inherent to the gravity method, is the impossibility of determining the interface absolute depths. However, the knowledge of the absolute depth at one single point of the interface by independent means allows the computation of all absolute depths. The computed residual component is transformed into an apparent density map. This transformation consists in calculating the intensity of several prismatic sources by linear inversion, assuming that the real sources are confined to a horizontal slab and have density contrasts varying only along the horizontal directions. The performance of the regional-residual separation method was assessed in tests using synthetic data, always producing better results as compared either with polynomial fitting by least-squares or with the spectral analysis method. The method for interpreting the regional component was applied to synthetic data producing interfaces very close to the true ones. The limit of resolution of the features being mapped depend not only on the degree of the fitting polynomial, but also on the limitation imposed by the gravity method itself. In interpreting the residual component, a priori information is needed about the depth and thickness of the slab confining the true sources. However, results of tests using synthetic data showed that reasonable estimates for the h6rizontal limits of the sources can be obtained, even when the depth and thickness of the slab are not known. The ambiguity involving depth to the top, thickness and the apparent density can be visualized by means of curves of apparent density as a function of the presumed depth to the top of the slab, each curve corresponding to a particular assumed value for the slab thickness. An analysis of the configuration of the curves allows a semi-quantitative interpretation of the real sources depths. The sequence of all three methods described above was applied to gravity data from northern Piauí and northwestern Ceará state. As a result, a crustal organization model was obtained consisting of crustal thickenings and thinnings related to a compressive event which caused the raise of dense, lower crust rocks to shallower depths. This model is consistent with surface geological information. Also, the .gravity interpretation suggests the continuity of the Northwestern Ceará Shear Belt for more than 200 km under the Parnaíba Basin sedimentary cover. Although the sequence of methods presented here has been developed for the study of large scale crustal structures, it could also be applied to the interpretation of smaller structures, as, for example, the basement relief of a sedimentary basin where the sediments have been intruded by mafic rocks.

Un fait marquant dans la structure des rifts et des bassins est la différence entre l'extension en surface, qui reste faible, l'amincissement crustal et l'amincissement lithosphérique (2 ou 3 fois plus important). Par ailleurs, la surrection des épaules est un phénomène tardif que le classique modèle d'étirement n'explique pas. Une perturbation thermique peut amincir rapidement la lithosphère, les mouvements crustaux qu'elle induit dépendent de la tectonique régionale. Si elle est extensive, le bassin est large et les épaules d'altitude moyenne. Sinon le graben est étroit et les épaules élevées, c'est le cas du rift Africain. Celui de suez a connu les deux stades, dans un premier temps, il a été couplé aux mouvements de la mer Rouge, mais actuellement l'ouverture du golfe d'Aqaba bloque celle de Suez. En surface cela se traduit par un fort rétrécissement de la largeur du Golfe. Un modèle combinant la propagation d'une anomalie thermique avec des variations de contraintes en surface permet de quantifier ce rétrécissement et la remontée actuelle des épaules. Une étude cinématique de la rotation des blocs permet de situer à 10 km la limite ductile/cassant dans la croute, l'hypothèse d'une ouverture dissymétrique le long de failles plates affectant le Moho parait irréaliste pour le golfe de Suez.

Ce travail axe sur trois aspects differents a pour objectif principal: l'analyse de la structure crustale a partir des donnees gravimetriques, l'etude des mecanismes de formation du bassin mesozoique et son evolution par rapport au contexte geodynamique de la region, la determination et l'analyse de l'etat thermique actuel et ses implications geodynamiques possibles. Ainsi, une carte gravimetrique synthetique et homogene a ete etablie pour l'ensemble de l'algerie. On y identifie les differents domaines structuraux majeurs qui portent l'empreinte de la structuration panafricaine de la croute au moins jusqu'a l'atlas saharien au nord, et qui ont subi l'influence des episodes tectoniques ulterieurs. Une importante et nouvelle zone de suture panafricaine a ete proposee. Ceci vient appuyer l'hypothese de la formation de la chaine panafricaine par accolement successif. L'etude de la subsidence du ne saharien aboutit a un schema d'evolution complexe de ce bassin au cours du mesozoique. Des mecanismes simples sont invoques et leur interaction rend l'analyse plus difficile. La determination du flux de chaleur permet d'etablir une cartographie de ce parametre pour l'ensemble de l'algerie. Elle met en evidence l'existence d'une importante anomalie thermique dans les bassins au nord du hoggar. Des evidences geophysiques et petrologiques permettent d'associer cette anomalie thermique a l'existence d'un manteau superieur anormal sous les bassins sahariens

Cette these est avant tout consacree au developpement et a l'application des methodes dites indirectes en gravimetrie et parmi elles la transformee en ondelettes. Ces methodes caracterisent la geometrie des sources qui generent les anomalies de pesanteur. Elles sont presentees et testees sur des anomalies synthetiques et sur deux leves acquis dans les cevennes et en himalaya. Les methodes dites indirectes en gravimetrie sans a priori ni sur la geometrie ni sur les densites des unites profondes, la modelisation en gravimetrie est limitee par la non-unicite des sources. Pour contourner cette difficulte, nous developpons une approche utilisant la transformee en ondelettes, qui permet une bonne separation spatiale et frequentielle des sources. L'analyse du champ ainsi transforme permet de determiner simultanement un indice de forme des sources, leur profondeur, leur extension verticale et leur pendage. Imagerie 3-d du massif granitique du m t lozere, cevennes dans le cadre du programme geofrance 3d, nous avons acquis et positionne plus de 1200 nouvelles mesures gravimetriques sur le massif granitique du m t lozere. Ces nouvelles donnees, contraintes a la fois par la geologie et l'application des methodes indirectes, nous permettent d'imager la structure 3-d du batholite. Par l'etude de sa geometrie et de ses relations avec l'encaissant, on propose un schema de mise en place du massif en liaison avec l'etat de contraintes regional. Apports a la connaissance de la zone himalayenne au sein du programme de cooperation franco-nepalais idylhim, nous avons realise deux profils gravimetriques au travers de l'himalaya du nepal. Ces donnees permettent 1/ a partir d'une analyse prealable en ondelettes, de modeliser les chevauchements qui structurent l'avant-chaine, et 2/ completees par des donnees indiennes et tibetaines, de contraindre des modelisations thermo-mecaniques a grande longueur d'onde. En particulier, l'hypothese d'une eclogitisation est ecartee sous l'hymalaya.

L'etude petrographique et geochimique de l'ophiolite de semail (massif salahi, oman) qui represente le plus vaste affleurement de lithosphere oceanique fossile permet de distinguer 3 phases de volcanisme, presentant des rapports ta/th anormaux qui sont interpretes comme les temoins d'une anomalie mantellaire regionale. Un modele d'evolution en contexte oceanique franc est propose. L'etude des differents episodes de circulation hydrothermale est egalement abordee

Si la modification actuelle du climat est attribuée aux émissions anthropiques, les émissions volcaniques ont toujours représenté un des moteurs principaux de son évolution. Elles correspondent à l'une des sources naturelles les plus importantes de soufre sur Terre, qui peut entrainer des perturbations du climat pendant plusieurs années. Cela est dû à l'oxydation du SO2 volcanique en aérosols de sulfate, qui ont la propriété d'interagir avec le rayonnement solaire incident. Cette étude vise à améliorer notre compréhension du devenir du soufre dans l’atmosphère au cours du temps, à partir d'une approche multi-isotopique sur les sulfates présents dans des dépôts volcaniques anciens et les encroutements noirs. L'étude conduite sur des dépôts volcaniques montre qu'à l'échelle des temps géologiques, les ignimbrites représentent une archive intéressante des oxydants de l'atmosphère. La différence de signature en ∆17O entre les sulfates anatoliens et péruviens suggère une implication différente de l'ozone dans leur formation. L'étude des encroutements noirs formés à différentes périodes a permis de comparer les voies d'oxydation du soufre en zone urbaine de nos jours et durant la période pré-industrielle. Leurs compositions isotopiques suggèrent que les contributions relatives des différentes voies d'oxydation ont changé depuis l’ère pré-industrielle. Enfin, de larges quantités de nitrate ont été découvertes dans des dépôts volcaniques. L'étude de ces nitrates a permis de montrer qu'ils résultent principalement de la fixation d'azote atmosphérique par les éclairs volcaniques
Present-day climate change is attributed to anthropogenic emissions, however, volcanic emissions have always played an essential role in climate evolution. They represent one of the most important natural sources of sulphur on Earth, that can lead to climate perturbation over years. This is due to volcanic SO2 oxidation into sulphate aerosols, which interact with incident solar radiations, thus modifying the Earth's radiative balance.Present-day sulphur oxidation pathways are relatively well-known, but they remainpoorly constrained in the past. This study aims to improve our understanding of the fate of atmospheric sulphur over geological times using a multi-isotopic approach on sulphates originating from volcanic deposits and black crusts.The study conducted on ancient volcanic deposits shows that ignimbrites represent an interesting archive of atmospheric oxidants, and certainly more reliable than volcanic ashfalls. The difference in ∆17O signature between Anatolian and Peruvian sulphates suggests a different involvement of ozone in their formation.The study of black crusts from the Parisian region and formed at different periods,leads to the comparison of sulphur oxidation pathways in urban area between the preindustrial era and the present-day. Their differences in isotopic composition suggest that the relative contributions of each oxidation channel have changed from pre-industrial to modern times.Finally, large amounts of nitrate have been found in volcanic deposits in Turkey andPeru. Our results show that most of the nitrates in volcanic deposits represent the end-product of atmospheric nitrogen fixation by volcanic lightning, occurring during explosive eruptions

The continental margin off the coast of Pakistan between the Murray ridge and the Gulf of Cambay has been studied in this work using gravity, magnetic and bathymetric data. Two dimensional gravity and magnetic models based on free-air gravity and residual magnetic data are developed along a north-south profile off the coast of Karachi. The purpose was to interpret the gross crustal structure of the region. A magnetic map has also been developed for the region between latitudes 20°N and 27°N and between longitude 60°E and 70°E. The gravity model extends to a distance of about 1200 km seaward south of Karachi. The seaward end of the gravity model is constrained by seismic refraction data which suggest the presence of typical oceanic crust. The Moho depth at this end of the profile is about 12 km. At the landward end of the profile A-A' the Moho depth is not constrained by seismic data. The gravity model suggests 27 to 17 km as the possible range of the depth of the Moho and a gradual thinning of the crust from land to sea. In addition, the gravity models as interpreted in this study show grabens at the distances of 350 and 450 km along the profile. If the graben-like structures are rift grabens formed during the rifting of India from Africa then transitional crust can be expected to extend to the 500 km mark along the profile A-A'. Two dimensional models for the magnetic data along the profile were also developed. These anomalies can be interpreted as due to oceanic crust or magnetic bodies embedded in transitional crust. The possibility that the observed magnetic anomalies are due to oceanic crust has been studied in detail in this work. The location of the observed magnetic anomalies with respect to marine magnetic anomaly (28) observed earlier on the Indian Ocean floor, were compared to a marine magnetic time scale. To get a reasonable correlation between the observed and theoretical anomalies requires a considerable amount of adjustment in the spreading rate of individual magnetic blocks. Also on the magnetic map the trend of the lineation of these anomalies is perpendicular to the continental margin instead of being parallel to the continental margin as expected for a rifted continental margin. The presence of horst-and-graben structures in the inland region suggests the rifted nature for the continental margin off Karachi than the sheared nature. This indicates that the lineations should be parallel to the margin. But the magnetic lineations are perpendicular to the continental margin and if they are from oceanic crust then they would suggest that the margin is a sheared margin, which contradicts the extensional structures observed inland. This makes it very unlikely that the source of these anomalies is oceanic crust. However, it is quite possible that the magnetic lineations observed in the map were parallel to the continental margin initially but later on the continent rotated clockwise along a fault landward of the magnetic lineation. This rotation is perhaps responsible for making the lineation perpendicular to the continental margin. One objective of this study was to locate the continent-ocean boundary, but with the available amount of data it is not possible to decide on the most appropriate source for the observed magnetic anomalies. Hence it was not possible to decide exactly on the location of continent-ocean boundary. However, on the basis of gravity and magnetic data it can be said that the continent-ocean boundary lies at a distance of 500 km or greater along the profile.
Graduation date: 1992

The West Antarctic Ice Sheet (WAIS) is mostly grounded in broad, deep basins (down to 2.5 km below sea level) that are stretched between five crustal blocks. The geometry of the bedrock, being mostly below sea level, induces a fundamental instability in the WAIS through the possibility of runaway grounding line retreat. The crustal environment of the WAIS further influences the ice sheet’s fast flow through conditions at the ice-bedrock boundary. This study focuses on understanding the WAIS by examining the subglacial geology (such as volcanoes and sedimentary basins) at the icebedrock boundary and the continent’s deeper crustal structure- primarily using airborne gravity anomalies. The keystone of this study is a 2004-2005 aerogeophysical survey over one of the most negative mass balance glaciers on the continent: Thwaites Glacier (TG). The gravity anomalies derived from this dataset- as well as gravity-based modeling and spectral crustal boundary depth estimates- reveal a heterogeneous crustal environment beneath the glacier. The widespread Mesozoic rifting observed in the Ross Sea Embayment (RSE) of West Antarctica extends beneath TG, where the crust is ~27 km thick and cool. Adjacent to TG, spectrally-derived shallow Moho depths for the Marie Byrd Land (MBL) crustal block can be explained by thermal support from warm mantle. I assemble here new compilations of free-air and Bouguer gravity anomalies across West Antarctica (from both airborne and satellite datasets) and re-interpret the extents of West Antarctic crustal block and their boundaries with the rift system. Airy isostatic gravity anomalies reveal that TG is relatively sediment starved, in contrast to the sediment-rich RSE. TG’s fast flow velocities could be sustained in this sediment poor environment if higher heat flux in MBL was providing an ample source of subglacial melt water to the glacier. The isostatic anomalies also indicate that TG’s outlet rests on a bedrock sill that will impede future grounding line retreat (up to ~100 km) and temporarily stabilize the glacier.
text

Geophysical surveys in the Gulf of California provided the data to construct contour maps of bathymetry, free-air anomalies and total field magnetic anomalies for the area north of 27° N. Major faults such as the Ballenas- Salsipuedes, Tiburón, Guaymas, and the South Cerro Prieto are clearly observable on these maps. Spectral analysis, using 2-D Fast Fourier Transform methods, of the magnetic anomalies north of 29° N, allowed the identification of at least three distinct magnetic source horizons. The shallowest depth magnetic horizon, with an average depth to the top of 3.1 km below sea level, is interpreted as the top of the magnetic basement. The intermediate depth magnetic horizon, with an average depth to the top of 5.3 km below sea level, may represent either a lithological discontinuity in continental crust, or a transition zone characterized by the intrusion of igneous rocks, faulting, and fracturing associated with rifling processes. Some lineaments observed in the contour map on this horizon are oriented about 15° counterclockwise from the expected orientation of faults. Other lineaments are almost perpendicular to those faults. The deepest magnetic horizon is not apparent at all locations. Computed depths to the bottom of the magnetized crust average 11.5 km below sea level. The depth to the bottom of the magnetic crust is interpreted as the depth of the Curie-point isotherm. Assuming a Curie-point temperature of 580°C and a thermal conductivity of 2.2 W/m °C, the calculated heat flow averages 114 mW/m². Using a two-dimensional Maximum Entropy Method (2DMEM) to obtain the power spectrum of the magnetic anomalies increased the horizontal spatial resolution of the depth determinations by a factor of 4. This method when used to compute the depth to the top of the intermediate horizon, shows an improvement in the delineation of structures. However, the other magnetic horizons and the depth to the bottom of the magnetic crust were not clearly observable using this technique.
Graduation date: 1988

The Queen Charlotte Fault system is a segment of the North America - Pacific plate boundary. From 45 Ma - 5 Ma, plate motion has been primarily translational. Since 5 Ma, transpression has been the dominant mode of interaction. The plate boundary west of the Queen Charlotte Islands is characterized by an approximately 30- km wide terrace, flanked to the west by a topographic trough and to the east by the seismically active Queen Charlotte Fault. At 53.4°N the fault bends eastward and the terrace becomes wider and discontinuous, forming triangular shaped highs and intervening lows. Approximately 300 km of multichannel seismic reflection and gravity data along and across the Queen Charlotte Fault off Dixon Entrance were collected as part of the ACCRETE experiment in 1994. Structural interpretation of the five new profiles reveals the presence of faults and folds within the terrace, which form an angle of 20° to the strike of the Queen Charlotte Fault. The direction of these structures corresponds to the trend of the plate boundary south of the bend and west of the Queen Charlotte Islands, implying that through complex compression and shear, material must have been carried from south to north along the margin during oblique plate motion. Based on this observation and on forward gravity modeling, which places limits on the possible plate configuration at depth, a four-dimensional model has been developed to explain the temporal and spatial evolution of structural styles in this region. Considering the amount of shortening that must be accommodated within the past 5 Ma (a maximum of 100 km), a model of an underthrusting Pacific plate is preferred over one of pure upthrusting. About 5-6 Ma ago, when transpression began, oceanic crust was flexed and thrust upward at the plate boundary to eventually reach a steady-state configuration of a subducting slab. Fractured basement rock and consolidated, deformed sediments underlie the terrace and form its foundation. As a result of strain partitioning, the terrace is now decoupled and moves both parallel to the continent and perpendicular to the underthrusting Pacific plate. North of the bend in the Queen Charlotte Fault, underthrusting north of it occurs obliquely along preexisting fractures at the base of the terrace. The repetitive pattern of triangular terrace slivers is the result of continuing uplift and shear along these trends. Active tectonism influences sediment dispersal and creates traps. A N-S trending fault was also identified in the trough segment and possibly involves oceanic basement. Its origin is thought to be due to distributed shear that was transmitted across the plate boundary. Sea-floor spreading magnetic anomalies trend north-south as well. Along these zones of weakness, synthetic strike-slip faults of a transpressional strain ellipse could has been initiated during early stages of subduction. Reactivation of such faults may occur when oceanic crust approaches the outer terrace boundary, as is the case in the study region. Gravity modeling confirmed the existence of thin (24 km) continental crust and an increase in oceanic Moho dip beneath the terrace, which is topped by unconsolidated sediments and underlain by material of near-basement densities. It could not be determined using gravity modeling whether oceanic crust exists beneath the continent, but if it does, it must be welded to the North American plate in shallow subduction.
Graduation date: 1998

Using gravity, magnetic, bathymetric and seismic refraction data, I have constructed a geophysical cross-section of the central part of the northern Gulf of California. The section exhibits a crustal thickness of 18 km and features an anomalous block of high density lower basement (3.15 g/cm³) which probably resulted from rifting processes during the opening of the Gulf. The magnetization of the upper basement ranges from 0.0005 to 0.0030 emu/cm³. Three different layers of sediments are modeled, ranging from unconsolidated (1.85 g/cm³) to compacted (2.50 g/cm³). I present a deconvolution method for automated interpretation of magnetic profiles based on Werner's (1953) simplified thin-dike assumption, leading to the linearization of complex nonlinear magnetic problems. The method is expanded by the fact that the horizontal gradient of the total field caused by the edge of a thick interface body is equivalent to the total field of a thin dike. Statistical decision making and a seven point operator are used to insure good approximations of susceptibility, dip, depth, and horizontal location of the source. After using synthetic models to test the inversion method, I applied it to the Northern Gulf of California using data collected in 1984 by the Continental Margins Study Group at Oregon State University. Fault traces, computed by the deconvolution, are plotted on a map. The faulting pattern obtained is in good agreement with that proposed by other workers using other methods. The depths to the top of the faults range from 4 to 5 km in the eastern part of the Gulf, where they may be interpreted as the top of the structural basement. Deeper estimates are obtained for the western part of the Gulf.
Graduation date: 1990

At collisional mountain ranges the tectonic history of crustal shortening and subsequent post-collisional erosion is preserved in the form of the presently observed gravity anomalies. In this study, models of erosion and isostatic rebound at various stages of collision illustrate the evolution of crustal structure, topography, and resulting gravity anomalies. The Ouachita Mountains of Arkansas, which show a low/high Bouguer gravity couple characteristic of the initial stages of collision, have undergone just 8 km of erosion during the process of completely rebounding the syn-orogenic crustal root. This minor rebound means that the Ouachitas retain a crustal geometry similar to the continental margin prior to collision, including thin transitional and oceanic crust. At more advances stages of collision Bouguer gravity anomalies show a broad low reflecting a thickened crustal root. The width of this low, which relates directly to the amount of crustal shortening, is retained during subsequent erosion and elastic rebound, but the amplitude decays gradually. Thus, the width and amplitude of the low can be used to estimate the degree of convergence and amount of erosion, respectively, for a specific mountain range. For the Scandinavian Caledonides results are consistent with 20 km of erosion following 200 km of crustal shortening. Following a larger magnitude of convergence, about 300 km, the southern Appalachians are estimated to have undergone 28 km of post-collisional erosion. Bouguer gravity profiles across the recently-active Alps compare with a model of 200 km of crustal shortening and 8 to 12 km of erosion. While the Alps have undergone a similar amount of shortening as that estimated for the Caledonides, erosion and post-collisional rebound is at an initial stage, such that a thick section of exotic crust still overlies the underthrusted European Platform. The results of these model comparisons suggest that the crustal geometry ofa collisional mountain range should be viewed as a consequence of the degree of crustal shortening as well as the amount of erosion and isostatic rebound. In models at moderate to advanced stages of shortening ( 200 km), and mature stages of erosion (e.g., Caledonides, Appalachians), the geometry of the crustal "suture" between overthrusting and underthrusting crusts is present as a shallow, subhorizontal de collement beneath the foreland. In the hinterland the suture abruptly steepens, a result of differential uplift during isostatic rebound. This crustal geometry, characteristic of seismic-reflection profiles across many ancient mountain belts, suggests: (1) that the "low angle detachment" observed beneath collisional mountain ranges was originally much deeper and steeper than it is at present; and (2) that steep-dipping seismic reflectors towards the hinterland represent the thrusted contact between converging crustal blocks, but have been steepened as a result of isostatic uplift following erosion.
Graduation date: 1992

Tese de mestrado, Geologia (Geologia Estrutural), Universidade de Lisboa, Faculdade de Ciências, 2019
O manuscrito agora submetido é constituído por duas partes. Uma primeira onde se procura contextualizar sucintamente a metodologia laboratorial de modelação análoga, tanto numa perspectiva histórica, como tendo em vista a sua relevância e utilidade em geologia estrutural. Uma segunda, mais extensa, consistindo no essencial do presente trabalho (entretanto submetido à revista Journal of Structural Geology), que incide na aplicação dessa mesma metodologia a um problema específico, conceptual, de geologia estrutural. O projecto experimental agora apresentado pretendeu estudar, com recurso a modelação análoga, o efeito da presença de heterogeneidades morfológicas e/ou reológicas na formação e modificação do padrão estrutural típico de uma zona de cisalhamento frágil supra-crustal. A heterogeneidade (anomalia) reológica considerada consiste num corpo com um comportamento mecânico (reológico) viscoso, de geometria oblata, disposto sub-horizontalmente (“inter-estratificado” nas rochas da crosta), e.g. correspondendo a um diapiro salino ou uma a uma câmara magmática (pré-tectónica) em arrefecimento. A heterogeneidade morfológica, por sua vez, corresponde a um relevo topográfico alongado (e.g. a uma crista ou cordilheira). O aparato experimental utilizado representa uma variação de uma experiência clássica de Riedel (e.g. Naylor, et al., 1986), consistindo numa caixa de deformação com duas placas basais, horizontalmente justapostas e deslizando lateralmente uma em relação à outra (materializando uma falha vertical de desligamento direito). No decurso da experiência, o movimento ao longo desta falha basal é transmitido a uma cobertura de areia sobrejacente, que acomoda a deformação implicada de um modo frágil. Esta configuração experimental pretende simular genericamente a reactivação de uma falha de desligamento num soco subjacente a uma cobertura sedimentar. Areia quártzica é utilizada como análogo mecânico da crosta superior, e silicone transparente (PDMS) é empregue para representar a anomalia viscosa supra-crustal. São investigadas experimentalmente as seguintes variáveis: a) presença/ausência de uma anomalia viscosa; b) presença/ausência de uma crista topográfica; c) diferente ângulo de interferência da crista topográfica com a direção da falha basal (90º ou 120º no sentido retrógrado). Em concordância, levaram-se a cabo três conjuntos de experiências nos quais se conjugam diferentes combinações destas variáveis: um primeiro no qual se considera somente a anomalia reológica (viscosa); um segundo, no qual a anomalia viscosa está ausente e a direcção da falha basal (DFB) intersecta uma crista topográfica segundo um ângulo de 90º ou 120º; e um terceiro, no qual se consideram, simultaneamente, a anomalia viscosa (interestratificada), e a crista topográfica (com as mesmas orientações anteriormente consideradas). Adicionalmente, foram ainda realizadas uma experiência na qual a anomalia reológica não foi coberta de areia (i.e., aflora ao nível da superfície da cobertura) e uma experiência de controlo, em relação à qual se se comparam os resultados obtidos, consistindo simplesmente numa experiência clássica de Reidel. Os resultados da experiência de controlo são compatíveis com aqueles obtidos noutras experiências do tipo Riedel (Naylor, et al., 1986; Dooley and Schreurs, 2012) e demonstram o mesmo desenvolvimento típico de estruturas, começando com o propagação de falhas sintéticas en-échelon (normalmente designadas de Riedel shears ou R-shears), com ângulos de ~17º-20º (sentido horário) em relação à DFB. Os R-shears tornam-se progressivamente inactivos, dando lugar ao desenvolvimento, a partir das suas extremidades (ou entre R-shears sobrepostos) de falhas sintéticas de baixo ângulo (<17º) (designadas de low-angle synthetic shears ou LAS shears), eventualmente estas últimas estruturas coalescem para formar uma zona de cisalhamento principal que afecta a totalidade do modelo (designada Y-shear) e que acomoda toda a movimentação, rejeitando as estruturas previamente desenvolvidas. A geometria helicoidal enraizada na descontinuidade basal, característica dos R-shears, é observada em corte, dando origem a um padrão estrutural de falhas “em flor” (flower structures). Os resultados do primeiro conjunto de experiências mostram uma interrupção da continuidade da zona de deformação frágil à superfície, revelando uma segmentação do correspondente padrão estrutural na área sobreposta à anomalia viscosa. Esta interrupção é marcada, a topo da anomalia, pela inibição da formação de R-shears e por um atraso significativo na propagação de LAS shears. Nesta mesma área, este tipo de estruturas acaba por formar-se de um modo mais disperso ou deslocalizado, evidenciando um padrão caracterizado por um maior número de falhas que acomodam individualmente menos movimentação, em claro contraste com o carácter mais localizado, ou discreto, do resto da zona de cisalhamento. Em corte observa-se que estes LAS shears enraízam no topo da anomalia viscosa. No caso do segundo conjunto de experiências, independentemente do ângulo (de 90º ou 120º) considerado para a interferência entre a crista e a DFB, observa-se sempre a formação de uma falha que corta este alto morfológico e que é mais longa, e acomoda mais movimentação, do que as restantes (que materializam a zona de cisalhamento frágil fora deste relevo). Nestas experiências, não é visível nenhum atraso na propagação de Riedel ou LAS shears através do alto morfológico. No terceiro conjunto, em ambas as experiências (interferência a 90º ou 120º), é visível uma obstrução à propagação de R-shears através da barreira morfo-reológica prescrita (compreendendo à anomalia viscosa e alto morfológico), bem como um atraso na propagação de LAS e Y-shears. Observa-se também a formação, em ambos os casos, de falhas normais em quadrantes opostos da anomalia viscosa, compatíveis com a direcção de tensão principal mínima (σ3) implicada no regime geral de cisalhamento simples. Estas falhas delimitam estruturas extensivas (pull-apart), que se verificam ser de maiores dimensões na experiência cujo ângulo entre a crista e a DFB é de 120º. Nesta experiência (120º), também se desenvolvem cavalgamentos, que afloram à superfície do modelo preferencialmente ao longo da base dos degraus morfológicos que delimitam a crista (nos quadrantes opostos, i.e., aproximadamente a 90º, das estruturas de pull-apart). Estes cavalgamentos estão ausentes, ou são incipientes, nas experiências em que se considerou a crista a 90º da DFB. Os cortes obtidos a partir dos modelos no estado final da deformação mostram que tanto as estruturas extensivas como as compressivas enraízam em profundidade na interface dúctil-frágil materializada pelo contacto vertical entre o silicone e a areia. Na experiência adicional também se observa o desenvolvimento em quadrantes opostos de estruturas extensivas e compressivas, associadas a diferentes orientações das interfaces de contacto entre a areia e o corpo viscoso. Este sofre deformação adoptando progressivamente uma geometria sigmoidal, com encurtamento acomodado à superfície pelo desenvolvimento de dobras de charneiras alinhadas, e estiradas, aproximadamente segundo a direcção do o eixo maior da elipse de deformação finita. Esta geometria sigmóidal do corpo viscoso é também inferida por via da reconstituição dos cortes seriados obtidos no final das experiências do primeiro e terceiro conjuntos de experiências. Em todas as experiências destes três conjuntos, os padrões estruturais, desenvolvidos nos domínios do modelo afastados das heterogenias morfológicas e/reológicas (i.e., fora da sua influência), são semelhantes àquele obtido na experiência de controlo. Destes resultados experimentais infere-se que a presença de um corpo viscoso intercalado numa cobertura frágil (supra-crustal) sobreposta a uma falha de desligamento basal, afetando um soco rígido subjacente, exerce um efeito de atenuação e deslocalização na propagação vertical da deformação implicada no movimento da falha. Isto é revelado nas experiências levadas a cabo pela ausência de R-shears a topo das anomalias viscosas, e pelo atraso no desenvolvimento à superfície de um padrão deslocalizado de LAS shears. O contraste reológico (entre o corpo viscoso e a areia), que controla o diferente modo de acomodação da deformação através da espessura da cobertura, produz também um efeito semelhante na horizontal, ditado pelo contacto lateral entre a anomalia viscosa e o meio frágilenvolvente condicionando, neste caso, a distribuição de estruturas extensivas e compressivas em torno de quadrantes opostos da referida anomalia. Na ausência de um corpo viscoso, o desenvolvimento preferencial de praticamente uma única estrutura principal (mais longa) a atravessar o alto morfológico é interpretado como o resultado da maior espessura da cobertura na zona topograficamente elevada, e é compatível com relações empíricas de proporcionalidade entre espessura da cobertura e comprimento das estruturas, anteriormente reportadas (e.g. Naylor, et al., 1986). Do conjunto de experiências em que se consideraram obstáculos morfo-reológicos, apenas na experiência com a crista topográfica a 120º da DFB se observam estruturas compressivas de direcção ortogonal a σ1. Este resultado mostra que o desenvolvimento deste tipo de estruturas é não apenas controlado pela existência das interfaces de contacto lateral (contraste reológico) entre o corpo viscoso e a areia, mas também, criticamente, pela orientação dos degraus morfológicas que delimitam as cristas. Uma crista topográfica representa uma transição abrupta (interface) entre domínios adjacentes com diferentes cargas litostáticas e compactação. Na experiência com a crista a 120º, a orientação mais ortogonal desta interface em relação a σ1 (75º) é interpretada como sendo mais favorável à formação e enraizamento de cavalgamentos segundo essa direção. O facto destas estruturas enraizarem na interface areia-silicone e aflorarem alinhadas com a base da crista evidencia a acção recíproca entre os dois tipos de obstáculos. Contrariamente, na experiência com a crista a 90º, a direcção dos degraus morfológicos correspondentes é mais oblíqua em relação a σ1 (45º), não propiciando a formação destas estruturas. Os resultados experimentais obtidos são ainda preliminarmente discutidos em vista de alguns exemplos naturais genéricos, nos quais se procura reconhecer os padrões estruturais revelados pelas experiências realizadas.

My dissertation research consists of two themes: (a) the analysis of Middle Jurassic - Early Cretaceous marine magnetic anomalies (M-anomalies) in order to construct a comprehensive geomagnetic polarity timescale and (b) the investigation of the upper oceanic crustal architecture using downhole geophysical logs. These themes were chosen to better understand how remotely-sensed geophysical signals elucidate the formation and evolution of oceanic crust. This revised Pacific-wide MGPTS model shows significant improvement in its reliability, exhibits global applicability, and highlights changes in the paleo-Pacific spreading regime. By integrating Atlantic Manomaly analyses with the new MGPTS model and reviewing previous seismic studies, we shed new light on the causes of a ubiquitously distributed ?Atlantic anomaly smooth zone? where little coherency among M5-M15 anomaly sequence is observed. For the second theme, I analyzed the architecture of 15 m.y. old superfast spreading East Pacific Rise crust drilled at Ocean Drilling Program Hole 1256D in the eastern Pacific. An intact upper oceanic crustal section was penetrated at this site to a depth of 1507 mbsf. In situ crustal architecture was mapped from resistivity imagery (electrofacies by Formation MicroScanner) combined with recovered cores and other logs. Highlights of this research are: (1) most of the extrusive section consists of massive flows and fragmented formations including breccias, which has important implications for the magnetic source layer and pathways of hydrothermal alteration; (2) the dike complex is composed of sheeted-dikes dipping away from the paleo-spreading axis consistent with submersible observations at other sites in the eastern Pacific; (3) the crustal construction processess from ridge axis to abyssal plain during 0-50 kyr time are consistent with previous seismic reflection studies based on the integration of our stratigraphy model with lava flow observations from the southern East Pacific Rise.