In paleomagnetism, practical measurements are rarely made using perfect, isolated, single-phase, ferromagnetic crystals. Experimental observations are typically made using magnetic materials formed by a variety of natural processes. In this thesis, we will look at bridging the gap between current numerical modelling capability and experimental observations. First, we work towards micromagnetic modelling of multi-phase magnetic materials, including magnetostriction, embedded in a rocky matrix, along with crystal defects. We present a derivation of the Boundary Element Method formulation used by the micromagnetics package, MERRILL, and provide an extension of this from single-phase materials to multi-phase. After discussing issues with previous approaches to modelling magnetostriction, we derive and present a more robust and flexible approach. This model of magnetostriction is suitable for non-uniformmagnetizations, for multi-phase materials, and for arbitrary boundary conditions, and can be incorporated into MERRILL.We then outline a method for extending our model to materials embedded in an infinite elastic matrix of arbitrary elasticity. Finally, we present a method for modelling the magnetic response of a material due to crystal defects, along with a concrete example of a magneto-dislocation coupling energy at a magnetite-ilmenite boundary where stress due to lattice misfit is eased by regular edge dislocations. Second, we work towards being able to verify micromagnetic models against nano-scale experimental data. To do this, we present two techniques for simulating electron holograms from micromagnetic modelling results, a technique capable of imaging magnetic structures at the nano-scale. We also present example electron holograms of commonly occurring magnetic structures in nano-scale rock and mineral magnetism, and highlight some distinguishing features, which may be useful for interpreting experimental electron holography data.

Un volumineux magmatisme anorogénique a recouvert une large partie (1.500.000 km2) du craton Amazonien à 1880 Ma et définit une province magmatique felsique qu'on appelle l'événement Uatumã. L'objectif de ce travail est d'étudier le paléomagnétisme ainsi que la pétrologie de ces roches afin de préciser le cadre spatio-temporel de cet événement et de définir la place du craton Amazonien au sein du Supercontinent Columbia. Deux régions d'études localisées dans le sud-ouest du craton Amazonien (Pará) ont permis de collecter les échantillons nécessaires : (1) la région de Tucumã où 16 filons felsiques, 7 filons mafiques, un filon gabbroïque et 3 sites du socle archéen ont été collectés. (2) la région de São Felix do Xingu où on a échantillonné 7 sites de laves rhyolitiques, 2 sites d'ignimbrites, un filon felsique et un site de brèches volcaniques qui appartiennent à la formation Santa Rosa. 6 sites de la formation Sobreiro (roches volcanoclastiques andésitiques) ainsi qu'un filon felsique de la suite Velho Guilherme ont aussi été collectés. Un des résultats majeurs de la pétrologie des filons felsiques de Tucumã (1880.9 ± 6.7 Ma U-Pb sur zircon) a été de montrer qu'ils représentent le système filonien associé à la formation volcanique Santa Rosa. L'aimantation rémanente des filons felsiques est portée par la magnétite et l'hématite. Cette hématite est syn- à post-magmatique et sa formation, à partir des fluides hydrothermaux, peut être quantifiée grâce certaines propriétés magnétiques. Les désaimantations (en champ alternatif, thermiques) montrent une composante A caractéristique de direction nord-ouest avec une inclinaison positive dont la moyenne par site donne un pôle paléomagnétique localisé à 52.9°S, 76.4°E (A95= 10.4°, K= 13.52). Une réaimantation régionale mésozoïque en relation avec les filons de la CAMP (Central Atlantic Magmatic Province) est observée dans cette région. Les meilleurs résultats paléomagnétiques ont été obtenus dans la région de São Felix de Xingu. Deux nouveaux pôles paléomagnétiques primaires, ont été déterminés: (i) Le pôle SF1 (319.7°E, 24.7°S, N= 10; A95= 16.9°) est obtenu pour des andésites et des rhyolites datés à 1877.4 ± 4.3 Ma (U-Pb zrn, LA-ICPMS), son origine primaire est confirmée par un test de contact inverse (> 1853 Ma). La pétrographie montre que la minéralogie magnétique de cette composante est l'hématite formée par des fluides hydrothermaux syn- à post-magmatiques. (ii) Le pole SF2 (220.1°E, 31.1°N, N= 15; A95= 9.7°) est déterminé par l'aimantation rémanente du filon felsique de la Suite Velho Guilherme, mais aussi par l'aimantation secondaire dans les échantillons de la formation Santa Rosa et Sobreiro. Un âge de 1853.7 ± 6.2 Ma (U-Pb zrn, LA-ICPMS) est calculé pour le filon felsique portant SF2, dont l'origine primaire est confirmée par un test de contact positif. Les pôles SF1 et SF2 sont très différents, malgré une différence d'âge de seulement ~25 Ma. Des résultats paléomagnétiques similaires ont été obtenus pour les pôles de même âge dans d'autres cratons (Inde, Supérieur (Laurentia), Slave (Laurentia), Kalahari, Baltica et Sibérie), et peuvent être expliqués par un événement de Vrai Dérive Polaire (VDP). Cette époque (~1880 Ma) est marquée par une forte activité du manteau, qui aboutit à la formation du Supercontinent Columbia, autour de 1850 - 1800 Ma. La formation de superpanaches est une conséquence possible de l'assemblage du supercontinent et de l'effet d'isolation thermique du manteau qui en résulte, ou bien lui est concomitante. Les superpanaches peuvent provoquer des perturbations de densité modifiant le tenseur inertiel de gravité de la Terre. Un rapide événement de Vrai Dérive Polaire (VDP) peut ainsi avoir eu lieu, ce qui va déplacer rapidement les continents et les superpanaches vers l'équateur. Ces événements peuvent être liés à une réorganisation du manteau dans son ensemble à la suite d'une période de faible activité magmatique entre 2400 et 2200 Ma. / An anorogenic magmatism covered a large part (1.500.000 km2) of the Amazonian craton at ca. 1880 Ma and defined a Silicic Large Igneous Province (SLIP) called the Uatumã event. The aim of this work is to study the paleomagnetism and petrology of these rocks to define the space-time framework of the Uatumã event and to try to elucidate the Amazonian craton evolution during the Columbia supercontinent amalgamation. Two regions were selected in the southwestern Amazonian craton (Pará) for sampling: (1) The Tucumã area where 16 felsic dikes, 7 mafic dikes, a gabbroic dike and 3 sites of the Archean basement were collected, and (2) the São Felix do Xingu area where, 7 sites of rhyolitic lava flows, 2 sites of ignimbrites, a felsic dike and a volcanic breccia belonging to the Santa Rosa Formation were sampled, and also 6 sites of the Sobreiro Formation (volcanoclastic rocks, andesitic) and one felsic dike of the Velho Guilherme Suite were collected. Petrology of the felsic dikes of Tucumã (1880.9 ± 6.7 Ma U-Pb zrn) showed that they represent the dike swarm associated with the Santa Rosa volcanic Formation. The remanent magnetization of the felsic dikes is carried by PSD magnetite and hematite. This hematite is syn- to post magmatic derived from hydrothermal fluids. Magnetic mineralogy can be used as a proxy to quantify the hydrothermal alteration. AF, thermal, LTD + AF and LTD + thermal demagnetizations show a northwest direction with a positive inclination (component A), whose site mean directions gives a paleomagnetic pole located at 52.9°S, 76.4°E, A95= 10.4 °, K= 13.52). However, this component seems to represent a remagnetization, probabily occurred at Neoproterozoic times. Another magnetic component (named component B) was also isolated for these rocks, and it was associated to a Mesozoic regional remagnetization related to the Central Atlantic Magmatic Province (CAMP). Yet, a third southwestern direction with low positive inclination (component C) was also isolated for some sites. This component was interpreted to be related with the ca. 1760 Ma Velho Guilherme magmatic intrusion. The best paleomagnetic results were obtained in the São Felix do Xingu area. Two new primary paleomagnetic poles have been determined: (i) SF1 pole (319.7°E, 24.7°S, N= 10; A95= 16.9 °) was obtained for andesites and rhyolites dated to 1877.4 ± 4.3 Ma (U-Pb zrn, LA-ICPMS), and its primary origin is confirmed by an inverse baked contact test (> 1853 Ma). Petrography shows that the magnetic mineralogy of this component is hematite formed by hydrothermal fluids syn- to post magmatic. (Ii) SF2 pole (220.1°E, 31.1°N, N= 15 ° A95= 9.7 °) was determined by the remanent magnetization of the felsic dike of the Velho Guilherme Suite but also as secondary magnetizations in samples of the Santa Rosa and Sobreiro Formations. An age of 1853.7 ± 6.2 Ma (U-Pb zrn, LA-ICPMS) is calculated for the felsic dike carrying SF2, whose primary origin is confirmed by a positive baked contact test. The SF1 and SF2 poles have a significant difference in angular distance, for a time interval of only ~25 Ma. Similar coeval paleomagnetic discrepancies were observed for other cratons (India, Superior (Laurentia), Slave (Laurentia), Kalahari, Baltica and Siberia), which can be explained by a True Polar Wander (TPW) event at ca. 1880 - 1860 Ma. This period is marked by a high mantle activity, which results in the amalgamation of the Columbia supercontinent, formed at ca. 1850 - 1800 Ma. Amalgamation of supercontinent may cause the formation of superplume and thermal insulation which can disturb mass distribution in mantle and alter the inertial gravity tensor of the Earth. A True Polar Wander (TPW) event may thus have taken place, which will move the cratons and the superplumes towards the equator. These conditions may be related to a reorganization of the whole mantle following a global magmatic quiescence between 2400 and 2200 Ma.

An understanding of the tectonic history of the Klamath Mountains is crucial for a valid paleogeographic reconstruction of the Pacific Northwest. However, prior to this study there were very few paleomagnetic (PM) data from the Klamath Mountains (KN), which resulted in conflicting interpretations about the role of the KM province in the tectonic evolution of western North America. Twenty-eight sites from five unmetamorphosed Middle Jurassic KM plutons with K-Ar ages ranging from 161 to 139 m.y.B.P. yielded stable PM results showing (1) a direction for the 160 in.y.B.P. Ashland pluton (D=324°, I=163°, α₉₅=8°, n=6) nearly concordant with the coeval expected direction (D=337°, 1=54°) and (2) clockwise rotated directions for the plutons of Grants Pass (D=045°, I=67°, α₉₅=12°, n=4), Greyback (D=083°,I=63°, α₉₅=9, n=9), and the Wooley Creek batholith and Slinkard pluton combined (D=037°, 160°, α₉₅=ll°, n=9). Tectonic interpretations of these PM data are difficult; two interpretations are offered to explain the observed directions. In the first, the mean PM direction of the four plutons with discordant directions (D=057°, 1=65°, α₉₅=7°, n=22) is restored to the expected 150 m.y.B.P. (the average K-Ar age for these four plutons) direction by rotation of a rigid block ~87° in a counterclockwise sense about a vertical axis (the possibility of tilt of these four plutons is disregarded in this interpretation). The Ashland pluton which shows no rotation is problematic. Either there was (is) a tectonic boundary west of the Ashland pluton, separating it from the rotation of the others, or the Ashland pluton was influenced both by clockwise rotation and tilt, the combined effect producing an essentially concordant PM direction. In the second interpretation we distinguish between the northern KN, intruded by the Grants Pas and Greyback Mountain plutons, and the southern region intruded by the Wooley Creek batholith and the Ashland and Slinkard plutons. The bases for this distinction are recent geologic and gravity studies which suggest that post-Middle Jurassic uplift of the domal Condrey Mountain Schist may have caused radially outward tilt of its adjacent terranes and plutons intruded therein, causing some of the observed discordances in their PM directions. Thus, in the second interpretation it is envisioned that (a) the northerly portion of the KM, intruded by the Grants Pass and Greyback plutons, was affected primarily by clockwise rotation about a vertical axis, and (b) discordant directions for the remaining plutons intruded farther south are due primarily to tilt in response to Condrey Mountain uplift. Based on the observed inclinations, there is no evidence of transport of the Klamath Mountain province along lines of longitude since Middle Jurassic time. Tectonic interpretations of the PM results of this study are consistent with significant post-Middle Jurassic clockwise rotation of the Klamath Mountains. The first interpretation above yields ~87° of clockwise rotation of the terrane examined. According to the second interpretation, a clockwise rotation of ~l00° is inferred from the average of the PM results of the northern Grants Pass and Greyback plutons. Therefore, 10° to 25° of clockwise rotation of the KM may have occurred prior to the formation of the Oregon Coast Range (~55 m.y.B.P.) and the two provinces may have rotated together since post-Lower Eocene time. / Graduation date: 1983

The anisotropy of magnetic susceptibility (AMS) was investigated in three Chinese Loess Plateau sedimentary sections along a W–E transect. Previously published models assumed that winter monsoons were responsible for the magnetic fabric formation of loess sequences. In our new interpretation, the stronger summer monsoons from the southeast played the major role in magnetic fabric orientation in the studied west and central parts of the Chinese Loess Plateau. The AMS was generated during the rainy summer monsoon when the sedimentary particles including magnetite were rearranged, settled, and fixed. We reconstruct the summer paleomonsoon routes for the last 130 kyr. These winds prevail from SE to NW but appear to be affected by regional topographic factors. Evidence in the world’s ocean current system indicates an abrupt cooling from 34.1 to 33.6 Ma across the Eocene-Oligocene boundary (EOB) at 33.9 Ma. I describe and date with magnetostratigraphy a section from the north Junggar Basin (Burqin section). Three fossil assemblages revealed through the EOB (34.8, 33.7, and 30.4 Ma) demonstrate that perissodactyl faunas were abruptly replaced by rodent/lagomorph-dominant faunas during climate cooling, and the changes in mammalian communities were accelerated by aridification in central Asia. Paleomagnetic studies of two sections of the northern Junggar Basin, China, are presented from Burqin and Tieersihabahe. Our paleomagnetic results demonstrate counterclockwise tectonic rotations in Burqin and Tieersihabahe (–17.2 ± 9.6° and –11.8 ± 6.1°, respectively) as well as considerable northward latitudinal displacement (12.2 ± 6.5° and 9.7 ± 4.1°, respectively) with respect to Europe. These results are consistent with the motions of contiguous blocks in the same geological time intervals (India, north and south China, Tarim, Amuria, and Kazakhstan). No significant intracontinental shortening or vertical-axis rotation is observed for the Junggar block from 40 Ma to 20 Ma. Our results reveal that the major compression and rotation between Junggar and northern Europe occurred after 20 Ma due to continuous penetration of India into Asia. We interpret the uplift of the Altay Mountains and the formation of the Lake Baikal rift system to be due to such intracontinental compression and relative rotations. / Geophysics

Der Baikalsee ist ein ideales Klimaarchiv für die Mitte Eurasiens. In dieser Arbeit wurde gesteinsmagnetische und paleomagnetische Analysen an hemipelagischen Sequenzen von vier Lokationen analysiert. Die Kerne erreichen ein Alter von maximal 300 ky. In Kombination mit TEM, XRD, XRF und geochemischen Analysen zeigt die gesteinsmagnetische Studie, dass detritischer Magnetit das magnetische Signal der glazialen Sedimente dominiert. Die magnetischen Signale der interglazialen Sedimente wurden durch diagenetische Prozesse verändert. Mittels HIRM können Hämatit und Goethit quantifiziert werden. Diese Methode eignet sich, den detritischen Eintrag in den Baikalsee abzuschätzen. Relative Paleointensitäten des Erdmagnetfeldes ergaben reproduizerbare Muster, welche in Korrelation mit gutdatierten Referenzproben die Ableitung eines alternativen Altersmodells für die Datierung der Baikalsedimente ermöglichten. Bei Anwendung des paleomagnetischen Altersmodells beobachtet man, dass die Abkühlung im Baikalgebiet und im Oberflächenwasser des Nordatlantiks wie sie aus den δ18 O-Werten planktonischer Foraminiferen abgeleitet werden kann, zeitgleich ist. Wird das aus benthischen δ18 O-Werten abgeleitete Altermdodell auf den Baikalsee angewandt, ergibt sich eine deutliche Zeitverschiebung. Das benthische Altersmodell repräsentiert die globale Veränderung des Eisvolumens, welche später als die Vänderung der Oberflächenwassertemperatur auftritt. Die Kompilation paleomagnetischer Kurven ergab eine neue relative Paleointensitätskurve “Baikal 200”. Mittels Korngrössenanalyse des Detritus konnten drei Faziestypen mit unterschiedlicher Sedimentationsdynamik unterschieden werden: 1) Glaziale Peroiden werden durch hohe Tongehalte infolge von Windeintrag und durch grobe Sandfraktion mittels Transport durch lokale Winde über das Eis charakterisiert. Dieser Faziestyp deutet auf arides Klima. 2) Während der Glazial/Interglazial-Übergänge steigt die Siltfraktion an. Dies deutet auf erhöhte Feuchtigkeit und damit verbunden erhöhte Sedimentdynamik. Windtransport und in den Schnee der Eisdecke eingetragener Staub sind die vorherrschenden Prozesse, welche den Silt in hemipelagischer Position zur Ablagerung bringen. 3) Während des klimatischen Optimum des Eeemian werden Grösse und Quantität des Silts minimal, was auf eine geschlossene Vegetationsdecke im Hinterland deutet. / Lake Baikal provides an excellent climatic archive for Central Eurasia as global climatic variations are continuously depicted in its sediments. We performed continuous rock magnetic and paleomagnetic analyses on hemipelagic sequences retrieved from 4 underwater highs reaching back 300 ka. The rock magnetic study combined with TEM, XRD, XRF and geochemical analyses evidenced that a magnetite of detrital origin dominates the magnetic signal in glacial sediments whereas interglacial sediments are affected by early diagenesis. HIRM roughly quantifies the hematite and goethite contributions and remains the best proxy for estimating the detrital input in Lake Baikal. Relative paleointensity records of the earth′s magnetic field show a reproducible pattern, which allows for correlation with well-dated reference curves and thus provides an alternative age model for Lake Baikal sediments. Using the paleomagnetic age model we observed that cooling in the Lake Baikal region and cooling of the sea surface water in the North Atlantic, as recorded in planktonic foraminifera δ18 O, are coeval. On the other hand, benthic δ18 O curves record mainly the global ice volume change, which occurs later than the sea surface temperature change. This proves that a dating bias results from an age model based on the correlation of Lake Baikal sedimentary records with benthic δ18 O curves. The compilation of paleomagnetic curves provides a new relative paleointensity curve, “Baikal 200”. With a laser-assisted grain size analysis of the detrital input, three facies types, reflecting different sedimentary dynamics can be distinguished. (1) Glacial periods are characterised by a high clay content mostly due to wind activity and by occurrence of a coarse fraction (sand) transported over the ice by local winds. This fraction gives evidence for aridity in the hinterland. (2) At glacial/interglacial transitions, the quantity of silt increases as the moisture increases, reflecting increased sedimentary dynamics. Wind transport and snow trapping are the dominant process bringing silt to a hemipelagic site (3) During the climatic optimum of the Eemian, the silt size and quantity are minimal due to blanketing of the detrital sources by the vegetal cover.

No description available.

No description available.

An anorogenic magmatism covered a large part (1.500.000 km2) of the Amazonian craton at ca. 1880 Ma and defined a Silicic Large Igneous Province (SLIP) called the Uatumã event. The aim of this work is to study the paleomagnetism and petrology of these rocks to define the space-time framework of the Uatumã event and to try to elucidate the Amazonian craton evolution during the Columbia supercontinent amalgamation. Two regions were selected in the southwestern Amazonian craton (Pará) for sampling: (1) The Tucumã area where 16 felsic dikes, 7 mafic dikes, a gabbroic dike and 3 sites of the Archean basement were collected, and (2) the São Felix do Xingu area where, 7 sites of rhyolitic lava flows, 2 sites of ignimbrites, a felsic dike and a volcanic breccia belonging to the Santa Rosa Formation were sampled, and also 6 sites of the Sobreiro Formation (volcanoclastic rocks, andesitic) and one felsic dike of the Velho Guilherme Suite were collected. Petrology of the felsic dikes of Tucumã (1880.9 ± 6.7 Ma U-Pb zrn) showed that they represent the dike swarm associated with the Santa Rosa volcanic Formation. The remanent magnetization of the felsic dikes is carried by PSD magnetite and hematite. This hematite is syn- to post magmatic derived from hydrothermal fluids. Magnetic mineralogy can be used as a proxy to quantify the hydrothermal alteration. AF, thermal, LTD + AF and LTD + thermal demagnetizations show a northwest direction with a positive inclination (component A), whose site mean directions gives a paleomagnetic pole located at 52.9°S, 76.4°E, A95= 10.4 °, K= 13.52). However, this component seems to represent a remagnetization, probabily occurred at Neoproterozoic times. Another magnetic component (named component B) was also isolated for these rocks, and it was associated to a Mesozoic regional remagnetization related to the Central Atlantic Magmatic Province (CAMP). Yet, a third southwestern direction with low positive inclination (component C) was also isolated for some sites. This component was interpreted to be related with the ca. 1760 Ma Velho Guilherme magmatic intrusion. The best paleomagnetic results were obtained in the São Felix do Xingu area. Two new primary paleomagnetic poles have been determined: (i) SF1 pole (319.7°E, 24.7°S, N= 10; A95= 16.9 °) was obtained for andesites and rhyolites dated to 1877.4 ± 4.3 Ma (U-Pb zrn, LA-ICPMS), and its primary origin is confirmed by an inverse baked contact test (> 1853 Ma). Petrography shows that the magnetic mineralogy of this component is hematite formed by hydrothermal fluids syn- to post magmatic. (Ii) SF2 pole (220.1°E, 31.1°N, N= 15 ° A95= 9.7 °) was determined by the remanent magnetization of the felsic dike of the Velho Guilherme Suite but also as secondary magnetizations in samples of the Santa Rosa and Sobreiro Formations. An age of 1853.7 ± 6.2 Ma (U-Pb zrn, LA-ICPMS) is calculated for the felsic dike carrying SF2, whose primary origin is confirmed by a positive baked contact test. The SF1 and SF2 poles have a significant difference in angular distance, for a time interval of only ~25 Ma. Similar coeval paleomagnetic discrepancies were observed for other cratons (India, Superior (Laurentia), Slave (Laurentia), Kalahari, Baltica and Siberia), which can be explained by a True Polar Wander (TPW) event at ca. 1880 1860 Ma. This period is marked by a high mantle activity, which results in the amalgamation of the Columbia supercontinent, formed at ca. 1850 1800 Ma. Amalgamation of supercontinent may cause the formation of superplume and thermal insulation which can disturb mass distribution in mantle and alter the inertial gravity tensor of the Earth. A True Polar Wander (TPW) event may thus have taken place, which will move the cratons and the superplumes towards the equator. These conditions may be related to a reorganization of the whole mantle following a global magmatic quiescence between 2400 and 2200 Ma. / Um grande magmatismo intraplaca cobriu várias áreas (1.500.000 km2) do Cráton Amazônico há 1880 Ma, o qual define uma grande província ígnea (SLIP) chamada coletivamente de \"evento Uatumã\". O objetivo deste trabalho é estudar o paleomagnetismo e a petrologia dessas rochas para definir o contexto espaço-temporal do evento Uatumã e a posição do cráton Amazônico dentro do Supercontinente Columbia. Duas áreas de estudo foram escolhidas para a amostragem, localizadas no sudoeste do cráton Amazônico (Pará): (1) A região de Tucumã, onde 16 diques félsicos, 7 diques máficos, um dique de gabro e 3 sítios da granodioritos do embasamento Arqueano foram coletadas. (2) A região de São Felix do Xingu, onde 7 sitios de lavas riolíticas, 2 sitios de ignimbritos, um dique felsico e um de brechas vulcânicas da Formação Santa Rosa foram amostrados. Seis sitios da Formação Sobreiro (rochas vulcanoclásticas andesíticas) e um dique felsico da Suite Velho Guilherme foram também coletados. O estudo petrológico em amostras dos diques felsicos de Tucumã (1880.9 ± 6.7 Ma U-Pb zrn) mostra que eles representam um sistema de siques associado à Formação vulcânica Santa Rosa. A magnetização remanente dos diques felsicos é portada por magnetita PSD e hematita. A hematita é sin- a pós-magmática e a mineralogia magnética pode ser usada para quantificar esta alteração hidrothermal. Desmagnetizações AF, térmica, LTD + AF e LTD + térmica mostram uma componente característica com direção noroeste e inclinação positiva (Componente A) para amostras de 16 sítios, cuja direção média é Dm= 330.5, Im= 27.9 (N= 16, 95= 11.4, R= 14.7, k= 11.47). O pólo paleomagnético calculado com a média dos PGVs está localizado em 52.9°S, 76.4°E (A95= 10.4°, K= 13.52). Entretanto, esta componente parece ser decorrente de uma remagnetização, provavelmente ocorrida durante o final do Neoproterozoico. Outra componente (chamada de Componente B) foi também isolada para estas rochas, a qual foi associada a uma remagnetização regional ocorrida durante a formação da Província Magmática do Atlântico Central (PMAC). Ainda, uma Terceira componente (C), representada por direções sudoeste e inclinações positivas baixas foi isolada para amostras de alguns sítios. Esta componente foi interpretada como sendo relacionada ao evento magmático da Suíte Intrusiva Velho Guilherme com idade de ~1860 Ma. Os melhores resultados, entretanto, foram obtidos para a região de São Felix do Xingu. Dois novos polos paleomagnéticos, considerados de origem primária, foram encontrados para o Craton Amazônico: O polo SF1 (319.7°E; 24.7°S; N= 10; A95= 16.9°) foi obtido para rochas félsicas e andesíticas, as quais foram datadas em 1877.4 ± 4.3 Ma (U-Pb zrn, LA-ICP-MS), sendo que sua origem primária é embasada em um teste de contato cozido inverso. A investigação petrográfica mostra que o portador magnético desta componente é atribuído à hematita, formada por processos hidrotermais tardi- a pós-magmáticos. O polo SF2 (220.1°E; 31.1°S; N= 15; A95= 9.7°) foi determinado para a componente de magnetização revelada para o dique da Suíte Velho Guilherme, Esta componente é também encontrada como componente secundária em amostras das formações Santa Rosa e Sobreiro, além de algumas amostras de sítios coletados na região de Tucumã (Componente C). Uma idade de 1853.7 ± 6.2 Ma (U-Pb zrn, LA-ICP-MS) foi atribuída à componente SF2 e sua origem primária é confirmada pelo teste de contato cozido positivo realizado para este dique. Os polos SF1 e SF2 são bem discrepantes, embora a diferença de idade destes polos seja de apenas 25 Ma. Resultados similares têm sido obtidos para polos de mesma idade de outros blocos cratônicos (India, Superior (Laurentia), Slave (Laurentia), Kalahari, Baltica e Sibéria), os quais podem ser explicados por um evento de deriva polar verdadeira (DPV) ocorrido nesta época, em decorrência de uma reorganização do Manto. Esta época (1880 Ma) é marcada por uma alta atividade do Manto, a qual culminou com a formação do Supercontinente Columbia, por volta de 1850 1800 Ma. A formação de superplumas e o isolamento térmico causado pela consequente formação do Columbia podem ter sido causas de perturbações de densidades que alteraram o tensor inercial da Terra e, consequentemente, um evento de DPV pode ter deslocado os continente e as superplumas para a região do equador. Estas condições podem estar ligadas a uma inteira reorganização mantélica que seguiu um período de pouca atividade magmática, ocorrido entre 2400 e 2200 Ma.

Ph.D (Geology) / The Karoo succession has economic significance through the exploitation of extensive coal deposits and in recent years has seen significant international interest due to potentially large shale gas resources. The thermal history of sedimentary basins affects the genesis of hydrocarbon deposits and it is therefore essential to model and reconstruct the geothermal variation across the Karoo Basin before evaluation of the hydrocarbon resources can take place. The main scientific questions related to the thermal history of the Karoo Basin are whether the emplacement of large volumes of magma was preceded by a large-scale lowgrade thermal doming as proposed for continental rift settings. Alternatively, was the Karoo thermal event restricted to the contact aureole of intrusives, as well as the question whether the intrusion of dolerite resulted in large-scale CO2 or CH4 degassing from coalbeds and carbonaceous shales based on similarities to other large igneous provinces? Magnetic techniques provide an alternative to more traditional methods to study the geothermal history of sedimentary basins (such as illite crystallinity and vitrinite reflectance), which are often associated with significant uncertainty. Three experiments using existing magnetic and palaeomagnetic methods were conducted to determine the peak temperatures reached by Karoo sedimentary rocks before and after the Karoo magmatic event. These experiments include the classic palaeomagnetic baked contact tests (magnetostratigraphy), analyses of the variation of magnetic susceptibility during repeated progressive heating (alteration index method) as well the variation of relative concentrations of fine grained pyrrhotite and magnetite in sedimentary strata relative to their distance from an intrusive (pyrrhotite/magnetite geothermometer). Additionally various magnetic fabric analyses were performed including a study of the variation in anisotropy of magnetic susceptibility (AMS). Although these techniques were successful in delineating the extent of the contact aureoles, only the alternating index (A40) had the ability to give estimated peak temperatures. Results indicate a general elevation of palaeotemperatures of the organic-rich sedimentary rocks of the Ecca Group to temperatures where hydrocarbons are normally converted into gas. Importantly, it is clear from this study that the greatest thermal effects of the sill intrusions on the sedimentary strata are limited to the contact aureoles, suggesting that there is an, as yet unquantified, potential for hydrocarbon resources remaining between these intrusions. A general increase in the palaeotemperatures from southwest to northeast across the basin was observed. This is mainly due to differences in thermal conductivity of the various lithologies across the basin from tight low porosity marine shales in the south and southwest towards more lacustrine mudstone and porous sandstone in the northeast.

The Western European Variscan orogenic belt is thought to represent the final in a series of Paleozoic continental collisions that culminated with the amalgamation of the supercontinent Pangea. The Iberian segment of the Variscan belt is characterized by Cantabrian orocline, which is 180º and convex toward the west. Several lines of evidence are at odds with classical interpretation of the Cantabrian orocline as the core of the much larger ‘Ibero-Armorican’ arc, suggesting instead that it is structurally continuous with a second more southerly and complimentary orocline. Paleocurrent data collected from the Lower Ordovician Armorican Quartzite of the deformed Iberian Paleozoic passive margin sequence confirm the existence of the so-called Central Iberian orocline. Structural continuity between the Cantabrian and Central Iberian oroclines suggests that they formed contemporaneously and in the same fashion. Mesoscale vertical-axis folds deforming slaty cleavage and shear fabric within the Ediacaran Narcea Slates have a dominant vergence toward the hinge of the Cantabrian orocline, suggesting that its formation was in part accommodated by a mechanism of flexural shear during buckling of a linear belt in response to an orogen parallel principle compressive stress. The Cantabrian-Central Iberian coupled oroclines therefore palinspastically restore to an originally linear belt 2300 km in length. Provenance analysis of detrital zircons sampled from the Armorican Quartzite along a 1500-km-long segment of the palinplastically restored Iberian passive margin indicate that it originated in a paleogeographic position stretching east-west along the northern limits of north African Gondwana, from the Arabian-Nubian Shield to the Saharan hinterland. Paleomagnetic data and the distribution of Variscan ophiolites support a model of mid-Paleozoic separation of the Variscan autochthon (Armorican continental ribbon) from north Gondwana preceding or in conjunction with a 90º rotation required to reorient the ribbon to a Late Carboniferous north-south trend. Formation of the Iberian coupled oroclines accommodated 1100 km of orogen parallel shortening. The Western European Variscan belt, North American Cordillera, and Eastern European Alpine system are orogens similarly characterized by both coupled oroclines and paleomagnetic inclinations that are significantly shallower than cratonic reference values. Palinspastic restoration of the Alaskan and Carpathian–Balkan coupled oroclines fully resolves inclination anomalies within the Cordillera and Eastern Alpine system, respectively. Inclination anomalies within the Iberian Variscan belt are only partially resolved through palinspastic restoration of the Iberian coupled oroclines, but the sinuous geometry of the belt is not yet fully deciphered. Oroclines within the Western European Variscan belt, not the orogen itself, provide the true record of Pangean amalgamation. / Graduate