Cette recherche exploite une combinaison de techniques de terrain, paléomagnétiques et de magnétisme des roches pour corréler et localiser la source de plusieurs ignimbrites de deux régions de Turquie qui appartiennent à deux grands systèmes acides : la région du Cappadoce (Anatolie centrale), qui a été précédemment étudiée par plusieurs auteurs, et la région de Afyon-Eskişehir (Anatolie occidentale), jusqu'à présent peu étudiée. L’examen de l’ignimbrite de Kızılkaya (région du Cappadoce), d'âge Miocène supérieur, a consisté en une étude stratigraphique et de magnétisme des roches de l'unité ignimbritique, qui est considérée comme une seule unité d’écoulement et de refroidissement. La magnétisation rémanente, la fabrique magnétique (AMS, AIRM et AARM) et la minéralogie magnétique ont été analysées pour 35 sites de 7 localités situées à des distances différentes de la source supposée par Le Pennec et al. (1998) et à différentes hauteurs stratigraphiques, pour un total de 444 spécimens. La signature magnétique rémanente n’est pas verticalement homogène dans les dépôts. Deux cas sont distingués : (1) une TRM stable, dont la direction est conforme aux données de la littérature précédente (Piper et al., 2002) et (2) deux composantes de magnétisation caractérisées par une température de blocage et des spectres de coercivité se recouvrant. Ces situations ont été enregistrées respectivement en présence d'un Ti-magnétite primaire, et en présence simultanée de Ti-magnétite et de phases magnétiques secondaires (Ti-magnétite altérée et / ou hématite). La fabrique AMS varie verticalement dans le dépôt, en fonction de l'angle entre la direction de la linéation magnétique K1 et du pôle de la foliation magnétique K3, trois types de fabriques ont été caractérisées : normale, oblique et transversale. Après avoir démontré l’origine primaire de la fabrique magnétique, l'AMS a été accentuée par élimination des échantillons caractérisés par des valeurs de densité différentes de plus de ±} 1σ de la valeur moyenne du site. Cette opération s'est traduite par l'élimination des fabriques obliques, qui sont interprétées par la présence locale de fragments de ponces et clastes lithiques dans les échantillons. Les données d'AIRM et AARM indiquent que la Ti-magnétite MD est le principal minéral magnétique porteur de la fabrique, ce que suggère que la fabrique magnétique peut être utilisée pour déterminer les directions d'écoulement. L’examen de la succession ignimbritique d’âge Miocène précoce-Miocène supérieur, exposée sur une zone de ≈14 300 km2 dans la région d'Afyon-Eskişehir, a consisté en une étude stratigraphique-sédimentologique et de magnétisme des roches, afin de localiser la source des ignimbrites en combinant les données du terrain avec les directions de fabrique magnétique. La mesure des sections stratigraphiques, qui ont été réalisées pour 76 localités, conduit aux corrélations stratigraphiques et à la production de cartes d'isopaques et d'isoplètes. La succession ignimbritique est constituée d'au moins trois éruptions majeures distinctes qui ont produit les ignimbrites de Incik, Sabuncu et Seydiler, différentes en âge, distribution, structure et caractéristiques texturales des dépôts. L’étude magnétique consistait en analyses de la minéralogie magnétique et de la fabrique magnétique (AMS et AIRM), qui a été déterminée pour 22 localités sur un total de 36 sites à différentes hauteurs stratigraphiques, soit 600 spécimens. Les données de terrain et les résultats de la fabrique magnétique indiquent deux zones sources. La source des ignimbrites de Incik et Sabuncu est identifiée dans la caldeira de Kırka, une vaste zone de ≈ 20 x 20 km de diamètre, où la présence d'un dôme résurgent a été détectée. La source de l'ignimbrite de Seydiler est identifiée à proximité du village de Bayat. (...) / This research exploits a combination of field, paleomagnetic and rock-magnetic techniques to correlate locate the source and investigate several ignimbrites from two selected regions of Turkey, belonging to two large silicic ignimbrite systems : the well-investigated Cappadocia region (central Anatolia) and the poorly studied Afyon-Eskişehir region (western Anatolia). Investigation of the Upper Miocene Kızılkaya ignimbrite (Cappadocia region) consisted in a stratigraphic rock-magnetic study of the ignimbrite unit, which is considered to be a single flow and cooling unit. Remanent magnetization, magnetic fabric (AMS, AIRM and AARM) and magnetic mineralogy investigations were performed at 35 sites in 7 areal distributed localities at different distance from the vent position inferred by Le Pennec et al. (1998) and different stratigraphic heights, for a total of 444 specimens. Magnetic remanences are not vertically homogenous through the deposits. Two cases are distinguished: (1) a stable TRM, whose direction is consistent with previous literature data (Piper et al., 2002) and (2) two magnetization components with overlapping blocking temperature and coercivity spectra. These situations have been referred respectively to the occurrence of only primary Ti-magnetite, and primary Timagnetite plus secondary magnetic phases (alterated Timagnetite and/or hematite). The AMS fabric varies vertically throughout the deposit; based on the angle between the direction of the magnetic lineation K1 and that of the foliation plunge K3, three types of fabric are evidenced: normal, oblique and transverse. After have attested a primary origin of the magnetic fabric, the AMS was firstly enhanced by discarding all specimens whose density values differ more than +/- 1σ from the site mean value. This resulted in the elimination of the oblique fabrics, which are interpreted as an orientation disturbance due to local occurrence of pumices and lithic clasts in the specimens. Measurement of the AIRM and AARM pointed out that MD Timagnetite is the main carrier of the fabric, which suggests that the magnetic fabric is a reliable proxy for flow directions. Investigation of the Early-Upper Miocene ignimbrite succession exposed in the region of Afyon- Eskişehir consisted in a stratigraphic-sedimentologic and rock-magnetic study of the ignimbrite deposits over a ≈14,300 km2 extended area, in order to locate the source by combining field data and magnetic fabric flow directions. Measurement of the stratigraphic sections, performed at 76 distributed localities, led to ignimbrite correlations and production of isopach and isopleth maps. The ignimbrite succession consists of at least three distinct eruptions that originated the Incik, Sabuncu and Seydiler ignimbrites, different for age, areal distribution, structure and textural features of the deposits. Magnetic investigation consisted in magnetic mineralogy and magnetic fabric analyses (AMS and AIRM), was performed at 22 distributed localities for a total of 36 sites at different stratigraphic heights and 600 specimens. Both field data and magnetic fabric results concur for two source areas. The source area of the Incik and Sabuncu ignimbrites is identified with the Kırka caldera, a ≈20 x 20 km extended area where the presence of a resurgent dome has been detected; source area of the Seydiler ignimbrite is identified in the proximity of the village of Bayat. Volume calculations based on field data pointed out a VEI index of 7 for the Incik and Seydiler ignimbrite, 6 for Sabuncu ignimbrite, evidencing Plinian eruptions; the correspondent magnitude M (Pyle, 2000) is greater than 7 for each ignimbrite.

The East African Rift System diverges in the Lake Natron Basin of Northern Tanzania and is a major zone of continental extension and crustal thinning with resulting in active tectonics and volcanism. The discovery of Acheulean technology in Olduvai Gorge and Peninj as well as the presence of significant volcanic centers, has made in the region subject to studies in various disciplines. However, lack of precise radiometric age constraints due to the complex geology of the region is a major drawback. The basin is bordered on the western side by an escarpment that contains thick sequences of volcanic (nephelinites, basanites, hawaiites, alkali basalts), volcaniclastic and lacustrine strata that predates 1.2 Ma. This thesis is based on 41 rock samples that were collected from two geological sections, the Endukai Kete (EK) and Waterfall (WF) sections and aims to establish a preliminary geomagnetic polarity time scale (GPTS) for the Natron Escarpment, together with establishing possible flow directions of the volcanic lavas within these sections. Nephelinites of EK section have an inferred NW-SE direction of flow, based on study of anisotropy of magnetic susceptibility. They record a normal polarity that most likely correspond to the Cobb Mountain Event (CMT; 1.187-1.208 Ma), although there is an 80-ka discrepancy between the CMT event and the dated lavas. The most probable source is the Mosonik that erupted nephelinitic lavas 1.28 Ma ago. The palagonitic tuff layer below the nephelinites displays reverse polarity and a NE-SW direction of flow. Due to the absence of approximately 200 m strata within the basanite series of the section, regional lithological correlation is used to constrain the GPTS pattern. Hajaro Beds of the Peninj Group to the north of the escarpment, postdates the Olduvai Event (1.71 to 1.86 Ma) and lacustrine strata of the escarpment for EK and WF sections are deposited over the same unconformity and share depositional similarities. Therefore, the lacustrine strata are correlative to Hajaro beds and the normal event observed within the basanite series of both sections is attributed to the Réunion Event (2.116 – 2.137 Ma). The establishment of a preliminary magnetostratigraphic sequence presented in this thesis demonstrate that the rift escarpment in northern Tanzania is suitable for paleomagnetic dating. Future studies should be conducted to establish a more detailed and constrained magnetostratigraphic section, which will be of great use in this part of the African Rift where radiometric dating has been challenging.

Thesis. 1976. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Vita. / Includes bibliographies. / Ph.D.

Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Vita. / Bibliography : leaves 223-233. / Ph.D.

Paleomagnetic data were collected from Miocene volcanic rocks in the Turtle Mountains, Clipper Mountain, Colton Hills, and Piute Range of the southern Basin and Range (SBR) province in southeastern California as well as in the Soledad Mountains of the Mojave block in southern California. The data from these two tectonic provinces yield significantly different paleomagnetic directions, which probably indicates the existence of a major crustal and /or lithospheric discontinuity in the area between the Barstow Basin and the Clipper Mountain. Comparing the mean direction from the SBR data to the Miocene expected direction indicates no statistically significant rotation (R = -0.2° ± 18.2°) or flattening (F = -6.5° ± 9.2°). A similar comparison for the Soledad Mountain data, which were combined with data of Burke et al. (1982) from the Barstow Basin, yields a significant rotation of -43.5° ± 12.9° and flattening of 19.3° ± 10.6° for the Mojave block. These Mojave block values may be exaggerated a few degrees due to inadequate averaging of secular variation and possible improper structural corrections.

No description available.

O magmatismo Penatecaua no Estado do Pará faz parte da Província Magmática do Atlântico Central (CAMP) cuja formação precedeu a ruptura do Pangea, o mais recente supercontinente. Nos arredores das cidades de Medicilândia, Placas, Rurópolis, Monte Alegre e Alenquer afloram uma grande quantidade de soleiras de diabásio e diques observados ao longo de estradas. Idades 40Ar/39Ar recentes atribuem a idade de ~ 201 Ma para o magmatismo CAMP nessa região. Estudo paleomagnético nessas intrusões foi realizado em trinta sítios de amostragem nas cinco áreas acima citadas. A magnetização remanente característica dos sítios estudados é de polaridade normal e foi identificada através de desmagnetizações por campos magnéticos alternados e térmicas e interpretada como termorremanente, adquirida na época do resfriamento da rocha. Os portadores magnéticos são magnetitas ou titano-magnetitas com baixo conteúdo de Titânio. O polo paleomagnético calculado com base em 20 sítios de amostragem e incorporando dados da literatura para o mesmo magmatismo, situa-se a 78 ºN 85.1 ºE (N = 25; 95 = 4.2; k = 47.78) e é coerente com polos de outras ocorrências ígneas da América do Sul já identificadas como pertencentes ao evento CAMP. Essa similaridade de resultados reforça as observações de que o CAMP na América do Sul foi um evento muito rápido e isso é notado claramente no registro magnético das rochas do Penatecaua. / The Penatecaua magmatism occurring in northern Brazil (Pará State) belongs to the so-called Central Atlantic Magmatic Province (CAMP), which preceded the Pangaea breakup. Near the cities of Medicilândia, Placas. Rurópolis, Monte Alegre e Alenquer diabase sills and dikes outcrop along roads. Recent 40Ar/39Ar dating give an age of ~ 201 Ma for the CAMP magmatism in the area. The paleomagnetic study on 20 sampling sites of the Penatecaua magmatism, revealed stable characteristic magnetization components through alternating magnetic field and thermal demagnetizations. The main magnetic carriers are magnetite or low-Ti titanomagnetites. The paleomagnetic pole based on the studied sites and incorporating more data from the literature for the same magmatism is located at 78 ºN 85.1 ºE (N = 25; 95 = 4.2; k=47.78) and is coherent to other South American CAMP paleomagnetic poles for which good age control is available. This similarity of results reinforces the observations that the CAMP in South America was a very fast event and this is noticed in the magnetic record of the Penatecaua rocks.

La géologie du bloc de Chine du Sud se caractérise par un important magmatisme mésozoïque dont la genèse des magmas, la mise en place des plutons et les contextes tectoniques et géodynamiques restent très disputés. Dans ce travail, 41 plutons d'âge Jurassique ont été visités. Ceux de Qitianling et Shibei ont été choisis pour des études détaillés. Les textures des granites sont magmatiques et dépourvues de déformation post-solidus. La modélisation gravimétrique révèle que le pluton de Qitianling est un lopolithe mis en place dans l'interface que représente la discordance dévonienne. Le pluton de Shibei est formé par accrétion de dykes le long d'une faille verticale. Les mesures des fabriques dans différents plutons montrent que la tectonique régionale ne contrôle pas leur mise en place. La mise en place du magma est contrôlée par sa viscosité et des structures héritées. L'étude paléomagnétique de deux plutons du même âge indique des mouvements relatifs ce qui implique que ce bloc ne peut pas être considéré comme rigide. Nos résultats montrent que le Jurassique était une époque de calme tectonique dans le secteur d'étude. Ces granites sont issus de la fusion de la croûte inférieure. Au Trias, les événements tectoniques qui ont épaissi la croûte continentale sont responsables du magmatisme jurassique. L'important contraste de rigidité du bloc de Chine du Sud est responsable de la localisation de la déformation dans sa partie SE. L'épaississement crustal contribue aussi à élever le flux de chaleur mais les magmas issus du manteau sont rares. Ainsi, les modèles de subduction océanique ou de rifting continental ne s'accordent pas avec nos résultats. / The Mesozoic geology of the South China Block (SCB) is marked by widespread magmatism. Disputes exist on magma genesis, tectonic and geodynamic contexts. The tectonic setting of the Jurassic magmatism is poorly investigated. 41 Jurassic plutons were investigated and the Qitianling and Shibei plutons were selected for detailed studies. Our study shows that the emplacement of J plutons is not related to regional tectonics. The granite textures are magmatic, devoid of post-solidus deformation. The magma emplacement is controlled by its buoyancy and the inherited structures or stratigraphic interfaces of country rocks. Gravity modeling reveals that the Qitianling pluton is a lopolith intruded in the stratigraphic uniformity and the Shibei pluton formed by dike-accretion intruded in a vertical tectonic structure. The fabric measurements on different Jurassic plutons show that the regional tectonics do not control their emplacement. Paleomagnetic results from the two contemporaneous plutons show a relative movement, implying that this block cannot be considered as a rigid one. Our results show that the Jurassic was a tectonically quiet epoch in our study area. There granites are mainly derived from partial melting of lower crust. The Triassic subduction-collison events that have trickened the continental crust are responsible for the Jurassic magmatism. Due to the high rigidity contrast of the SCB, the deformation was localized on its SE part. The crustal thickening may increase the thermal contribution of the mantle, but few mantle materials are observed. So, the slab subduction or intracontinental rifting models are not supported by our studies.

This research exploits a combination of field, paleomagnetic and rock-magnetic techniques to correlate locate the source and investigate several ignimbrites from two selected regions of Turkey, belonging to two large silicic ignimbrite systems : the well-investigated Cappadocia region (central Anatolia) and the poorly studied Afyon-Eskişehir region (western Anatolia). Investigation of the Upper Miocene Kızılkaya ignimbrite (Cappadocia region) consisted in a stratigraphic rock-magnetic study of the ignimbrite unit, which is considered to be a single flow and cooling unit. Remanent magnetization, magnetic fabric (AMS, AIRM and AARM) and magnetic mineralogy investigations were performed at 35 sites in 7 areal distributed localities at different distance from the vent position inferred by Le Pennec et al. (1998) and different stratigraphic heights, for a total of 444 specimens. Magnetic remanences are not vertically homogenous through the deposits. Two cases are distinguished: (1) a stable TRM, whose direction is consistent with previous literature data (Piper et al., 2002) and (2) two magnetization components with overlapping blocking temperature and coercivity spectra. These situations have been referred respectively to the occurrence of only primary Ti-magnetite, and primary Timagnetite plus secondary magnetic phases (alterated Timagnetite and/or hematite). The AMS fabric varies vertically throughout the deposit; based on the angle between the direction of the magnetic lineation K1 and that of the foliation plunge K3, three types of fabric are evidenced: normal, oblique and transverse. After have attested a primary origin of the magnetic fabric, the AMS was firstly enhanced by discarding all specimens whose density values differ more than +/- 1σ from the site mean value. This resulted in the elimination of the oblique fabrics, which are interpreted as an orientation disturbance due to local occurrence of pumices and lithic clasts in the specimens. Measurement of the AIRM and AARM pointed out that MD Timagnetite is the main carrier of the fabric, which suggests that the magnetic fabric is a reliable proxy for flow directions. Investigation of the Early-Upper Miocene ignimbrite succession exposed in the region of Afyon- Eskişehir consisted in a stratigraphic-sedimentologic and rock-magnetic study of the ignimbrite deposits over a ≈14,300 km2 extended area, in order to locate the source by combining field data and magnetic fabric flow directions. Measurement of the stratigraphic sections, performed at 76 distributed localities, led to ignimbrite correlations and production of isopach and isopleth maps. The ignimbrite succession consists of at least three distinct eruptions that originated the Incik, Sabuncu and Seydiler ignimbrites, different for age, areal distribution, structure and textural features of the deposits. Magnetic investigation consisted in magnetic mineralogy and magnetic fabric analyses (AMS and AIRM), was performed at 22 distributed localities for a total of 36 sites at different stratigraphic heights and 600 specimens. Both field data and magnetic fabric results concur for two source areas. The source area of the Incik and Sabuncu ignimbrites is identified with the Kırka caldera, a ≈20 x 20 km extended area where the presence of a resurgent dome has been detected; source area of the Seydiler ignimbrite is identified in the proximity of the village of Bayat. Volume calculations based on field data pointed out a VEI index of 7 for the Incik and Seydiler ignimbrite, 6 for Sabuncu ignimbrite, evidencing Plinian eruptions; the correspondent magnitude M (Pyle, 2000) is greater than 7 for each ignimbrite.

Les cratères d'impact hypervéloces sont les structures morphologiques les plus abondantes à la surface des corps planétaires telluriques du système solaire, sauf sur Terre où ils sont effacés par les processus de surface. La structure interne des cratères d'impact de type complexe ne peut être étudiée en détail que sur Terre par des études géophysiques et géologiques de terrain. De telles approches - combinées à de la modélisation - peuvent révéler comment le processus de cratérisation, la composition des roches cibles, l'érosion et d'autres processus post-impact peuvent conduire aux anomalies géophysiques observées, qui peuvent également être détectées par des données satellitaires sur d’autres planètes. La cartographie du champ magnétique, les mesures gravimétriques, les sondages électromagnétiques (EM34), les analyses paléomagnétiques, le magnétisme des roches et les techniques pétrographiques sont utilisées. Pour la première fois, nous révélons que la structure de Tunnunik récemment découverte présente des anomalies de gravité négative et de champ magnétique positif typiques, ce qui nous aide à reconsidérer l'étendue de la fracturation dans les roches cibles. La structure d’Haughton, moins érodée que Tunnunik, montre des signes d'une aimantation augmentée au centre de son soulèvement central, ce qui est causé par l’altération hydrothermale induite par l’impact. Le paléomagnétisme aide à contraindre les âges différents des deux impacts des lacs à l’eau claire au Québec. Ce travail a des implications importantes pour notre compréhension du processus de cratérisation dans le système solaire, notamment en ce qui concerne l'étude des surfaces planétaires. / Hypervelocity impact craters are the most abundant morphologic features on rocky planetary bodies of the solar system, except on Earth where they are erased by surface processes. The internal structure of complex impact craters can only be studied on Earth by using ground-truth geophysical and geological studies. Such approaches - combined with modeling - can reveal how impact cratering, target geological composition, erosion and other post-impact processes can lead to the observed geophysical anomalies, which could also be detected by remote geophysical data on other planetary surfaces. Magnetic field mapping, gravimetry measurements, electromagnetic soundings (EM34), paleomagnetic analyses, rock magnetism and petrography techniques are used. For the first time, we reveal that the recently-discovered Tunnunik impact structure has typical negative gravity and positive magnetic field anomalies, which help us to reconsider the brecciation extent in the target rocks. The Haughton crater, which is less eroded than Tunnunik, shows evidence for an enhanced-magnetization in the core of the central uplift, caused by impact-generated hydrothermal alteration. Paleomagnetism helps to constrain the different ages of the East and west clearwater lake impacts. This work has important implications for our understanding of impact-cratering in the solar system, especially concerning the study of planetary surfaces.