Le domaine allochtone de la chaîne Neoproterozoïque Araçuaí met en jeu de grandes quantités de magma, de la fusion partielle et un gradient thermique élevé, ce que caractérise cette chaîne comme un orogène chaud. La suite tonalitique Galiléia, mise en place dans des métasédiments et déformée à l'état magmatique, représente un énorme batholite qui a fortement influencé le comportement mécanique de la croûte moyenne. L'anisotropie de susceptibilité magnétique (ASM) mesurée à travers le batholite et utilisé comme proxy de la petrofabrique, associé à une étude de la minéralogie magnétique, a permit de définir le comportement paramagnétique de la suite Galiléia et de mettre en évidence une déformation complexe en 3D. Les structures développées dans le magma visqueux résultent d'une combinaison de tectoniques tangentielles induites par la compression, et de forces gravitaires découlant du poids de la croûte sus-jacente. La dynamique du batholite est compatible avec celles déjà décrites pour des roches ductiles d'orogènes chauds. Les datations U/Pb sur zircon et monazites et 40Ar/39Ar sur amphiboles, muscovites et biotites ont permit la caractérisation de l'évolution t hermique du batholite et de contraindre la durée de la déformation. Le batholite Galiléia s'est mis en place à ~580 Ma, au cours d'un important événement magmatique, tectonique et thermique. Les températures sont restées hautes durant les premiers ~50 Ma de l'évolution thermique, favorisant une déformation constante du batholite à l'état magmatique, pendant plusieurs dizaine de millions d'années. De telles hautes températures et une telle déformation stable durant de si longues périodes sont des caractéristiques qui semblent communes au orogènes chaud. Le refroidissement lent estimé à 10°C/Ma après ~500 Ma indique l'exhumation a été très lente, probablement due à l'érosion uniquement. / The allochtonous domain of the Neoproterozoic Araçuaí belt involves large amounts of magma, widespread partial melting, granulitic facies and high geotherm, characterising this belt as a hot orogen. The Galiléia tonalitic suite, emplaced within host metasediments and deformed at magmatic state, represents a huge batholith that strongly influenced the mechanical behaviour of this middle crust. The anisotropy of magnetic susceptibility (AMS) measured through this batholith and used as a petrofabric proxy, combined to a detailed magnetic mineralogy investigation, permitted to characterize the paramagnetic behaviour of the Galiléia suite and therefore to highlight a complex 3D strain deformation. The observed structures developed within the viscous magma resulted from a combination of tangential tectonics induced by the compression, and gravitational forces arising from the load of the overlying crust. The kinematics of the batholith is compatible with that already described for ductile rocks of hot orogens. U/Pb dating on zircons and monazites together with 40Ar/39Ar dating on amphiboles, muscovites and biotites permitted to define the thermal evolution of the Galiléia batholith and its host metasediments and constrain the timing of the deformation. The Galiléia batholith emplaced during an important magmatic, tectonic and thermal event at ~580 Ma. Temperature remained high during the first ~50 Ma of the thermal evolution, promoting a seemingly constant deformation of the batholith at magmatic state during several tens of millions years. Such high temperature conditions and stable deformation kinematics during protracted periods of time are supposed to be characteristic of hot orogen. The slow cooling rate of ~10°C/Ma evidenced after ~500 Ma probably indicate a very slow exhumation probably only conducted by erosion.

Two experimental seismic surveys were collected in the 1991 LITHOPROBE Trans-Hudson Orogen (THO) data acquisition program. The purpose of the coincident dynamite and vibroseis reflection surveys was to compare crustal images obtained using high-fold low-energy and low-fold high-energy sources. On single-fold field records, signal amplitudes from explosive sources are consistently 50 dB higher than on the corresponding vibroseis records. The vibroseis final stack exhibits better defined upper-crustal reflectivity due primarily to the higher fold. However, at lower-crustal and Moho levels, the dynamite data provides images which are equal or superior to those obtained from the vibroseis data. The dynamite source not only allowed deeper signal penetration but also succeeded in mapping of a number of subcrustal reflections not identified in previous vibroseis data. These new seismic images indicate that the crustal root is not simple depression on the upper-mantle as was inferred initially but a broad (3 s) zone of reflectivity that dips west and extends more than 10 km below the younger regional Moho. Moreover, the dynamite data also indicates that diffraction patterns, detected at lower crustal and Moho depths, have large apertures which permitted proper migration of these lower crustal events. Four vibroseis expanding spread profiles (ESP) were also acquired during the data acquisition program to obtain more detailed and accurate velocity structure. These profiles, with a maximum offset of 18 km, were centered on areas where prominent crustal reflectivity was detected by the regional vibroseis survey. The small source stepout distance (100 m) generated high-fold ($>$30) data. Extensive modeling was carried out to estimate the offset range within which each traveltime approximation and velocity analysis technique may be implemented. The results reveal that velocity estimation becomes more robust and accurate when crustal seismic surveys utilize longer offsets than commonly used. These larger source-receiver separations, however, must be generally limited to offset/depth ratios not exceeding 1.5 when conventional velocity analysis techniques, based on the hyperbolic moveout assumptions, are implemented. Besides the semblance method two velocity estimators adapted to crustal studies, namely the covariance and the $\tau$ - p techniques, were tried. The former yielded the highest resolution followed by the semblance and the $\tau$ - p methods. Resolution of the semblance estimator for a maximum offset of 36 km is equal to that of the covariance method with a corresponding offset of 18 km for mid-crustal reflectors. The advantages provided by the long-offset data acquisition include increased S/N ratio and a greater number of traces with sufficiently large moveouts whichimproved velocity resolution, especially below mid-crustal depths. To achieve similar advantages in a regional crustal reflection survey would require the adoption of longer spread lengths than those presently implemented in standard data acquisition procedures.

University, Diss., 2007--Frankfurt (Main). / Zsfassung in engl. und dt. Sprache.

Zugl.: Göttingen, Universiẗat, Diss., 1995.

In the Paleoproterozoic Trans-Hudson Orogen, a well exposed section of mid-crust on Hall Peninsula, southeastern Baffin Island, offers an opportunity to improve our understanding of mid-crustal tectonothermal processes in hot, collisional orogens. Additionally, more robust age constraints on the tectonic history of Hall Peninsula are important for plate tectonic reconstructions of the North Atlantic region. Recent mapping shows that the section comprises Archean crystalline basement overlain by Paleoproterozoic supracrustal rocks, which host felsic plutons on the western peninsula. There is a westward increase in peak metamorphic grade, from amphibolite- to granulite-facies, and three regional deformation events are recognized (D1, 2, 3). Equilibrium phase diagram modeling constrained by garnet compositions in pelite indicates peak conditions of ~720–740°C on the eastern peninsula and ~850°C further west, with pressures of ~6.25–7.35 kbar. Modeling and petrographical evidence suggest subsequent cooling, decompression, growth of retrograde biotite and, on the eastern peninsula, retrograde muscovite. In situ U-Pb monazite dating (~450 analyses) and U-Pb zircon depth profiling (~90 analyses) resolve the timing of regional metamorphism and crustal shortening between ca. 1860–1820 Ma, coincident with the accretion of crustal blocks and arc terranes during the amalgamation of the orogenic upper (Churchill) plate. Regionally-occurring ca. 1800–1750 Ma monazite domains and zircon rims are interpreted to result from fluid-assisted dissolution-reprecipitation. They likely record the terminal collision with the lower-plate Superior craton and post-orogenic thermal activity, possibly related to the emplacement of pegmatitic syenogranite dykes. The new data strengthen formerly tentative correlations with southern Baffin Island, West Greenland and northern Labrador. 40Ar/39Ar thermochronology on muscovite, biotite and phlogopite suggests that Hall Peninsula underwent slow cooling at rates of ~1–2.5ºC/Myr after peak metamorphism, remaining hotter than ~400°C until ca. 1670–1660 Ma. Analogous thermochronological ages from elsewhere in the Trans-Hudson Orogen imply orogen-wide slow cooling. Despite significant crustal thickening and elevated paleotemperatures, the Hall Peninsula crustal section does not record evidence of orogenic collapse, implying that it may not be a hallmark of all hot, thickened orogens.

No description available.

Petrographic and age analysis of sandstones, detailed structural analysis and gravity modeling were conducted to investigate the origin of the Gondwana Sequence in the Timor Region, and to better constrain the tectonic evolution of the active Banda Arc. Our field studies and U/Pb zircon age analysis helped assign most units to either Asian or Australian affinity. Detrital zircon from uplifted Banda forearc units (Asian affinity) have U/Pb ages as young as 80 Ma (Standley and Harris, in press). In contrast, analysis of detrital zircon from Gondwana Sequence sandstones accreted to the Banda Arc from Savu to East Timor are no younger than 234.6 ± 4.0 Ma, and have peak ages at 301 Ma and 1873 Ma with some Archean ages. These age constraints provide a reliable new application for distinguishing rocks units as Asian or Australian affinity. Petrographic and provenance analysis of Triassic Australian affinity greywacke units yield QFL abundances consistent with a proximal, syn-rift, intracratonic or recycled orogen source, from the northeast. The Mount Isa region to the east has the most similar peak U/Pb zircon ages to the Gondwana Sequence. However an extension of this terrane to the west, which would have rifted away during Jurassic breakup, is required to account for the immaturity of the sandstones. Structural measurements of Gondwana Sequence units accreted to the Banda Arc show a northwest - southeast paleo and current maximum stress direction, and vergence mostly to the southeast. Individual thrust sheets are 3 km thick and account for 50% total shortening. The deformational grain of Timor is a hybrid of the east-west strike of Banda Arc and northeast-southwest strike of incoming Australian continental margin structures. The Banda forearc, which is 200 km wide north of Savu, progressively narrows towards East Timor. In order to constrain the location of the forearc, three area-balanced structural models were tested against the gravity field of the Banda Arc. The best fit model requires internal shortening and under-stacking of the forearc beneath the arc, which may account for the cessation of volcanism and uplifted coral terraces north of East Timor.

Knowledge on lithosphere extensional mechanisms has greatly benefited from studies made both at presentday rifted margins and onshore fossil analogues. Nevertheless, the spatial and temporal evolution of the processes leading to continental break-up and oceanic crust formation remains poorly constrained. The Bay of Biscay and Pyrenees is used in this study as a natural laboratory to investigate the formation and reactivation of rift systems. A new offshore-onshore approach is developed and applied to identify, characterize and map the rift domains inherited from the Bay of Biscay opening and partly integrated into the Pyrenean orogen. This mapping reveals the complex architecture of European-Iberian plate boundary resulting from a strongly polyphased evolution. Several rift systems spatially distinct are preserved at different evolutionary stages. An important segmentation partially inherited from the pre-rift structuration controls the formation of the rift systems, an observation that has important implications for regional kinematic restorations. Several steps in compressional deformation can be distinguished and related to the rift inherited architecture. Reactivation is initiated in the exhumed mantle domain. Following the subduction of hyperthinned crust, continental collision processes are controlled by the proximal and necking domains acting as buttresses. These results emphasize the role of pre-rift inheritance for the spatial evolution of rift systems and the importance of the rift-related architecture to unravel the formation of collisional orogen.

Cette thèse présente une étude structurale qualitative et quantitative du socle paléozoïque des Pyrénées. Elle se base sur une étude de terrain et une compilation exhaustive et inédite des structures, harmonisées à l'échelle de toute la chaîne des Pyrénées. A partir de cette base de données, nous avons construit le champ de déformation régional varisque produit au Paléozoïque supérieur (310-295 Ma). On montre que lors du raccourcissement régional majeur, la déformation est partitionnée entre une croûte supérieure s'épaississant en régime transpressif et une croûte inférieure fluant latéralement. Une zone d' " attachement " a permis de maintenir la cohérence cinématique entre ces deux domaines aux comportements structuraux contrastés. Le fluage latéral au sein de la croûte inférieure permet l'emplacement de dômes extensifs dans ce contexte convergent. Le refroidissement progressif de la croûte favorise i) le couplage mécanique progressif entre ces deux domaines et ii) une localisation de la déformation dans des zones de cisaillement transpressives verticales d'échelle crustale. Bien que situées dans l'avant-pays de la chaîne varisque, les Pyrénées enregistrent une déformation typique d'une lithosphère anormalement chaude. Ce contexte thermique est l'expression d'une délamination progressive du manteau lithosphérique initiée dans les zones internes de l'orogène et qui a permis la fermeture de la syntaxe ibéro-armoricaine ainsi que l'échappement latéral de la croûte ductile pyrénéenne pris dans cette syntaxe. L'analyse quantitative du champ de déformation à partir d'outils géostatistiques montre que ce champ résulte d'une déformation assimilable à un cisaillement pur produit par un raccourcissement NS tandis que la composante dextre de la transpression se concentre aux bordures de plutons ou de dômes. Ainsi, la mise en place de ces objets structuraux gouverne l'hétérogénéité du champ de déformation. La cohérence du champ de déformation varisque met en question l'existence de nappes de socle lors de la construction du prisme orogénique pyrénéen à partir du Crétacé supérieur. En dehors de la bordure ouest et sud de la Zone Axiale, le socle n'est affecté que par des réactivations mineures, y compris dans les Pyrénées centrales où le chevauchement de Nogueres ne peut être enraciné. Ce chevauchement correspondrait plutôt à un décollement dans les séries paléozoïques supérieures, impliquant des quantités de raccourcissement moindres durant la collision pyrénéenne que celles calculées précédemment. / We present a qualitative and quantitative structural study of the Paleozoic basement of the Pyrenees. Based on new observations and an extensive compilation of structural data that were harmonized at the scale of the orogen, we built the regional-scale Variscan strain field. The main Variscan deformation was partitioned between transpressional thickening of the upper crust and the laterally flowing lower crust. An attachment zone acted to maintain the kinematic coherency between these two structural domains. Lateral flow of the lower crust accompanied emplacement of syn-convergence extensional domes. Regional cooling progressively increases the degree of mechanical coupling between the two structural levels and enhanced strain localization in steep regional transpressive shear zones. Though part of the foreland of the Variscan orogen the Pyrenees developed strain patterns typical of hot crusts/lithospheres. Such a hot thermal context is due to lithospheric mantle delamination that initiated beneath the hinterland of the orogen and allowed the Iberian-Armorican syntax to close and extrude the soft Pyrenean crust trapped in it. Geostatistical analysis of the strain field suggests that it results bulk pure shear deformation with a dextral transpressional component restricted to the boundaries of plutons and domes. Emplacement of such structural objects governed the degree of heterogeneity of the strain field. The coherency of the strain field argues against the stacking of large basement nappes in the Axial Zone of the Pyrenees during building of the orogenic prism since the Late Cretaceous. Only the southern and western parts of the Axial Zone are affected by "Alpine" thrusting and only minor local reactivation occurred elsewhere in the Axial Zone. Therefore, the Nogueres thrust cannot root into the Axial Zone but is a decollement within Late Paleozoic series. This suggests a lower amount of Alpine shortening in this part of the Pyrenees than previously estimated.

<p> The area of study is a small area of allochthonous rocks structurally located immediately above an Archean antiform in the Wopmay Orogen of the Northwest Territories. The rocks of the study area represent various units of the Akaitcho Group epicontinental rift fill deposits. All rocks are allochthonous, lying structurally above basement and autochthonous cover. </p> <p> Within the allochthonous rocks, three phases of deformation and a metamorphic culimnation are evident. The first episode (Dl) involved thrusting and folding of the allochthon over the Slave Craton, coinciding with peak metamorphic conditions. This phase of deformation was followed by an episode of coaxial, thick-skinned folding (D2), producing the large scale folds of basement and cover witnessed in the area. A late cross-folding event (D3), has provided up to 6 km of structural relief in the study area. The deformational history thus recorded in the Wopmay Orogen is similar to that documented in other orogenic belts such as the eastern Alps and Gape Smith Belt in Quebec. </p> <p> Metamorphism in the study area is hot side up Buchan type. A progression is observed in pelitic assemblages from muscovite-sillimanite grade to sillimanite-K feldspar grade, from east to west across the study area. A retrogression has produced sillimanite quartz pods (faserkiesel), its occurrence has permitted a relocation of the pre-existing prograde isograd. </p> <p> These finds are consistent with the current tectonic model for the area. </p> / Thesis / Bachelor of Science (BSc)