Deformation in southern Alaska is controlled by the accretion and partial subduction of the Yakutat terrane as margin-parallel motion transitions to subduction. Recent studies have shown that deformation in the St. Elias orogen, at the northern end of the terrane, accommodates a large portion of convergence, but deformation at the eastern and southern margins remains more poorly constrained. Rapid recent sedimentation (> 1cm/yr) and glacio-isostatic uplift rates (> 3 cm/yr) in the Fairweather corridor highlight short-term vertical deformation at the eastern margin; however, the relationship between these rates and long-term deformation is less well known. New low-temperature cooling ages are reported along the eastern flank of the St. Elias orogen, placing constraints on vertical deformation over the past few million years. Young cooling ages (< 3 Ma) occur in a broad zone, extending along the onshore length of the strike-slip Fairweather fault. These ages indicate that protracted convergence has been accommodated in the Fairweather corridor. Average (~1 mm/yr) and peak (~3 mm/yr) late Cenozoic exhumation rates are similar to rates to the north, and suggest that the orogen is actually boomerang-shaped in map view. If ~1 mm/yr exhumation has been steady, the onset of rapid exhumation is constrained to post-12 Ma, but likely occurred at 5 Ma with changes in climate and plate obliquity. Although cooling ages reveal no coherent regional pattern relative to known structures, they indicate the margin accommodates a significant component of pure shear and is slip-partitioned. The resolved magnitude of convergence in the Fairweather corridor also indicates that Yakutat terrane motion is rotated from Pacific plate motion, and likely requires significant slip on the Transition fault at the southern edge of the Yakutat terrane. Although million-year exhumation rates are rapid, they are slower than short-term rates related to deglaciation. / Master of Science

The influence of erosion on uplifting orogens has been demonstrated to be a primary force in landscape development. An understanding of fluvial erosion in mountain belts is fairly well documented, but the impact of glacial erosion is yet to be fully recognized. The uplift of the Chugach-St. Elias Mountains over the last 5-6 Ma under the influence of intense glaciation provides a unique setting to study the impact of glacial erosion on landscape development. The range has been built by rapid convergence (~5 cm/yr) of the Yakutat terrane with North America. Climatic forcing of northward-driven storms has created a disproportionate glacier distribution across strike, where extensive piedmont glaciers (low equilibrium line altitudes) cover the windward side of the range and small isolated glaciers (high equilibrium line altitudes) occupy the leeward side. If glacial erosion is greatest at the equilibrium line altitude, then glaciers will act as "buzzsaws" there to limit topographic development. Exhumation would therefore be expected to increase towards the coast. If glacial erosion is not dominant, exhumation would be expected to increase away from the coast towards the core of the range, where fault dip angles are high and deep crustal rocks are exposed. To determine the impact of long-term glacial erosion on exhumation of the Chugach-St. Elias Mountains, samples were collected along and across the strike of the range and analyzed by the apatite radiogenic helium (AHE) technique. Samples previously dated using the apatite fission track (AFT) method and located adjacent to our field area were also included in the analyses. The low-temperature sensitivity of these thermochronometers allows exhumation rates to be determined for shallow crustal depths. Both glacial and tectonic processes have influenced exhumation of the range. Exhumation rates increase to the south and east towards the collision zone, but coastal rates (0.36-2.5 mm/yr) are significantly higher than inland samples (0.038-0.24 mm/yr). These rates indicate that coastal glaciation plays a dominant role in landscape development and suggest that short-term erosion rates inferred from sediment yields are exaggerated. Although the exhumation rates are lower than expected, the correlation of exhumation patterns, glacier distribution, and equilibrium line altitude supports the "glacial buzzsaw hypothesis". / Master of Science

No description available.

En utilisant l'épitaxie par jets moléculaires à basse température, nous avons élaboré des couches de (Ge,Mn), contenant des nanostructures ferromagnétiques, sur deux types de substrats GaAs d'orientation (001) : des substrats GaAs « epiready » (échantillons « Ga-GeMn »), et des substrats encapsulés par de l'arsenic amorphe (échantillons « As-GeMn »). Dans les échantillons Ga-GeMn, nous obtenons la formation de nanocolonnes riches en Mn ; celles-ci sont parallèles entre elles, ou enchevêtrées, suivant la morphologie de surface initiale. Les mesures de magnétométrie révèlent deux phases magnétiques : les nanocolonnes ferromagnétiques avec une température de Curie de 150 K, et la matrice de germanium, rendue paramagnétique par la présence de Mn dilué. Les mesures de magnétotransport montrent que ces couches sont de type p, et révèlent un l'effet Hall anormal (AHE) et plusieurs contributions à la magnétorésistance : une magnétorésistance géante négative, à basse température, la magnétorésistante orbitale, parabolique, et une contribution supplémentaire à faible champ. Un calcul des propriétés de magnétotransport a été commencé en s'appuyant sur des hypothèses de la structure de bande entre les inclusions riches en Mn et la matrice semiconductrice de type p : celui-ci montre que la présence d'AHE dans les inclusions donne naissance à un AHE sur tout l'échantillon, mais aussi à un mécanisme de magnétorésistance qui rend compte de cette contribution (que nous appelons magnétorésistance Hall). Dans les échantillons As-GeMn, la diffusion de l'arsenic change le mode de croissance, avec une décomposition spinodale qui perd son caractère bidimensionnel pour devenir tridimensionnelle, avec la formation d'agrégats riches en Mn (température de Curie de l'ordre de 50 K) et d'agrégats de la phase ferromagnétique connue Ge3Mn5. La compensation entre Mn (accepteur) et As (donneur) gouverne les propriétés de transport. Dans les couches de type n, une forte anisotropie de la magnétorésistance est observée, dont nous montrons qu'elle est due à des effets de localisation faible. Une autre contribution à la magnétorésistance est observée, que nous suggérons d'attribuer à une magnétorésistance tunnel à travers la jonction Schottky qui se forme à l'interface entre les inclusions riches en Mn, qui sont métalliques, et le semiconducteur Ge de type n.

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No description available.