Mineralised shear zones in Otago are often truncated by regional low-angle faults, which juxtapose schist of different metamorphic grade. The Footwall Fault and the Hyde-Macraes Shear Zone are one example for this kind of tectonic setting, and are the subject to this study. Although, the mechanisms for the development of the mineralised thrust-origin shear zones are well studied, the relationship to the truncating faults is still poorly understood. Currently, the truncating low-angle faults are assumed to be related to crustal extension, starting in the early Cretaceous after the schist passed the ductile-brittle transition. This study presents new kinematic data for the Footwall Fault, suggesting development of normal sense movement under ductile conditions due to an abundant shear band cleavage in the footwall, which dynamically recrystallises quartz grains. However, brittle high-angle normal faults truncating these shear bands indicate either reactivation of normal sense movement after passing the ductile-brittle transition or continuous normal sense movement during the transition. Furthermore, this study presents a model, which suggests a regional scale rolling hinge development, consisting of an array of individual low-angle normal faults along the boundary of the textural zone change from TZ IV to TZIII, and strike-parallel high-angle faults at the NE margin of the Otago schist.

Mineralised shear zones in Otago are often truncated by regional low-angle faults, which juxtapose schist of different metamorphic grade. The Footwall Fault and the Hyde-Macraes Shear Zone are one example for this kind of tectonic setting, and are the subject to this study. Although, the mechanisms for the development of the mineralised thrust-origin shear zones are well studied, the relationship to the truncating faults is still poorly understood. Currently, the truncating low-angle faults are assumed to be related to crustal extension, starting in the early Cretaceous after the schist passed the ductile-brittle transition. This study presents new kinematic data for the Footwall Fault, suggesting development of normal sense movement under ductile conditions due to an abundant shear band cleavage in the footwall, which dynamically recrystallises quartz grains. However, brittle high-angle normal faults truncating these shear bands indicate either reactivation of normal sense movement after passing the ductile-brittle transition or continuous normal sense movement during the transition. Furthermore, this study presents a model, which suggests a regional scale rolling hinge development, consisting of an array of individual low-angle normal faults along the boundary of the textural zone change from TZ IV to TZIII, and strike-parallel high-angle faults at the NE margin of the Otago schist.

Thesis (M. Sc.)--University of Canterbury, 2007. / Typescript (photocopy). "September 2007." Includes bibliographical references (leaves 83-89). Also available via the World Wide Web.

The recognition of fault-perturbed stress fields is an important tool in areas of mineral or hydrocarbon exploration. The Bowie underground coal mines of western Colorado expose a reverse-reactivated growth fault that perturbed the stress field during cleat (fracture) formation, rotating cleat orientations up to 500 m on both sides of the fault. Two unusual fracture types are found only in coal adjacent to the fault: (1) concentric cleats, highly curved fractures that form blocks resembling balls or eggs and (2) horsetail cleats, striated surfaces that superficially resemble shattercones and result from shear failure in coal. Numerical models created with the boundary element program Poly3D were used to estimate the magnitudes and orientations of the paleostress axes during cleat formation, taking into account the depth of burial, 3-D fault orientation, elastic rock parameters, and far-field stress states. When the elastic rock parameters and modeled orientations of the stress axes are held constant, the relative stress ratio, R = (σ_1 — σ_2)/(σ_2 — σ_3), uniquely determines the orientations of fractures forming in the fault-perturbed stress field. Comparison of the models with systematic observations on both sides of the fault allows the selection of a best-fit model. If the depth of overburden during fracture formation is known, this technique can be used to estimate the magnitude of σ_1 in fault-perturbed areas. The rotated face cleats and unusual, fault-related cleat types provide unequivocal evidence of a fault when (1) the fault predates cleat formation and (2) the far-field horizontal stress during cleat formation is oblique to fault strike. In addition, the varying spatial association of these fault-perturbed cleat styles with the fault may provide a qualitative estimate of fault location while mining. Pre-existing faults also strongly control reactivation-related folding, which at formed several low-amplitude folds, including a footwall fold. Igneous sills in three Bowie coal seams show strong preference for fault zones. The recognition of similar fracture trends in other mining or exploration areas is a valuable tool that may significantly reduce economic or human cost by helping to mitigate fault-related hazards and highlighting potentially productive zones in faulted reservoirs.

La formation des gisements métallogéniques en général et uranifères en particulier, dans les bassins paléoprotérozoïques, dépend de la migration des fluides riches en divers éléments (U, Cu, Fe, etc.). L’objet de cette thèse a été de définir le contexte tectonique, sédimentologique et diagénétique associées aux gisements bitume--‐uranium du bassin de Franceville. L’étude tectonique réalisée met en évidence des failles de transfert N180--‐170, héritées de la tectonique archéenne et des failles normales longitudinales N110--‐120. Ces deux familles de failles compartimentent le bassin de Franceville en plusieurs sous--‐bassins de subsidence variable. Les failles longitudinales N110--‐120° contrôlent la mise en place d’anticlinaux de mur et des synclinaux de toit synsédimentaires (i.e. discordances progressives). Les gisements d’uranium du bassin de Franceville, se localisent au niveau des anticlinaux de mur des failles normales. L’étude sédimentologique du bassin caractérise la distribution spatiale des paléoenvironnements de dépôt. Quatre grands environnements de dépôts sont respectivement mis en évidence : fluviatile (formation FA inferieur), deltaïque (formation FA moyen), tidal (formation FA Supérieur) et marin profond (formation FB). La distribution des facies sédimentaires à la transition FA--‐FB est responsable de la mise en place de barrières de perméabilité. Les barrières de perméabilité sont responsables de l’augmentation de la pression fluide, qui favorise la mise en place des structures de surpression fluide (dykes, stylolites, veines de quartz), au voisinage des anticlinaux de mur contrôlés par les failles normales. Les différences de pression dans le bassin favorisent la migration des fluides uranifères et des hydrocarbures, des zones profondes du bassin vers les anticlinaux de mur. Les structures de fracturation hydraulique vont contrôler la mise en place des bitumes et des minéralisations d’uranium associées. / Metallogenic deposits within paleproterozoic basins depend on generation and migration of fluids. The aim of this study is to provide a better understanding of tectonic, sedimentological and diagenetic setting of the uranium deposits in the Franceville basin and to characterize hydraulic fracturing impact on fluid migration processes in sandstone reservoirs.Tectonic study define the N180-170° transfer faults, associated with Archean tectonic and the N110-120° longitudinal normal faults. These two fault directions split the Franceville basin into small sub-basins. The longitudinal normal faults are associated with footwall anticlines and hanging wall synclines. The uranium deposits of Franceville basin are located in footwall anticlines of longitudinal normal faults.Sedimentological analysis allows to describe four depositional environments: Fluvial (lower FA), deltaic (middle FA), tidal (upper FA), and open marine environments (FB). Facies distribution in the FA-FB transition promotes the establishment of permeability barriers. These latter are responsible of the increase in fluid pressure and of the formation of fluid pressure structures (dykes, stylolites, quartz veins), in footwall anticlines of longitudinal normal faults. Increase in fluid pressure allows the migration of uranium-fluids, and hydrocarbon from the deep basin to the footwall anticline. Hydraulic fracturing processes lead the precipitation of uranium mineralization, associated with bitumen, in microfractures.

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