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No description available.

The Rouyn-Noranda area in the center of the Abitibi subprovince is composed of comparatively unmetamorphosed Archean volcanics of Blake River Group (ERG). Rouyn-Nornada is one of the lozenge shaped structural blocks in the area bounded by the Porcupine Destor Fault (PDF) zone and the Larder Lake Caddilac Fault (LCF) zone. 160 samples were collected from 29 different sites selected to lie in the center of the block, at or near the PDF zone and accross the PDF zone in the neighboring lozenge. Stable magnetizations have been obtained from the central relatively unstrained parts of the lozenges. These magnetizations show some improvement in precision statistics after structural corrections implying that they are pre-folding and probably Archean in age. Sites along the PDF zone show scattered magnetizations, which do not agree at sample or site level. The Zone I (Sites in the center of the Rouyn-Noranda block) structurally corrected mean direction (D=154.0, I=-95.0, k=19.0, a95 =19.0, N=6 sites) differ from the Zone III (Sites from the center of the adjacent block) mean direction (D=214.0, I=-49.0, k=16.0, a95 =92.0) by 60+/-37° implying that the Rouyn-Norand block have rotated anti-clockwise by 60+/-37° about a vertical axis relative to the adjacent structural block. / Thesis / Master of Science (MS)

This thesis presents the results of a combined magnetic fabric and palaeomagnetic analysis of lower crustal rocks exposed in the Oman (Semail) ophiolite. This has long been an important natural laboratory for understanding the construction of oceanic crust at fast spreading axes and its subsequent tectonic evolution, but magnetic investigations in the ophiolite have been limited. Analyses presented here involve using: (i) magnetic anisotropies as a proxy for magmatic petrofabrics in lower crustal rocks in order to contribute to outstanding questions regarding the mode of accretion of fast-spread oceanic crust; and (ii) classical palaeomagnetic analyses to determine the nature of magnetization in these rocks and gain further insights into the regional-scale pattern of tectonic rotations that have affected the ophiolite. The extensive layered gabbro sequences exposed in the Semail ophiolite have been sampled at a number of key localities. These are shown to have AMS fabrics that are layer-parallel but also have a regional-scale consistency of the orientation of maximum anisotropy axes. This consistency across sites separated by up to 100 km indicates large-scale controls on fabric development and may be due to consistent magmatic flow associated with the spreading system or the influence of plate-scale motions on deformation of crystal mushes emplaced in the lower crust. Detailed analysis of fabrics in a single layer and across the sampled sections are consistent with either magmatic flow during emplacement of a melt layer into a lower crustal sill complex, or traction/drag of such layers in response to regional-scale stresses (e.g. mantle drag). Together, results support formation of the layered gabbros by injection of melt into sill complexes in the lower crust. New anisotropy data from the overlying foliated gabbros sampled at two key localities also provide insights into the style of melt migration at this crustal level. Fabrics are consistent with either focused or anastomosing magmatic upwards flow through this layer, reflecting melt migration beneath a fossil axial melt lens. Previous palaeomagnetic research in lavas of the northern ophiolitic blocks has demonstrated substantial clockwise intraoceanic tectonic rotations. Palaeomagnetic data from lower crustal sequences in the southern blocks, however, have been more equivocal due to complications arising from remagnetization. Systematic sampling resolves for the first time a pattern of remagnetized lowermost gabbros and retention of earlier magnetizations by uppermost gabbros and the overlying dyke-rooting zone. Results are supported by a positive fold test that shows that remagnetization of lower gabbros occurred prior to Campanian structural disruption of the Moho. NW-directed remagnetized remanences in the lower units are consistent with those used previously to infer lack of significant rotation of the southern blocks. In contrast, E/ENE-directed remanences in the uppermost gabbros imply a large, clockwise rotation of the southern blocks, of a sense and magnitude consistent with that inferred from extrusive sections in the northern blocks. Hence, without the control provided by systematic crustal sampling, the potential for different remanence directions being acquired at different times may lead to erroneous tectonic interpretation.