M.Sc. (Geology) / Please refer to full text to view abstract

M.Sc. / Please refer to full text to view abstract

The mechanism of emplacement of ultramafic intrusions has been a controversial subject for many years. This thesis summarizes evidence favoring the intrusion of ultramafic rocks as magmas. Experiments were conducted to study the effect on the melting behavior or ultramafic rocks of carbon dioxide, sodium, sulphur and water at temperatures up to 925°C. and pressures up to 2125 atmospheres. No melts were produced but it is felt that the evidence presented justifies further work on the problem. / Applied Science, Faculty of / Mining Engineering, Keevil Institute of / Graduate

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The western Piedmont of Pennsylvania is underlain by the Octoraro and Peters Creek Formations, and these formations were juxtaposed during Late Paleozoic dextral strike-slip shearing. North of the shear zone, the Octoraro Formation contains evidence for two metamorphisms and deformations prior to strike-slip shearing, whereas south of the shear zone the Peters Creek Formation contains evidence for only one. The discordance in metamorphic and deformational history across the shear zone suggests the now juxtaposed rocks originated in different parts of the orogen. A minimum of 150 km of orogen parallel dextral offset is proposed for the shear system based on palinspastic reconstruction of the Cambrian-Ordovician shelf edge between northern Maryland and southeastern New York. The Peters Creek Formation consists of three lithofacies: 1) graded metasandstone beds, 2) meta-quartz-pelite, and 3) massive metasandstone lenses within the graded bedded sequences. The occurrence of interlayered greenstone in lithofacies 1) suggests rift related deposition. These rift clastics consist of two submarine turbidite-fan systems defined by thick sequences of interlayered feldspathic metasandstone and schist, separated by a region underlain of quartz-schist. Comparison of the Peters Creek Formation with known Iapetan rift clastics in the central Appalachians of Virginia suggests the Peters Creek deposits are also related to Iapetan rifting. Transpressional structural models have been applied to oblique convergence tectonics, with the coeval development of contractional and transcurrent structures. Late Paleozoic post-Taconian deformation in the north-central Appalachian Piedmont of Pennsylvania and Maryland is characterized by two stages of dextral transpression. Stage one comprises a map-scale ductile conjugate shear zone pair (the Rosemont and Crum Creek shear zones) that developed at amphibolite facies. These conjugate shear zones were later overprinted, during stage two, by greenschist facies dextral shear zones that flank broad upright antiforms. Conjugate shear-pair criteria were applied to these structures to constrain the paleo-principal compressive stress orientations. During stage one σ1 and σ3 were shallowly plunging, with σ2 steeply plunging. During stage two σ1 and σ2 were shallowly plunging, with σ3 steeply plunging. The structural evolution and associated change in stress array suggests unroofing during transpression, consistent with the decrease in metamorphic grade. Post-transpressional deformation produced a pair of conjugate cleavages in the lower Susquehanna River region, and determined orientations of the principal compressive stresses suggest Late Paleozoic extension, possibly related to gravitational collapse. Previously published orthogonal collision and tectonic assembly models for the north-central Appalachian Piedmont are incompatible with the new data. Oblique collision tectonics resulted in complex dextral transpressional deformation and large orogen parallel displacement of crustal blocks. Tectonic models that do not include the transpressional orogen component of the tectonic history should be seriously reconsidered. / Ph. D.

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Both the SW South China Sea and southern Beaufort Sea represent areas of extended continental crust, located on continental margins associated with oceanic spreading centres and zones of continent ocean transition (COT). Multichannel seismic reflection data are interpreted to characterize the COT in the SW South China Sea and the Southern Beaufort Sea. Based on the modelling and subsidence analysis, these two areas, SW South China Sea and southern Beaufort Sea, are compared with each other and the process of formation of hyper-extended crust in marginal oceanic basins can be conceptually modelled. It is noted that the initial weak thinning of the continental crust happened and the ductile middle/lower crust is coupled with the brittle upper crust. As extension continued, the continental crust is thinned down to ~10km, which is in accord with depth-dependent lithosphere thinning. Major crustal thinning is unlikely to result from brittle, high-angle normal fault in the upper crust. The degrees of lower crustal extension are so high and very high amounts of lower crustal extension, presumably achieved by ductile flow, would be required to have affected the crust within the COT. The seafloor spreading centre existed in the area adjacent to the research regions, so the extension within the COT occurred prior to the onset of seafloor spreading and the lower ductile flow is away from the continent and towards the oceanic crust. The interpretations require that the continental lithosphere prior to seafloor spreading must have been very weak given the evidence for significant lower crustal flow, inferred shallow depth of the brittle ductile transition and the fact that the COT continued to extend after the cessation of seafloor spreading.

published_or_final_version / Earth Sciences / Master / Master of Philosophy