Electronic Thesis and Dissertation Repository


Doctor of Philosophy




Desmond Moser


The Vredefort and Sudbury impact basins in South Africa and Canada, respectively, are currently the world’s oldest and largest impact structures. Over a hundred years of study on both impacts has still not answered all the questions surrounding these sites. The 2019 Ma Vredefort structure is thought to have an original diameter of 300 km, however, due to erosion all that is left of the structure is the basement of the central uplift. Small pockets and dykes of melt rock still remain but in the case of the gabbronorite its origin remains to be proven. The Sudbury structure is 1850 Ma, with an original diameter of approximately 250 km, orogenic deformation has preserved the impact melt, including mafic-ultramafic inclusions found at the base of the melt sheet. The origin of these inclusions are still not fully understood. In this work, new light is shed on impact melting through detailed field mapping and application of new geochemical and micro-imaging techniques (e.g. FEG-SEM Electron Backscatter Diffraction, colour cathodoluminescence, SIMS and Laser ablation ICP-MS) to evaluate mineral assemblages and U-Pb dating minerals from both sites. At Vredefort, dykes and lenses of a gabbronorite body were studied and determined to be the age of the impact as well as Lu-Hf values in concordance with zircons formed from melting of the target material, however, the whole rock chemistry suggests a mantle origin for the melt. Zircons from the mafic-ultramafic inclusions in the Sudbury Sublayer were analysed for evidence of shock and found to have igneous-like textures and no planer or remodeling features. This suggests that the mafic-ultramafic inclusions formed at the time of impact. Both sites show strong evidence for late modification stage adjustment in the central uplift and crater floor, and raises questions about the crystallization and modification of impact basins. Further understanding of these processes and the microstructures formed during these events could lead to new bench marks for identifying old impact craters on Earth and for understanding crater dynamics on other stony bodies in the solar system.

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