D.Phil. / The amalgamation of SW Gondwana after the break-up of Rodinia supercontinent during the Neoproterozoic-early Palaeozic was one of the most active tectonic periods of the earth history and its geological evolution remains controversial. Recently, diverse hypotheses such as mantle plume activity, orthogonal continent-continent and strike-slip collisions according to different models try to explain the complex evolution of the Pan-African Brasiliano orogens and the associated sedimentary basins. In order to get insight of the SW Gondwana reconstruction, provenance analyses were performed on two Neoproterozoic-early Palaeozic sedimentary units: (1) the Arroyo del Soldado Group representing a —5000 meter thick platform succession unconformably overlying the mainly Archaean to Neoproterozoic rocks of the Rio de la Plata Craton in Uruguay and, (2) the Nama Group, a —2000 meter thick shallow marine to fluvial deposit interpreted as a foreland basin in response to tectonism in the adjacent northern Damara and western Gariep Orogenic Belts and unconformably overlying the mainly Mesoproterozoic rocks of the Kalahari Craton in Namibia. Several techniques including petrography, heavy mineral analysis, geochemistry, Sm-Nd isotope analysis and zircon dating were applied to both sedimentary basins. The petrographic, heavy mineral analyses and geochemical results from the Nama Group indicate a recycled upper crust composition characterized by metamorphic and granitic sources and minor mafic rocks. Palaeocurrent analyses of the chromian spinet bearing sandstones of the Nama Basin point to a volcanic island arc source located in the Damara Belt. Detrital zircon dating of the Nama Group display major peaks of Neoproterozoic and Mesoproterozoic ages suggesting a provenance from the Damara/Gariep Belts and their basements. Palaeocurrents from the west and the dominance of Neoproterozoic-Cambrian detrital zircon ages (76%) in the "Molasse" stage of the foreland evolution probably indicate exhumation of the felsic volcanic arc root which probably occurred after the time indicated by the younger zircon dated at 531 ±9 Ma. The petrographic and geochemical results from the Arroyo del Soldado Group indicate a recycled upper crust composition characterized by source diversity composed of granite-gneissic and mafic-metamorphic rocks. On average, Nd isotopes account for negative ENd values and TDM ages in a range of variation found elsewhere within SW Gondwana. Detrital zircon dating indicate sources dominated by Palaeoproterozoic (1.7-2.0-2.2 Ga) and subordinate Archaean ages (2.5-2.9-3.5 Ga). The scarcity of Mesoproterozoic and Neoproterozoic zircons and palaeocurrent directions towards the east indicate that the Arroyo del Soldado Group was fed by detritus from the Rio de la Plata Craton favouring a passive margin tectonic setting for their deposition. Deformation of the Arroyo del Soldado Group took place ca. 530 Ma, after strike-slip collision with an African affinity terrane. Finally, based on the palaeogeographic evaluation, the provenance of Nama foreland basin and the passive margin deposit of the Arroyo del Soldado basin suggest that continent-continent collision of the Kalahari/Congo Cratons with the Rio de la Plata Craton and the Cuchilla Dionisio Pelotas Terrane most likely occurred due to strike slip accretion related to a component of N—S shortening in the period between 530 and 495 Ma.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:9855 |
| Date | 10 September 2012 |
| Creators | Blanco Gaucher, Gonzalo Homero |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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