Paleomagnetic record and magnetic mineralogy of neoproterozoic sedimentary successions of West Gondwanaland
This thesis investigates the paleomagnetic record and magnetic mineralogy of Neoproterozoic sedimentary successions across West Gondwanaland, aiming to constrain the timing, mechanisms, and geodynamic context of large-scale remagnetization that occurred on the paleomegacontinent. By integrating pale...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2025 |
| País: | Brasil |
| Institución: | Universidade de São Paulo (USP) |
| Repositorio: | Biblioteca Digital de Teses e Dissertações da USP |
| Idioma: | inglés |
| OAI Identifier: | oai:teses.usp.br:tde-18072025-204857 |
| Acceso en línea: | https://www.teses.usp.br/teses/disponiveis/14/14132/tde-18072025-204857/ |
| Access Level: | acceso abierto |
| Palabra clave: | Estratigrafia Geofísica Geologia Magnetismo de rochas Mineralogia magnética Paleomagnetismo |
| Sumario: | This thesis investigates the paleomagnetic record and magnetic mineralogy of Neoproterozoic sedimentary successions across West Gondwanaland, aiming to constrain the timing, mechanisms, and geodynamic context of large-scale remagnetization that occurred on the paleomegacontinent. By integrating paleomagnetic, rock magnetic, and mineralogical datasets from three key stratigraphic unitsthe Maieberg Formation (Congo Craton), Nama Group (Kalahari Craton), and Corumbá Group (Amazonian Craton)this work provides a multi-craton assessment of secondary magnetizations attributed to the final stages of West Gondwanaland amalgamation. In the Maieberg Formation (635 Ma), a cap carbonate sequence overlying Marinoan glacial diamictites in northern Namibia, two distinct remanence components were isolated: C1, interpreted as a depositional or post-depositional magnetization, and C2, a thermochemical remanence associated with fine-grained authigenic magnetite. Paleolatitude estimates derived from C1 suggest deposition at ~33°S, providing the first quantitative constraint on the low-latitude position of the post-Snowball Earth Congo Craton. Our dataset shows a clear distinction in domain state, coercivity and mineralogical aspects between C1- and C2-bearing samples, suggesting that C2 represents a remagnetization overprint, likely associated with magnetite precipitation mediated by clay diagenesis and subsequently affected by thermal relaxation. We then investigate the Nama Group, a terminal Ediacaran siliciclastic-carbonate succession located in the foreland of the Damara-Gariep Belts. Drill-core samples were subjected to comprehensive rock magnetic analyses. A pervasive reverse-polarity remagnetization component (C2) was isolated, carried by SD magnetite and monoclinic pyrrhotite, with unblocking temperatures ranging from 320450°C. Mineralogical and coercivity spectra indicate a mixture of non-interacting SD grains and superparamagnetic particles. Our findings argue against a chemical remagnetization origin and instead support a thermoviscous acquisition mechanism under prolonged, low-grade heating conditions. The remagnetization (490470 Ma) is interpreted as regional and post-orogenic, imposed during the waning stages of the Damara orogeny. The next focus was on the Corumbá Group (Amazonian Craton), a Neoproterozoic carbonatesiliciclastic sequence exposed in the Southern Paraguay Belt. Magnetic experiments reveal a remanence carried by SD pyrrhotite and magnetite, with a significant superparamagnetic fraction. The overall observations are consistent with thermally relaxed diagenetic assemblages. Paleomagnetic results show a 4090° counterclockwise vertical-axis rotation relative to expected cratonic reference directions, providing critical evidence for late-stage deformation synchronous with or postdating remagnetization. The blocking temperature distribution and paleomagnetic directions suggest a thermoviscous remanent magnetization acquired during regional burial heating and subsequently blocked during tectonic uplift between ~500480 Ma. These constraints redefine the timing of deformation in the Southern Paraguay Belt and support a post-orogenic remagnetization mechanism common to the other cratonic margins of West Gondwanaland. Finally, to further test the physical viability of a thermoviscous remagnetization mechanism, we developed a thermotectonic model specifically using data from the Nama Group. Raman spectroscopy of organic matter was conducted on shale samples, with spectral parameters and calibrated temperature estimates indicating peak burial temperatures between 300 and 350°C, consistent with lower greenschist facies conditions. These results were integrated with magnetic thermochronometry modeling based on Néel relaxation theory. The resulting blocking temperature distributions and geological relaxation times demonstrate that magnetization acquired during orogenic heating could have progressively locked in during post-tectonic cooling at rates of approximately 1°C/Myr. The convergence of magnetic, thermal, and geochronological evidence supports a unified thermotectonic model in which the widespread remagnetization of West Gondwanaland occurred during tectonic stabilization phase through isostatic rebound following the Pan-AfricanBrasiliano orogenies. Altogether, this thesis establishes a unified, multi-craton framework for understanding the remagnetization of West Gondwanaland as a thermotectonic phenomenon. Through a detailed description of the remanence carriers and by demonstrating the physical feasibility of long-duration thermal remagnetization under geologically realistic heating and cooling scenarios, this work provides a new physical basis for interpreting the enigmatic, widespread secondary magnetizations observed in sedimentary basins across this megacontinent. |
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