Thesis defense:
Student: Thales Pescarini
Program: Geophysics
Title: “Paleomagnetic Record and Magnetic Mineralogy of Neoproterozoic Sedimentary Successions of West Gondwanaland"
Advisor: Prof. Dr. Ricardo Ivan F. da Trindade - IAG/USP
Judging Comitee:
- Prof. Dr. Ricardo Ivan Ferreira da Trindade – IAG/USP
- Prof. Dr. Andrew John Biggin - University of Liverpool
- Prof. Dr. Mathew Michael Domeier - University of Oslo
- Prof. Dr. Augusto Ernesto Rapalini - Universidad de Buenos Aires
- Profa Dra. Daniele Cornellio de Paiva Caldeira Brandt – IAG/USP
Abstract
This thesis investigates the paleomagnetic record and magnetic mineralogy of Neoproterozoic sedimentary successions across multiple cratonic margins of West Gondwanaland, aiming to constrain the timing, mechanisms, and tectonic context of a large-scale remagnetization event linked to the final stages of supercontinent assembly. By integrating paleomagnetic, rock magnetic, and mineralogical data from three key units—the Maieberg Formation (Congo Craton), Nama Group (Kalahari Craton), and Corumbá Group (Amazonian Craton)—this work provides a multi-craton assessment of secondary magnetizations associated with the Pan-African–Brasiliano orogenic system. In the Maieberg Formation (~635 Ma), a cap dolostone overlying Marinoan glacial diamictites in northern Namibia, two distinct components were identified: C1, interpreted as a depositional or early post-depositional magnetization, and C2, a thermochemical remanence carried by fine-grained authigenic magnetite. Paleolatitude estimates from C1 place the Congo Craton at ~33°S after the Snowball Earth glaciation, representing the first quantitative paleogeographic constraint for this setting. Differences in coercivity, domain state, and mineralogy between C1 and C2-bearing samples indicate that C2 reflects a remagnetization overprint related to clay diagenesis and subsequent thermal relaxation. In the Nama Group, a terminal Ediacaran foreland basin succession adjacent to the Damara-Gariep Belts, drill core samples were analyzed using detailed rock magnetic methods. A pervasive reverse-polarity remagnetization (C2) was isolated, carried by SD magnetite and monoclinic pyrrhotite with unblocking temperatures between 320 and 450 °C. The magnetic signal includes both stable SD grains and a significant superparamagnetic fraction. Coercivity spectra and organic matter thermal maturity suggest that this remagnetization resulted from thermoviscous acquisition under long-duration, low-grade burial heating. The inferred age (490–470 Ma) aligns with the waning stages of the Damara orogeny and supports a post-orogenic regional remagnetization. The Corumbá Group, exposed along the southern Amazonian Craton in the Paraguay Belt, consists of a Neoproterozoic carbonate–siliciclastic sequence. Rock magnetic experiments show remanence carried by SD pyrrhotite and magnetite with a strong superparamagnetic contribution. Paleomagnetic directions reveal vertical-axis rotations of 40–90° counterclockwise, indicating late-stage tectonic overprinting concurrent with or postdating remagnetization. The observed blocking temperature distributions and mineral assemblages support a thermoviscous mechanism during burial heating, with magnetization locked in during tectonic exhumation between ~500–480 Ma. To assess the physical viability of thermoviscous remagnetization, thermal and magnetic constraints from the Nama Group were integrated into a thermotectonic model. Raman spectroscopy of organic matter in interbedded shales indicates peak burial temperatures between 300 and 350 °C, consistent with lower greenschist facies. These temperatures, combined with blocking temperature distributions calculated from Néel relaxation theory, show that magnetization acquired during orogenic heating could stabilize progressively during post-orogenic cooling at ~1 °C/Myr—consistent with geological exhumation rates. Altogether, this thesis presents a unified thermotectonic model for the remagnetization of West Gondwanaland. By detailing the mineralogical nature of remanence carriers and demonstrating the physical feasibility of long-duration thermal remagnetization under realistic geological conditions, this work provides a new, quantitative framework for interpreting widespread secondary magnetizations in Neoproterozoic sedimentary basins across this ancient supercontinent.
Palavras- chave: Paleomagnetism, Rock magnetism, Magnetic mineralogy, Gondwana, Neoproterozoic, Ediacaran, Cambrian, Paleogeography, Remagnetization.