Full text
| |
| Author(s): |
Joao Pedro Rodriguez Pinto
Total Authors: 1
|
| Document type: | Master's Dissertation |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Instituto Astronômico e Geofísico (IAG/SBD) |
| Defense date: | 2024-06-14 |
| Examining board members: |
Daniele Cornellio de Paiva Caldeira Brandt;
Gelvam Andre Hartmann;
Filipe Altoé Temporim
|
| Advisor: | Daniele Cornellio de Paiva Caldeira Brandt |
| Abstract | |
The evolution of the continental blocks that formed Gondwana is not a consensus due to the scarcity of paleomagnetic poles for the Neoproterozoic period (900-600 Ma). Paleomagnetic studies provide information about the latitude and orientation of rocks relative to the Earth\'s rotational axis at the time of their formation. This study aimed to determine a paleomagnetic pole for mafic dikes of Itapé (650 Ma), located in the Itabuna-Salvador-Curaçá Orogen, southeast of the São Francisco Craton. A total of 274 specimens were collected from 20 sites near Itapé city for the studies of anisotropy of magnetic susceptibility, magnetic mineralogy and paleomagnetism. Analysis of the magnetic mineralogy revealed the presence of magnetic minerals with low coercivities that saturate at fields around 400 mT. Thermomagnetic curves, hysteresis, and First Order Reversal Curves diagrams indicate the dominance of pseudo-single domain magnetites as the primary carriers of magnetization in the rocks. The anisotropy of magnetic susceptibility exhibited a normal behavior in most sites, with the maximum and intermediate axes predominantly oriented NE-SW, aligning with the dike directions. Anisotropy of anhysteretic remanent magnetization displayed consistent behavior with the anisotropy of magnetic susceptibility. Both thermal and alternating field demagnetizations yielded similar results. Most demagnetizations were analyzed using great circles. Two baked contact tests yielded inconclusive results. The virtual geomagnetic poles showed a high dispersion. A 45° cutoff angle was considered the most appropriate for excluding spurious data and including data related to the normal paleosecular variation of the field. Directional analysis provided a mean direction: Dm= 257.9°, Im= 20.5° (N=9, k=7.6, 95=20.0°), and a paleomagnetic pole with a 45° cutoff angle of colatitude at 14.4°S, 238.2°E (K=16.4, A95=13.1°). The pole aligns with an age coherent with geochronology results, when compared with the apparent polar wander path of the Congo-São Francisco Craton, suggesting a primary magnetization record. Assuming primary magnetization, the high dispersion of the dataset may be attributed to an unstable behavior of the geomagnetic field during that period. Simulations using statistical paleosecular variation models indicated that the observed behavior in the Itapé data may be linked to a low intensity of the geomagnetic field, consistent with results from other studies suggesting a less intense dipolar mean field for this period. (AU) | |
| FAPESP's process: | 22/13120-3 - Paleomagentic and Anisotropy of Magnetic Susceptibility study of mafic dikes from Itapé |
| Grantee: | João Pedro Rodriguez Pinto |
| Support Opportunities: | Scholarships in Brazil - Master |