Magnetic fabric of a basaltic dyke swarm associated with Mesozoic rifting in northeastern Brazil |
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| Institution: | 1. ISEL and IDL (Univ. Lisbon), Lisbon, Portugal;2. University of Lisbon, Lisbon, Portugal;3. Institute of Geophysics AS CR, v.v.i, Bo?ní II/1401, 14131, Prague 4, Czech Republic;4. Paléomagnétisme, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, 94107 Saint-Maur cedex, France;5. Laboratory of Natural Magnetism, Institute of Geophysics, ETH-Zürich, CH-8092 Zurich, Switzerland;6. EMEPC, 2770-047 Paço d''Arcos and University of Évora, D. Geociências and C. Geofísica de Évora, 7000-671 Évora, Portugal;7. Laboratory of Neutron Diffraction, Department of Solid State Engineering, Faculty of Nuclear Sciences and Physical Engineering, CTU in Prague, Czech Republic;1. IDL-FCUL, Instituto Dom Luíz, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, Lisbon, Portugal;2. IPMA, Instituto Português do Mar e da Atmosfera, Lisboa, Portugal;3. Institut de Minéralogie, de Physique des Minéraux et de Cosmochimie, Sorbonne Universités-Pierre and Marie Curie, University of Paris 06, UMR CNRS 7590, Muséum National d''Histoire Naturelle, IRD UMR 206, Paris, France;4. Departamento de Geologia, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, Lisbon, Portugal;5. Faculty of Sciences-Semlalia, Geology Department, Cadi Ayyad University, Prince Moulay Abdellah Boulevard, P.O. Box 2390, 40 000 Marrakech, Morocco |
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| Abstract: | The anisotropy of magnetic susceptibility (AMS) has been studied in a 120 km long, Early Cretaceous tholeiitic dyke swarm emplaced during the early stages of rifting and opening of the equatorial Atlantic Ocean. The vertical dykes filled a set of E-trending fractures that cut the structural grain of the Precambrian basement of northeastern Brazil at a high angle. These strongly magnetic rocks contain pseudo-single domain, Ti-poor magnetite and secondary maghemite as revealed by thermomagnetic and hysteresis data. The contribution of the paramagnetic and the high coercivity antiferromagnetic fractions to the bulk susceptibility is less than 1.2%. The dykes generally show well-clustered AMS principal directions. The plunge of the magnetic lineation varies from nearly subvertical in the center of the swarm to horizontal in the west. The strike of the magnetic foliation is generally oblique to the dyke wall and exhibits a curved trend at the regional scale. This fabric pattern suggests that the magma source that fed the dykes was situated in the center of the swarm, which is presently below Tertiary sandstones. |
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