HR: 1340h
AN: GP33A-0104    [Abstracts]
TI: Tectonic Fabric Revealed by AARM in the Unmetamorphosed Proterozoic Mafic Dike Swarm from the Salvador Area (Bahia State):São Francisco Craton, NE Brazil.
AU: * Raposo, M B
EM: irene@usp.br
AF: Geosciences Institute,São Paulo University, Rua do Lago 562, São Paulo, SP 05508-80 Brazil
AB: Magnetic fabric and rock magnetism studies were performed in 25 mafic dikes of the unmetamorphosed Proterozoic (~1.02Ga) dike swarm from Salvador (Bahia, State NE Brazil). This area lies in the north-eastern part of the São Francisco Craton. The dikes crop out along the beaches and in quarries around Salvador city, and cut across both the amphibolitic dikes and granulites. They range from a few centimeters up to 50 m wide and show two main trends N140-170 and N100-120 with vertical dips. Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). AMS tensors represent the contribution of all rock-forming minerals whereas AARM fabrics are given by ferromagnetic minerals. Rock magnetism properties indicate that 2-5 μm pseudo-single-domain grains of almost pure magnetite carry the magnetic fabrics. The main AMS fabric recognized in the swarm is Normal, in which K_max-K_int plane is parallel to the dike plane and the magnetic foliation pole (K_min) is perpendicular to it. This fabric is interpreted as due to magma flow, and analysis of the K_max inclination permitted to infer that the dikes were fed by horizontal or subhorizontal flows(K_max < 30°). This interpretation is supported by structural field evidence found in six dikes. On the other hand, for all dikes the AARM tensors are not coaxial with AMS fabrics and show a magnetic foliation (A_max-A_int plane) oriented to N30-40E, suggesting that magnetite grains were rotated clock-wise from dike plane. The orientation of AARM foliation is similar to the orientation of a system of faults in which the Salvador normal fault is the most important. These faults were formed during Cretaceous rifting in the Recôncavo-Tucano-Jatobá assemblage which corresponds to an aborted intra-continental rift, now filled with non-marine sediments, formed during the opening of the South Atlantic. Therefore, the AARM fabric found for the dikes is probably tectonic in origin and suggests that the dike swarm was affected by the important tectonic event responsible for the break-up of the Gondwanaland.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1540 Rock and mineral magnetism
DE: 1599 General or miscellaneous
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005