ELSEVIER Physics of the Earth and Planetary Interiors 87 (1995) 183-196 PHYSICS OFTHE EARTH AND PLANETARY INTERIORS Anisotropy of magnetic susceptibility in the Ponta Grossa dyke swarm (Brazil) and its relationship with magma flow direction Maria Irene Bartolomeu Raposo *, Marcia Ernesto Instituto Astron6mico e Geofi~ico, Universityof S~o Paulo, C.P. 9638, 01065-970S~o Paulo, Brazil Received 13 July 1993; revision accepted 24 June 1994 Abstract Measurements of anisotropy of magnetic susceptibility (AMS) in 95 mafic dykes (mainly tholeiites 10-200 m in width) from the Mesozoic Ponta Grossa swarm, Southern Brazil, revealed two main types of magnetic fabric. Type I fabric (plane K1-K 2 parallel to the dyke plane) represents magma flow within the dykes, whereas Type II (plane K1-K 3 parallel to the dyke plane) is compatible with a fabric pattern reflecting vertical compaction of the magma column. Fabric Type I dominates (51% of the dykes) within the swarm, whereas Type II (38% of the dykes) concentrates mainly in the western region where the dykes intrude sediments. Considering the dykes with Type I fabrics, it is concluded that 58% of the dykes were fed by horizontal or sub-horizontal (K 1 inclinations less than 30°) magmatic flow, and 42% were fed by inclined to vertical (K1 inclinations more than 3&) magma flow. The latter are more frequent in the southeastern part of the swarm, suggesting a magma source close to this area, although there may have been other sources in other regions where dykes with inclined flow and distinct chemical characteristics are also found. 1. Introduction In recent years there has been growing interest in the investigation of dyke emplacement pro- cesses, not only because dykes represent former conduits for the passage of magma from deeper levels of the Earth to the surface but also because dyke swarms give information about tectonic pro- cesses thatdeformed the lithosphere. The flow of magma inside fractures is one issue of basic im- portance in understanding how continental swarms developed. Magma flow directions are traditionally investigated by means of petro- * Corresponding author, e-mail: mernesto@fox.cce.usp.br. graphic fabric (e.g. Komar, 1972; Shelley, 1985; Ross, 1986; Greenough et al., 1988), oriented vesicles (e.g. Coward, 1980) and fingers, grooves or lineations (e.g. Baer and Reches, 1987). How- ever, these methods are not useful when the petrographic fabric is poorly defined. Even when the fabric is strongly developed, observations made on oriented thin sections under the micro- scope are tedious, and methods based on field observations of flow structures must be used with care, as a distinction between lineations and grooves is not always evident or these indicators are simply absent. The anisotropy of magnetic susceptibility (AMS) has long been a useful tool to investigate problems in sedimentology, tectonics and igneous 0031-9201/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0031-9201 (94)02970-9