Magnetic fabric and palaeomagnetic analyses of the Plio–Quaternary calc–alkaline series of Aegina Island, South Aegean volcanic arc, Greece

Abstract

The South Aegean volcanic arc represents the magmatic expression of active subduction of the African plate beneath Eurasia in the region of Greece. Plio–Quaternary calc–alkaline (dacite and andesite) lava flows exposed on the island of Aegina have been sampled for palaeomagnetic analyses to determine whether the western arc area has experienced vertical axis rotations. In addition, these units allow an assessment of the use of magnetic fabric (anisotropy of magnetic susceptibility; AMS) data as indicators of lava flow directions in silicic extrusive rocks, providing a complement to the existing literature on AMS of basaltic flows. The magnetic mineralogy of both dacites and andesites is dominated by fine-grained magnetite/Ti-poor titanomagnetite. Sampled flows are characterised by oblate magnetic fabrics at site level which are co-axial to flow surfaces, although most sites also contain specimens with prolate AMS ellipsoids. Where magnetic lineations are developed, they are systematically aligned either parallel or orthogonal to average (exposure-scale) lava flow directions. Magnetic lineations in these rocks, therefore, may be interpreted in terms of flow directions only when the latter are known a priori. Observed variability in the directions of remanent magnetisation recorded by the Aegina lava flows may be explained wholly in terms of geomagnetic variations. Several sites record spot readings of the geomagnetic field during polarity transitions. Scatter of directions at the remaining sites is consistent with that expected from secular variation. Post-eruption tilting of magnetisation vectors is shown to be limited. The in situ mean direction of magnetisation of the non-transitional sites is indistinguishable from the geocentric dipolar field used as a reference direction for these young (<5 Ma) rocks. This indicates that the island has not experienced significant Plio–Quaternary tectonic rotations, in contrast to other parts of the Aegean volcanic arc. The magnetic data also further constrain the timing of volcanic activity in the western part of the South Aegean arc, when used in conjunction with existing radiometric and stratigraphic ages. The most recent volcanic products are shown to correlate with the Pleistocene part of the Matuyama polarity epoch. The Aegina volcanic centre has, therefore, been inactive for at least the last 720 000 yr, in contrast to the historical activity further to the east on Santorini.