First palaeomagnetic results from the Cycladic Massif, Greece, and their implications for Miocene extension directions and tectonic models in the Aegean

Abstract

The Aegean region is characterised by major fault-bound blocks with markedly contrasting structural trends. Recent models account for these variations either through differential block rotation or through a process of radial extension. The Cycladic Massif in the back-arc region to the Hellenic trench represents a critical area where apparently divergent extension directions are observed. This contribution provides the first palaeomagnetic results from this area. Magnetic remanence and AMS data are presented for Middle Miocene granodiorites on the islands of Mykonos and Naxos. These rocks are exposed in the footwalls of major extensional detachments within the metamorphic core complexes that characterise the Cycladic Massif. A clear correlation between the orientation of an L-S fabric developed within the intrusions and the magnetic fabric data is observed, demonstrating the presence of a ‘normal’ magnetic fabric of extensional origin. The mean extension directions inferred from the orientations of the maximum axes of the magnetic susceptibility ellipsoids are markedly divergent at 064° (Mykonos) and 351° (Naxos). Northeasterly in situ magnetic remanences are observed on Mykonos, whereas those on Naxos are directed to the NNW. Neither direction corresponds to the appropriate palaeomagnetic reference direction for the Miocene. Standard palaeomagnetic tilt corrections cannot be applied at these sites because of the lack of palaeohorizontal markers. Deviations of in situ remanences away from the reference direction are potentially due to rotation around any one of an infinite number of net tectonic rotation axes. Each of these may be arbitrarily decomposed into components of tilting and vertical axis rotation. Geologically realistic solutions which restore the in situ magnetisations back to the reference direction may be found by consideration of regional structural trends, assuming that tilting is controlled by the major block-bounding faults. These solutions indicate significant components of differential vertical axis rotations across the Cyclades. Mykonos is bound by NW-SE trending regional faults and is shown to have rotated clockwise by ca. 22°. In contrast, Naxos is bound by ENE-WSW trending structures and has rotated anticlockwise by ca. 33°. The apparent divergence of extension directions inferred from both the magnetic anisotropy and structural data is resolved by correcting the mean AMS data for subsequent rotation using the rotation solutions derived from the magnetic remanence data. The data imply that the primary Middle Miocene extension direction in the Cycladic Massif was nearly uniform along a NNE-SSW azimuth. We consider that the uniform extension direction and opposing senses of block rotation determined for this portion of the Cycladic massif are most consistent with a recent model describing deformation within the Aegean in terms of a system of broken, fault-bounded slats. Within this scenario, Mykonos occupies a position on a clockwise rotating slat which continues northwest, at least as far as Evvia. A major ENE-WSW trending fault between the islands, previously identified on shallow seismic sections, separates this domain from an anticlockwise rotating slat which includes Naxos.