Tectono-metamorphic evolution of Syros and Sifnos islands (Cyclades, Greece)

Miocene exhumation of metamorphic rocks in the Aegean Sea is partly a consequence of post-orogenic extension. If the postorogenic mechanism of exhumation is rather well understood, the earlier syn-orogenic Eocene exhumation is still largely enigmatic. Previous authors have argued in terms of extension or compression. New structural and petrological data on Sifnos and Syros islands show that exhumation of high pressure-low temperature (HP-LT) rocks involves crustal-scale extensional ductile shear zones during the Eocene. We observe a continuum of top-to-the-NE and -E ductile shear from the Eocene (in the blueschist facies) to the Miocene (in the greenschist facies). This deformation is distributed in the eclogites and blueschists, whereas it is rather localised along ductile shear zones in the greenschists. Eclogites, which are preserved only at the top of the structural pile, are exhumed with a 'cold' retrograde P-T path. In the lower part of the structural pile we observe a progressive retrogression of eclogites in blueschist then greenschist facies. This lower part of pile is subsequently exhumed with P-T paths showing a nearly isothermal decompression before cooling. P-T-t-deformation data suggest that the Cycladic blueschists are progressively exhumed by a continuum of accretion at the base of the orogenic wedge, and by a partly non-coaxial extensional deformation above, distributed during the syn-orogenic stage, then localised during the post-orogenic stage. We then compare the mechanism of syn-orogenic exhumation of Crete and the Cyclades and we discuss a simple geodynamic scenario for the Aegean domain and the external Hellenides which accounts for (1) the southward migration of the Hellenic trench and arc during the Cenozoic; (2) the P-T-t-deformation data for the Cycladic blueschists and the Phyllite-quartzite nappe; and (3) the transition from syn-orogenic to post-orogenic in the Cyclades. (C) 2001 Elsevier Science B.V. All rights reserved.