Tectonics of the Syros blueschists (Cyclades, Greece): From subduction to Aegean extension
Résumé
On Syros, high-pressure metamorphism affects a lithological pile that is composed of, from base to top: (1) the Komito-Vari granitic basement, (2) a margin sedimentary sequence that is predominantly made of marbles and schists (the Pyrgos and Kastri units), and (3) the Kambos metaophiolitic mélange. The tectonic history occurred in three main stages. During the first stage, in the mid-Eocene, the Kambos oceanic unit was thrust southward on top of the sedimentary pile. Top-to-the-south-southwest ductile senses of shear are synchronous with prograde high-pressure metamorphism and associated with this thrusting event. The second stage corresponds to a top-to-the-northeast ductile shear that affects the whole metamorphic pile and is synchronous with the metamorphic retrogression from eclogite to greenschist facies. However, the Kambos oceanic unit remained partly undeformed, as shown by significant volumes containing undeformed lawsonite pseudomorphs. No major extensional detachment related to this exhumation event outcrops on the island. The localized semibrittle to brittle deformation of the third stage is associated with the postmetamorphic development of (1) a ramp-flat extensional system at the island scale, whose southward minimum displacement is estimated at approximately 7 km, and (2) two sets of steeply dipping strike-slip faults with a normal component, trending either east-west or around north-south, indicating that the mean stretching and shortening directions are trending NNE-SSW and ESE-WNW, respectively. This sequence of major tectonic events and their relationship to metamorphism are interpreted within the framework of the subduction of the Pindos Ocean and then of the Adria continental passive margin.
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