Abstract:

The Attic-Cycladic metamorphic-core complex province was formed during Miocene to recent regional back-arc extension in response to slab-rollback and gravitational collapse. The development of several metamorphic-core complexes along extensional low-angle detachment faults resulted in the exhumation of lower-plate rocks and the tectonic denudation of the upper plate that is only sparsely exposed throughout the Aegean Sea. One of these areas is in southern Syros where the tectonic configuration includes three tectonometamorphic units separated by two detachment faults. The lower-plate Cycladic Blueschist Unit underwent late Cretaceous-Eocene HP/LT metamorphism and subsequent greenschist-facies overprint during Oligo-Miocene times. The tectonically overlying greenschist-facies Upper Unit is a tectonic sliver between the Cycladic Blueschist Unit and the structurally highest unit, the Vari Unit, which is dominated by quartzofeldspathic gneisses that are affected by four distinct deformation events (D1-D4). The late Cretaceous upper greenschist-facies D1 episode is related to late Cretaceous-Eocene subduction while D2-D4 fabrics were formed in response to late Cenozoic extension and exhumation. Field relations and structural data suggest that the late Miocene brittle Vari Detachment juxtaposes both Vari Unit and Upper Unit against the Cycladic Blueschist Unit. Mylonitic rocks observed at the base of Vari Unit along the contact with Upper Unit and zircon and apatite (U-Th)/He thermochronometry corroborate this scenario with lower-plate Cycladic Blueschist Unit rocks recording rapid cooling at $\sim$. 8-10. Ma. In contrast, cooling ages from the Vari Unit cluster at $\sim$. 13-15. Ma, indicating rapid exhumation coeval with activity along the Livadi Detachment on the neighboring island of Tinos. These data document new evidence that Vari Unit was affected by a major middle Miocene phase of exhumation and an older semi-brittle episode, both unrelated to late Miocene detachment faulting, revealing that crustal extension occurred in multiple stages and in a protracted episodic fashion similar to other parts of the Cyclades. © 2012 Elsevier B.V.