Exhumation kinematics of the Cycladic Blueschists unit and back-arc extension, insight from the Southern Cyclades (Sikinos and Folegandros Islands, Greece)

摘要

Current models for the Oligo-Miocene postorogenic back-arc extension of the Aegean domain suggest that stretching is accommodated by two bivergent detachment systems of opposing shear sense. The coexistence in the Eocene of a top-to-the-south thrust at the base of the Cycladic Blueschists unit and top-to-the-north extensional shear zones at the roof raises the problem of differentiating synorogenic and postorogenic deformations with similar directions and shear senses. Based on structural field data, this study shows that the postorogenic deformation recorded in the Southern Cyclades is extremely asymmetric as the Cycladic Blueschists unit is pervasively affected by top-to-the-north shearing deformation distributed on four main shear zones. All activated in greenschist-facies conditions, some of these shear zones operated in the brittle regime during the final part of the exhumation. The Cycladic Blueschists/Cycladic Basement contact displays clear polyphased deformation with the preservation of top-to-the-south thrust kinematics. Thermal structure of the Cycladic Blueschists unit with regard to position of ductile shear zones was retrieved using the Raman Spectroscopy of Carbonaceous Material peak-metamorphic temperatures. This study shows a series of major metamorphic gaps accommodating an upward and stepwise decrease of more than 200 degrees C within the Cycladic Blueschists unit. Pressure-temperature estimates show that only lower parts of the Cycladic Blueschists unit recorded approximately 18-20kbar for 530 degrees C peak conditions. While flanking the West Cycladic Detachment System, which shows a top-to-the-south shear sense, the Southern Cyclades are dominated by a top-to-the-north noncoaxial shearing. Deformation is therefore genuinely asymmetric in the center of the Aegean domain.