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

Abstract

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°C within the Cycladic Blueschists unit. Pressure-temperature estimates show that only lower parts of the Cycladic Blueschists unit recorded approximately 18-20 kbar for 530°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.

The geological signature of a slab tear below the Aegean

“…The same sense of shear is also found on Naxos and Paros further south. Although no detachment crops out on Folegandros, Sikinos or Sifnos and the northern part of Ios, syn-greenschist and partly brittle shearing deformation is recorded with the same top-to-the north or northeast sense of shear (Huet et al, 2009;Augier et al, 2014). A more symmetrical pattern of detachments, top-tothe north and top-to-the south, has been described further south within the massif itself (Hetzel et al, 1995;Gessner et al, 2001).…”

Synkinematic skarns and fluid drainage along detachments: The West Cycladic Detachment System on Serifos Island (Cyclades, Greece) and its related mineralization

“…The Cycladic Continental Basement Unit crops out in the central and southern parts of the Cyclades Islands on Naxos, Paros, Ios, Serifos and Sikinos (Andriessen et al, 1987;Grasemann and Petrakakis, 2007;Augier et al, 2015). This unit usually consists of granite,…”

CO 2 flow during orogenic gravitational collapse: Syntectonic decarbonation and fluid mixing at the ductile-brittle transition (Lavrion, Greece)

“…The North Cycladic Detachment System NCDS) and the West Cycladic Detachment System (WCDS) observed in Lavrion area (this study), accommodate post-orogenic exhumation of the Cycladic Blueschists Unit. Modified after Jacobshagen (1986); Grasemann et al (2012);Berger et al (2013) andAugier et al (2015). b. Geological map of the Lavrion peninsula (modified afterCarras, 2001 andScheffer et al, 2016).…”