Thrust or detachment? Exhumation processes in the Aegean: Insight from a field study on Ios (Cyclades, Greece)

摘要

Whether back-arc extension in the Aegean is symmetric or asymmetric at the regional scale is a key question to the understanding of the dynamics of back-arc extension. New field observations, structural analysis of finite and instantaneous strain, and their relationship with metamorphism on Ios island (Cyclades) lead us to investigate the significance of the South Cycladic Shear Zone (SCSZ). Two successive tectonometamorphic events have been documented in the hanging wall and the footwall of the SCSZ. First, penetrative top-to-the-south shearing is observed in the upper Cycladic Blueschists and the lower Cycladic Basement close to their contact. It is expressed in the hanging wall by shear bands and pressure shadows associated to blueschist facies minerals, contemporaneous with overthrusting of the Cycladic Blueschists over the basement unit. Second, large extensional top-to-the-north shear zones lately crosscut the whole stack. These constraints allow us to reinterpret the kinematics of the Aegean domain during synorogenic and postorogenic periods. When replaced in the reconstructed geometry of the Hellenic Eocene orogenic wedge, the observed top-to-the-south thrusting is coeval with extensional shear zones observed in Syros that could represent the roof of the Cycladic Blueschists. An extrusion structure is then proposed for the exhumation of the Cycladic Blueschists in the subduction zone. Top-to-the-north asymmetric extension together with the Andros-Tinos-Mykonos and Paros-Naxos detachments can be correlated to Oligo-Miocene extension in the back-arc domain.