Crustal-scale geometry and kinematics of late-orogenic extension in the central Aegean (Cyclades and Ewia Island)

Abstract

The Aegean continental domain is known to be the site of widespread “back-arc” extension since at least the Middle Miocene, overprinting structures related to the Mesozoic-Cenozoic Hellenic orogeny. Features attributed to early thrusting include the overall ductile deformation within two broad belts that have suffered HP/LT metamorphism across the Aegean. This study summarizes the results of a structural study in the central Aegean area (Cyclades and Evvia Island), examining in particular the relationship between ductile and brittle deformation on a regional scale. Extension appears to be responsible for most of the ductile deformation within HP rock units that have experienced penetrative greenschist-facies and higher-grade metamorphic overprinting. On each island studied, progressive extensional deformation has occurred through the development of a major normal-sense detachment zone down to depths of about 18–25 km. The geometry of the extensional system on the scale of the central Aegean is described, taking into account the characteristic features of the regional-scale ductile strain field. At least two-probably three-subparallel major north-dipping detachment zones are identified, trending NW-SE in the northwestern Cyclades and E-W in the southeastern Cyclades, both with initial dips estimated at around 30–45°. The present geometry of the extensional system implies that these detachment zones and related metamorphic core complexes interfere with each other. Two main evolutionary models are considered which can account for such a kinematic link between parallel detachment zones. Structural data favour the sequential development of synthetic major detachments zones in a direction opposite to the sense of dip of the detachments. Early (?Oligocene-Lower Miocene) detachment-type extension in the area studied is related to a late-orogenic post-thickening “back-arc” setting.