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International audienceLow-angle normal faults accommodate a large part of continental post-orogenic extension. Besides the intrinsic rheological characteristics of the continental crust that may lead to the formation of shallow-dipping shear zones at the brittle–ductile transition, the role of pre-existing low-angle structures such as large thrusts has been proposed by several authors. We explore this question with the example of the North Cycladic Detachment System (NCDS) that is composed of a series of distinct detachments cropping out on the islands of Andros, Tinos and Mykonos, separating the Cycladic Blueschists in the footwall from the Upper Cycladic Nappe in the hanging wall. We show that these extensional structures are part of a single large-scale structure (more than 200 km along strike) that reactivates the Vardar suture zone. It extends eastward on Ikaria and westward offshore Evia and Thessalia where it probably connects to recent shallow-dipping normal faults evidenced on published seismic reflection profiles. The NCDS started its activity in the Oligocene concommitantly with the Aegean extension, and was still active in the Late Miocene. It has exhumed a series of metamorphic domes from southern Evia to Mykonos below low-angle detachment systems, made of low-angle normal faults and low-angle ductile shear zones. The ductile shear zones and the faults were created with a low dip and they kept the same attitude throughout their exhumation. We identify three main detachments that are part of a continuum of extension on the NCDS : Tinos detachment, Livada detachment and Mykonos detachment. A fourth detachment (Vari detachment) is the reactivation of an Eocene exhumation-related structure. Deformation in the footwall is characterized by intense stretching and flattening. Using the spatial evolution observed along strike from Andros to Mykonos we construct a history of formation of the NCDS starting with the reactivation of former thrusts leading to the exhumation of high-temperature metamorphic domes. The Aegean example shows that reactivation of earlier shallow-dipping discontinuities can play a fundamental role in continental post-orogenic extension

[1] 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-TinosMykonos and Paros-Naxos detachments can be correlated to Oligo-Miocene extension in the back-arc domain. Citation: Huet, B

The Monviso ophiolite Lago Superiore Unit constitutes a well-preserved, almost continuous upper fragment of oceanic lithosphere subducted at c. 80 km depth, thereby providing a unique opportunity to study mechanical coupling processes and meter-scale fluid-rock interactions occurring at such depths in present-day subduction zones. It is made of (i) a variably thick (50-500m) section of eclogitized basaltic crust (associated with minor calcschist lenses) overlying a 100-400m thick metagabbroic body and of (ii) a c. 1km thick serpentinite sole. We herein focus on the three major eclogite-facies shear zones found at the top of the unit, at the boundary between basalts and gabbros, and between gabbros and serpentinites, respectively. Strain