Ductile extension and the formation of the Aegean Sea

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Abstract The Aegean Sea and Tyrrhenian sea areas offer almost unique examples of actively collapsing orogens where the internal velocity field, kinematic boundary conditions and seismic activity can be compared and where older structures formed during the early stages of the same process are exhumed. We study these processes along two major transects. The first runs from Mt Olympos to the centre of the Cyclades, it is perpendicular to the direction of extension and offers the opportunity to study the brittle-ductile transition by direct observation of rocks and deformation. A gradient of finite exhumation along the transect has brought various crustal levels to the surface, and the localization of deformation during exhumation along major detachments has preserved older penetrative structures formed below. As these outcrops are distributed along the strike of a single crustal-scale tilted block the observed structures probably corresponds to a single tectonic event, and we can sample the associated deformation at increasing depth along the transect. We propose a stratification of deformation regimes associated to extension: localized normal faults in the brittle crust, distributed extensional shear bands with a partition between domains of noncoaxial and coaxial flow below 8–10 km, and distributed coaxial flow below 20 km. Flat-lying extensional shear bands extend brittle normal faults within the brittle-ductile transition. The second transect runs from Crete to Naxos and Mykonos and is parallel to the main direction of extension. It shows the transition from tectonic accretion and syn-orogenic extension near the subduction front to post-orogenic extension in the backarc region. The exhumation of metamorphic rocks proceeds in two stages: synorogenic ‘cold’ exhumation along detachments in the upper part of the accretionary complex, with a good preservation of HP-LT parageneses, and post-orogenic exhumation in a warmer environment in the backarc region with a complete retrogression of HP-LT parageneses. The 3D finite strain field is then compared to recent space geodesy data and the mechanism of crustal collapse discussed.