DUCTILE CRUST EXHUMATION AND EXTENSIONAL DETACHMENTS IN THE CENTRAL AEGEAN (CYCLADES AND EVVIA ISLANDS)

The Aegean continental domain is known to be the site of widespread '' back-arc'' extension since at least 13 Ma, on the basis of seismotecto nic, stratigraphic and fault analysis studies. This extension is docum ented to overprint structures related to the Mesozoic-Cenozoic Helleni c orogeny. Features attributed to early thrusting include the overall ductile deformation within two broad belts that have suffered HP/LT me tamorphism across the Aegean. This study presents a structural analysi s of the central Aegean area (Cyclades and Evvia Islands), examining i n particular the relationship between ductile and brittle deformation, both in the field and on a regional scale. Extension appears to be re sponsible for most of the ductile deformation within HP rock units tha t have experienced penetrative greenschist facies and higher grade met amorphic overprinting. On each studied island, progressive extensional deformation has occurred through the development of a major normal-se nse detachment zone down to depths of about 18-25 km. Large displaceme nt along the detachment zone accounts for rapid cooling and exhumation of ductile lower crust to form a local metamorphic dome or core compl ex. Structural and stratigraphic features support a progressive migrat ion of normal faulting away from the dome axis, and a rotation of prev iously active faults toward low dips, as in kinematic models recently suggested for the development of extensional detachment systems. All t he studied domes, except that seen on los Island, show a dominant top- to-the north or north-east sense of shear, while on the southern flank of many of them, an opposite sense of shear is observed, displaying t he same progressive evolution from ductile to brittle rock behaviour. This opposite sense of shear is thought not to result from shearing al ong a major conjugate detachment zone, as in some recent models, but f rom the accommodation in the ductile crust of upward bending of the br ittle upper crust in the footwall of the north-dipping detachment. Ava ilable radiometric and stratigraphic data indicate an early minimum ag e (22-19 Ma) for the onset of extension. The relationship between earl y metamorphic domes and shallow-dipping detachments, on one hand, and Messinian-Quaternary steep normal faults and grabens, on the other han d, is best explained with the progressive and continuous development o f new normal faults away from the domes axes, rather than with a two-s tage evolutionary model (core-complex stage, then Basin-and-Range stag e) of the type invoked for the North American Cordillera.