This paper presents a study of the ductile and brittle deformation on Naxos and Paros islands (Cyclades, Greece). Previous maps and studies of the two islands have shown that a major low-angle fault zone separates surface rocks above the contact from an initially deep-seated unit below, showing a metamorphic evolution from high to low pressures. Structural analysis, as well as available stratigraphical, metamorphic, and geochronological data taken together demonstrate that this fault zone is a major normal-sense detachment zone dipping to the north. Rapid denudation of footwall rocks subsequent to high temperature metamorphism, at an estimated rate of 1.8--9.5 mm/yr, attests for tectonic unroofing during regional-scale top-to-the north ductile shearing. The change from ductile to brittle behavior of the footwall rocks together with a progressive localization of high strain intensity deformations just below the hanging wall is explained by the progressive cooling of the uprisen footwall of the detachment. Mio-Pliocene clastic sediments in the hangingwall represent the infilling of half grabens opened in between major normal faults that are synthetic to the underlying ductile shear zone. These sediments are as old as (Aquitanian-25 Ma), or younger than the earliest recognized evidence of ductile extension in the footwall. This provides a minimum age for the onset of extension in the Cyclades, which appears significantly older than maximum ages reported up to now (13--5 Ma).

Structural data strongly suggest that the detachment fault was initially rather low dipping (≈35¿). An evolutionary model is proposed, in which migmatite domes in the footwall correspond to the uprise of the lower ductile crust between two separating upper crustal blocks, during a process of asymmetric boudinage of the crust. This detachment model applies to a previously thickened continental lithosphere, which then suffers thermal relaxation and weakening, allowing extensional deformation to reach a climax during and subsequent to high temperature metamorphism. In the Cyclades, crustal-scale extension started after Early Cenozoic thrusting, while the crust was still thick, or less likely, before late underthrusting below the present surface. ¿ American Geophysical Union 1993