STRUCTURE AND KINEMATICS OF UPPER CENOZOIC EXTENSIONAL DETACHMENT ON NAXOS AND PAROS (CYCLADES ISLANDS, GREECE)

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 separa tes surface rocks above the contact from an initially deep-seated unit below, showing a metamorphic evolution from high to low pressures. St ructural analysis, as well as available stratigraphical, metamorphic, and geochronological data taken together demonstrate that this fault z one is a major normal-sense detachment zone dipping to the north. Rapi d denudation of footwall rocks subsequent to high temperature metamorp hism, at an estimated rate of 1.8-9.5 mm/yr, attests for tectonic unro ofing during regional-scale top-to-the north ductile shearing. The cha nge from ductile to brittle behavior of the footwall rocks together wi th a progressive localization of high strain intensity deformations ju st below the hangingwall is explained by the progressive cooling of th e uprisen footwall of the detachment. Mio-Pliocene clastic sediments i n the hangingwall represent the infilling of half grabens opened in be tween major normal faults that are synthetic to die underlying ductile shear zone. These sediments are as old as (Aquitanian-25 Ma), or youn ger than the earliest recognized evidence of ductile extension in the footwall. This provides a minimum age for the onset of extension in th e Cyclades, which appears significantly older than maximum ages report ed up to now (13-5 Ma). Structural data strongly suggest that the deta chment fault was initially rather low dipping (almost-equal-to 35-degr ees). An evolutionary model is proposed, in which migmatite domes in t he footwall correspond to the uprise of the lower ductile crust betwee n 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 relaxado n and weakening, allowing extensional deformation to reach a climax du ring 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.