Résumé
The Miocene Hellenic subduction zone in the Aegean retreated with time to the south implying <br />that accreted high-pressure rocks shifted from a fore-arc position to a back-arc position. The <br />Cycladic islands in the central Aegean became part of the magmatic arc in the Late Miocene and are <br />now in a back-arc position. They are famous for their blueschists and extensional detachments. It is <br />widely assumed that exhumation of the blueschist unit from depths of 50-60 km was accomplished <br />by detachment faulting in a back-arc setting. On Crete, the exhumation of the Miocene high-<br />pressure rocks was achieved by normal faulting in a fore-arc position. This raises the question as to <br />whether exhumation of the Cycladic blueschists was mostly accomplished when these rocks were <br />still in a fore-arc setting. To answer this question we need to constrain the timing of detachments <br />and arc related magmatism, define how many detachments exist, determine their slip rates and <br />estimate the amount of displacement. Only with this type of data it will be possible to assess the <br />extent to which these faults contributed to the exhumation of the blueschists. <br />Using consistent apatite and zircon fission-track and apatite (U-Th)/He ages from sample <br />transects parallel to the tectonic transport direction of major extensional detachments on eight <br />Cycladic islands (Samos, Ikaria, Tinos, Mykonos, Naxos, Paros, Serifos and Ios), I have been able <br />to estimate the timing, rate and extent of slip movement for each detachment. The time constraints <br />on shear zones (ductile part of the extensional fault systems) indicate that the Selçuk shearing on <br />Samos started to operate >21 Ma followed by the Tinos and Naxos/Paros shear zones at ~21-20 Ma <br />while at about 15-10 Ma, when the granites intruded, a number of detachments started to operate <br />(Kerketas on Samos, Messaria/Kallithea on Ikaria/Samos, Mykonos, Serifos and Ios) or remained <br />active (Tinos and Naxos/Paros extensional fault system). This intimate relationship between arc-<br />related magmatism and extensional detachments (especially for the Messaria/Kallithea detachments <br />on Ikaria/Samos, as well as Mykonos, Serifos, and Ios detachments) was aided by relatively high <br />thermal gradients and extensional stresses caused by an extensional boundary condition related to <br />the subduction-zone retreat. This induced rapid cooling of the footwalls at ~75-25°C/Myr and fast <br />slip rates of ~8-7 km/Myr. No specific pattern of the extension timing and/or slip rate have been <br />recognized in the Aegean, linked to location of the islands within the arc. <br />This study has also shown that the Naxos extensional fault system is unique in the Aegean. The <br />Naxos detachment exhibits a slightly faster minimum slip rate and cooling rate at ~9-8 km/Myr and <br />~108°C/Myr related to the higher temperature condition during the formation of the Naxos <br />extensional fault system. Furthermore, the slip rate increases across the brittle/ductile transition <br />from ~6 km/Myr to ~9-8 km/Myr owing to the intrusion of a huge granodiorite close to the brittle <br />fault zone. On Ikaria, the slip rate on the Messaria extensional fault system is constant at ~8 <br />km/Myr across the brittle/ductile transition because the Ikaria granodiorite intrusion was <br />synchronous with onset of shearing. Generally, it is assumed that the extensional shear zones rooted <br />at the brittle/ductile transition while the Naxos extensional shear was rooted in the lower crust. <br />The data also show that the detachments accomplished a minimum offset from ~53 km (Ikaria) to <br />~12 km (Tinos) related to rock exhumation of less than 10 km on Tinos, Ikaria and Mykonos. <br />Therefore, Miocene normal faulting in the Aegean did not cause much exhumation of the <br />blueschists. However, the fast-slipping Miocene normal faults were the primary agents for opening <br />of the Aegean Sea. <br />Although there are differences between the studied detachments with regard to timing, slip rate <br />and depth of formation, there are important similarities among the Cycladic detachments especially <br />for the intra-arc period between ~15-5 Ma.