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Constraining the tectonic evolution of extensional fault systems in the Cyclades (Greece) using low-temperature thermochronology

Year: 2004
Type: dissertation
Abstract: The Miocene Hellenic subduction zone in the Aegean retreated with time to the south implying that accreted high-pressure rocks shifted from a fore-arc position to a back-arc position. The Cycladic islands in the central Aegean became part of the magmatic arc in the Late Miocene and are now in a back-arc position. They are famous for their blueschists and extensional detachments. It is widely assumed that exhumation of the blueschist unit from depths of 50-60 km was accomplished by detachment faulting in a back-arc setting. On Crete, the exhumation of the Miocene high- pressure rocks was achieved by normal faulting in a fore-arc position. This raises the question as to whether exhumation of the Cycladic blueschists was mostly accomplished when these rocks were still in a fore-arc setting. To answer this question we need to constrain the timing of detachments and arc related magmatism, define how many detachments exist, determine their slip rates and estimate the amount of displacement. Only with this type of data it will be possible to assess the extent to which these faults contributed to the exhumation of the blueschists. Using consistent apatite and zircon fission-track and apatite (U-Th)/He ages from sample transects parallel to the tectonic transport direction of major extensional detachments on eight Cycladic islands (Samos, Ikaria, Tinos, Mykonos, Naxos, Paros, Serifos and Ios), I have been able to estimate the timing, rate and extent of slip movement for each detachment. The time constraints on shear zones (ductile part of the extensional fault systems) indicate that the Selçuk shearing on Samos started to operate >21 Ma followed by the Tinos and Naxos/Paros shear zones at ~21-20 Ma while at about 15-10 Ma, when the granites intruded, a number of detachments started to operate (Kerketas on Samos, Messaria/Kallithea on Ikaria/Samos, Mykonos, Serifos and Ios) or remained active (Tinos and Naxos/Paros extensional fault system). This intimate relationship between arc- related magmatism and extensional detachments (especially for the Messaria/Kallithea detachments on Ikaria/Samos, as well as Mykonos, Serifos, and Ios detachments) was aided by relatively high thermal gradients and extensional stresses caused by an extensional boundary condition related to the subduction-zone retreat. This induced rapid cooling of the footwalls at ~75-25°C/Myr and fast slip rates of ~8-7 km/Myr. No specific pattern of the extension timing and/or slip rate have been recognized in the Aegean, linked to location of the islands within the arc. This study has also shown that the Naxos extensional fault system is unique in the Aegean. The Naxos detachment exhibits a slightly faster minimum slip rate and cooling rate at ~9-8 km/Myr and ~108°C/Myr related to the higher temperature condition during the formation of the Naxos extensional fault system. Furthermore, the slip rate increases across the brittle/ductile transition from ~6 km/Myr to ~9-8 km/Myr owing to the intrusion of a huge granodiorite close to the brittle fault zone. On Ikaria, the slip rate on the Messaria extensional fault system is constant at ~8 km/Myr across the brittle/ductile transition because the Ikaria granodiorite intrusion was synchronous with onset of shearing. Generally, it is assumed that the extensional shear zones rooted at the brittle/ductile transition while the Naxos extensional shear was rooted in the lower crust. The data also show that the detachments accomplished a minimum offset from ~53 km (Ikaria) to ~12 km (Tinos) related to rock exhumation of less than 10 km on Tinos, Ikaria and Mykonos. Therefore, Miocene normal faulting in the Aegean did not cause much exhumation of the blueschists. However, the fast-slipping Miocene normal faults were the primary agents for opening of the Aegean Sea. Although there are differences between the studied detachments with regard to timing, slip rate and depth of formation, there are important similarities among the Cycladic detachments especially for the intra-arc period between ~15-5 Ma.
Source: HAL
Language: English
Open Access status: green
OpenAlex ID: W2256012514
Cites: 259
Cited by: 15
Related to: 10