A low-angle ductile shear zone associated with two brittle detachment faults of Late Miocene to Pliocene age has been mapped on Ikaria Island. The ductile shear zone formed in the uppermost ductile crust under mid-greenschist facies conditions. Ductile shearing was not by simple shear and involved a component of vertical shortening. Cooling in the footwall of the extensional fault system was rapid and the offset was of the order of 70 km or even more. The dominant shear sense was top-to-the-NNE; during footwall unloading and doming of the extensional fault system minor late-stage antithetic shears developed.
The progressive superposition of ductile, ductile-brittle and brittle structures in the footwall of the Messaria extensional fault system, brittle deformation in the hangingwall and the decrease of cooling ages parallel to the northward slip direction of the hangingwall reflects progressive southward migration of footwall exhumation and is typical for extensional fault systems above metamorphic core complexes.
Initial movement in the ductile Messaria shear zone of the Messaria extensional fault system at ~14 Ma was accompanied and aided by the intrusion of synkinematic granites (Figure 20a) and a relatively high thermal field gradient of ~30°C km-1. The Messaria extensional fault system operated from ~350-400°C to at least 80°C between ~14-3 Ma. Because biotite in samples from the northern portion of the shear zone was stable during mylonitisation, temperatures were slightly higher than 400°C in the northern part of Ikaria. T-t paths indicate rapid cooling as the footwall was exhumed to the surface. The Messaria detachment probably rooted at the brittle/ductile transition and the Messaria shear zone in the underlying ductile crust (Figure 20a). The fact that the cooling rates of both the S-type Xylosirtis granite and the metapelite of the Ikaria nappe are largely similar is thought to be due to a relatively deep intrusion of the small granite during extensional shearing and that both rocks units were then exhumed and cooled together. The steeper cooling curve for the large I-type Raches granite is interpreted to reflect intrusion into higher crustal levels but according to the zircon ages concurrently with the intrusion of the S-type Xylosirtis granite at ~14 Ma.
Figure 20. Three stage evolution
Three stage evolution of extensional deformation on Ikaria. (a) The inception of Messaria extensional fault system was aided by the intrusion of the S-type granite and later also by the intrusion of the large I-type granits in the magmatic arc of the Hellenic subduction zone. Note that the ~10 Ma old granodiorite and the 11 Ma old volcanic rocks on Samos Island (Weidmann et al., 1984) are of the same age. We envisage that subduction-zone retreat caused intra-arc extension at this stage. The simplified arrows show accelerated corner flow in the asthenosphere behind the retreating subduction zone, which caused relative plate divergence in the intra- and back-arc area. Asthenospheric flow in the arc and back-arc region behind retreating slabs is forced to accelerate in the horizontal to fill the free space caused by the retreating subduction zone causing plate divergence. (b) The Messaria shear zone moved into brittle crust and is juxtaposed with Messaria detachment. The magmatic arc probably shifted southward. (c) Late-stage updoming of the footwall as a response to unloading.
Average slip rates at the Messaria extensional fault system were ~6-9 km Myr-1 and imply a displacement of ~>70 km. This displacement and the maximum depth of 15 km for the onset of extensional faulting yields a dip angle of ~15°, or even less, for the Messaria extensional fault system. A prerequisite for the development of low-angle faults appears to be that they have to reactivate earlier fault planes. Pre-extension thrusts were not observed on Ikaria. However, the occurrence of the high-pressure Messaria nappe above the non-high-pressure rocks of the Ikaria nappe suggests that the contact between these two nappes was a thrust, probably the Cyclades-Menderes thrust. If so, extensional reactivation of this thrust plane would be in line with the low-angle nature of the Messaria extensional fault system.
How does the Fanari detachment relate to the Messaria extensional fault system? The occurrence of allochtonous Pliocene sediments in the hangingwall of the Fanari detachment indicates that movement on it continued until or commenced after ~5 Ma. Because movement on the Messaria extensional fault system lasted until ~3 Ma, it appears feasible to assume that the Fanari detachment is a brittle fault in the hangingwall of Messaria extensional fault system and hence is ultimately related to the latter (Figure 20b).
The kinematic indicators in the Messaria shear zone and the brittle Messaria and Fanari detachments together with the spatial trend of footwall cooling ages indicates a general top-to-the-NNE sense of movement. We envisage that the late-stage top-to-the-SSW shear-sense indicators reflect minor antithetic extensional structures related to footwall unloading and updoming of Ikaria (Figure 20c).
Because most of the Aegean region is under water any regional correlations are necessarily speculative. Scaling relationship for faults suggests that large-scale structures such as the Messaria extensional fault system must continue laterally over great distances as far as Samos Island (Figure 3). For us a correlation of the Fanari and Kallithea detachments with the Messaria extensional fault systems appears to be reasonable. Both detachments have the same shear sense and have non-metamorphic units in their hangingwall, which contain Pliocene sediments. Inception of the Kallithea detachment is fairly well dated at ~10 Ma age (Ring et al. 1999b) and a granodiorite dike in its footwall yielded a zircon fission track age of 7.3±0.6 Ma (Sa7) (Ring et al. this volume). If it was accepted that the Fanari detachment is related to the Messaria extensional fault system, then the Kallithea detachment would also be part of the Messaria extensional fault system.
Ductile extensional shearing on Samos has no relationship to the brittle Kallithea detachment. One zircon fission track age of 14.1±0.8 Ma (Sa9) (Ring et al. this volume) from the Basal unit suggests that the Kerketas extensional system is slightly older than the Messaria extensional fault system on Ikaria. Ductile extension and exhumation of the Ampelos nappe below the Selçuk extensional system lasted until in the Early Miocene as indicated by zircon fission track ages of 20-18 Ma (see above), which consistently young northeastward in the direction of hangingwall slip. The zircon fission track ages indicate that the Selçuk and Kerketas extensional systems are unrelated to each other and also show that both extensional systems are unrelated to the Messaria extensional fault system.