extensional Messaria shear zone and associated brittle detachment faults, Aegean Sea, Greece, The

Journal of the Geological Society, Jul 2005 by Kumerics, Christine, Ring, Uwe, Brichau, Stéphanie, Glodny, Johannes, Monié, Patrick

Abstract:

Structural, thermochronological and metamorphic data are used to elucidate the tectonic nature and evolution of the ductile extensional Messaria shear zone and the associated brittle Messaria and Fanari detachment faults, which exhumed their footwall from mid-crustal depths on the island of Ikaria in the Aegean. Thermobarometric data indicate that the Messaria shear zone formed at 350->400 °C and 3-4 kbar (i.e. at a depth of c. 15km). Normal faulting was accompanied by the intrusion of two granites, which together with the thermobarometric data indicate a relatively high thermal field gradient of 25-35 °C km^sup -1^. Zircon and apatite fission-track and apatite (U-Th)/He ages demonstrate rapid cooling in the footwall of the Messaria detachment from c. 400 °C to c. 40 °C between 11 and 3 Ma. Age-distance relationships of the data suggest that the Messaria shear zone and the Messaria detachment slipped at apparent rates of c. 69 km Ma^sup -1^. Kinematic indicators show a consistent top-to-the-NNE shear sense for the extensional faults. However, at the southern part of the Messaria detachment some late-stage shear-sense indicators are top-tothe-SSW and are assumed to be associated with updoming of the footwall. Numerous deformed pegmatite veins in the Messaria shear zone allow the reconstruction of deformation and flow parameters. The mean kinematic vorticity number ranges from 0.13 to 0.80, indicating that shearing deviated significantly from simple shear; that is, extensional shearing was associated with vertical ductile thinning, which contributed to tectonic exhumation. Finite strain shows oblate geometries and axial ratios of the finite-strain ellipse in sections parallel to tectonic transport and normal to the mylonitic foliation range from 1.8 to 19.9. We calculate, using a 1D numerical model, that vertical ductile thinning contributed c. 20% to exhumation during extensional shearing. Normal faulting was the major agent exhuming the footwall from c. 15 km depth.

Keywords: Aegean Sea, Greece, exhumation, extension, ductile flow.

The extending Hellenide orogen in the Aegean Sea of Greece (Fig. 1) exposes a number of spectacular brittle low-angle normal faults (detachments) (Lister et al. 1984; Lee & Lister 1992; Gautier & Brun 1994; Ring et al. 2001a). On some Aegean islands, the brittle detachments are associated with underlying extensional ductile shear zones and examples of these ductile-tobrittle extensional fault systems have been reported from the Cycladic islands of Naxos (Buick 1991) and Ios (Vandenberg & Lister 1996). It has been suggested that detachment faulting was the primary agent achieving >250 km of extension since the Miocene (Ring et al. 20030). Some workers have even claimed that Miocene extensional faulting fully accomplished the exhumation of the Cycladic blucschist unit in the central Aegean (Lister et al. 1984).

The subhorizontal foliation in the Cycladic blueschist unit suggests that ductile extensional shearing was accompanied by vertical shortening and therefore was not simple shear. Evidence that natural shear zones deviate significantly from progressive simple shear and are characterized by pronounced shortening perpendicular to the shear zones has been shown by a number of studies (e.g. Coward 1976; Choukroune & Gapais 1983; O'Hara 1990; Simpson & DePaor 1993; Bailey et al. 1994; Ring 1999). These studies indicate that ductile flow contributes to the exhumation of metamorphic rocks in the footwalls of extensional and contractional shear zones. For extensional shear zones this implies that tectonic exhumation involves two processes: (I) normal faulting; (2) vertical ductile thinning (Ring et al. 1999α). The relative importance of these two processes during exhumation is a matter of debate.

On the island of Ikaria in the eastern Aegean Sea, a low-angle extensional ductile shear zone and two associated brittle detachment faults are well exposed. We have studied this extensional fault system in detail. Numerous deformed pegmatite veins in the shear zone allow the reconstruction of deformation and flow parameters (Passchier 1990). In combination with metamorphic petrology and gcochronological work this allows us to constrain the amount of vertical ductile thinning associated with extension and thus to evaluate fully the tectonic significance of extensional shearing.

Geological setting

Previous research has distinguished several tectonic zones in the Hellenides characterized by rock type, stratigraphy, tectonometamorphic history and pre-orogenic palaeogeography (Robertson et al. 1991). The Cycladic zone is fringed to the north by the oceanic Vardar-Izmir-Ankara suture zone and the continental Lycian-Pelagonian zone (Fig. 1). The dominant tectonic unit of the Cycladic zone is the Cycladic blueschist unit, which comprises from top to bottom three composite nappes: (1) an ophiolitic mélange; (2) a Permo-Carboniferous to latest Cretaceous passive-margin sequence (blueschist series of Vandenberg & Lister 1996); (3) a Carboniferous basement, which also occurs as slices in the passive-margin sequence. The Cycladic blueschist unit is overlain on some islands by the Upper Unit, which is a non- to weakly metamorphosed ophiolitic nappe that contains Pliocene sediments. In some windows in the Cycladic zone, the Basal Unit, as part of the External Hellenides, crops out below the Cycladic blueschist unit (Godfriaux 1968; Avigad & Garfunkel 1989). The Cycladic blueschist unit and the External Hellenides were part of the Adriatic microcontinent. A major difference between the Aegean and adjacent western Turkey is that in the latter the Menderes nappes instead of the External Hellenides form the lowermost tectonic unit. In contrast to parts of the External Hellenides, the Menderes nappes do not show Tertiary high-pressure metamorphism (Gessner et al. 2001 ; Ring et ai 19996, 2001è; Régnier et al. 2003). The Mcnderes nappes are part of the Anatolian microcontinent, which collided with Eurasia further east than Adria (see tectonic reconstruction of Gessner et al. 2001, fig. 16). A distinctive feature of the Anatolian microcontinent is that its basement formed at c. 550 Ma (Gessner et al. 2001, 2004).