The extensional Messaria shear zone and associated brittle detachment faults, Aegean Sea, Greece
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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. 15 km). 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−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. 6–9 km Ma−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-to-the-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.