The Vari Unit in the hanging wall of the West Cycladic Detachment System (Agios Georgios, Greece): A small island with a big message

The timing and kinematics of low-angle normal faulting in the Cyclades (Aegean region, Greece) has been a matter of debate, mainly because the detachments arch over the islands and only remnants of the fault systems and small klippen of hanging-wall rocks are preserved. The small (4.3 km²) island of Agios Georgios, 20 km south of the Attica Peninsula, lies structurally above the West Cycladic Detachment System (WCDS). The island consists of metavolcanic greenschists and grey pelitic schists defined by an Ms+Chl+Ep/Czo+Ab±blue Amp assemblage. Zoned blue amphiboles occur both within albite porphyroclasts and the matrix in greenschists, and in pelitic schists. A locally strongly deformed granitoid dominates the east end of the island, which also contains zoned amphiboles (partly relict magmatic?), actinolite often replacing hornblende and two generations of white mica. The granitoid yields a zircon U-Pb date of 248.2 ± 1.0 Ma (MSWD: 0.83) with Variscan inheritance. Dynamically recrystallized feldspar in both the granitoid and host rocks suggests upper greenschist to amphibolite facies conditions. The white mica in the host rock forms bundles that define an anastomosing foliation, and although strongly deformed in many parts, stretching lineations and shear-sense indicators show variable orientations. New ⁴⁰Ar/³⁹Ar geochronology on the mica yields c. 60-47 Ma dates, and new zircon (U-Th)/He dates indicate cooling <200°C at c. 21 Ma. The new geochronology and lack of a strong top-to-SW directed deformation, diagnostic of the WCDS, indicates that the island lies well above the detachment system.

Thirty km to the NNE, along a section perpendicular to the strike of the WCDS, lies the small (18.5 km²) island of Makronisos, where the WCDS and the footwall are exposed. The island consists of greenschists and pelitic/graphitic schists with quartzites and blue-grey mylonitic marbles, and metabasites containing blue amphiboles. The structurally highest level consists of white to pale grey/reddish ultramylonites up to 40 m thick lying on the central ridge of the island and along the NE and NW coasts. Strongly clustered stretching lineations and macro-/microscopic shear-criteria indicate a top-to-SSW shear sense. Deformation mechanisms in quartz (LT-bulging) and calcite (recrystallization) indicate deformation at ~300°C. Albite porphyroclasts may preserve an older foliation and an earlier, higher grade metamorphism. This record is consistent with cooling during top-to-SSW deformation. New ⁴⁰Ar/³⁹Ar analyses on white mica yield ages of 22-15 Ma, markedly younger than the ages from Ag. Georgios. Published zircon (U-Th)/He dates from the Western Cyclades footwall record cooling at 14-9 Ma. The relict status of the HP-mineral assemblages suggests Makronisos is part of the Lower Cycladic Blueschist Nappe (i.e. below the Trans Cycladic Thrust) and hence the ultramylonites are interpreted as the footwall of the early ductile high strain zone forming the WCDS. The older dates and distinctly different structural styles on Ag. Georgios indicate that as the hanging wall to the WCDS it is comparable to the Vari Unit on Syros, likely an extensional allochthon of the Pelagonian block.