Field Guide to the West Cycladic Detachment System on Kythnos, Greece

Kythnos lies between Kea and Serifos in the Western Cyclades, both of which were critical in developing an understanding of the Miocene top-to-the-SSW deformation associated with the West Cycladic Detachment System (WCDS). On Kythnos, the Cycladic Blueschist Unit (CBU; WCDS footwall; probable metamorphosed equivalent to the Pindos Unit) preserves relict parageneses of an M1 Eocene blueschist-facies peak metamorphism within retrograde greenschist facies assemblages. The north and east of the island have an ENE-WSW trending structural grain of M1 age; this gradually rotates to a NNE-SSW trend in the south and west, associated with Miocene M2 movement on the WCDS. The structural evolution also reflects extensional deformation during cooling, from the ductile to brittle fields. Southern Kythnos has a well-defined lithostratigraphy (de Smeth 1975) of epidote schists with rare marble conglomerates (Kanala Formation) overlain by sericite-quartz-albite-graphite schists with some Cr-rich mica/chlorite layers (Upper & Lower Flabouria Formations). Between these sericite-quartz-albite-graphite schist formations, blue-grey calcite marble mylonites and quartz–white-mica–calcite schists/mylonites occur (Petroussa Formation); the blue-grey marble thins from ~14 m thick in SE Kythnos to one or more layers, each <0.75 m thick, near the WCDS in SW Kythnos. Another unit of quartz–white-mica–calcite schists/mylonites with lenses/boudins of blue-grey marble mylonites/ultramylonites (Mavrianou Formation) occurs along the west coast of southern Kythnos, overlying the Upper Flabouria Formation; these are similar to the high-strain part of the Petroussa Formation and are locally overlain by epidote schists (Episkopi Formation). However, the distribution of the Mavrianou Formation on the crest of southern Kythnos is different to that proposed by de Smeth (1975), with several isolated small outcrops. An impure grey-brown to blue-grey calcite marble (ultra)mylonite (Rizou Formation) lies directly under the WCDS in SW Kythnos, underlain by albite “gneisses” (Laner 2009, Lenauer 2009); this is possibly equivalent to the Mavrianou Formation. The application of this stratigraphy becomes more uncertain in the northern half of the island. The WCDS comprises marble ultramylonites (Rizou Formation), with top-to-the-SSW shear criteria, overlain on a sharp detachment surface by brittle-deformed quartzites (Aghios Dimitrios Formation), taken to be part of the Upper Cycladic Unit (Pelagonian Unit; although it might be an imbricate of the Cycladic Blueschist Unit). Calcite-quartzite ultracataclasites are locally developed at the detachment surface.