Don’t leave me hanging: Where is the hanging wall in the western Cyclades (Greece)?

Miocene crustal extension in the Cyclades resulted in the development of major low-angle detachment systems, typically juxtaposing the Cycladic Blueschist Unit (CBU) in the footwall against the Pelagonian Unit (PU) in the hanging wall. Although the footwall CBU is well-documented, the PU hanging wall remains poorly understood in the western Cyclades due to extensive erosion. This study presents new geological, geochronological, and thermochronological data from two uninhabited islands, Makronisos and Agios Georgios, to better constrain the architecture and displacement of the West Cycladic Detachment System (WCDS). Our results demonstrate that Makronisos constitutes part of the WCDS footwall, with Raman spectroscopy of carbonaceous material temperatures of ~450°C indicating it belongs to the Lower Cycladic Nappe of the CBU. The island preserves intense top-to-SSW shear strain localized within marble ultramylonites, correlative with structures on neighboring Kea. New white mica 40Ar/39Ar ages indicate Middle to Late Miocene deformation, consistent with CBU exhumation documented elsewhere along the WCDS. Conversely, Agios Georgios represents a hanging-wall remnant situated structurally above the WCDS. It comprises Triassic granitic orthogneiss and metasediments that underwent upper greenschist to amphibolite facies metamorphism. New U-Pb zircon dating confirms an Early Triassic magmatic protolith, and 40Ar/39Ar and zircon (U-Th)/He data reveal a Late Cretaceous-Paleogene metamorphic overprint followed by cooling to shallow crustal levels by c. 20 Ma. The lithological and tectonometamorphic evolution of Agios Georgios closely matches the Vari and Akrotiri units of Syros and Tinos, respectively. This correlation extends the known distribution of Late Cretaceous-Paleogene Pelagonian domain remnants ~100 km westward. The divergent thermal histories, Miocene cooling in the footwall versus Paleogene cooling in the hanging wall, constrain a total WCDS displacement of 20-30 km between 20 Ma and 8 Ma, yielding an estimated slip rate of 1.5-2.5 mm/yr.