Tectonic juxtaposition of blueschists and greenschists in Sifnos Island (Aegean Sea)-implications for the structure of the Cycladic blueschist belt

In the Cyclades, kilometre-thick high-pressure rock sequences, displaying blueschist- and eclogite-facies mineral assemblages, overlie a rock sequence which was thoroughly overprinted in the greenschist and amphibolite facies during its exhumation. Both rock sequences, subducted and metamorphosed at high-pressures in Eocene times, have been considered by previous workers to have been exhumed as a coherent rock unit. In contrast, it is suggested here that the preserved high-pressure rock sequences were exhumed more rapidly and prior to the underlying greenschists. Significant metamorphic, structural and geochronological discontinuities exist across the blueschist-greenschist contact and field evidence suggests that the high-pressure metamorphic rocks in Sifnos are tectonically juxtaposed above the greenschists. These two rock sequences were juxtaposed by a low-angle fault subsequent to the Oligocene-Miocene greenschist-facies overprint. Published geochronological data and petrological criteria are used to show that the high-pressure sequence cooled below 350°C when the rocks now immediately underlying it suffered a greenschist-facies overprint at temperatures of ca 450°C. The section inferred to absorb this temperature difference is now missing and it is suggested that it has been cut out by the low-angle fault.