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 degrees C when the rocks now immediately underlying it suffered a greenschist-facies overprint at temperatures of ca 450 degrees 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.