The Cycladic Blueschist Unit of the Hellenic subduction orogen: Protracted high-pressure metamorphism, decompression and reimbrication of a diachronous nappe stack

Abstract

We discuss the assembly and tectonochronology of the Cycladic Blueschist Unit (CBU) in the central Aegean Sea region, Greece, as a composite, subduction-related, high-pressure (blueschist to eclogite facies) stack of nappes. Our work is based on a review of geochronologic data from the Cycladic islands of Tinos, Syros and Sifnos. In addition, we present a large set of 34 new, internally consistent Rb-Sr multimineral isochron age data from these islands that can be linked to distinct metamorphic stages and episodes of ductile deformation. The reviewed and new ages provide remarkably similar results indicating that the age data and their interpretation is robust. We find that three distinct, significantly diachronous, CBU high-pressure nappes were sequentially accreted between ~55 and ~30 Ma, with a fourth ~24-22 Ma old high-pressure belt, the Basal Unit, below the CBU. Each CBU nappe was exhumed soon after its high-pressure imprint and experienced greenschist-facies overprinting with partial metamorphic reworking ~10 Ma after high-pressure accretion. At the time the structurally higher high-pressure nappe was exhumed, an underlying nappe was accreted and metamorphosed at high-pressure conditions. Our work suggests that the high-pressure nappes were considerably exhumed as rather coherent units during sustained lithospheric shortening in a subduction channel by return flow. The early Oligocene underthrusting of the External Hellenides to the south of the Cyclades, and tectonically below the CBU, is interpreted to have reduced the taper of the Hellenic subduction wedge, which caused significant shortening across the Cyclades region and an important phase of out-of-sequence thrusting that emplaced the non-high-pressure upper units above the CBU. Our analysis shows that this shortening event also caused significant reimbrication within the CBU along the recently discovered Trans Cycladic Thrust.