The Early Cretaceous to Miocene tectonic evolution of NW Cyclades based on 40Ar/39Ar multiple single crystal dating from Andros

Abstract

The Cycladic Archipelago in Greece preserves widespread evidence for high-pressure (HP) metamorphism and subduction and extension tectonics, during the evolution of a NE dipping subduction zone. How and at what rates various HP rocks were subducted and exhumed and how the associated complex deformation patterns originated remains controversial. We collected 26 samples from the Cycladic Blueschist Unit (CBU), the overlying Makrotantalon Unit (MU) and their contact zone on Andros for white mica single grain 40Ar/39Ar dating and combined the results with field and petrological observations to constrain the tectonic evolution of the NW Cyclades. To investigate the thermal state during tectonic evolution and the duration of deformation, we conducted 40Ar/39Ar multiple single crystal dating on different grain size fractions of white mica from the same sample. Our results show three distinct episodes of deformation: D0 is only locally preserved within the MU and the underlying contact zone and derives from the Early Cretaceous (128-119 Ma) Vardar ophiolite obduction above the Pelagonian margin, driving metamorphism and deformation of the passive margin. The white mica grains from the MU present Early Cretaceous ages of ~128 Ma (500-1000 mm) and ~117 Ma (120-250 mm). We suggest that the 11 Ma difference indicates a slower cooling rate. D1 recorded in the contact zone and in the CBU, indicating that the CBU was exhumed along this contact zone from subduction depths during the Eocene (52-48 Ma). The white mica grains from the CBU present Eocene ages of ~52 Ma (500-1000 mm) and ~48 Ma (120-250 mm), with a gap of 4 Ma suggesting a faster cooling rate. D2 is characterized by ductile-to-brittle deformation preserved in the contact zone and within the CBU, indicating they accommodated extension from around 28 Ma, with the initiation of slab rollback in the eastern Mediterranean. The contact zone between the MU and CBU exhumed greenschist-facies rocks of the CBU as a detachment until 21 Ma, when the rocks crossed the brittle-ductile transition and initiated brittle deformation, characterized by top-to-NE normal faulting. We further redefine the structure of North Andros. In our study, we merge the MU and the ophiolites of the upper unit exposed in North Andros, previously thought to be separate units, in one unit equivalent of the Pelagonian unit. The contact between the MU and CBU, previously thought to be a thrust, is remapped as a brittle-ductile detachment fault between this one Pelagonian unit and the lower CBU and represents the NW extension of the Tinos, Mykonos detachments. This contact has accommodated syn-orogenic exhumation the CBU below the Pelagonian unit during Eocene HP metamorphism, and accommodated regional extension as part of the North Cycladic Detachment System during the Miocene.