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Abstract

The geometry of large-scale extensional deformation in the southwestern Cyclades archipelago is a controversially discussed
issue. Recent studies suggest a bivergent (i.e., coeval top-to-the-S and top-to-the-N shear) geometry of early Miocene
extensional deformation. On Sikinos Island, top-to-the-S shear structures in the Cycladic basement are overprinted by a
prominent ductile-to-brittle top-to-the-N extensional shear zone in the Cycladic basement at its contact with the tectoni-
cally overlying passive-margin sequence of the Cycladic Blueschist Unit. Hitherto, the top-to-the-S shear structures have
been related to thrusting of the passive-margin sequence onto the Cycladic basement. Rb–Sr multimineral dating constrains
the age of retrograde, lower greenschist-facies top-to-the-S shearing at 23.20 ± 0.25 Ma. This age is identical to published
40 Ar/ 39 Ar ages of ~ 23 Ma for a lower greenschist-facies top-to-the-S extensional shear zone on nearby Folegandros Island.
Therefore, an interpretation of the top-to-the-S shear zone in the Cycladic basement of Sikinos as resulting from extensional
deformation is consistent with the regional scale context of crustal extension invoked for the Cyclades in the early Miocene.
On Folegandros Island, top-to-the-S extension occurs at the top of the passive-margin sequence of the Cycladic Blueschist
Unit, whereas on Sikinos coeval top-to-the-S extension occurs near the top of the underlying Cycladic basement. Our new
Rb–Sr age of ~ 23 Ma indicates that top-to-the-S fabrics reflect the earliest Miocene extensional structures. They are inti-
mately related to the top-to-the-N extensional structures forming an early Miocene bivergent extensional hinge zone at the
southern end of the Cyclades archipelago.