Evolution of a passive crustal-scale detachment (Syros, Aegean region): Insights from structural and petrofabric analyses in the hanging-wall

Abstract

New detailed (micro-)structural investigations, quartz petrofabric analyses and geological/structural mapping in southeast Syros (Cycladic massif, Aegean region) allow us to place new constraints on the tectonic evolution of the Uppermost unit, which occupies the immediate hanging-wall of the crustal-scale Vari Detachment. We show that the Eocene ‒ Oligocene deformation history in the hanging-wall of this detachment is associated with SW-directed ductile shearing. This history includes an early distributed constrictional deformation expressed by transport-parallel upright folds, L-tectonites and cleft-girdles quartz c-axis fabrics that were formed at temperatures ∼500 °C. Ductile deformation progressively localized at the bottom of the Uppermost unit leading to the formation of a greenschist-facies mylonitic zone under plane strain conditions inferred from Type-I cross-girdles quartz c-axis fabrics. The ongoing mylonitization was also associated with temporally increasing pure shear component of deformation coupled with cooling from ∼500 °C to ∼400 °C. We suggest that the Vari Detachment represents a passive normal-sense roof fault resulted from the NE-directed ductile extrusion of the Blueschist unit (footwall) at middle Eocene ‒ Oligocene times. In the middle Miocene, the activation of the brittle SSW-directed Late Vari Detachment enhanced the brittle exhumation of both the Blueschist unit and the Vari Detachment.