Current models for Miocene backarc extension of the Aegean region generally suggest that stretching was accommodated mainly by NE-dipping low-angle normal faults with N to NE sense of shear. A crustal-scale low-angle normal fault system trending over a length of more than 200 km forms the North Cycladic detachment system, which records a NE-directed normal shear sense separating the Cycladic Blueschist unit in the footwall from the Upper Cycladic unit in the hanging wall. Based on new structural field data, we propose the existence of another large-scale low-angle normal fault system, the West Cycladic detachment system, which is exposed on Kea, Kythnos, and Serifos, strikes over a length of at least 100 km, and has a possible extension to the SE, where the existence of a South Cycladic detachment system has been recently postulated. The West Cycladic detachment system shares many similarities with the North Cycladic detachment system, with the notable exception that the structure dips toward the SW with top-to-the-SSW kinematics. New 40Ar/39Ar and U-Th/He thermochronological data suggest that the West Cycladic detachment system accommodated extension throughout the Miocene. Since both the North and the West Cycladic detachment systems were active until the late Miocene but exhibit opposing shear sense, we propose that a large part of the stretching of the Aegean crust was accommodated by these two bivergent crustal-scale detachment systems.

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