Strain development and kinematic significance of the Alpine folding on Andros (western Cyclades, Greece)

The general trend of both fold axes and stretching lineation in the Cycladic Blueschist Unit is NE-SW to NNE-SSW. This orientation forms a large angle (almost perpendicular) with respect to the Hellenic trend that is inferred from the main thrusts on mainland Greece. Thus, the kinematic significance of the stretching parallel folding in the Cycladic Blueschist Unit is non-trivial. Since within the western Cyclades, the NE-trending folds are best exposed on the island of Andros, it is a key locality for understanding the timing, style and kinematic significance of folding. Here we show that the NE-trending folds on Andros formed within the stability field of glaucophane, after the peak high-pressure metamorphism and simultaneously with the early stage of retrogression. The axes-parallel stretching was non-rotational; it started during the NE folding at blueschist-facies conditions, and continued long afterward and well into the retrograde greenschist overprint. Furthermore, we present the result of a finite strain calculation which shows that the large NE folds could not have been reoriented at ~ 90° as previously thought. Instead it is suggested that these folds formed under constrictional strain regime during regional NE-SW extension, and represent coeval transverse NW-SE shortening and vertical thinning. This implies that NE extension and southwest directed rollback of the active margin prevailed in the western Aegean between the Eocene and early Miocene.