Ductile and brittle shortening, extension-parallel folds and maintenance of crustal thickness in the central Aegean (Cyclades, Greece)

In contrast to previous studies that concentrated on the two-dimensional crustal strain in the central Aegean region (Cyclades), it is shown that NNE-SSW extension via ductile and brittle stretching and low-angle detachments was accompanied and/or alternated with horizontal shortening perpendicular to the stretching direction since at least the early Miocene. Roughly E-W directed ductile shortening produced large-scale overturned and upright folds having axes parallel or slightly oblique to the stretching lineation. Upright folding and arching of low-angle normal faults occurred above the brittle-ductile transition and brittle E-W compression culminated with vertical axis block rotations, strike-slip faults and minor thrusts. Since at least the early Miocene, the structure of the Cycladic massif evolved through alternating and/or coeval increments of horizontal shortening and vertical thinning associated with an approximately constant NNE-SSW stretching. Exact magnitudes of the vertical and horizontal deformational components are difficult to assess. Nevertheless, we note that extensional tectonics that affected the Cyclades during the last 15-20 m.y. have produced no net crustal thinning. We suggest that crustal thickness was maintained by extension-parallel folds, which represent true contractional structures, and that crust was fed into the extended region from its margins.