Block rotations and continental extension in the central Aegean Sea: palaeomagnetic and structural evidence from Tinos and Mykonos (Cyclades, Greece)

The Aegean Sea, floored by an attenuated continental crust, is a Mediterranean-type back-are basin formed above the subducting African plate. We investigated post-12 Ma block rotation in relation to back-are extension in the area of the western Cyclades (central Aegean Sea). On Tinos island, NW-trending dacitic dikes and a Miocene monzogranite penetrated an Alpine metamorphic series. K-Ar analyses of 6 dike samples yielded an average age of 11.5 +/- 0.4 Ma. The palaeomagnetic analyses of the dikes and the monzogranite reveal 23 degrees vertical-axis clockwise rotation. We assume that the dacitic dikes intruded vertical (as they are today) and that no tilt correction is required. This is supported by the fact that no inclination anomaly is revealed when the palaeomagnetic data is compared with a tilt-corrected pole position from Kimi (Evia). The absence of tilting implies that post-12 Ma crustal thinning in the area of Tinos island was minor. On Mykonos, a low-angle normal fault dipping 30 degrees to the NE juxtaposes a series of Oligo-Miocene molasse sediments above a 10-12-Ma granite. Published palaeomagnetic data revealed a considerable amount of inclination flattening and 22 degrees clockwise rotation of the footwall granite. We show that tilting predated vertical-axis rotation, and that the (now) low-angle normal fault originated steeper, dipping ca. 54 degrees. Planar-rotational fault geometry indicates extension of ca. 60%. In spite of the significant variations in the magnitude of post-12 Ma upper crustal extension across the Tinos-Mykonos transect, crustal thickness below the entire area changes only slightly. This indicates that upper-crustal levels were decoupled from the lower crust. The widespread Middle Miocene granitic magmatism in the Cyclades suggests that flow in the lower crust may have had a dominant role in maintaining the smooth Moho. Assuming that the western Cyclades constitute a set of NW-striking crustal blocks, it is estimated that vertical-axis rotations increased the N-S dimension of the area by ca. 31% (e.g. 25 km). The exact timing of this clockwise rotation is not well constrained, but it postdated the formation of low-angle normal faults and tilting, and may have taken place in the Late Miocene. (C) 1998 Elsevier Science B.V. Ail rights reserved.