BLOCK ROTATIONS AND CONTINENTAL EXTENSION IN THE CENTRAL AEGEAN SEA -PALEOMAGNETIC AND STRUCTURAL EVIDENCE FROM TINOS AND MYKONOS (CYCLADES, GREECE)

The Aegean Sea, floored by an attenuated continental crust, is a Medit erranean-type back-are basin formed above the subducting African plate . We investigated post-12 Ma block rotation in relation to back-are ex tension in the area of the western Cyclades (central Aegean Sea). On T inos island, NW-trending dacitic dikes and a Miocene monzogranite pene trated an Alpine metamorphic series. K-Ar analyses of 6 dike samples y ielded an average age of 11.5 +/- 0.4 Ma. The palaeomagnetic analyses of the dikes and the monzogranite reveal 23 degrees vertical-axis cloc kwise rotation. We assume that the dacitic dikes intruded vertical (as they are today) and that no tilt correction is required. This is supp orted by the fact that no inclination anomaly is revealed when the pal aeomagnetic data is compared with a tilt-corrected pole position from Kimi (Evia). The absence of tilting implies that post-12 Ma crustal th inning 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 palaeom agnetic data revealed a considerable amount of inclination flattening and 22 degrees clockwise rotation of the footwall granite. We show tha t tilting predated vertical-axis rotation, and that the (now) low-angl e normal fault originated steeper, dipping ca. 54 degrees. Planar-rota tional fault geometry indicates extension of ca. 60%. In spite of the significant variations in the magnitude of post-12 Ma upper crustal ex tension 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 Mioc ene granitic magmatism in the Cyclades suggests that flow in the lower crust may have had a dominant role in maintaining the smooth Moho. As suming that the western Cyclades constitute a set of NW-striking crust al 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 r eserved.