Aegean crustal thickness inferred from gravity inversion. Geodynamical implications

Since Oligo–Miocene times, the Aegean domain has undergone regional extension due to the southward retreat of the Hellenic subduction zone. Boundary conditions have been more recently modified by the westward extrusion of Anatolia. A new map of the Aegean crustal thickness inferred from gravity inversion is proposed here to better constrain the variations in space and time of crustal thinning that has accumulated since Oligo–Miocene times. Moho topography is obtained by inversion of satellite marine gravity data. Data are first corrected for terrain anomalies and deep mantle effects (African subducting slab). They are then filtered between 50 and 300 km to avoid short wavelength intracrustal effects. Results are consistent with previous 2D geophysical studies (seismic refraction, receiver functions) and show that an overall regional isostatic compensation of the crust holds for the Aegean area, with a mean crustal thickness of 25 km. Three different provinces (North Aegean, Cyclades and Cretan Sea) can be identified. Thinner crust is observed both in the North Aegean region (NE–SW trending of thinning, with crustal thickness lower than 24 km) and in the Cretan Sea (crustal thickness of 22–23 km). Between these two regions, the Cyclades are marked by a rather flat Moho at 25 km. A two-stage model of the Aegean extension could well explain the observed crustal thickness variation. From Oligocene to middle Miocene, gravitational collapse of the Hellenides, due to the southward retreat of the African slab, reduced the Aegean continental crust from 50 km (by reference to continental Greece and Anatolia) to a mean value of 25 km at the scale of the whole Aegean. From upper Miocene to present, the westward extrusion of Anatolia modified the extension and the associated crustal thinning in the North Aegean domain. During this second episode, crustal thinning related to the southward retreat of the African slab tends to localize in the Cretan Sea. The Cyclades likely behave as a rigid block translated toward the south–west without significant deformation, in agreement with the GPS velocity field and the lack of major earthquakes.