Crustal structure beneath the Cyclades Metamorphic Core Complex, Aegean Sea, Greece

The velocity structure of the crust and sediments of the Cyclades Metamorphic Core Complex (CMCC), central Aegean Sea, Greece, was obtained from active seismic observations. The CMCC comprises blueschists resulted from subduction during Eocene-Oligocene, and greenschists formed during exhumation and magmatic/volcanic intrusions during Miocene. Crustal thickness below the Myrtoon Sea is 23 km, thinning below Andros Island to 20 km and, gradually thickening towards the Chios Basin to 23 km, resulting to a dome structure. The upper crust is inhomogeneously structured with high Vp-velocity of 6.4 to 6.8 km/s, exposed to surface levels, followed by a velocity inversion of 5.8 to 5.5 km/s. Geometry of the velocity units indicates that, during crustal shortening, delamination of crustal layers involved in the subduction was significant. The high velocity units exposed on the islands correspond to pelagic volcano-sedimentary rocks and ophiolites of the Pindos-Cyclades oceanic domain, overthrusting low velocity units of the External Carbonate Platform of the Hellenides developed above continental crust that followed into the subduction. Internal thrusting and isoclinal folding have produced thickening of the Cycladic blueschists bearing nappes and duplication of the nappe pile with younger thrusting beneath Andros Island, resembling the structure of Northern Attica. Below the Chios Basin, velocity units are associated with the Internal Carbonate Platform of the Hellenides developed above the Pelagonian continental basement. Below the crust/mantle boundary we mapped reflectivity separating low velocity zones (7.5 to 7.9 km/s) up to 65 km depth. This low velocity upper mantle, associated also with low density, contributes to the isostatic equilibrium of the CMCC. Sediments of 2.6 to 3.6 km thickness are observed within post tectonic basins underlain by extensional detachments. The high velocity (4.8 to 5.3 km/s) sediments at the lower part of the surficial basins are most probably klippen of the Internal Carbonate Platform.