Thinned continental crust below northern Evoikos Gulf, central Greece, detected from deep seismic soundings

In March 1996, a wide aperture reflection/refraction profiling (WARRP) seis mic survey was carried out across the Maliakos-Sporades and northern Evoiko s basins, central Greece. Two onshore/offshore seismic lines were recorded, using 14 ocean bottom seismographs (OBS) and 10 land stations. As an energ y source, we used one sleevegun of 60 l volume, operating at 120-bar pressu re. The results acquired by kinematic, two-point raytracing modelling of th e time sections provided evidence of a thinned, stretched continental crust , of only 20 km thickness, below the central part of the northern Evoikos B asin. This basin was developed by the separation of the island of Evia-with continental crust of approximately 30 kin thickness-from the Greek mainlan d, by transtension and stretching of the crust, forming the north Evia and the Sperchios valleys. Along the Maliakos-north Sporades Basin, the crust t hins from 34 kin at the mainland to 22 km below the Sporades Basin. The cru st at the northern part of the island of Evia and the Trikeri Straits is 30 -32 kin thick and of normal continental structure, it is separated by an in tercrustal discontinuity between an upper and lower crust. The sediments th icken significantly towards the Sporades Basin, where they obtain maximum t hicknesses of the order of 8-10 km. The igneous and metamorphic crust thins significantly in the basin maintaining however its continental character a long the entire section. The transition from the Maliakos-Trikeri Straits t o the Sporades Basin is controlled by a steep listric fault that downthrows the basin crust by more than 6-7 km. The crustal transition of the thinned Sporades Basin to the 30-km thick crust of the northern Evia and the Malia kos Straits is poorly understood, Simple or pure shear stretching cannot ha ve developed this margin, and we favour the assumption that it denotes the limit of two different crustal domains; these were merged together during a compressive tectonic episode, prior to the present-day extension and stret ching of the Aegean Sea that was activated 5 Ma ago. (C) 2001 Elsevier Scie nce B.V All rights reserved.