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

Abstract

In March 1996, a wide aperture reflection/refraction profiling (WARRP) seismic survey was carried out across the Maliakos–Sporades and northern Evoikos basins, central Greece. Two onshore/offshore seismic lines were recorded, using 14 ocean bottom seismographs (OBS) and 10 land stations. As an energy source, we used one sleevegun of 60 l volume, operating at 120-bar pressure. The results acquired by kinematic, two-point raytracing modelling of the time sections provided evidence of a thinned, stretched continental crust, of only 20 km thickness, below the central part of the northern Evoikos Basin. This basin was developed by the separation of the island of Evia—with continental crust of approximately 30 km thickness—from the Greek mainland, by transtension and stretching of the crust, forming the north Evia and the Sperchios valleys. Along the Maliakos–north Sporades Basin, the crust thins from 34 km at the mainland to 22 km below the Sporades Basin. The crust at the northern part of the island of Evia and the Trikeri Straits is 30–32 km thick and of normal continental structure; it is separated by an intercrustal discontinuity between an upper and lower crust. The sediments thicken significantly towards the Sporades Basin, where they obtain maximum thicknesses of the order of 8–10 km. The igneous and metamorphic crust thins significantly in the basin maintaining however its continental character along the entire section. The transition from the Maliakos–Trikeri Straits to 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 Maliakos Straits is poorly understood. Simple or pure shear stretching cannot have 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 stretching of the Aegean Sea that was activated 5 Ma ago.