Lithospheric structure along a transect from the Northern Apennines to Tunisia derived from seismic refraction data

The region of the Mediterranean Sea has gone through a complicated history of evolution and it has been tectonically active until the present. Continental collision zones with the evolution of orogenies, isolated continental fragments, young marine basins partly overlying oceanic crust, and subduction zones are found close together. The general stress distribution of the whole region is controlled by the relative motion between Africa and Eurasia. However, movements of small blocks can be locally more significant. The whole region of the Mediterranean Sea has been the subject of many geological and geophysical studies. Nevertheless, there are still many open questions to answer which are important for a better understanding of its tectonic evolution. A series of seismic wide-angle reflection and refraction experiments was carried out between the Northern Apennines in northern Italy and Tunisia as part of the Southern Segment of the international European Geotraverse project (EGT). Several of these experiments were designed for a high resolution survey of the crustal structure in the marine areas. The method of two-dimensional raytracing was used for the interpretation of these mostly N-S oriented profiles and several other profiles in Sardinia and the Tyrrhenian Sea. Limited information was obtained also about the lower lithosphere beneath Corsica and Sardinia from data at recording distances of up to 580 km. The crustal structure of the Corsica-Sardinian block in the center of the segment is laterally rather homogeneous and has at a depth of 16 to 19 km a clear boundary to the lower crust except for southern Sardinia. In general the seismic velocities increase at this boundary from 6.1 to 6.3 km/s to 6.6 to 6.8 km/s. The crust-mantle boundary is bowl-shaped with its greatest depth of about 32 km beneath southern Corsica and northern Sardinia. The crustal thickness decreases towards the surrounding marine basins to 18 km in the Ligurian Sea north of Corsica, to 21 km in the Sardinia Channel south of Sardinia, and to 15 km in the Tyrrhenian Sea east of northern Sardinia. The contact zones between the crust of the Corsica-Sardinian block and the crust of Liguria and of Tuscany north and east of Corsica are very complex but of different types. The Ligurian Sea represents a stretched continental crust with less indication of an oceanization in the area of the EGT than in the Sardinia Channel. The structure has been strongly influenced by the asymmetrical opening of the Proven9al Basin, the progressive rifting northeasterly directed, and the counter-clockwise rotation of the Corsica-Sardinian block. The adjacent Ligurian crust in the north has a thickness of only 20 to 25 km with sedimentary layers reaching to a depth of about 10 km. Previous interpretations show an overlapping of the crustal units caused by the collision of Corsica and Tuscany. This could not be confirmed. However, the detailed structure of the transition in the northern Tyrrhenian Sea from the 32 km thick Corsican crust in the west to the 21 km thick crust under Tuscany in the east could not be determined well enough from the available data. The transition to the central Tyrrhenian Sea east of Sardinia is similar to the passive continental margin of the Atlantic type. Clear differences in the average seismic velocity and crustal structures are recognized north and south of a basement high in the central Sardinia Channel between Sardinia and Tunisia. A velocity of more than 6.0 km/s is already reached beneath 1 to 3 km thick sediments in the northern part but only at a depth of 8 km in the southern part. The velocity does not increase significantly down to the Moho beneath the southern Sardinia Channel. An oceanization of the thinned continental crust