Seismic stratigraphy and tectonic style variation along the Eastern Mediterranean Ridge Accretionary Complex (results of re-interpretation of the MS-multichannel seimsic dataset)

We analyse four multichannel seismic profiles, collected by OGS from 1971 to 1979 along the outer part of the Mediterranean Ridge Accretionary Complex, in order to understand the nature of the deformation affecting African plate sediments as they approach the Africa/Europe plate boundary. Tectonic style as revealed by our data and previous studies varies considerably along the plate boundary and this variation suggests the division of the margin in three provinces characterised by different structural patterns and deformation rates: 1) Near orthogonal terminal subduction-initial collision (Ionian and Sirte foredeeps); 2) Incipient continental collision (collision of continental promontories of the African margin off the Cyrenaica Peninsula); 3) Oblique terminal subduction-initial collision (Herodotus foredeep); This work aims to relate the tectonic features revealed by the seismic profiles to the geodynamic and stratigraphic parameters of the convergent margin. These are essentially related to the geometry of subduction (rate of convergence and obliquity) and to the lithology of the incoming sedimentary section as well as the accumulation rate in the foredeep. In the Sirte province accumulation rate in the foredeep is relatively low and deformation is related to thrusting and folding, which account for sediment shortening, and uplift at the outer deformation front. In the provinces of oblique terminal subduction, the rate of normal convergence is lower, the sedimentary section is thicker, and the foredeep fed by the Nile river has an higher syntectonic sedimentation rate. Here deformation produces a broad band of large wavelength and high amplitude folds. Folding in this area is produced both by plate convergence related shortening and by movement of salt triggered by differential loading of the prograding Nile deep sea fan. In the area of incipient continental collision the foredeep is a V-shaped trough as the abyssal plain has been completely subducted and the folded sediments of the accretionary complex rest directly on the African continental slope. Such differences in structural style are explained by great differences in strain rate, that in the Herodotus foredeep province has been calculated as at least four times weaker than in the western sector. Two factors contribute to decrease strain rate: syn-contractional accumulation and normal component of the effective convergence rate. The former increases from west to east and possibly causes overpressuring on the décollement level, salt mobilisation, landward and seaward vergent folding, and a very low surface taper in the Herodotus foredeep. The latter decreases from west to east and favours thrusting in the western sector.