Crustal image of the Ionian basin and accretionary wedge

The interpretation of the CROP-MARE deep seismic lines has revealed the structure of the Ionian crust and the geometry of the subduction beneath the Calabrian Arc. CROP seismic lines crossing the Ionian abyssal plain and the south-eastern Tyrrhenian, through offshore southern Calabria, return an image of a well-developed SE-vergent accretionary wedge and the NW-dipping oceanic basement. In the abyssal plain, the Ionian basement images a highly reflective, layered body and a transparent and unstratified band with overlapping hyperbolae, corresponding to an oceanic crust; the oceanic Moho deepens northwards from 9 to more than 10 s/TWT in a few kilometres. The crystalline crust is still coupled with the oldest sedimentary layers. In the intermediate Ionian sector the crust progressively deepens with a complex trajectory, appearing offset by a WNW-ESE lateral (?) discontinuity. The Moho develops at about 11 s/TWT. In approaching the SE Calabrian offshore area the oceanic crust becomes steeper as the Moho discontinuity occurs at more than 14 s/TWT. The upper crust, strongly deformed in some tectonic slices, is overlain by a 2 s/TWT thick sedimentary rock-imbricated wedge, in turn underlying a 3 s/TWT thick seismically transparent to chaotic body, correlatable to the adjacent Calabrian units. The reflection seismic lines imagery shows that both sedimentary and crystalline Ionian crustal bodies are progressively detached from their substrate more deeply and markedly towards NW. As a consequence, the sedimentary and oceanic crust units appear imbricated to form the SE verging accretionary wedge. The subduction hinge zone is seismically imaged in the area where the Calabrian crystalline units overthrust the deformed oldest sediments deposited on the Ionian crust. The occurrence of an oceanic crust certainly favours subduction in this area, generating the Aeolian volcanic arc and the deep seismicity in the south-eastern Tyrrhenian, as well as the ascent of the Etna magmas. The comparison with the crustal setting of the adjacent continental crustal sectors (Sicily and Southern Apennines) highlights the importance of the crustal and lithospheric heritage of the downgoing foreland. The convergence of two continental crusts causes more difficulty in the subduction of the Sicilian crust with respect to the Ionian sector where a greater convergence rate facilitates both a southward advancing of the deformation front (arcuate shape of the Apenninic front) and a vertical separation between the Ionian and Sicilian crusts. The surface expression of this behaviour is the shorter propagation of the Sicilian frontal accretion and the building of a chain with a greater topographic relief than the accretionary wedge of the Calabria-Ionian sector.