Pre-stack depth migrated (PSDM) seismic transects across the Calabrian Arc: a Miocene-Pleistocene arcuate complex accretionary wedge in the Ionian sea

The Calabrian Arc (CA) is the most impressive arcuate feature of the Central Mediterranean sea and connects the E-W trending Sicilian Maghrebian belt with the NW-SE trending Southern Apennines defining the African-Eurasian plate boundary in the Ionian sea. The CA attained its geometry mostly in the interval between Middle-Late Miocene to Pleistocene and its evolution is related to the opening of the back-arc Tyrrhenian basin, in response to the SE retreat of the subduction zone caused by the sinking of the Mesozoic Ionian oceanic lithosphere along a steeply inclined Benioff plane. In this geodynamic frame, the submerged portion of the CA is a key area to study subduction and collisional processes in detail. We reconstructed the regional architecture of the accretionary complex from MS-OGS lines and CROP deep seismic profiles acquired during 70s and 90s in the Ionian Sea. We preliminary re-processed MCS seismic data at ISMAR-BO, through a sequence that involves velocity analysis, Dip Move Out (DMO), velocity analysis after DMO, stack and time migration. Three CROP seismic lines were further processed at the Marine Geodynamics Department of the IFM-GEOMAR (Kiel) in the frame of the European EC-IHP project. We obtained full pre-stack depth-migrated (PSDM) seismic sections through an iterative migration procedure (The SIRIUS/GTX, Migpack software package) that uses seismic velocities constrained by focusing analysis and common reflection point gathers. We have thus obtained a very accurate velocity model that includes both lateral and vertical velocity variations in agreement with interpreted geological cross-sections. The offshore external part of the CA is represented by a 300 km wide accretionary wedge, bordered by the Malta and Apulia Escarpments. Its active front extends southward close to the Medina Bank and is dissected by a prominent NNW-SSE striking tear fault located about 50 km E of the Malta Escarpment. The accretionary wedge, whose geometry and evolution are driven by the slow convergence between the African and the Eurasian Plates (estimated in 5 mm/yr or even <5 mm/yr), is characterized by a sub-horizontal décollement, and consists of African Plate sediments, scraped off from the thick (up to 10 km) Mesozoic and Cenozoic sedimentary cover of the descending plate and piled up along low angle thrust faults. The surface of the accretionary complex is marked by a gentle and irregular slope that sinks the water depth down to more than 4000 m. The outer portion of the slope (approximately from 3000 m to 4000 m of water depth down to the Ionian abyssal Plain) displays shallow strong deformation triggered by the presence of a very thick sequence of Messinian evaporites that have been accreted since Messinian. In this outer zone, the contractional deformation is driven by a detachment surface located at the base of the evaporites. The inner portion of the slope (approximately from 2000 m to 3000 m depth) exhibits a steeper topographic gradients associated with pronounced gravity anomalies and is the site of the pre-Messinian accretionary wedge. Here, the basal detachment cuts to deeper levels leading to the formation of complex thrust system, chaotic units and mud volcanoes. This region seems to be devoid of evaporites but thick accumulation of Messinian deposits occurs in restricted basins on top of the pre-Messinian wedge. Landward, between the upper slope and the onland Calabria (approximately at depths less than 2000 m or in the inner plateau of the CA), a backstop, made of Calabrian-affinity nappes, is partly overlain by back-thrust units of the pre-Messinian wedge and by the NE-SW trending Crotone-Spartivento fore-arc basin. Beneath most of the accretionary wedge the Mesozoic subducting crust is 18-20 km thick and seems to have an oceanic character, but quite anomalous if compared to typical oceanic crust because of velocity profiles and locally-preserved features typical of a passive margin.