New insights into the stratigraphic-structural setting of the outermost Calabrian accretionary wedge (NW Ionian Sea ) based on recently-migrated reflection data

External Calabrian Arc (hereinafter ECA) is a SE-verging accretionary wedge developed above the convergent plate boundary between Africa and European plates, related to the subduction of the Ionian plate beneath the Calabrian plate: sediments have been scraped off the subducting Ionian plate and piled up along thrust faults opposite of the European region. Although regional geophysical studies conducted in the past 20 years suggest constraints on the internal structure of the ECA (e.g. CATALANO & SULLI, 2006), few, if any, of these studies discussed in detail its thin frontal portion, i.e. the transition of the ECA with the Ionian abyssal plain. We believe that this is the reason why a major questions remain unanswered: 1) are the Messinian evaporirtes composed of salt so that they behave as a weak décollément level? 2) how the occurring Messinian evaporites influence the tectonic style and geometry of the wedge? Till now seismic character of the Messinian evaporites in terms of reflectivity patterns and deformational structures have not yet been convincingly related to its stratigraphic layering and its rheology. That is because the rough seabed topography (otherwise known as the “cobblestone topography”, HERSEY, 1965) at the ECA southernmost front and the occurrence, at depth, of diffraction hyperbolae coming from the Messinian evaporitic sequence have strongly hampered the study of the outermost ECA internal structure. The afore-mentioned questions are addressed here through a recently-migrated set of existing multichannel seismic reflection profiles from the NW Ionian Sea, around the front of the wedge (Fig. 1). A new detailed seismostratigraphic analysis allows us to better define the seismic stratigraphy of the Messinian evaporite deposits and to analyze the role played by Messinian evaporites in controlling the tectonic style of the thin frontal portion of the wedge. Dataset To constrain better the main features of the thin frontal part of the ECA, we use post-stack time- migrated multichannel seismic profiles of the CROP (CROsta Profonda) Project (Fig. 1). Application of migration techniques to the CROP dataset focused on imaging, in detail, the shallow subsurface in the 1.0–2.0 s ⁄TWT range. We focused on the velocity distribution with depth, by utilizing the results of the Expanding Spread Profile (ESP) data supplied by De Voogd et al. (1992). The result was an improvement of the data quality and enhancement of the signal-to-noise ratio of the data. Seismic Interpretation The seismostratigraphic analysis of the seismic profiles calibrated with the refraction data (DE VOOGD et al., 1992) suggest a general bipartition of the Messinian unit in the outer ECA, consisting of a transparent subunit at the bottom and a layered subunit at the top. Wave-like deformation and salt-based compressional structures characterize the folded but overall unfaulted lower subunit, suggesting ductile deformation of this subunit, submitted to diffuse flow. Thrusting and brittle deformation characterize the overlying subunit which contains faults soling out at its base. Both the difference in seismic facies and the difference in deformational style allow a better definition of the unit‟s stratigraphic layering, which consists of the reflection-free and plastic Messinian salt layer below and the alternation of marls and gypsum above.Locally, the two Messinian subunits are not well-imaged and a chaotic facies occurs. Here, a different deformational style is evidenced by the occurrence of a series of double-verging imbricated thrust sheets of the whole Messinian unit At the south-easternmost border of the study area (Fig. 1), the Messinian unit shows two superposed imbricated packages; the upper one, with a chaotic facies and a thickness smaller than the lower one, appears markedly detached from the underlying one. Due to both the laterally discontinuous occurrence and the reduced thickness of Messinian upper package, we interpret the upper body as the result of large gravitational glide tectonics over Messinian deposits. This large-scale instability of the area, since the Late Messinian, could be due to a progressive increase in the wedge slope steepness, consequent to the thickening of a sub-critically tapered wedge. However, it could be hypothesized that the salt tectonics have also played a role in the emplacement of the chaotic succession. In this case, the gravity-gliding tectonics might well reduce the top of the taper, contributing to lowering the taper value detected for the ECA. Concluding remarks Post-stack time migration of Ionian CROP seismic data provides new and interesting constraints for structures and processes characterizing the outermost ECA. - A more detailed stratigraphy framework of Messinian evaporite deposits than previously known, consisting generally of gypsum and marls overlying salt, is given. The “upper” evaporite layer shows evidence of brittle deformation while the “lower” evaporite layer acted as a globally ductile layer, also through the development of salt-cored thrusting structures. - Lateral variation in composition and thickness of Messinian evaporite deposits reflects a change in style of compressional structures, with the development of double-verging thrust faults offsetting the whole Messinian sequence. - A gravitational glide tectonics characterized the outermost ECA since Late Messinian, due to both the salt occurrence and a wedge slope too steep to support a skinny evaporite-based top layer (a near-surface "olistostrome"). - Migration of data permitted also to map more carefully the ECA leading edge that advances substantially farther to the south than shown on previously reconstructions (see e.g. CATALANO et al., 2001; FINETTI, 2005).