GNGTS 2013 - Atti del 32° Convegno Nazionale

The reprocessing of the CROP profiles in the region, let us to apply the wave equation datuming technique (Barison et al. , 2011; Giustiniani et al. , 2013) extracting information previously hidden by approximate static corrections and surface noise. From picked first arrivals, a tomography inversion created a near surface velocity model by iterative ray-tracing and travel time calculations. Then, the wave equation was applied to move shots and receivers to a given datum plane, removing time shifts related to topography and to near-surface velocity variations. Basically, WED is the process of upward or downward continuation of the wave- field between two arbitrarily-shaped surfaces; a process useful to attenuate ground roll, enhance higher frequencies, increase the resolution and improve the signal/noise ratio. The new outputs show evidence of regional continuity of high amplitude horizons, a better fitting with the surface geological setting and with well data, resembling the local industry investigations; the results confirm the role of overpressured fluids, better define the tectonic setting as well as the contribution to the reflectivity of lithology and of hydrothermal fluids or thermo-metamorphic minerals. The wave equation datuming is now applied to the SIRIPRO profile crossing central Sicily from the Tyrrhenian Sea to the Sicily Channel. This reprocessing requires large computers, parallel processors, because of the long listening times. The profile was acquired without additional complementary geophysical tools, like new wide-angle experiments, offshore- onshore continuation, … It shows the crustal flexure from the Hyblean foreland towards the Caltanisetta through, accompanied by crustal thinning: from an already thinned crust (around 30 km the Moho depth on coast of the Sicily Channel) to about 16-17 km thickness of the same interval beneath Caltanisetta. North of the collision zone the Maghrebides chain includes units of the African margin in a similar way to the crustal setting in the North-Algeria facing the Algerian basin. It is necessary to better resolve the structures of Maghrebides chain, north of the Caltanisetta through in the collision zone, where a Moho uplift is possible, as confirmed by the gravity anomalies and old deep refraction data. Again a bended continuous crust-mantle interface from hinterland to foreland is suggested, with delaminations and the decoupled mantle lithosphere still locally subducted and recycled in the asthenosphere, but more complex structures cannot be excluded. Moreover, contraction events are currently active at the toe of the North Sicilian margins, possibly accounting for the initiation of a new underthrusting/ subduction zone along the southern margin of the Tyrrhenian basin, that will accomodate further convergence between Africa and Europe. A crustal flexure has been observed moving from the Hyblean domain to the Ionian deep basin, where the crust-mantle transition interval is characterized by a curious high amplitude reflectors in the low frequency band (around 10 Hz). The crustal thinning from the Sicily channel to the Ionian basin across the Malta Escarpment is shown in reflection seismic profiles as well as the presence of a crustal transcurrent fault accounting for a major dextral lateral escape of the Calabria Block during the Plio-Quaternary opening of the Tyrrhenian Sea. The fault separates the eastward thinning continental crust from the deep Ionian domain, which crust is proposed as oceanic from many authors. The interpretation of the seismic data is still under debate and the presence of a wide oceanic domain or of an area still belonging to a distal portion of the North African continental margin cannot be confirmed or plainly denied. Following the exposed Tethian ophiolitic suture (in Albania, Western Greece, Crete, Western Turkey, …) the Tethian ocean has been already recycled within the asthenosphere. The Cretaceous rifting, presently recognized offshore Libia, is a possible origin of the thinning of the crust of a north African margin widely extended in the Ionian Sea with the presence of pull-apart basin up to oceanic openings; once more, the eclogitization of the upper mantle can account for the high velocities and high densities necessary to justify the positive Bouguer anomalies. The tectonic loading of the Africa foreland lithosphere by the Calabrian (and Aegean) arcs is not sufficient to account for the 4 km water depth in the Ionian abyssal plain. Other forces, like slab pull, lateral push of the asthenosphere upwelling in the Tyrrhenian XVII GNGTS 2013 L ectio M agistralis