GNGTS 2016 - Atti del 35° Convegno Nazionale

214 GNGTS 2016 S essione 1.2 sections of the V P , and V P /V S models obtained, respectively. In the obtained V P image (Fig. 1) well resolved areas (RDE ≥ 0.4 SF ≤ 3.5; DWS ≥ 100) are mostly located in the center of the model. From the surface down to 4 km, we observe a central low V P volume that well correlates with the known geology consisting of a package of Meso-Cenozoic carbonate rocks and thinner marl levels (Bianchi et al. , 1987). From 6 to about 16 km, the most significant feature is a central eastern high V P anomaly that occurs with a remarkable variation from 4.8 to 6.9 km/s, showing a dome-shaped basement (see cross sections in Fig. 1). As for V P , the V P /V S model (Fig. 2), is well resolved (RDE ≥ 0.4 SF ≤ 3.5 DWS ≥ 100) only in the central eastern part of the investigated region. From the surface down to 4 km depth, we notice clear V P /V S contrasts well matching with V P anomalies. From about 6 km to 19 km, the central eastern area shows high V P /V S overlapping the high V P region. The attenuation along a ray path is quantified by the t* operator which is evaluated, following Eberhart-Phillips and Chadwick (2002), by fitting the amplitude spectrum decay of P-waves (e.g. De Gori et al., 2005). A total of 2596 t* observations were inverted for the 3D Q P structure, by using the SIMULPS12 algorithm modified for attenuation by Rietbrock (2001). The medium was parameterized with the same 3D grid nodes and velocity values obtained by the 3D V P tomography, assuming an initial Q P = 300, which is reported in literature as the average value in the crust of south-eastern Sicily (e.g., Giampiccolo et al. , 2003; de Lorenzo et al. , 2004). Well resolved areas, delimited by RDE≥ 0.35 SF ≤ 4.5 and DWS > 100, are located in the central eastern sector of the study area (Fig. 3). The most prominent feature is a high Q P (> 300) dome-shaped volume that extends from 6 km to about 20 km. Above 6 km, a very small central portion is characterized by low Q P that correlates with low V P and high V P /V S . At greater depths, in the eastern sector, high Q P values are mainly found in coincidence with high V P and low V P /Vs volumes. Conversely, in the central area high Q P values are associated with high V P and high V P /Vs regions (Figs. 1 and 2). The joint interpretation of both tomographies and the petrophysical interpretations of velocity and attenuation anomalies provide some constraints for the nature and composition of the lithosphere beneath the region. In particular, the present tomographies allow us to extend the punctual outlook deriving from Hyblean xenoliths found in diatremes, representing natural inverse drillings (from the bottom to the surface), to the whole Hyblean lithosphere. Moreover, although the oceanic nature of the Hyblean lithosphere is currently strongly debated, new density and degree of serpentinization models retrieved here from V P values, suggest that the structure and composition of the Hyblean lithosphere may differ from the traditional Africa continental plate. References Barberi G., Cosentino M.T., Gervasi A., Guerra I., Neri G. and Orecchio B.; 2004: Crustal seismic tomography in the Calabrian Arc region, south Italy, Phys. Earth Planet. Int., 147, 297–314. Ben-Avraham Z., Boccaletti M., Cello G., Grasso M., Lentini F., Torelli L. and Tortorici L.; 1990: Principali domini strutturali originatisi dalla collisione nogenico-quaternaria nel Mediterraneo centrale, Mem. Soc. Geol. It., 45, 453–462. Bianchi F., Carbone S., Grasso M., Invernizzi G., Lentini F., Longaretti G., Merlini S. and Mostardini F.; 1987: Sicilia orientale: Profilo geologico Nebrodi-Iblei, Mem. Soc. Geol. It., 38, 429–458. Brancato A.; 2005: Reconstruction of seismogenic structures by high-precise relative location of microearthquake hypocenters occurred in southeastern Sicily (Italy). Ph.D. thesis, Department of Earth Science, University of Naples “Federico II”, Naples, Italy. BrancatoA., Hole J.A., Gresta S. and Beale J.N.; 2009: Determination of seismogenic structures in southeastern Sicily (Italy) by high-precision relative relocation of microearthquakes, Bull. Seismol. Soc. Amer., 99, 1921–1936. Burollet F.P., Mugniot J.M. and Sweeney P.; 1978: The geology of the Pelagian Block: the Margins and Basins of Southern Tunisia and Tripolitania, in The Ocean Basins and Margins, edited by A.E.M. Nairm, W.H. Kanes, and F.G. Stehli, pp. 331–359, The Western Mediterranean. Plenum Press, New York. Catalano R., Di Stefano P., Sulli A. and Vitale F.P.; 1996: Paleogeography and structure of the central Mediterranean: Sicily and its offshore area, Tectonophysics, 260, 291–323