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the tethyan roots of southeastern siciLy (itaLy): an interdiscipLinary insight from new 3d seismic tomographiesand petrophysicaL inferences E. Giampiccolo1, A. Brancato1, F.C. Manuella2, S. Carbone2, S. Gresta2, V. Scribano2

1 Istituto Nazionale di Geofisica e Vulcanologia – Osservatorio Etneo, Catania, Italy2 Dip. Scienze Biologiche, Geologiche e Ambientali, Sezione di Scienze della Terra, Università di Catania, Italy

A number of geological and geophysical investigations were carried out to determine the structure and composition of lithosphere of the Hyblean Plateau (south-eastern Sicily) (e.g., Finetti et al., 2005 and references therein). Detailed datasets were gathered for the Hyblean upper crust, consisting of a Meso-Cenozoic sedimentary-volcanic sequence (Bianchi et al., 1987). However, no direct information is known for the buried Permo-Triassic basement of this region (Bianchi et al., 1987; Finetti et al., 2005), which is believed the uplifted emerged portion of the Pelagian block, as the northern offshoot of the Africa continental plate (Burollet et al., 1978, Ben-Avraham et al., 1990; Lentini et al., 1994), extending from Tunisia to Sicily. On the other hand, a 30-year study of deep-seated xenoliths embedded in tuff-breccia deposits of Hyblean diatremes (e.g., Scribano 1987; Scribano et al., 2006a,b), Late Tortonian-Lower Messinian in age (Suiting and Schmincke, 2010), led Manuella et al. (2013, 2014, 2015) to propose a new lithospheric model for southeastern Sicily, and neighboring areas. This model, which is based on an interdisciplinary study of petrologic and geochemical data from Hyblean xenoliths and volcanic rocks, with recent geophysical datasets (Giampiccolo et al., 2003; Brancato, 2005), is consistent with the hypothesis proposed by Scribano et al. (2006a,b) and the palaeogeographic model developed by Vai (1994, 2003) and Catalano et al. (1996), attesting that Hyblean Permo-Triassic basement, is a remnant of the slow-spreading Permian Tethys Ocean (Manuella et al., 2015a) as a part of the Oman-Iraq-Levantine-Sicily-Texas oceanic seaway.

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Geophysical investigations of the crustal structure of the Hyblean Plateau include several velocity tomography studies, in which different techniques and datasets were considered (e.g. Di Stefano et al., 1999; Barberi et al., 2004; Scarfì et al., 2007; Brancato et al., 2009; Musumeci et al., 2014).

Fig. 1 – P-waves velocity model resulting from the 3D inversion. S-N and W-E vertical sections along the traces reported at 0 km level are also shown. On the 0 km layer the main structural features are reported with grey lines. The zones with RDE ≥ 0.4 SF ≤ 3.5; DWS ≥ 100 are shown with thick black, dotted white and dashed white contour lines, respectively.

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In the present work we propose new 3D VP and VP/VS models and, for the first time, a 3D P-wave attenuation image (QP) for southeastern Sicily, obtained by using a large dataset of local seismic events that occurred in the time span 1994–2013. Measurements of attenuation of seismic waves are important indicators of Earth heterogeneities, not easily amenable to study using only seismic velocities (i.e., Fehler et al., 1992; Mitchell, 1995).

We used the SIMULPS12 (Thurber, 1983). algorithm to jointly invert a total of 4448 P and 2876 S absolute arrival times. Based on the P and S ray paths of the selected data obtained by

Fig. 2 – VP/VS model resulting from the 3D inversion. S-N and W-E vertical sections along the traces reported at 0 km level are also shown. The zones with RDE ≥ 0.4 SF ≤ 3.5; DWS ≥ 100 are shown with thick black, dotted white and dashed white contour lines, respectively.

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the pseudo-bending method (Um and Thurber, 1987), we used a regular horizontal grid with 10 km×10 km node spacing, covering an area of 110 km×110 km. In the vertical directions we used a grid spacing of 2 km, covering a depth range from the surface to 24 km, as in Brancato et al. (2009). A minimum 1D velocity model was used as input for the 3D inversion, with the initial VP/VS fixed at a constant value of 1.78 at all nodes, based on previous studies in the investigated area (Musumeci et al., 2003; Piana Agostinetti and Amato, 2009).

Figgs. 1 and 2 show the horizontal slices at different depths and the N-S and W-E cross

Fig. 3 – QP model resulting from the 3D inversion. S-N and W-E vertical sections along the traces reported at 0 km level are also shown. The zones with RDE≥ 0.35 SF ≤ 4.5 and DWS > 100 shown with thick black, dotted white and dashed white contour lines, respectively.

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sections of the VP, and VP/VS models obtained, respectively. In the obtained VP 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 VP 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 VP 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 VP, the VP/VS 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 VP/VS contrasts well matching with VP anomalies. From about 6 km to 19 km, the central eastern area shows high VP/VS overlapping the high VP 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 QP 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 VP tomography, assuming an initial QP = 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 QP (> 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 QP that correlates with low VP and high VP/VS. At greater depths, in the eastern sector, high QP values are mainly found in coincidence with high VP and low VP/Vs volumes. Conversely, in the central area high QP values are associated with high VP and high VP/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 VP values, suggest that the structure and composition of the Hyblean lithosphere may differ from the traditional Africa continental plate.

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