GNGTS 2013 - Atti del 32° Convegno Nazionale

cumulative scarp, which is completely draped by the Holocene lava flows. Along this segment, the stratigraphy of the volcanic products on the two sides of the structure is well exposed, providing several constraints for determining the age and the displacement-rate of the fault (Profile 2 in Fig. 1). The cross-section evidences that most of the total vertical displacement (180 m), corresponding to the offset of the top of the pre-Tyrrhenian alkaline lavas (> 125 ka), pre-dates the emplacement of the Tyrrhenian alkaline lavas of the Valle del Bove centers, showing radiometric age of about 93 ka (Branca et al. , 2007). They unconformably cover the Acireale Lahars (132-125 ka) that reach a maximum thickness of about 200 m on the hangingwall of the fault, while they are reduced to a few meters preserved by erosion in the footwall of the fault. The later vertical displacements affect the Tyrrhenian alkaline lavas and the deposits of the Milo Formation (60-40 ka). At the northern termination, the fault segment, as well as the southeastern segment, shows a 160 m-high tectonic scarp. Also in this case, the scarp offsets a volcanic succession that includes the pre-Tyrrhenian and the Tyrrhenian alkaline lavas, ranging in age from 128 to 93 ka. The tectonic scarp is by-passed by the 40 ka-old basal levels of the Wurmian alkaline lavas, that cross undisturbed the fault. The field evidence, collected along the entire Santa Tecla Fault Zone would constrain, thus, an activity of the fault that must be confined in the 125-40 ka. Santa Venerina Fault. The Santa Venerina Fault was described by Azzaro (2004); Azzaro et al. (2009) as a NW-SE oriented, 5 km long seismogenic fault responsible for at least two main historical shocks (1879, Mm=4.3; 2002, Mm=4.1), associated with ground fracturing. Because of the absence of any surface evidence of the structure, the fault has been traced on the basis of the distribution of the coseismic ground fractures that are cyclically renewed. These outline an alignment that splays from the northern tip of the NNW-SSE oriented Acireale segment of the Acireale-S.Alfio fault (Azzaro et al. , 2009; 2012; Monaco et al. , 2010; Barreca et al. , 2013). The last episode of reactivaction of the Santa Venerina Fault, which occurred during the 2002 event, was characterized by the development of two distinct discrete fracture zones: the former originated at the southeastern end of the structure, in the localities of San Giovanni Bosco and Dagala Canne, the latter developed at the northeastern termination of the “hidden” fault, across the village of Santa Venerina (Fig. 2). The southern fracture zone was composed of two distinct clusters of closely spaced N-S to N170° oriented fractures, ranging in length from few hundreds to about 500 m that flanked a discrete segment of the northern termination of the Acireale Fault. This latter, characterized by oblique (dextral) motion (Monaco et al. , 1997) clearly cut through very recent lava flows, forming a buried fault scarp by-passed by historical (1329) lavas (profile 3 in Fig. 1). The two clusters of coseismic fractures developed parallel to the main structure, on both the hangingwall and footwall of the fault, showing kinematics consistent with that of the major structure. The northern fracture zone reactivated a length of 1 km of the termination of the “hidden” fault, along which closely spaced N10°-20° oriented fractures, ranging in length from 100 to 300 m, originated. The fracture zone crossing Santa Venerina has been investigated in detail through a grid of 9 cross-sections, based on integration of field data, from 1:5000 geological mapping, and several logs from bore-holes. The collected data provided sufficient information to outline a 3D geometry of the topography that have been concealed by the recent alkaline lava flows, related to the modern activity of Mt. Etna (< 15 ka; Recent Mongibello; Gillot et al. , 1994)(Fig. 2). In the 3D reconstruction, a sharp 20 m-high scarp marks the trace of a buried NE-SW normal fault, separating the western and eastern sectors of the village (profile 1 and 2 in Fig. 2). The shallow stratigraphic horizons characterising the hangingwall of the structure have been analysed by three aligned, 90 m-deep bore-holes (profile 2 in Fig. 2) that are aligned from the buried fault line towards the east. The borehole S1, located immediately to the south- east of the fault trace, crosses the fault, thus constraining the southeast-dipping geometry of the structure. The deeper portion of the log, related to the horizons uplifted in the footwall of the fault, is represented by the pre-Tyrrhenian alkaline lavas that rest on a paleosoil draping 44 GNGTS 2013 S essione 1.1