GNGTS 2013 - Atti del 32° Convegno Nazionale

of the modern stratovolcano is constituted by products, ranging in age from about 40 to 15 ka, while the recent volcanic covers consist of products emitted from the active feeding systems, in the last 15 ka (Gillot et al. , 1994; Branca et al. , 2007). A distinction between the Wurmian alkaline lavas and the Holocene lavas is here proposed on the basis of their position with respect to the deposits of the Chiancone alluvial fan (Calvari and Groppelli, 1996). The conglomerates of the “Chiancone” actually belong to two distinct sequences, separated by an erosional surface. The older sequence, confined in age in the last 15 ka (Guest et al. , 1984; Calvari and Groppelli, 1996), forms most of the alluvial fan that is unconformably draped by a thin horizon made up of the younger sequence, showing a radiometric age of 7.6 ka B.P. (Calvari and Groppelli, 1996). Fiandaca Fault. The Fiandaca Fault represents a major tectonic alignment that controls the boundary between the Acitrezza Ridge and the Giarre Basin. The fault is marked by a dramatic offset of the top of the Middle Pleistocene marly-clay, from about – 500 m b.s.l. (Cassinis et al. , 1970), within the basin, to 210 m a.s.l., in the Acitrezza Ridge. The fault motion was also responsible for the emergence of the 460 ka-old tholeiitic pillow-lavas, which were emitted at a minimum depth of about 400 m b.s.l. (Corsaro and Cristofolini, 2000) and for the huge vertical displacement between the pre-Tyrrhenian alkaline lavas resting on the Acitrezza Ridge and those cropping out along the Timpa of Acireale (Fig. 1). On the downthrown sector of the fault, the Basal Clastic Wedge accumulated, with a thickness of about 400 m, comparable with the depth of deposition of the underlying marly clay. The fault is concealed by the marine terrace of the OIS 5.5 (125 ka), which crosses undisturbed the structure. It directly covers the batial marly clays of the Acitrezza Ridge, while it rests on subaerial volcanic succession, including the transitional basalts (225 ka) and the overriding 190 m-thick pre-Tyrrhenian alkaline lavas (140-132 ka), on the downthrown sector. Here, the thickness of the pre-Tyrrhenian sub-aerial products constrains a minimum apparent amplitude (>210 m) of the Tyrrhenian transgression (125 ka) that is at least 70 m wider than the actual absolute rise of the sea-level (136 m; Chappel and Shackleton, 1986). The Fiandaca Fault is buried beneath the Tyrrhenian alkaline lavas and the overriding recent lava flows. However, its trace has been reconstructed taking into account the distribution of the outcrops of the Early-Middle Pleistocene marly-clays, representative for the extent of the upthrown sector, and the isopachs of the lava succession (Cassinis et al. , 1970), significant to discriminate the volcanic cover accumulated on the downthrown sector. The resulting fault trace matches the location of the historical coseismic fracture zones which has been referred to the Fiandaca “hidden” fault (Azzaro, 1999). The relations of the fault with the different stratigraphic units are illustrated along a NNE-SSW oriented cross- section (profile 1 in Fig. 1). The profile shows that the Tyrrhenian alkaline lavas and the overriding recent lava flows, being very thick on the top of the Basal Clastic wedge within the Giarre Basin and thinner on the Acitrezza Ridge, have obliterated a tectonic scarp that cumulated before the Tyrrhenian. Considering all the described geological constraints, we can conclude that the fault activity along the Fiandaca Fault was confined in the 460-125 ka interval, during which it accommodated the whole relative vertical motions along the southwestern margin of the Giarre Basin. Santa Tecla Fault Zone. Adjacent to the Fiandaca Fault, the Santa Tecla Fault Zone (Fig. 1) consists of two main left-stepping NW-oriented segments. The southeastern segment, showing a length of about 6 km, splays from the Acireale Fault, to the north of Acireale. Along a short length of the fault, at its southeastern termination, a tectonic scarp as high as 160 m is exposed. Along the scarp, the fault offsets a volcanic succession that includes the pre-Tyrrhenian and the Tyrrhenian alkaline lavas. The scarp is clearly bypassed by the Holocene lava flows, that cross undisturbed the structure, obliterating every morphological evidence of the fault motion. The trace of the fault has been, thus, reconstructed following the trend of historical coseismic ground fractures (Azzaro, 1999). The northwestern segment, showing a length of 4 km, developed in the area of Zafferana. This segment is marked by a 42 GNGTS 2013 S essione 1.1