GNGTS 2013 - Atti del 32° Convegno Nazionale

epiclastic deposits. These have been correlated with the top of Basal Clastic Wedge of the Giarre Basin that, in the hangingwall, has been detected by geophysical data (Cassinis et al. , 1970) at an elevation of about 150 m lower. The entire stratigraphic succession encountered in the boreholes S2 and S3, located more to the east, can be clearly referred to the volcanic succes- sion that accumulated on the hangingwall of the fault, on top of the Basal Clastic Wedge. The stratigraphy and the geometry of the successions involved on both the footwall and the hang- ingwall of the fault, here designed as Santa Venerina-Giarre Fault (SGF in the inset of Fig. 1), are illustrated by the two cross-sections (Fig. 2). They show that the base of the Tyrrhenian al- kaline lavas was vertically displaced by the fault for about 150 m and that the accommodation space due to motion along the SGF has been mainly filled with the Milo Formation that forms a wedge widening towards the fault. Finally, the structure appears to be bypassed by huge vol- umes of the volcanic products of the Recent Mongibello (< 15 ka) and the associated deposits of the two cycles of the Chiancone alluvial fan, which cross undisturbed the structure. The bore-hole data, combined with field information, thus suggest an age of the fault, starting from 125 ka B.P.. The end of the fault activity pre-dates the 15 ka and more probably was confined at the age of the top of the Milo Formation (40 ka). However, the buried fault played a major role in the evolution of the paleotopography reproduced in Fig. 2. The structure controlled the pre-15 ka river entrenchment that was confined at the footwall of the structure, while in the hangingwall was dominating the deposition. This topography has influenced the distribution of the volcanic products of the last 15 ka. In the western sectors of the village, where the coseis- mic fractures developed, the recent volcanic cover, alternating with several alluvial horizons, are channelized within WNW-oriented valleys that are modeled on a succession including the Acireale Lahars, the Tyrrhenian alkaline lavas and the Milo Formation (profile 1 in Fig. 2). The subsurface data, fitting well the surface evidence, show a clear continuity of the different stratigraphic units, which exclude the existence of a rooted fault beneath the surface co-seismc fracture zone. The data rather point out that the fracture zone affects the volcanic products and the alluvial deposits infilling the axis of a buried valley. Discussion and conclusions. According to the new geological data on the eastern flank of Mt. Etna, the coseismic fracture zones that have been referred to “hidden” faults developed in almost two distinct conditions. The fracture zones aligned along the Fiandaca and the Santa Tecla faults are actually connected to buried tectonic structures. In both the cases, along the rooted faults, huge Late Quaternary vertical displacements can be recognised, even if the end of their activity can be referred to 125 and 40 ka B.P., respectively. This implies that there is no a direct connection between the active coseismic ground deformation with the long-term evolution of the two structures. This is also demonstrated by the evidence that the lava flows of the last 40 ka by-passed the two faults, obliterating their older morphological expression. Moreover, none new morphotectonic feature generated and cumulated on these recent volcanic products. The cyclical renewal of the active fracture zones, thus, did not produce an appreciable cumulative permanent ground deformation. On the other hand, the explanation of the active fracture zones developed along the Santa Venerina fault is even more problematic, as our data would exclude a connection between the ground deformation and a deep-seated NW-SE oriented fault. Paradoxically, the most unclear situation could provide the explanation for the genesis of the “hidden” faults at Mt. Etna. If the development of the coseismic fracture zones of the 2002 event is considered, we can distinguish along an apparent single NW-SE alignment, designed as Santa Venerina “hidden” fault, two distinct sets of structure. The former consists of theN170° fractures that, being developed in the San Giovanni Bosco area with the same trend and kinematics of the main Acireale Fault, can be referred to the remobilization of a discrete length of the major structure. The latter set of fractures, in the area of Santa Venerina, developed where the geological data inhibit to locate a rooted fault. A genesis of the fractures different from tectonics is also suggested by their unusual geometry that deviates from that of the modeled shear zones, because of the high angle orientation of the single fractures (N 10-20°) compared to the trend of their alignment (N 150°). The detailed 46 GNGTS 2013 S essione 1.1