GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.2 141 Over the whole TP, a chaotic seismic facies topped by a highly reflective horizon, locally showing internal reflectivity and projecting acoustic shadows below, suggests the occurrence of scattered volcanic bodies; these overlie the pre-Pliocene substratum or are interbedded within the Plio-Quaternary succession, or else are exposed at the seafloor. The magmatic intrusions are mostly localized along and between the main faults, as highlighted by seismic blanking and arching of the reflectors. Magnetic and gravity data, together with the forward magnetic and gravity models along the seismic lines validate the reliability of the seismic interpretation. In particular, the reduced- To-the-Pole (RTP) map highlights the high magnetic signature of the TP area, with respect to the surroundings. The nearshore TP is characterized by scattered, very high-frequency RTP anomalies, related to shallow intrusive bodies, which also crop out along the coast (Aci Trezza islets), and to the submarine continuation of coastal lava flows. The compression and related uplift affecting the coastal sector of TP (Branca et al. , 2014) and the Catania-Motta Sant’Anastasia area, are the direct effect of the transpressive deformation induced by the north- and south-verging thrusts of the strike-slip related push-up. Therefore, it is not strictly due to the active frontal folds of the Apenninic-Maghrebian chain (Catalano et al. , 2017 among others) as also suggested by the lack of allochthonous seismo-stratigraphic units, or by the Etna edifice gravitational spreading (Borgia et al. , 1992). The onshore sub-parallel Acireale andAci Catena faults, together with the westward back-tilted blocks comprising among them, appear as the surficial effect of the volcanic pile creeping failure and coseismic rupture above push-up splays, explaining the presence of active extensional structures in an area affected by important shortening. These faults, together with other well-known structures affecting the eastern flank of Mt. Etna (Fig. 1) represent preferential shallow structures locally conditioning seaward sliding. This movement is less pronounced in the southeastern sector of the volcano up to the onshore continuation of AL, because it is hampered by the transpressive deformation, further lifting the pre-existing TP morpho-structural high (and its onshore continuation). In contrast, it is enhanced to the north, probably favoured by the presence of a tectonically lowered sector. Thus, this process superimposes on the pre-existing morpho-structural setting of the substratum, even though is also triggered by magmatic intrusions. Although several studies reported on volcanic activity associated with transpressional tectonics (Tibaldi et al. , 2010), the current deformation of TP apparently does not fit with local volcanism, as magmatic processes are usually favoured by a tensional regime. To resolve this ambiguity and explain the rapid northward shifting of Etna’s eruptive vents, we propose a model that connects the time-space evolution of volcanism with strike-slip tectonics; we suggest that migration of releasing and restraining zones along strike-slip faults may control the temporal and spatial distribution of volcanism. The WNW-ESE trending dextral “Southern Etna Shear Zone”, offshore centred on ATL, produced scattered and fissure-type volcanism at local releasing zones, both off and onshore, from the TP area to the low SW portion of Etna. Indeed, the deeper magmatic intrusions run strictly along the ATL and the oldest onshore volcanic findings (> 320 ka, Branca et al. , 2011) are aligned along the shear zone. Following the model by Wakabayashi et al. (2004), migration of releasing and restraining regions along the “Southern Etna Shear Zone” favoured new fissure-type volcanism (> 110 ka) and hampered the former one, with positive tectonic inversion affecting the previous releasing zones (e.g. the push-up observed on TP). Releasing zones migrated also northwards along the NW-SE trending “Central Etna Shear Zone”, of which the offshore expression results in AL, favouring volcanism along the Acireale coast up to the exit of Valle del Bove. This second shear zone may represent a splay fault of ATL or a step-over belt between the “Southern Etna Shear Zone” and a possible analogous dextral sub-parallel structure to the north (“Northern Etna Shear Zone”). At about 105 ka, volcanism along the “Southern Etna Shear Zone” ceased, likely also due to the ongoing indentation of the northwestward moving Hyblean block; a further migration of releasing and restraining zones