Architecture, tectonic control and instability of the submarine continental margin offshore Mount Etna, Italy

The Island of Sicily (southern Italy) is located at the collision zone of the Eurasian Plate and the subducting African Plate. It is one of the most active areas in terms of seismicity in Europe; the trend of the subduction zone and its subducting mechanism, however, are still debated. The structural unrest of the area is documented by large plate ruptures in historical times like the prominent 1693, 1783 and 1908 earthquakes. Especially the 1908 Messina earthquake (~ 80,000 casualties) is of major interest as it was associated with the worst tsunami Italy experienced in the historical time (~2000 casualties). It is still debated whether a plate rupture at the seafloor or a massive submarine landslide triggered the tsunami, which hits the coast of East Sicily and Calabria. Related to this geodynamic setting, the area is also famous for its active volcanism. The Aeolian Island in the Tyrrhenian Sea form the highly active backarc-volcanism of the Calabrian subduction regime. Another type of volcano is observed directly at the shore of East Sicily – Mt Etna. Mt Etna is not only Europe’s largest volcano edifice with a recent height of 3323 m a.s. l, furthermore it is well known for its frequent eruptions. Compared to Earth’s history, Mt Etna is a relatively young volcano with an active volcanic history of ~500 ka. The origin of Mt Etna is still debated in several theories have been proposed (e.g., asymmetrical rifting processes, intersection of structural elements, hot spot, extensional tectonics). Mt Etna’s edifice can be traced mostly onshore and is directly adjacent to the continental margin of the Ionian Sea east of Sicily. Mt Etna is well known for its onshore edifice flank instability, as its eastern and southern flanks are gliding towards the continental margin in the east. The part of the flank gliding to the east is bound by prominent volcano-tectonic fault systems. The continental margin offshore of the gliding flank is characterized by a prominent bulge, which is not found further to the north or south. The onshore flank movement is well documented and monitored, but the potential continuation of this process towards the continental margin is not well investigated due to a lack of offshore data. In order to investigate and evaluate the phenomenon of volcano flank instability and its link to continental margin instability, a new high resolution new 2D/3D hydro/seismo-acoustic and geological dataset was acquired during RV METEOR research cruise M86/2 in December 2011/January 2012. The new dataset shows that not only the volcano edifice reveals an instability; furthermore, the entire continental margin east of the gliding sector of Mt Etna shows indications for recent extensional tectonics related to gravitational spreading. Whereas the northern boundary of the moving volcano flank is well defined by the sharp Pernicana-Provenzana Fault onshore, the offshore continuation reveals a diffuse grade of deformation and cannot be traced as a distinct sharp boundary. In contrast, the offshore southern boundary of the moving flank is clearly imaged by the new data set. It is identified as a right lateral oblique fault north of Catania Canyon. Two anticlines are present directly in front of the continental margin bulge. These anticlines are bound to half-graben basins towards the east and mark the eastern limits of the volcano flank and continental margin instability. As volcano-tectonic structures like the southern boundary fault can be traced from the volcano edifice across the continental margin towards the continental toe, the whole system is considered as a coupled volcano flank / continental margin gravitational instability and collapse. During M86/2 a new high-resolution 3D seismic cube at the center of the continental margin between the funnel shaped depositional system of Valle di Archirafi and a prominent amphitheater-like headwall, was collected. By assessing this 3D seismic data, a secondary spreading center was mapped at the center of the continental margin affected by the continental margin gravitational instability. It is located at a structural high, directly adjacent to the massive amphitheater headwall. The structural high hosts a set of normal faults, dipping towards steep amphitheater’s headwall. These faults show indications for recent and ongoing activity, as most of the fault planes are striking out at the seafloor. This fault activity makes to structural high to one of the most striking areas in terms of possible future hazards, related to submarine landslides, in the entire survey area. Next to mass transport deposits, traced in the funnel shaped Valle di Archirafi, the entire continental margin and the continental toe is strongly overprinted by a variety of mass transport deposits in the upper 750 m of the sedimentary record. A typical succession of mass transport deposits directly overlain by tephra layers indicate that volcano induced seismicity and flank deformation prior to an eruption act as important trigger mechanisms for catastrophic slope failures on the continental margin east of Mt Etna. The near-surface mass transport deposits are relatively small but seismic data illustrate significantly larger buried mass transport deposits, which were most likely triggered by similar volcanic processes in the past. The new dataset does not support the theory that the 1908 tsunami was triggered by a massive submarine landslide north of Mt Etna’s continental margin. The proposed slide deposit is overlain by a ~150 m thick succession of in-situ strata. The mass transport deposits seems to be the product of an ancient landslide event and has not occurred just ~100 years ago. All these findings imply a recently active coupled volcano edifice / continental margin system, which results in instable slopes and submarine mass movements. These findings even increase the geo-hazard potential of the area, which is already very high due to the ongoing seismic activity.