Gravitational slope-deformation of a resurgent caldera: New insights from the mechanical behaviour of Mt. Nuovo tuffs (Ischia Island, Italy)

Ischia Island (Italy) is an impressive example of the rare phenomenon of caldera resurgence. The emplacement and replenishment of magmas at shallow depth resulted in a vertical uplift of about 900 m, concentrated in the western portion of Mt. Epomeo (789 m a.s.l.). As a consequence of this uplift, the island has experienced several slope instabilities at different scales since the Holocene, from shallow mass movements to large rock and debris avalanches. These mass wasting events, which mobilised large volumes of greenish alkali-trachytic tuff (the Mt. Epomeo Green Tuff, MEGT), were strictly related to volcano-tectonic activity and the interaction between the volcanic slopes and the hydrothermal system beneath the island. Deep-Seated Gravitational Slope Deformation (DSGSD) at Mt. Nuovo, located adjacent to densely populated coastal villages, is an ongoing process that covers an area of 1.6 km2. The Mt. Nuovo DSGSD involves a rock mass volume of 190 Mm3 and is accommodated by a main shear zone and a series of sub-vertical fault zones associated with high-angle joint sets. To improve our understanding of this gravity-induced process, we performed a physical (porosity and permeability) and mechanical (uniaxial and triaxial deformation experiments) characterisation of two ignimbrite deposits - both from the MEGT - that form a significant component of the NW sector of Mt. Epomeo. The main conclusions drawn from our experiments are twofold. First, the presence of water dramatically reduces the strength of the tuffs, suggesting that the movement of fluids within the hydrothermal system could greatly impact slope stability. Second, the transition from brittle (dilatant) to ductile (compactant) behaviour in the tuffs of the MEGT occurs at a very low effective pressure, analogous to a depth of a couple of hundred metres, and that this transition is likely moved closer to the surface in the presence of water. We hypothesise that compactant (porosity decreasing) behaviour at the base of the layer could therefore facilitate slope instability. Although our results show that transient exposure to 300 °C does not influence the short-term strength of the tuff, we speculate that the high in-situ temperature could increase the efficiency of brittle and compactant creep and therefore increase the rate of slope deformation. Taken together, our experimental data highlight a potentially important role for the hydrothermal system (that reaches a minimum depth of ~ 1 km) in dictating the DSGSD at Mt. Nuovo. An understanding of this deformation process is not only important for the proximal coastal villages, at risk of engulfment by a large debris avalanche, but also for the towns and cities along the coast of the Gulf of Naples that are at risk to a secondary consequence of such an avalanche - a tsunami wave.