Basement sliding and the formation of fault systems on Mt. Etna volcano

The influence of faulting on the eruptive mechanisms of Mt. Etna has been intensively studied, especially regarding the importance of regional tectonics, magma pressure, gravitational spreading and east flank instability. Here we examine the influence of an additional process: the wholesale sliding of the Etna massif along its sloping basement (Murray et al., 2018). Using laboratory analogue experiments, we create a series of model volcanoes on sloping basements, with obstructions to represent the mountains and hills surrounding Etna, and an unconstrained downslope edge to represent the unbuttressed seaward slopes. We find that analogues of all the Etna fault systems can be produced in the same model. Furthermore, we find that the relative velocities of transcurrent faulting and extension of each model flank fault system match those of Mt. Etna in every case. We also find convincing evidence that gravitational spreading of the summit cone, combined with downslope sliding, controls the position of future eruptive vents around the summit, by creating faults and fractures that form paths of least resistance for magma intrusions (Pezzo et al., 2020). The intruding magma in turn augments fracture opening by an order of magnitude, in a feedback process that dominates within the north-south summit graben. We conclude that gravitational spreading and sliding are the dominant processes in creating faults at Etna, and that these two processes, augmented by magma pressure, are responsible for the rapid seaward movement of the eastern slopes, tectonically cut off from the western flanks by the summit graben. The influence of regional tectonism is up to two orders of magnitude lower. The conceptual model derived here could make an important contribution to the investigation and monitoring of eruptive, seismic and landslide hazards, by providing a unified mechanical system that can be used to understand deformation.