Dyke-induced graben formation in a heterogeneous succession on Mt. Etna: Insights from field observations and FEM numerical models

The most common way of magma transfer towards the surface is through dyking. Dykes can generate stresses at their tips and the surrounding host rock, initiating surficial deformation, seismic activity, and graben formation. Although scientists can study active deformation and seismicity via volcano monitoring, the conditions under which dykes induce grabens during their emplacement in the shallow crust are still enigmatic. Here, we explore through FEM numerical modelling the conditions that could have been associated with dyke-induced graben formation during the 1928 fissure eruption on Mt. Etna (Italy). We use stratigraphic data of the shallow host rock successions along the western and eastern sections of the fissure that became the basis for several suites of numerical models and sensitivity tests. The layers had dissimilar mechanical properties, which allowed us to investigate the studied processes more realistically. We investigated the boundary conditions using a dyke overpressure range of 1–10 MPa and a local extensional stress field of 0.5–2 MPa. We studied the effect of field-related geometrical parameters by employing a layer thickness range of 0.1–55 m and a variable layer sequence at the existing stratigraphy. We also tested how more compliant pyroclastics, such as scoria, (if present) could have affected the accumulation of stresses around the dyke. Also, we explored how inclined sheets and vertical dykes can generate grabens at the surface. We propose that the mechanical heterogeneity of the flank succession and the local extensional stress field can largely control both the dyke path and dyke-induced graben formation regardless of the tested dyke overpressure values. Similarly, soft materials in the stratigraphy can greatly suppress the shear stresses in the vicinity of a propagating dyke, encouraging narrow grabens at the surface if only the fracturing condition is satisfied, while inclined sheets tend to form semigrabens, respectively. Finally, we provide some insights related to the structural evolution of the 1928 lateral dyking event. All the latter can be theoretically applied in similar case studies worldwide.