Analysis of dike-induced stresses and deformation: new insights from Mt. Etna and SW Iceland

During eruptions, magma arrives at the surface through vertical dikes. However, dikes often arrest in the crust, not feeding an eruption. Moreover, their emplacement causes a concentration of tensile and shear stresses in the host rock, that can lead to fracturing and/or faulting at the topographic surface. The relation between magma intrusion and dike-induced surface deformation has been analysed in the last decades through analytical, analogue and numerical models, but mostly considering an elastic and homogeneous half-space. Realistic data should therefore be considered to overcome this limitation.

For this reason, in this work, field structural data are used as inputs for numerical modelling. Three case studies, all affected by shallow dike emplacement, are considered: the 1928 and 1971 eruptive fissures on Mt. Etna (Italy), and the Younger Stampar eruptive fissure in SW Iceland. On Mt. Etna, dike-induced brittle deformation is visible at the surface, in plan view for both cases, and in section view for the 1971 case study. In Iceland, two dikes (one feeder and one arrested) are exposed along a cliff; the tip of the arrested dike is only 5 m below the surface, but no brittle deformation is observed. These case studies allow us to investigate the parameters that i) favour/inhibit dike-induced brittle deformation at the surface, ii) affect the geometry of dike-induced graben faults, and iii) promote dike arrest at shallow depths.

We collected structural data integrating classical fieldwork and remote sensing analyses, using high-resolution 2D and 3D models, reconstructed through Structure from Motion photogrammetry from drone images and historical aerial photos. We used all these structural data as inputs for 2D Finite Element Method numerical models, using the software COMSOL Multiphysics. In the models, sensitivity analyses were run to analyse the parameters that affect dike propagation and the induced surface deformation.

This work underlines the role of layering on the formation of stress barriers and on the distribution of dike-induced stresses, that concentrate in stiffer materials and are suppressed in softer layers. Furthermore, it confirms the effects of dike overpressure and inclination on its propagation and on dike-induced stresses. Topography also affects the dike propagation path and the geometry of dike-induced graben faults. Finally, the role of lateral compression induced by nearby previous intrusions is investigated, showing how this can promote dike arrest and the lack of brittle deformation.