Dike-arrest vs dike-propagation and associated surface stresses: an example from the Younger Stampar eruption (13th century), Reykjanes Peninsula, SW Iceland

Understanding the factors that affect dike propagation and dike arrest in the shallow crust, and subsequently control the associated dike-induced surface deformation is fundamental for volcanic hazard assessment. In this work, we focus on two dike segments associated with the Younger Stampar eruption (1210-1240 AD) on the Reykjanes Peninsula (SW Iceland). Both segments (spaced 30 m apart horizontally) were emplaced in the same heterogeneous crustal segment composed of lavas and tuffs. Here, the first dike to be emplaced fed a lava flow, while the second dike became arrested 5 m below the free surface without producing any brittle surface deformation. Therefore, this area represents an ideal case study to analyse the conditions that promote dike arrest or, alternatively, dike propagation to the surface. The outcrop also provides further examples of the absence of brittle deformation around a dike arrested just below the surface. For this work, we collected structural data from the dikes and the heterogeneous layers as well as from the nearby crater rows associated with the Stampar eruptions. We integrated our field observations with a high-resolution 3D model reconstructed from UAV-collected pictures through Structure-from-Motion photogrammetric techniques. These 3D model data were then used as inputs for Finite Element Method (FEM) numerical models through the COMSOL Multiphysics® software (v5.6). We performed a range of sensitivity tests to investigate the role of dike overpressure (Po= 2 - 4 MPa), the mechanical properties of the host rock (e.g., Young's modulus), and the layering of the crustal segment subject to horizontal extension and compression boundary conditions.Our multidisciplinary structural analyses show that the Stampar crater rows is consistent in strike with the orientation of the volcanic system of the Reykjanes Peninsula, as well as the other historic and prehistoric eruptive fissures in the region. Furthermore, our numerical models indicate that the layering and the dissimilar mechanical properties of the host rock contributed to the arrest of non-feeder dike and the associated absence of brittle deformation at and above its tip. In particular, the layering (stiff lava flow on top of soft tuff) magnifies (concentrates) the compressive stress induced by the earlier feeder dike which cuts through an existing lower part of the surface lava flow. The horizontal compressive stress, in turn, is one reason for the very low overpressure of the non-feeder when it approached the tuff-lava contact, hence its arrest at the contact. Our studies can be applied to other dike-fed volcanic areas in Iceland and worldwide.