Dike-arrest vs dike-propagation: new insights from the Younger Stampar eruption (13th Century), SW Iceland

Unravelling the parameters that control dike arrest and dike propagation in the shallow crust, and subsequently the associated dike-induced deformation at the surface, is of paramount importance in volcanology. This is because dikes can select among many different paths towards the surface and either stall in the crust or, alternatively, feed volcanic eruptions. In this work, we study two vertical dikes exposed in the sea-cliffs of the Reykjanes Peninsula (SW Iceland). Both dikes are associated with the Younger Stampar eruption (1210-1240 CE) and were emplaced in the same crustal segment, which includes lava flows and tuff layers. Although one of them fed a lava flow at the surface, the other dike, located at a distance of 30 m from the feeder, became arrested only 5 m below the surface of the active rift zone without inducing any brittle deformation. Hence, this outcrop represents an ideal case study to investigate the factors that favor dike arrest versus dike propagation, as well as the conditions that affect dike-induced brittle deformation. We collected detailed structural data on the dikes and identified the stratigraphic sequence of the outcrop. We mapped the nearby crater rows of the Stampar eruptions and reconstructed a highresolution 3D model through drone images and Structure from Motion (SfM) techniques. These data became inputs for 2D Finite Element Method (FEM) numerical models, using the COMSOL Multiphysics® software (v5.6), to explain mechanically the dike arrest and why there is no brittle deformation at the surface induced by the arrested dike. We tested the role of dike overpressure (Po = 2-4 MPa), the stiffness (Young’s modulus) of the layers, and the presence of an extensional or a compressional tectonic stress field. Our structural data show that the strike of the Younger Stampar crater row is consistent with the strike of nearby historic and prehistoric eruptive fissures, as well as the orientation of the volcanic systems of the Reykjanes Peninsula. Our numerical models indicate that dike-induced compressive stress (caused by the earlier feeder dike), together with the contrasting stiffness of the layers, can explain the arrest of the later dike and the lack of brittle deformation at the surface. Specifically, the presence of a stiff lava flow on top of a soft tuff concentrated the feeder dike-induced compressive stress in the lava flow, favoring dike arrest at the tuff-lava contact. These results have implications for hazard studies in other volcanic areas, particularly as regards dike arrest at shallow depths, in Iceland and worldwide.