Sonja H. M. Greiner: The Influence of Pre-Existing Fractures and Tectonic Stress on Magmatic Dikes

Abstract

Magmatic dikes form an integral part of volcanic systems and transport magma from depth towards the surface. Dike propagation through the Earth’s crust is affected by the mechanical properties of the crust, which is important to better understand potential effects on volcanic activity. This thesis investigates the influence of crustal heterogeneity on magmatic dikes, focusing specifically on the interaction between magma and pre-existing weaknesses and on the influence of tectonic stress. Basaltic dikes exposed in moderately fractured hyaloclastite in the extinct Dyrfjöll volcanic system, NE Iceland, showed that dikes can follow existing fractures, change strike when intersecting them without propagating into a fracture or be arrested in front of a fracture. Laboratory models of intrusions into pre-faulted crust demonstrate that the host rock cohesion and the strength contrast between intact and faulted host rock strongly control if and how faults affect intrusions. Faults additionally affect the amplitude and pattern of intrusion-associated surface deformation. Finite Element models simulated dike opening in a tectonic stress field. Comparison of these to surface deformation associated with the 2021 February-March Fagradalsfjall dike, SW Iceland, show that tectonic stress can be a sufficient driving mechanism for dike opening. The relative amount of predicted opening and shearing of the dike plane is consistent with expectations based on geological models and the area’s obliquely-spreading tectonic setting. This thesis demonstrates the complexity of dikes interacting with heterogeneous crust and the potential of considering multidisciplinary research as a key to advance understanding of such interactions.