Doctoral defence in Geophysics - Sonja H. M. Greiner

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Doctoral candidate: Sonja H. M. Greiner

Title of thesis: The Influence of Pre-existing Fractures and Tectonic Stress on Magmatic Dikes

Opponents:
Dr. Meredith R. Townsend, Assistant Professor at the Department of Earth and Environmental Sciences, Lehigh University, USA
Dr. John Browning, Associate Professor at the Department of Mining Engineering and Department of Structural and Geotechnical Engineering, Pontificia Universidad Católica de Chile, Chile

Advisor: Dr. Freysteinn Sigmundsson, Research Scientist at the Institute of Earth Sciences, University of Iceland

Other members of the doctoral committee:
Dr. Steffi Burchardt, Professor at the Department of Earth Science, Uppsala University, Sweden
Dr. Olivier Galland, Researcher at the Faculty of Mathematics and Natural Sciences, University of Oslo, Norway
Dr. Halldór Geirsson, Associate Professor at the Faculty of Earth Sciences, University of Iceland

Chair of Ceremony: Dr. Guðfinna Th Aðalgeirsdottir, Professor and Vice Head of the the Faculty of Earth Sciences, University of Iceland

This is a joint doctoral degree with Uppsala universitet, Sweden.

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.

About the doctoral candidate

Sonja Greiner was born in 1995 in Marburg an der Lahn in Germany and grew up in the nearby town of Amöneburg, where she graduated from the Gymnasium Stiftsschule St. Johann in 2014. She completed her undergraduate studies at the Philipps-Universität Marburg and received a BS in physics in 2018. Sonja began her MS studies in geophysics at the University of Iceland in August 2019 and completed them in June 2021. She began her doctoral studies in July 2021 and as part of this lived in Reykjavík (Iceland), Uppsala (Sweden) and Olso (Norway).