Abstract
Volcano-tectonic processes in the central part of Iceland, covered by the Vatnajokull glacier, are investigated by inversion of focal mechanisms. Working on a large catalogue of focal mechanisms determined by the Icelandic Meteorological Office, we used a damped regional-scale stress inversion method to obtain an insight of kilometric variations of the stress field. To evaluate the resolution and the stability of this stress field solution, we computed checkerboard tests, stress field models and error propagation tests. Stress field models showed a continuous stress regime between normal and strike-slip faulting, associated with a high stress shape ratio (i.e.; σ1 ≈ σ2). Two main directions of σhmin were evidenced: the first one was in agreement with the regional spreading direction of Iceland and the second one was deviated, being almost perpendicular to the first one. The deviated stress direction is sustained through the 20 year time-span of the study around the Barðarbunga and Grimsvotn central volcanoes while the spreading direction remains predominant around the Hamarinn volcano. This result supports the hypothesis that this volcano lacks collapse caldera and shares a fissure swarm with the larger Barðarbunga volcano. On a smaller temporal scale, during the 1996 volcanic crisis, a bimodal distribution of σhmin showed two opposite strike-slip regimes where the deviated direction dominated. Because these two states of stress T1 and T2 show stress regimes away from the Andersonian positions, P, B and T axes, the rapid flip between these two regimes may be associated with the progressive melt intrusion of a dyke.
Highlights
At the intersection of the North Atlantic spreading ridge and the Icelandic hotspot, Iceland is a interesting area to investigate the interactions in space and time between tectonics and volcanic activity
A second stress analysis was carried out to evaluate the state of stress during a volcanic crisis and the results were compared to the regional stress field
Determining the stress field from focal mechanisms is not straightforward and raises several questions: what is the volume in which crustal stress can be considered to be homogeneous? What is the number and quality of the required data? How can the presence of different types of focal mechanisms within the volume and the geometry of the active tectonic systems be explained? Paleostress analyses highlighted that the separation of a heterogeneous dataset into spatially and temporally homogeneous subsets is fundamental to avoid artefacts in stress solutions: such computed solutions can only represent a compromise between different homogeneous subsets and are inconsistent with the actual stress field (e.g., Célérier et al, 2012)
Summary
At the intersection of the North Atlantic spreading ridge and the Icelandic hotspot, Iceland is a interesting area to investigate the interactions in space and time between tectonics and volcanic activity. Using a large number of focal mechanisms provided by the Icelandic Meteorological Office (IMO), we computed a first order stress field to characterize the crustal stress. This stress field results from volcanic, tectonic processes and isostasic rebound of the crust. A second stress analysis was carried out to evaluate the state of stress during a volcanic crisis and the results were compared to the regional stress field
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