Obliquity and rifting: Interaction of faulting and magma propagation during volcano-tectonic events in North Iceland using UAV-based structural data

Volcano-tectonic systems involve a relation between magma propagation and faulting that is fundamental in volcanology research. Earth’s upper crust is often modelled as homogeneous and elastic. However, fracturing and reactivation of pre-existing structures plays a key role in volcano-tectonic processes and magma propagation. Moreover, obliquity affects > 70% of Earth’s rifts. This study aims at investigating inherited structures’ role on magma propagation in extensional settings, subject to different degrees of opening obliquity.

We performed a detailed and extensive structural mapping based on UAV imagery and field observations in the North Volcanic Zone, choosing representative rift segments that have likely a cyclic nature and display different obliquity degrees. We selected four zones within the Askja and Bárðarbunga volcanic systems, delimited by the Fjallagjá graben to the North and the Holuhraun graben to the South. Structures progressively bend from an almost N-S orientation in the North to a rather NE-SW to the South, while the strain field orientation of the rift shows a constant extension vector’s azimuth of ~104°. Recently, the 2021 Fagradalsfjall volcano-tectonic event show an extreme case of high obliquity end-member system along the plate boundary.

We did a detailed morphostructural analysis of the processed imagery (~3 cm/px DEMs and ~2cm/px orthomosaics) and analysed fracture orientations, sense of opening and the effect of topography on the rift segments. The strength of the obliquity signal increases going from North (where no clear obliquity dominance is observed) to South (where Holuhraun shows distinct obliquity with a left lateral sense of shear), following the curvature of the overall rift segments. The processed imagery revealed typical structures related to volcano-tectonic processes, such as monoclines, open fractures, nested grabens with fault scarps that suggest reactivation, and intrusions oblique to the graben shoulders. For example, in the northern zone, we observe that eruptive fissures are ~ parallel to the main orientation of the plate boundary extension, but ~10°-20°consistently oblique to the enclosing graben shoulders.

Our observations help constraining the stress configuration and their evolution during intrusions.
The aim is to unveil the processes that govern magma propagation in a fractured crust at divergent plate boundaries from depth to the surface, which exert a fundamental influence on eruptions locations.