Tectonic framework and fault structures in the Fagradalsfjall segment of the Reykjanes Peninsula Oblique Rift, Iceland

The two volcanic eruptions of 2021 and 2022 at Fagradalsfjall in SW Iceland occurred within the Reykjanes Peninsula Oblique Rift, a segment of the complex boundary in Iceland between the North America and Eurasia Plates. Two of the plate boundary segments are highly oblique to the overall plate velocity vector, i.e., the Reykjanes Peninsula and the Grímsey oblique rifts. They contain both volcanic systems and seismogenic strike-slip faults. Oblique spreading leads to extensive volcanism and large earthquakes, a combination that is otherwise uncommon in Iceland. The fissure swarms of individual volcanic systems contain normal faults and fissures, arranged en echelon along the plate boundary. The fissure swarms fade out as they extend into the plates on either side. These volcano-tectonic rift structures on the Reykjanes Peninsula are overprinted by sets of parallel, N-S striking transcurrent faults that generate the largest earthquakes in the zones, up to M 6.5. Their surface expressions are en echelon fracture arrays and push-up structures. The distance between them varies from 0.3 to 5 km. They are most prominent in the areas between the overlapping fissure swarms, and together they form a bookshelf-type fault system taking up the shear component of plate movements across the oblique rift zones. The Fagradalsfjall volcanic system is located between the fissure swarms of the Svartsengi and Krísuvík fissure swarms. It lacks its own fissure swarm, which is otherwise one of the characteristics of Icelandic volcanic systems. We present maps of surface fracturing structures of the area and interpret them as the result of strike-slip displacement on underlying N-S faults. About 20 faults are implied along a 8-km-long section of the plate boundary. In addition to these bookshelf-type faults, several areas have been identified where earthquakes appear to line up along ENE-WSW-striking, fault-like structures. These structures have so far only been seen at the surface in one place despite a thorough search. Taken together, the N-S and the ENE-WSW faults form a conjugate set of faults. The implied tectonic stress field has a horizontal maximum principal stress with a N45°E orientation, and a minimum principal stress with a N135°E orientation, perpendicular to the fissure swarms on the peninsula and the dike intrusion that preceded the Fagradalsfjall eruption in 2021. It is postulated that bookshelf faulting is one of the characteristics of unstable or immature plate boundaries.