Faulting activity during the 2021 oblique rifting event in the Reykjanes Peninsula (SW Iceland)

The Reykjanes Peninsula is located at the boundary between the North American and the Eurasian plates. The Peninsula is experiencing an oblique rifting episode that started with a strong earthquake swarm the 24 February 2021, preceded by sparse swarm occurrences in the decade before. This activity has been followed by a volcanic eruption the 19 March 2021 and by three other eruptions in 2022 and 2023 after ~800 years of quiescence in this region. These series of events offer a unique opportunity to explore the relation between tectonic, faulting and volcanic activity in a highly oblique rift zone. Here we focus on faulting activity during the onset period of the ongoing rifting episode, from February to March 2021. We use an extensive dataset of field observations and high resolution drone orthomosaics and DEMs over a ~30 km2 area to map, quantify and characterize the widespread ground fracturing associated with the oblique rifting activity. We mapped ~20000 ground cracks of metric to decametric length, centimetric extensional offset, and clear dextral shear component, well-correlated to major earthquakes. Most fractures show en-chelons structures globally aligned along NS-striking fault zones up to 3-4 km long. By analyzing the timing of the ground fracturing thanks to our repeated field observations, seismic data and InSAR images, we relate most ground fractures to earthquakes of Mw 5.0 that occurred in the month preceding the March 2021 Fagradalsfjall eruption. Using optical image correlation from drone data and air photos, we further characterize in unprecedented details several NS fault zones that were reactivated up to three times during the event and were already existing before. Our results show dominant strike-slip features, atypical in rift zones, that highlight the geometry of a bookshelf-mode activity along a diffuse and highly oblique plate boundary. These findings further question the relation between tectonic and magmatic activity at mid-oceanic ridges. Thanks to our immediate on-site response in early March 2021, we witnessed extensive ground fractures that have been quickly lost within a year due to erosion or recovered by lava flows, pointing out a potential under-representation of diffuse fracturing when studying oblique volcanic systems.