Rift structure and development: The Krafla Fissure Swarm (Northern Iceland)

Understanding rift evolution is important for seismic and volcanic hazard assessment, as highlighted by recent dyking events in Iceland. We analysed 2507 structures constituting the N-S Krafla Fissure Swarm (KFS) through the ArcticDEM, 12 original orthomosaics, derived from historical aerial photos, and field reconnaissance. At a scale of 1:50,000, we identified and studied 323 eruptive fissures, 1412 extension fractures, 759 normal faults, and 13 caldera rim faults. Structures show a decrease in intensity northward and southward from the Krafla Caldera. Fault slip profiles are predominantly asymmetric, showing cumulative offsets tapering outward from the caldera. Fault slip rates, measured along normal faults cutting post-LGM units, range from 0.06 to 6.55 mm/yr, and decrease outward from the Krafla Caldera. These patterns resulted from repeated dykes propagating laterally from the Krafla magma chamber along the rift, a gradual deepening of the dyke tip, and a lower occurrence and thinning of dykes with distance. An along-rift asymmetry is given by more abundant extension fractures in the northern portion of the KFS and more normal faults in its southern part, together with a decrease in rift width from an average of 7.5 km in the south to 6 km in the north, and a decrease of dilation values, from 32.5 m to 14 m in the same direction. The KFS is subject to a double process of rift development given by the local effect of dykes protruding from the Krafla magma chamber, and the regional stress field determining a general northward development of the rifts in northern Iceland.