Rift development at the Askja Fissure Swarm, Iceland

Understanding how faulting and dyke intrusion interact to control rift development in volcanotectonic systems remains a key challenge, particularly in areas where multiple magma sources coexist. Here, we contribute to this topic by providing a high-resolution structural dataset for the Askja Fissure Swarm (AFS), one of the major rifts of the Northern Volcanic Zone of Iceland (NVZ), where central volcanoes and their associated dyke-fissure swarms act as magma plumbing systems and long-term eruption sources. We describe the Holocene structures of the AFS through the analysis of high-resolution remote sensing data and virtual field reconnaissance. We developed an inventory of 3749 individual structures, including dykes and fractures, mapped at a scale of 1:50,000, and systematically measured their strike, dip direction, length and kinematics. We recognized 395 eruptive fissures, 2301 extension dry fractures (fissures), and 1038 normal faults, of which 536 dip eastward and 502 westward. Along the main faults, we reconstructed the slip profiles, with the aim of evaluating the direction of along-axis fault and rift propagation. The overall dataset exhibits a predominant N–S to NNE–SSW strike, with an average strike of N17.5°E. Structure lengths vary from 10 m up to 13 km, with a mean of 445 m. Normal faults are generally longer than other structures, with a mean of 1042 m. With distance to the central volcanoes, we find the number of extension fractures and faults, their lengths and the slip profiles decrease. We interpret these characteristics as the effect of multiple dykes that laterally propagated outward from the magma chambers below Askja and Hrúthálsar central volcanoes. We also observed variation of fracture intensity and strike where the AFS intersects possibly hidden inherited transversal structures. Looking at the whole AFS, the tapering of fault slip profiles shows a dominant northward propagation of faults. This may indicate the superimposition of a regional northward propagation of the rift on the local effects of dykes and magma chambers, consistent with the northward spread of the NVZ.