Faulting and fissuring in active oceanic rift: Surface expression, distribution and tectonic–volcanic interaction in the Thingvellir Fissure Swarm, Iceland

Iceland brings exceptional opportunity for analysing extension related to rifting of the Mid-Atlantic ridge, especially revealing fresh structural patterns in active fissure swarms. Post-glacial fracture systems of the Thingvellir rift segment of the West Volcanic Zone (WVZ) and interaction with holocene lava flow overlapping are analysed in detail in this paper. We mapped 5390 fractures at metric to kilometric scales in order to realise a precise structural map, a representative fault length distribution analysis and some statistical calculations in terms of fault length/number growth rates from Holocene to recent time. Mapping and 3-D geometrical analysis of faults and fissures are based on use of photogrammetric techniques, GPS positioning at ground control points and validation from geological field work. This approach allowed us to measure the vertical throw distribution along 52 faults with a precision around 0.5–1 m. Most of these faults have symmetric serrated fault-displacement profiles; however some of them have profiles offset to the north or south. Fault vertical offset as a function of the age of the hosting lava flows are presented too. Finally, from the study of 70 transverse topographic profiles and the fault offset analysis, we propose a propagation model for Holocene fissure development, partly controlled by Pleistocene tectonic inheritance. Our model takes into special account alternating volcanic events and faulting. Simple fissure zones with small hangingwall monocline or more complex scarp zones with graben and larger hangingwall monocline developed. Because of lava flow accumulation during the rift extension, estimating the amount of extension based on the present-day morphology would have led to severe under-evaluation.