Oblique Rift Opening and Graben Collapse in the 2014-2015 Bárðarbunga (Central Iceland) Rifting Event

During the past decade, rifting events and episodes have provided new insights about the mechanisms that generate ground deformation, faulting and eruptions at divergent plate boundaries. However, our knowledge is still limited on, for example, the influence of pre-existing structures, the relations between dike intrusions and seismicity, and about near-field deformation. Here we show evidence for oblique rift opening related to a graben collapse during a rifting event originating from the Bárðarbunga caldera (central Iceland). This event led to the largest basaltic eruption in Iceland for more than 200 years. From seismicity and high-resolution radar imagery (TerraSAR-X and COSMO-SkyMed) we reveal the timing and kinematics of the graben formation and other near-field deformation. We use radar amplitude image offsets and earthquake fault plane solutions (over the same period) to show that significant left-lateral shear took place in addition with the opening across the graben. This suggests that pre-existing fracture zone and two centuries of far-field strain accumulation controlled the intrusion pathway. Our results imply that pre-existing fracture zones, which are rarely considered in dike propagation studies, play a key role in controlling both rift opening obliquity and fault plane rupture geometries.