This model shows a 30-m-wide graben (covered by the road), which represents a typical effect of extensional tectonics across the Theistareykir Fissure Swarm, in the Northern volcanic Zone of Iceland (Pasquaré Mariotto et al., 2020; Bonali et al., 2019). The 3D model visualizes, with exceptional detail, the two sets of opposite-dipping normal faults that border the graben, as well as the low-lying floor of the volcano-tectonic structure.
Such normal faults show a pure extension component: in fact, moving towards their termination, they gradually transition to extension fractures. Furthermore, the topographic surface in the low-lying floor is dipping towards the inner graben, with the same dip as that of the normal faults that originated it. Such geometry is unusual, if compared to the one that is usually associated with faulting in non-volcanic rocks, where the tilt is opposite to the fault dip. Both the low-lying floor and the upper part of the graben are characterized by a series of smaller fractures parallel to the normal faults. The model is also instrumental in highlighting that tectonic subsidence across the graben floor has developed in a differential fashion, as attested by the fault system to the WNW (upper part of the figure), marked by a greater offset than its counterpart to the ESE. The lava deposit affected by the graben is 2.4 ky old (Saemundsson et al., 2012); the road is brand new and is not affected by the deformation. This graben can be the result of the extensional tectonic regime working in the Northern Volcanic Zone of Iceland, where the overall spreading directions is about N106°, with an opening of about 2 cm/yr (Hjartardóttir et al., 2016).

Credits: UAV-based survey and 3D DOM by Fabio Marchese; funding is from MIUR project ACPR15T4_00098 ( Model description by Alessadro Tibaldi and Federico Pasquaré Mariotto.

• Bonali, F. L., Tibaldi, A., Marchese, F., Fallati, L., Russo, E., Corselli, C., & Savini, A. (2019). UAV-based surveying in volcano-tectonics: An example from the Iceland rift. Journal of Structural Geology, 121, 46-64.
• Hjartardóttir, R., Einarsson, P., Magnusdóttir, S., Bjornsdóttir, Þ. and Brandsdóttir, B. (2016) Fracture systems of the Northern Volcanic Rift Zone, Iceland: an onshore part of the Mid-Atlantic plate boundary. In: Wright, T. J., Ayele, A., Ferguson, D. J., Kidane, T., Vye-Brown, C. (eds), Magmatic Rifting and Active Volcanism. Geological Society, London, Special Publications, 420, 297-314.
• Mariotto, F. P., Bonali, F. L., & Venturini, C. (2020). Iceland, an Open-Air Museum for Geoheritage and Earth Science Communication Purposes. Resources, 9(2), 14.
• Saemundsson, K., Hjartarson, A., Kaldal, I., Sigurgeirsson, M.A., Kristinsson, S.G. and Vikingsson, S. (2012) Geological map of the Northern Volcanic Zone, Iceland. Northern Part 1: 100.000. Reykjavik: Iceland GeoSurvey and Landsvirkjun.