###### Abstract  
In the present work, we demonstrate how drone surveys coupled with structure-from-motion (SfM) photogrammetry can help to collect huge amounts of very detailed data even in rough terrains where logistics can affect classical field surveys. The area of study is located in the NW part of the Krafla Fissure Swarm (NE Iceland), a volcanotectonic rift composed of eruptive centres, extension fractures, and normal faults. The surveyed sector is characterized by the presence of a hyalocalstite ridge composed of deposits dated, on a stratigraphic basis, to the Weichselian High Glacial (29.1-12.1 ka BP), and a series of lava flows mostly dating back to 11-12 ka BP. The integration of remotely sensed surveys and field inspections enabled us to recognize that this segment of the Krafla rift is made of grabens arranged en-echelon with a left-stepping geometry. A major graben increases in width in correspondence of the hyalocalstite cone; we interpret this geometry as resulting from the mechanical contrast between the stiffer lava succession and the softer hyalocalstites, which favours the development of concentric faults. We also measured a total extension of 16.6 m and 11.2 m along the fractures affecting the lava units, and a total extension in the hyalocalstites of 29.3 m. This produces an extension rate of 1.4 mm/yr in the Holocene lavas and 1.7 \(\pm\) 0.7 mm/yr in the Weichselian hyalocalstite deposits. The spreading direction we obtained for this area is N97.7\({}^{\circ}\) E, resulting from the av. of 568 opening direction values. structure from motion; photogrammetry; rift; Iceland; normal faults +
Footnote †: journal: Applied  
Frequently, crustal extension rates have been determined indirectly from plate tectonic models, such as in the Eastern Africa rift system [3; 4]. Uncertainty in the quantification of crustal extensional rate and direction derives also from the fact that, frequently, hard logistic conditions do not allow to take a large enough number of measurements along a fault or along an extension fracture in order to have a complete perception of the strain field. The most precise reconstruction of the strain field, in fact, can be achieved only by collecting a huge amount of horizontal dilation values, for both normal faults and extension fractures, together with fault slip profiles [5], sometimes referred to also as \"displacement-length profiles,\" that represent the measurement of the scrap height as a function of the distance along the fault. Such profiles can be considered as the most complete picture of fault-deformation, as well as a proxy to record the different phases of fault growth [6; 7]. Examples on the use of slip profiles can be found elsewhere in the world, especially as application to the assessment of fault or rift propagation direction [6; 7; 8; 9; 10; 11; 12; 13; 14]. Recent examples of the use of structural data to define the shallow crustal extension in a rift can be found in [5; 15; 16; 17], including a multidisciplinary approach based on field data and analogue modelling [18]. The difficulties mentioned above that can arise from collecting quantitative data, both along normal faults and extension fractures, have been recently overcome by the use of unmanned aerial vehicles (UAVs), coupled with structure-from-motion (SfM) photogrammetry, which can fly over rough or dangerous terrains allowing to reconstruct extremely detailed digital surface models (DSMs) and orthomosaics of the areas. The latter are used to collect plenty of high-resolution structural data. This methodology has been increasingly used in different branches of geological sciences [19], and also in active tectonics studies: after the first attempts with balloons [20], UAV-based images have been used to recognise, map and study active faults [17; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30]. Because of the high resolution of the resulting models, it is also possible to collect quantitative data, including dilation values, along fractures and faults [17], even in immersive virtual reality [31].