This dataset contains supplementary materials to the manuscripts “Interpreting inverse magnetic fabric in Miocene dikes from Eastern Iceland” by Trippanera et al., (submitted to JGR) and “Anatomy of an extinct magmatic system along a divergent plate boundary: Alftafjordur, Iceland” by Urbani et al. 2015. These works present an extensive multi-scale and multi-disciplinary study focused on the magnetic fabric of dikes belonging to the Alftafjordur volcanic system in Eastern Iceland. Eastern Iceland is one of the most suitable places to analyze the roots of the volcanic systems that are composed of central volcanoes and fissure swarms. We sampled 19 NNE-SSW oriented dikes (for a total of 383 samples) belonging to the exhumed fissure swarm portion of Alftafjordur volcanic system, aiming at understanding the direction of magma propagation in the swarm by using Anisotropy of Magnetic Susceptibility (AMS) analysis. However, most of the samples (80% out of the measured cores) show an inverse geometric magnetic fabric (kmax is perpendicular to the dike margins and sub-horizontal)- therefore the study of the flow direction is complicated. Nevertheless, this result poses the problem of why the geometrically inverse fabric is present and widespread in the whole dike swarm. In order to understand the origin of this inverse fabric, besides standard AMS measurements, we also performed additional analysis such as different field and temperature AMS, Anisotropy of Anhystheretic Remanent Magnetization (AARM), Hysteresis loops and First-order reversal curves (FORC), Scanning Electron Microscope (SEM) and Optic microscope images analysis. This dataset includes the following materials: • Location of the sampled sites (.kml) • AMS measurements at room temperature by using H=300 A/m for all samples (.ran) • AMS measurements at room temperature by using H=200 A/m and H=600 A/m for selected samples (.ran) • AMS measurements at different temperature (from 20 to 580 ℃) for selected samples (.ran) • AARM measurements for selected samples (.ran) • DayPlots data for selected samples (.xls or .csv) • SEM and Optical microscope images of thin sections of selected samples. AMS and AARM data can be opened through Anisoft open-source software provided by Agico (Chadima and Jelinek, 2009; https://www.agico.com/text/software/anisoft/anisoft.php). Data have been acquired at: Roma Tre University (Rome, Italy), Istuto di Geofisica e Vulcanologia (INGV, Rome, Italy) and Laboratoire des Sciences du Climat et de l'Environnement, CEA, CNRS, UVSQ (Gif-sur-Yvette Cedex, France). For the interpretation of the data refer to Urbani et al., 2015 and Trippanera et al., (submitted). The description of each dataset is provided in the description file.