Characterisation of slow-spreading ridges: the Theistareykir and Fremrinamar rifts, Northern Volcanic Zone (Iceland)

The Northern Volcanic Zone (NVZ) of Iceland identifies the northernmost portion of the emerging Mid-Atlantic plate boundary. Its peculiar location enables us to observe the main features that characterise slow-spreading ridges directly on the field. Here we present the first outcomes on the characterisation of the structure of the NVZ, focusing on two out of the seven volcanic rifts that constitute the area, namely the Theistareykir and the Fremrinamar.

We linked field surveys and remote sensing analysis to identify and classify all the features constituting these areas (i.e. normal faults, extension fractures, and eruptive fissures) and to collect quantitative data. Information regarding structures’ azimuth, length and vertical offset (for the normal faults subset) were collected to obtain a comprehensive characterisation of the rifts’ surficial deformation. Moreover, fault-slip profiles have been realised, to evaluate the along-axis rift propagation within the studied fissure swarms.

For both rifts normal faults represent the most abundant subset, also characterised by the highest length values. The analysis of faults’ dip directions highlights a clear preponderance of E-dipping scarps within the Theistareykir rift against a predominance of W-dipping ones within the Fremrinamar rift, supporting the hypothesis that they represent rift shoulders. Within both rifts, we observed a decrease in the surficial deformation moving away from the rifts’ central volcanoes. Additionally, the interpretation of the fault-slip profile displayed the tendency of the along-axis rift deformation to move away from the central volcanoes. Within the Theistareykir, the rift intersects with the Husavik-Flatey transform Fault (HFF), generating structures showing right-lateral strike-slip components.  Within the Fremrinamar rift, however, the intersection of the rift within the Grimsey Oblique Lineament leads to a local re-orientation of structures’ strikes, without any strike-slip component. These data corroborate the hypothesis that the GRL represents the offshore propagation of the Fremrinamar rift towards the Kolbeinsey Ridge, thus having a different behavior than the HFF.

Our study could contribute to the general understanding of how slow-spreading ridges develop, with a particular focus on how volcanic and tectonic processes concur in defining rift structures.