Extension direction re-orientation in the oceanic rift of Iceland, and comparison with continental rifts

In this study we report the results of a structural analysis carried out on fault systems that form the active rift segments of two areas of Iceland, namely (1) the northern part of the Eastern Rift segment (Northern Volcanic Zone, NVZ) and (2) the Western Rift segment, that include the Western Volcanic Zone (Thingvellir area) and the Reykjanes Peninsula. In the N-S-trending NVZ (mainly Krafla volcanic area), the obtained extension directions trend around ESE. At Thingvellir, the extension directions trend ca. NW-SE and are perpendicular to the main NE-SW-trending faults. In the ENE-WSW-trending tectono-magmatic segments of the Reykjanes Peninsula, our data show a marked difference with the GPS data and spreading directions. The extension directions trend roughly orthogonal to the strike of the boundary faults in all the investigated sectors. Extension shows an overall southward increase in divergence from the regional directions indicated by GPS vectors. This behavior has been compared with the continental Main Ethiopian Rift of East Africa, where structural studies show a re-orientation of the extension direction between the rift margins and the internal sectors, resulting in a divergence between local and regional extension directions. Aimed at investigating the causes of these differences, we performed a new series of analogue models by varying different parameters such as rift obliquity and width. Our findings suggest that rift width exerts a primary control on the boundary-axis variation of the extension direction. In particular, significant differences in fault orientation and local direction of extension between the margins and the rift axis are observed in wider rift valleys (e.g., continental rift of Ethiopia); conversely, fault orientation and local extension direction are much more homogeneous in narrower rift valleys (e.g., rifts of Iceland), where a variations in the direction of extension from the margins to the rift axis are not observed.