Rift jump process in Northern Iceland since 10 Ma from 40Ar/ 39Ar geochronology

Abstract

Due to the westward American–Eurasian plate boundary migration, the rift zone location in Northern Iceland imposed by the stable Icelandic hotspot has jumped eastward. The present-day North Volcanic Zone of Iceland is thus shifted about 100 km to the east with respect to the Kolbeinsey Ridge. A Miocene paleo-rift location was proposed along the Húnaflói-Skagi axis. Unconformities inside the lava pile on both sides of the present-day rift as a result of the new rift formation have been used to date the rift jump, but timing is still controversial. Whether extinction of the paleo-rift was synchronous with initiation of the new rift or the two rifts were active during some time period is unknown. Using the 40Ar/ 39Ar radiochronology method, 37 dykes were dated along a 350 km profile parallel to the divergent plate motion direction (i.e., ESE–WNW) crossing the present rift, the unconformities, and the inferred paleo-rift. Our results reveal that the paleo-rift axis was not located along the Húnaflói-Skagi axis, but 60 km to the east. This previously unknown paleo-rift is named the Skagafjördur paleo-rift. It remained active until approximately 3 Ma. The activity of the present North Volcanic Zone started about 8–8.5 Ma, intruding 9–9.5 Myr rocks located at least 10 km east of the Skagafjördur paleo-rift axis. Consequently, the two rifts were simultaneously active for 5–5.5 Myr. During this time period, their accretion rates were nearly equal but with asymmetrical opening, the largest rates occurring on external flanks. These results are coherent in the Icelandic context and permit a first reconstruction of the evolution of Northern Iceland since 10 Ma.