Holocene vertical deformation outside the active rift zone of north Iceland

Abstract

In Iceland, the main deformation activity occurs not only in the active rift zone where volcanism and seismicity coexist in fissure swarms but also in the transfer zones (north and south) where the main part of the seismic activity is located. Many studies described extension and vertical displacement associated with rifting and magmatism inside this rift zone. They have classically assumed that no deformation occurs outside the rift zone. However, few studies have already been conducted outside the rift zone. This work focuses on an area west of the rift zone in the Akureyri area, which displays an anomalous topography. We illustrate an ENE tilting of 0.23° since approximately 17,000 years ago using ancient lacustrine shorelines mapped in detail with a differential GPS survey and analysis of SPOT images. This degree of tilting corresponds to a vertical displacement of 160 m, i.e., a deformation rate of 1 cm/year, which is close to values measured in the active zone. This long wavelength deformation is compatible both with (i) the topographic anomaly of the Akureyri area, and with (ii) the very diffuse microseismicity that affects northern Iceland. The origin of this tilting is discussed and the different hypotheses (glacial loading response, rift uplift, transform zone effect, thermal doming, local magmatic intrusion) are reviewed. Our results, structural data and the topography are more consistent with a local magmatic intrusion inducing local doming. This work is the first attempt at quantifying long-term deformation outside the rift zone in Iceland.