Plate Spreading and Rapid Uplift Observed by GPS in Iceland

Iceland provides a unique area to study a multitude of geodynamic phenomena due to its location astride a divergent plate boundary influenced by an active mantle plume. The biggest effort to measure crustal deformation in Iceland was undertaken in two nationwide GPS campaigns (termed ISNET) in 1993 and 2004. The network consists of about 120 campaign sites evenly distributed over the whole country. The data provide the first velocity and deformation rate maps of the whole country. Several M>5 earthquakes, volcanic intrusions and eruptions occurred during the time spanned by the two surveys. The network is, however, not dense enough to capture the local deformation caused by these events. Rather, it provides a broad map of the deformation field in Iceland. The horizontal ISNET velocities indicate that the spreading is mainly accommodated within ~100 km wide areas across the active volcanic zones in north and south-central Iceland. The plate boundary in southwest Iceland is complex, with oblique spreading on the Reykjanes Peninsula, transform motion in the south Iceland seismic zone and a small component of rifting accommodated in the western volcanic zone. A more detailed view of the temporal and spatial variation in velocities in southwest Iceland has been obtained from repeated GPS campaigns in denser networks [e.g. Árnadóttir et al., JGR, 2006]. The vertical ISNET velocities indicate significant uplift rates (~1 cm/yr) over a large part of central and southeastern Iceland, providing a more coherent view of the extent of the area of uplift than has been obtained from previous GPS studies. Glacio-isostatic adjustments, due decreasing ice load in Iceland since 1890, are likely to contribute to the high vertical velocities. The large extent of the uplift area, however, suggests additional sources. Deep magma accumulation, intrusions, or increased activity in the mantle plume are possible sources that may also contribute to the long wavelength uplift signal.