Fast and partitioned postglacial rebound of southwestern Iceland

Abstract

International audienceLocated both on the Mid-Atlantic Ridge and above a mantle plume, Iceland is subject to horizontal and vertical motions. Many studies described these deformations in terms of rifting episodes that have combined both extensional tectonics and magmatism. However, few studies have described the glacio-isostatic response induced by the retreat of the Weichselian ice cap. The melting of this ice cap induced a postglacial rebound for the whole of Iceland that may be controlled by the geodynamic setting and the rheological layering of the lithosphere. This study is devoted to (1) understanding the Holocene rebound on the southwestern coast and (2) estimating the asthenosphere viscosity and depth beneath Iceland. Two stages of holocene evolution were determined by means of GPS profiles, morphological observations, and data compilation. The first stage corresponds to a vertical uplift of 67.5 to 157.5 m. It started at 10,000 years BP and ended at 8500 years BP implying uplift rates between 4.5 and 10.5 cm/a. It was a quick isostatic response to the fast ice retreat. The second stage had vertical motion of tens of meters with a probable tectonic origin and started at 8500 years BP. The uplift rate is 1 to 2 orders of magnitude slower than the one during the first stage. Uplift partitioning during the first stage was controlled by the thermal state of the lithosphere, the highest geothermal flux inducing the maximum uplift rates. The relaxation time for uplift provides a viscosity estimate of 5.4­5.8 — 1019 Pa s for the asthenosphere. This value is similar to those determined for glacial areas in different continental contexts. However, the flexural wavelength indicates a shallower asthenosphere than that occurring in continental domains. Therefore this study highlights a coupling between the thermal structure of the Icelandic asthenosphere and the glacial rebound

Impacts of post‐glacial rebound on landslide spatial distribution at a regional scale in northern Iceland (Skagafjörður)

“…The rates of maximal uplift during this time were 15-30 cm/yr, 10 times more than the average vertical movements in the most seismically active zones of the planet (Mörner et al, 2000). Seismicity can thus be influent on landslide distribution, especially through shaking processes, or through differential uplift, faulting and weakening of the bedrock (Biessy et al, 2008).…”

“…More generally, both jointing (at local scale) and faulting (at slope scale) during uplift weaken the internal strength of bedrock, favoring water seepage, and thus making the displacement of material easier and keeping the area unstable (Hippolyte et al, 2006;Hippolyte et al, 2009). Seismicity can thus be influent on landslide distribution, especially through shaking processes, or through differential uplift, faulting and weakening of the bedrock (Biessy et al, 2008). Secondly, the uplift can generate movements along faults and cracks and can trigger strong earthquakes (Davenport et al, 1989;Lagerbäck, 1992;Fjeldskaar et al, 2000;Mörner et al, 2000;McColl et al, 2012).…”

“…The most impressive evidence of both deformation and seismicity is exhibited not only in the regions of greatest ice thickness but also in the regions marginal to the Late Pleistocene ice sheets . The magnitude and response time of glacial-rebound is related to the ice load, unloading rate, and the asthenosphere viscosity (Biessy et al, 2008). +900 m; Van Vliet-Lanoë, 2005) because of the thin and hot lithosphere; flexure is slower and lower in the context of a thick and rigid lithosphere (e.g.…”

“…However, a wide timeframe (between 12 400 and 6100 yr cal. The onset of intense landsliding in the area begun after the maximum uplift rate was reached, and continued while uplift rates had drastically declined (Norðdahl and Pétursson, 2005;Biessy et al, 2008) (Figure 14). In the Hofsós area and southward, at least six landslides occurred between 9700 and 4211 ± 31 yr cal.…”

“…Canadian and Scandinavian shields) (Firth and Stewart, 2000;Larsen et al, 2004;Martinec and Wolf, 2005). The magnitude and response time of glacial-rebound is related to the ice load, unloading rate, and the asthenosphere viscosity (Biessy et al, 2008). In Iceland, the largest rebound appears to have occurred near the coast, where rapid unloading and the conditions of the asthenosphere favored high rebound ( Figure 5).…”