Glacial rebound and plate spreading: results from the first countrywide GPS observations in Iceland

Abstract

S U M M A R YIceland is one of the few places on Earth where a divergent plate boundary can be observed on land. Direct observations of crustal deformation for the whole country are available for the first time from nationwide Global Positioning System (GPS) campaigns in 1993 and 2004. The plate spreading across the island is imaged by the horizontal velocity field and high uplift rates (≥10 mm yr −1 ) are observed over a large part of central and southeastern Iceland. Several earthquakes, volcanic intrusions and eruptions occurred during the time spanned by the measurements, causing local disturbances of the deformation field. After correcting for the largest earthquakes during the observation period, we calculate the strain rate field and find that the main feature of the field is the extension across the rift zones, subparallel to the direction of plate motion. Kinematic models of the horizontal plate spreading signal indicate a slightly elevated rate of spreading in the Northern Volcanic Zone (NVZ) (23 ± 2 mm yr −1 ), while the rates at the other plate boundary segments agree fairly well with the predicted rate of plate spreading (∼20 mm yr −1 ) across Iceland. The horizontal ISNET velocities across north Iceland therefore indicate that the excessive spreading rate (>30 mm yr −1 ) observed by GPS in 1987-1992 following the 1975-1984 Krafla rifting episode was significantly slower during 1993-2004. We model the vertical velocities using glacial isostatic adjustment (GIA) due to the recent thinning of the largest glaciers in Iceland. A layered earth model with a 10-km thick elastic layer, underlain by a 30-km thick viscoelastic layer with viscosity 1 × 10 20 Pa s, over a half-space with viscosity ∼1 × 10 19 Pa s can explain the broad area of uplift in central and southeastern Iceland. A wide area of significant residual uplift (up to 8 mm yr −1 ) is evident in north Iceland after we subtract the rebound signal from the observed rates, whereas the Reykjanes Peninsula and the Western Volcanic Zone (WVZ) appear to be subsiding at a rate of 4-8 mm yr −1 . We observe a coherent pattern of small but significant residual horizontal motion (up to 3 mm yr −1 ) away from Vatnajökull and the smaller glaciers that is most likely caused by glacial rebound. Our study demonstrates that the velocity field over a large part of Iceland is affected by deglaciation and that this effect needs to be considered when interpreting deformation data to monitor subglacial volcanoes in Iceland.

Changes in the southeast Vatnajökull ice cap, Iceland, between ~ 1890 and 2010

“…Even though there have been surges in the larger outlets of Vatnajökull (Björnsson et al, 2003), they have not affected the ice divides of the studied southeast outlet glaciers during the study period. Uplift rates around Vatnajökull in the last 20 years have been on the order of 10-30 mm a −1 , highest around the edge of the ice cap (Árnadóttir et al, 2009;Auriac et al, 2013). We do not however, account for this change of the bed elevation in the glacier surface DEMs, as it is smaller than the vertical error estimate.…”

A two‐magma chamber model as a source of deformation at Grímsvötn Volcano, Iceland

“…The overall retreat of the icelandic glaciers is causing a mechanical response in the Earth crust [Pagli et al, 2007;Arnadóttir et al, 2009;Pinel et al, 2007]. As a first measurable consequence, an uplift of over 20 mm/yr is currently occurring around Vatnajökull ice cap [Arnadóttir et al, 2009]. The Vatnajökull ice cap covers several active volcanoes.…”

Geophysical constraints on the dynamics of spreading centres from rifting episodes on land

“…When these sites were reoccupied in 1990, they revealed spreading rates across the plate boundary of up to 6 cm yr -1 , around three times higher than the long-term, far-field average 67 . Average rates, at distances of 50 km and more, had in the 1993-2004 period returned to approximately background levels 68 . The deformation data have been modelled using a variety of simple viscous 67,69 or viscoelastic 70,71 rheologies, with no magma movement.…”