Extension across a divergent plate boundary, the Eastern Volcanic Rift Zone, south Iceland, 1967–1994, observed with GPS and electronic distance measurements

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Abstract
The average spreading rate in south Iceland, 19 mm/yr, is distributed over two parallel rift zones. We measured a Global Positioning System (GPS) network of 42 stations in the eastern zone in 1994. This network consists of stations measured with GPS in 1986 and 1992, stations in a 60 km long distance profile measured several times since 1967, and a few new stations. The 1994 GPS data were processed using the Bernese software version 3.5, and the average position uncertainties are about 3 mm in horizontal components and about 8 mm in the vertical component for baselines up to 100 km. Comparison with results of former GPS campaigns gives a uniform extension, steady in time, perpendicular to the spreading axis. A strain rate of 0.12±0.01 μstrain/yr is observed across the 100 km wide network, or an extension of about 12 mm/yr. Minor deformation is observed in direction parallel to the spreading axis. Observations along the distance profile, which lies across the rift zone, gave a significant contraction during the period 1967–1977 but gave an extension during 1977–1994. This extension is about 138±47 mm and is mainly accommodated in the western part of the profile. The observed extension across the rift zone during 1986–1994 can be simulated with a simple model of infinitely long dike intrusions into an elastic layer overlying a viscous layer. This model is not able to simulate observed contraction along the distance profile 1967–1977. The observed irregularities along the western part of the distance profile coincide in space and time with volcanic and tectonic unrest near the Hekla volcano. The disturbances are probably caused by some common underlying process leading to crustal deformation, eruptions, and earthquakes.
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