EXTENSION ACROSS A DIVERGENT PLATE BOUNDARY, THE EASTERN VOLCANIC RIFT-ZONE, SOUTH ICELAND, 1967-1994, OBSERVED WITH GPS AND ELECTRONIC DISTANCE MEASUREMENTS

The average spreading rate in south Iceland, 19 mm/yr, is distributed over two parallel rift zones. We measured a Global Positioning System (GPS) netswork of 42 stations in the eastern zone in 1994. This networ k 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 abou t 3 mm in horizontal components and about 8 mm in the vertical compone nt for baselines up to 100 km. Comparison with results of former GPS c ampaigns gives a uniform extension, steady in time, perpendicular to t he spreading axis. A strain rate of 0.12 +/- 0.01 mu strain/yr is obse rved 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 1967-1917 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 s imulated with a simple model of infinitely long dike intrusions into a n elastic layer overlying a viscous layer. This model is not able to s imulate observed contraction along the distance profile 1967-1977. The observed irregularities along the western part of the distance profil e coincide in space and time with volcanic and tectonic unrest near th e Hekla volcano. The disturbances are probably caused by some common u nderlying process leading to crustal deformation, eruptions, and earth quakes.