The continental margin in Iceland — A snapshot derived from combined GPS networks
Introduction
For the past seven decades, geodetic measurements have been carried out to determine the deformations which are related to tectonical processes of the mid-oceanic plate boundary in Iceland. The tectonical setting in Iceland is quite complex due to the interaction between the hot spot and the Mid Atlantic Ridge. A detailed description of this tectonical setting is given within this volume (Riedel and Ebbing, 2008-this issue). The main features for our study are outlined in Fig. 1.
The earliest geodetic triangulation network was established in 1938 across the rift zone in the northeastern part of the island (Niemczyk, 1943). Its purpose was to verify the assumption of moving continental plates (Wegener, 1929). The first re-measurement of this network including electro-optical distance measurements was carried out in 1965 (Gerke, 1967). The re-measurement had been repeated ten times by 1987 (Heumann, 1972, Ritter, 1982, Wendt, 1988, Möller, 1989). The results were not always reliable because of the accuracy of the measurements as well as the state of the art of the deformation analysis techniques at a time. Accordingly, a spreading of about 2 cm/yr – like it is well known nowadays and predicted by several global plate models for this part of the ridge (De et al., 1990, DeMets et al., 1994, Sella et al., 2002, Gripp and Gordon, 2002, Kreemer et al., 2003) – was not easy to detect. Comparable geodetic campaigns can be found in the southwest of Iceland. But there were no measured connections between these network results because they are separated in space and time.
Since 1986 GPS networks have been established to cover larger areas (Hackman, 1991, Foulger et al., 1993). But even the larger networks are still separated. Several institutions set up their own networks to solve specific questions (Jahn, 1992, Sigmundsson et al., 1995, Magnusson et al., 1997, Alex et al., 1999, Hreinsdóttir, 1999, Árnadóttir et al., 2001). Every institution chose a different subset of stations for observation. Quite often a concrete pillar as a station monument and one or more brass bolts as ground marker are assembled in an area of only a few metres on top of the same lava outcrop. Several of the unknown tie connections between the assembled foundations have been measured during the 1999 GPS campaign in southwestern Iceland (Perlt and Heinert, 2006). For the combination of the GPS networks the model in this article assumes an identical motion for all remaining unknown tie connections. At the beginning of this investigation the following questions arose:
• Is it possible to combine the results of the separated networks to get a field of tectonical motions over the whole island?
• Are the motions of a combined solution the same as those of the single local campaigns?
• How is the spreading of about 2 cm/yr distributed over the various tectonic units of Iceland?
To answer these questions, a subset of the GPS measurement campaigns between 1986 and 2002, see Table 1, has been taken into consideration to create a representative linear velocity field. The mathematical approach of a multi-epoch deformation analysis with linear motions has been adapted (Altamimi et al., 2002, Perlt and Heinert, 2006) for this purpose.
A linear motion model is sufficient to approximate the tectonics of all of Iceland. This model combines several networks with their characteristic network geometry and their chosen station subset.
The first adaptation of this multi-epoch deformation analysis is the determination of numerically stable blocks, which should coincide with the expected tectonical stability. The assumption of one velocity that is valid for a set of stations in combination with the assumption of linearity reduces the number of parameters in the model.
The second adaptation is the reduction of significant non-linear motions due to up-doming volcanoes or co-seismic deformations.
Section snippets
The eastern rifting zones
The first measurement campaigns across the Northern Volcanic Zone (Fig. 1) did not yield accurate and reliable displacements (Gerke, 1967, Heumann, 1972). In spite of these accuracy problems, the early measurements proved to be necessary a priori information for the rifting episode at the Krafla Volcanic System that followed in 1975–84. The later campaigns were able to measure a rifting effect of about 9m compared to these first measurements (Möller and Ritter, 1980, Möller, 1989), see Fig. 2.
Concept and data base
A combined model with the use of coordinate sets from several networks has a lot of advantages compared with a set of solutions of separately computed two-epoch deformation analyses (Perlt and Heinert, 2006). First of all, possible systematic errors associated with the two epoch analysis strategy are minimized. These systematic errors are caused by different geodetic datums, by the measurement methods or the subjective selection of stable blocks. Secondly, a consistent deterministic – in our
Results
The combination of the local results allowed the estimation of a medium-term velocity field for Iceland.
As a first main result of this investigation, the field of relative velocity vectors, see Fig. 7, is derived from the coordinate sets and synthetic or – as available – real covariance information of the GPS campaigns in Iceland between 1986 and 2002. If the value for the 1σ standard deviation exceeds the vector length the vector is indicated to be insignificant.
Besides the homogenized
Conclusions
The linear multi-epoch deformation model on the basis of 20 combined GPS campaigns is able to derive the medium-term deformation field of the spreading in Iceland. The combined solution quantifies the spreading for the volcanic zones. The full spreading rate between the major lithospheric plates, represented by the IGS (International GNSS Service) permanent stations Reykjavík and Höfn, can be amounted to 21.2 mm/yr ± 3.0 mm/yr in direction N118.4 ± 8.4° E.
The Hreppar microplate has to be viewed as
Acknowledgments
The GPS Campaign in 1995 of the Institut für Geodäsie und Photogrammetrie (IGP) was carried out in cooperation with the Nordic Volcanological Institute and the Science Institute of the University of Iceland. Our thanks are directed to all members of this campaign, especially to Prof Páll Einarsson and Dr Freysteinn Sigmundsson. The GPS Campaign in 1999 was carried out in cooperation with the National Land Survey of Iceland (LMÍ) and with friendly support of the Science Institute and the
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