Jökull


Jökull - 01.12.1994, Page 20

Jökull - 01.12.1994, Page 20
Fig. 1 Schematic tectonic map of the Hengill triple junction showing the Hengill, Hromundartindur (Hrom) and Grens- dalur systems and GPS points of our network. Dashed lines show the Hengill and Hromundartindur topographic highs and the extinct Grensdalur system. Inset shows the regional setting. The neovolcanic zones are shaded. 1. mynd. Yfirlitskort af sprungusveimum við þrjár eld- stöðvar á Hengilssœðinu. Punktalínur eru umhverfis hœstu fjöll í Hengils- og Hrómundartinds-eldstöðvunum og hina kulnuðu Grensdalseldstöð. Punktar sýna mœlistöðvar fyrir GPS-gervitunglastaðsetningar. Á litla kortinu eru gliðn- unarsvœði landsins skyggð. tivity. The interaction of the tectonic units that meet there, and deformation within the area are not well understood and little data on crustal movements are available. The area therefore provided a prime target for study using GPS geodesy. In 1991 a 30 x 25 km network of 23 ground con- trol points was established in the Hengill area and sur- veyed using GPS. Data were collected during eight- and twenty-hour sessions and the diurnal effect of ocean loading on baseline lengths was assessed to in- vestigate the magnitude of this possible error source. Sub-centimetre location accuracies were achieved in the horizontal for all points within the network. The effect of ocean loading was found to be much smaller than that of other errors for standard GPS field sur- veys and lines up to 75 km long in South Iceland and therefore to comprise a negligible error source in our survey. The Hengill GPS network will be remeasured after a few years or following a major tectonic event such as an earthquake in the SISZ. TECTONICS OF THE HENGILL AREA The accretionary plate boundary comes onshore in Iceland at the Reykjanes Peninsula and strikes N70°E for 85 km, as far as Mt. Hengill (Fig. 1, inset). The Hengill system comprises the southemmost part of the N35° trending Western Volcanic Zone (WVZ). The SISZ forms a 90 km long EW zone of historical de- stmctive earthquakes connecting the southem ends of the WVZ and the Eastem Volcanic Zone (EVZ). The WVZ and the EVZ are thought to be dying and propa- gating rifts respectively and the SISZ releases strain that accumulates in the rotating microplate between them by dextral strike-slip movement on NS faults (Einarsson, 1988; Phipps-Morgan and Kleinrock, 1991; Foulger et al., 1993). The SISZ is thought to be migrating south with the southerly propagation of the EVZ. Propagating-dying rift systems on the sea-floor are generally a few km broad whereas this analogous structure in Iceland is approximately 90 km broad. The Hengill area thus represents a very unusual kind of triple junction. Crustal accretion within the triple junction occurs along the Hengill volcanic system, which is dominat- ed topographically by Mt. Hengill (Fig. 1). Two older systems also lie within the area, the Hromundartindur and Grensdalur systems, which have been rendered almost inactive by migration of volcanism and spreading to the Hengill system. Six postglacial emp- tions have occurred within the area. A continuously seismically active high-temperature geothermal area encompasses the whole volcanic complex, which is thought to be fueled by volcanic intrusions and partial melt (e.g., Foulger, 1988a,b). South of the volcanic complex, in Ölfus, the westem end of the SISZ forms a zone of destructive historic seismicity. Southerly migration of the SISZ through the Grensdalur vol- canic system a few hundred thousand years ago may have triggered the migration of spreading within the Hengill triple junction. Interaction of the discrete tec- 18 JÖKULL, No. 44

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