Jökull


Jökull - 01.12.1980, Page 47

Jökull - 01.12.1980, Page 47
dilation in a section near the west end of the Vogar fissure swarm (partly outside the map in Fig. 2). His section is 4 km long and inter- sects 12 fractures with a total dilation of 17.65 m. T. Einarsson (1965) believes that all the major fractures, in the Holocene lavas on the peninsula, have formed near the beginning of that period. Jónsson (1978) has carried out the most detailed geological investigation in the area. Some of his results can also be found in Jakobsson et al. (1978). Various tectonic studies have been under- taken in later years. Tryggvason (1968, 1970, 1974a, 1974b) measured vertical movement across the Vogar fissure swarm. He estimates the maximum subsidence, during the 10000 years since the lava formed, as 50 m. The measurements are only relative, i.e. not refer- red to sea level. Movement is irregular; both subsidence and uplift occur, but the general tiiting of the area is 2 microradians per year. Nakamura (1970) made a study of the fissure swarms on the western part of the peninsula. He concludes that the Reykjanes Peninsula is an oblique spreading ridge. Brander et al. (1976) measured horizontal movement. In a profile near the west end of the peninsula they inferred an average 9 mm dilation per year, during the interval 1968—1972. New measurements were carried out in the summer 1979, but the results have not been published yet. . Seismology has given important infor- mation. Pálmason (1971) has examined the crustal structure of Iceland from explosion seismology. Some of his profiles are in the Reykjanes Peninsula. The main results for that area are as follows: the surface layer (layer 0), composed of a mixture of young lavas and hyaloclastites, is 0.35 to 0.54 km thick. Below are layers 1 and 2. Both are es- sentially flood basalts, reaching down to depth of about 3 km. Layer 3, below this depth, is composed of metabasalts, and is probably to be equated to the oceanic layer. At a depth of about 8.5 km layers 4 takes over. It represents the upper mantle. Fjarthquakes are frequent on the Reykjanes Peninsula. Most of the rele- vant data is given in Klein et al. (1977) and P. Einarsson (1979). The majority of the earth- quakes occur within a 1 —2 km wide zone that runs along the peninsula, parallel to the zone of volcanism. The focal depth usually varies between 2 and 5 km, the maximum being about 10 krn. METHODS Air photographs in scale 1:35700 were used to map the Vogar fissure swarm and for most of the measurements on the fractures. Detailed field work was also carried out. Its main objective was a detailed study of in- dividual fractures and to test the accuracy of the laboratory methods. The accuracy of the data given in this paper is 0.5 m for width and throw of fractures, and about 1 ° for orien- tation of fractures. THE VOGAR FISSURE SWARM - RESULTS General comments For various reasons, the Vogar fissure swarm is the most appropriate for detailed measurements. In the first place, it lies ahnost Fig. 3. Location of profiles across the Vogar fissure swarm. (3) is column number and (3) is profile number. — Mynd 3. Staðsetning sniða yfir sþrunguþyrpmguna við Voga. (3) er númer dálks og Q) er númer sniðs. Önnur tákn eru eins og á mynd JÖKULL 30. ÁR 45

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