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Jökull - 01.01.2017, Qupperneq 51

Jökull - 01.01.2017, Qupperneq 51
Leó Kristjánsson and Jónsson growth of various forms of secondary magnetite, in basalts buried to more than 2.5–3 km below the orig- inal surface of the crust. This level coincides roughly with a stage of hydrothermal alteration characterized by epidote, chlorite and prehnite (Hall, 1985 and ref- erences therein, cf. Pálmason et al., 1979, Fig. 14). Further studies of the opaque mineralogy of available deep-drilling material from the Reykjanes peninsula and locations elsewhere in the country may reveal ev- idence of such secondary magnetite. AEROMAGNETIC RESULTS IN 1985–1986 AND 1991–1992; DATA PROCESSING Sigurgeirsson’s surveys in 1968–1980 did not include the Faxaflói bay. A new survey was undertaken in 1985–1986 to cover this area and a few others, at al- titudes around 950 m. Flight lines were headed 37◦ west of north. Their spacing was 3–4 km, and po- sitions were determined by means of a Loran-C re- ceiver. Measurements of the field strength were made once every 300 m. Most of the lines reached to sev- eral km south of the Reykjanes peninsula, thus con- necting to the area of a 1973 marine survey described by Kristjánsson et al. (1989). In 1991–1992 five additional lines were flown at 6 km spacing south-west of Faxaflói, parallel to those of the previous survey. Some cross-check lines were also added. Measurements were again made at about 300 m intervals, with positions determined by a GPS receiver. Details of these surveys and on the acquisition of magnetic maps of the whole of Iceland may be found in Jónsson and Kristjáns- son (1991), Kristjánsson and Jónsson (1996) and at http://www.lso.is/Magn-vefur/index.htm. In our analyses of magnetic field records from West Iceland and offshore (Kristjánsson and Jónsson, 1996, 1998), results from the Reykjanes peninsula were only shown on a small-scale map and not dis- cussed. The main purpose of this paper is to provide a view of the magnetic field over the peninsula in as much resolution as the available material allows. The data from the above two surveys have now been combined with those of Sigurgeirsson’s (1970a,b) survey in 1968. The latter data set was ob- tained by digitizing positions and field readings from his analog records at approx. 500 m intervals. All the aerial survey lines from these projects as well as some east-west lines from the 1973 marine survey are shown in Figure 2. They were used to create a digital grid of the Reykjanes peninsula by means of a krig- ing algorithm, with 8 km search radius and 1 km node spacing. No correction was attempted for different survey altitudes. Figure 3 displays the grid as a color image superimposed on a volcano-tectonic map of the peninsula (Á.R. Hjartardóttir, pers. comm. 2018). SOME FEATURES ON THE MAGNETIC MAP OF THE PENINSULA Passing from west to east, Figure 3 shows first a pos- itive WSW-ENE anomaly lineation with three peaks. The smallest of these is seen at the geothermally ac- tive SW-tip (Reykjanes) of the peninsula and to the southwest. A larger one, also within the Reykjanes swarm of Jakobsson et al. (1978) may also be linked to well-known high-temperature areas. A still more prominent anomaly covers the central part of their Grindavík swarm. An elongated anomaly coincides more or less with the Krísuvík swarm of Jakobsson et al. (1978) which includes a region with several high-temperature lo- calities. The northeastern part of the anomaly tends towards a northerly strike, and so do the next two anomalies. While both of them overlap partially with Jakobsson et al.’s Bláfjöll swarm, they are clearly sep- arate entities. The most easterly positive anomaly shown in Figure 3 overlaps the Hengill swarm of Jakobsson et al. and its geothermal areas. However, it is much wider especially in its central and southwest- ern parts. It reaches a high peak above the Skálafell hill, probably to some extent due to extrusive vol- canics. The main body of the anomaly occurs near the western end of the South Iceland seismic zone. An extended negative anomaly at the right-hand edge of Figure 3, presumably due to basement for- mations from the Matuyama chron, passes out to sea and turns southwest. It is interrupted by a pos- itive anomaly which coincides with the Ingólfsfjall mountain. A series of normally magnetized lavas in its southern part was mapped by Kristjánsson et 46 JÖKULL No. 67, 2017
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