Jökull - 01.01.2017, Page 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