Jökull - 01.01.2009, Blaðsíða 47

A new study of paleomagnetic directions in the Miocene lava pile, NW Iceland
It would be interesting to see if similar results are ob-
tained in outcrops of this group which Preston traced
around the end of Patreksfjörður; he also located it
within or just southeast of our profile KH, in the main
valley of Fossfjörður (Figure 1). The normally mag-
netized flows RS 1–7 near the Breiðafjörður coast
may represent the top of the polarity zone PS 8–24.
Figure 4. a) Virtual geomagnetic poles (VGPs) in
flows TR 1–3 (reverse) and TS 1–11, Tálknafjörður.
b) VGPs in flows VS 1–34, Patreksfjörður. Excur-
sions to low latitudes occur in flows 3–8 and 28–30.
–Mikið flökt segulskautsins í tveim sniðum.
With RS 8–15, PS 25–29, and especially profile
HK southeast of Patreksfjörður, we may have reached
the thick zone of reverse flows which Friedrich (1966)
called “Ar”. The distance of some 10 km from PS to
HK is however uncomfortably large, and additional
profiles need to be sampled in the (somewhat faulted)
region between them. One can do no better at present
than assuming that this reverse zone reaches all the
way up through the sampled flows MH 26–37 and is
also represented by the rather thin flows JF 1–43 of
McDougall et al. (1984).
The normal-polarity flows MH 38–52 would then
correspond to Friedrich’s (1966) “Bn” zone, and so
would (according to Friedrich) the thin flows JF 44–
105 of McDougall et al. (1984).
OVERALL PALEOMAGNETIC
RESULTS; ROCK MAGNETISM
The arithmetic average remanence intensity in the
lavas after 10 mT demagnetization is 4.2 A/m, some-
what higher than observed in previous surveys on
lavas of 1–15 m.y. age in Iceland (Kristjánsson, 2002;
Kristjánsson et al., 2003). Results from profiles MH
and AB are not included in this average. Median de-
structive fields during AF treatment exceed 20 mT
in the majority of cases. Initial susceptibilities were
measured in one sample from each flow of the 2004–
2008 collection and profile MH, yielding an average
of about 0.017 SI units, a little lower than the average
found in the survey of Kristjánsson et al. (2003).
Out of our 364 stable lavas collected in 2004–
2008, 256 are normally magnetized, i.e. their rema-
nence directions correspond to virtual geomagnetic
poles lying north of the Equator. The excess of
normal-polarity lavas is in part due to sampling at sev-
eral locations in the almost 1 km-thick mostly normal
series in the middle of the column in Figure 3.
The most noticeable positive aeromagnetic
anomaly (see e.g. Figure 7 of Kristjánsson, 2008)
over the Northwest peninsula reaches most of the way
from Breiðafjörður to Ísafjarðardjúp. In the area south
of Arnarfjörður it is 10–20 kilometers wide, with an
axis lying roughly between our profiles RS and AC
and an amplitude of 500 nT. This anomaly may be in
part due to the thick normal-polarity zone mentioned
in the previous paragraph, but the anomaly has a more
northerly strike than is assumed here for the lava pile,
i.e. around 30◦ East of North instead of 60–70◦.
The mean remanence direction in all the lavas (N
= 364, after inversion of reverse directions) has a dec-
lination of 1◦ East and an inclination of +74.4◦, vector
sum R = 331.5. This corresponds to a slightly “far-
sided” average pole, as the central axial dipole field
JÖKULL No. 59 47