Jökull - 01.01.2009, Page 41

A new study of paleomagnetic directions in the Miocene lava pile, NW Iceland
feldspar phenocrysts is variable, so that up to a third of
the entire present collection should perhaps be called
“sparsely feldspar-porphyritic”.
Table 1: Geographic coordinates of the lowest flow
in each of the sampling profiles, and tectonic tilt cor-
rections used. – Staðsetning neðstu hrauna í hverju
jarðlagasniði, og halli.
North West dip ◦
TE 65◦46’.259 24◦01’.664 2
VS 65◦35’.842 23◦58’.770 4
VR 65◦36’.261 24◦00’.404 4
HV 65◦36’.693 24◦11’.734 3
HM 65◦34’.296 24◦06’.899 3
FH 65◦35’.117 24◦08’.456 3
PS 65◦30’.802 23◦51’.342 4.5
TS 65◦37’.486 23◦53’.573 3
FD 65◦38’.687 23◦59’.625 3
AJ 65◦41’.868 23◦38’.611 3
AI 64◦41’.445 23◦35’.846 3
AC 65◦42’.620 23◦40’.817 3
TV 65◦35’.337 23◦46’.740 4.5
AK 65◦37’.393 23◦35’.140 5
RF 65◦37’.742 23◦27’.918 4.5
KH 65◦36’.175 23◦32’.365 4.5
HK 65◦29’.696 23◦38’.498 5
HP 65◦33’.936 24◦03’.425 4
KV 65◦36’.881 24◦18’.810 3
RS 65◦27’.397 23◦54’.375 4.5
TW 65◦38’.161 23◦50’.491 3
MH 65◦29’.785 23◦28’.684 4
AB 65◦39’.512 23◦38’.393 3
In Figure 2, the presence of interbasaltic sedi-
ments is indicated with a filled triangle if they are
noted to be of 30 cm or more in thickness. It should be
kept in mind that such sediments, along with the sco-
riaceous top and bottom parts of lava flows, are very
commonly covered by scree or soil.
Cores of generally 4–8 cm length and 2.45 cm di-
ameter were collected by a portable two-stroke engine
with a water-cooled diamond bit. In most cases, four
cores were collected per flow. These were oriented by
a Brunton compass clinometer and by sighting on dis-
tant geographic objects or the Sun. The accuracy of
the inclination and declination measurements is of the
order of 1–2◦ in each.
One specimen of 2.1–2.2 cm length was cut from
each core for paleomagnetic measurements. These
were made in an Institut Dr. Förster static four-probe
fluxgate magnetometer at the University of Iceland.
The remanence direction was measured before alter-
nating field (AF) demagnetization in a Molspin de-
magnetizer, and after demagnetization treatment at
10, 15, 20, 25 and 30 mT peak fields. In a few in-
stances, the treatment was repeated or extended to ob-
tain improved average directions. For the measure-
ments following each step, an average was computed
from the individual sample directions in a custom-
ary way (see Kristjánsson et al., 2003). The average
with the smallest 95% confidence radius (α95) was
selected; in cases where two steps gave similar α95-
values, that at the higher field was used. In the present
collection, any secondary remanence was generally
removed by the 10 mT treatment, and the 30 mT de-
magnetization step turned out to be quite unnecessary
in almost all cases. Extended AF treatment to 80 mT
was applied to seven pilot samples, one becoming un-
stable at 30 mT but no change in remanence directions
was experienced in the others.
All directions were corrected for the estimated
tectonic tilt, which according to Kristjánsson et al.
(1975) increases from 2–3◦ in the oldest lavas to 5◦ in
the younger parts of the area of Figure 1. During the
present field work, these values were checked by esti-
mates of apparent dip at several locations. The down-
dip direction used in these corrections is 150◦–160◦
East in agreement with Kristjánsson et al. (1975) and
McDougall et al. (1984).
The total number of lavas sampled in 2004–2008
was 365. This number does not include the small
crumbly outcrops TE 1 and 9 where only one sam-
ple was cored in each. One flow, RS 14, gave quite
unreliable results. Additionally, about 20 individual
samples were rejected because of instability or sus-
pected orientation errors; replacements were obtained
in some of the lavas involved (in profile HK).
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