Jökull - 01.01.2001, Síða 33

Paleomagnetic studies in Skarðsheiði, South–Western Iceland
Leó Kristjánsson
Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík
Ágúst Guðmundsson
ÁGVST Geological Services, Ármúla 4, 108 Reykjavík
Abstract – We have mapped a well–exposed composite profile through the lava pile in the western part of the
Skarðsheiði mountain, and made detailed laboratory measurements of paleomagnetic remanence directions in
oriented samples from 92 lava sites from this profile. The lower 55 lavas or so consist of a partially recorded
normal–polarity zone, overlain by a thick reverse–polarity zone. The latter which includes a series of por-
phyritic lavas, was named R5 by Einarsson (1957). The upper part of our profile contains a normal–polarity
sequence of about 20 flows (N5 of Einarsson) and a partially recorded reverse zone. Franzson (1978) and
others have correlated the R5–N5 boundary with a similar boundary in the Akrafjall mountain and with the
transition between the Gilbert and Gauss geomagnetic chrons. We have also sampled 29 lavas in two shorter
profiles farther east in Skarðsheiði, at the boundary between the R5 and N5 polarity zones. We confirm the
observation of Wilson et al. (1972) and Kristjánsson and Sigurgeirsson (1993) that several lava flows with
transitional directions occur at this boundary at the northern side of the mountain. Only a few such flows
are found on the south slopes of Skarðsheiði. At least three short geomagnetic excursions are recorded in the
R5 zone. The lava flows are good material for paleomagnetic direction measurements, although hydrothermal
alteration has affected the lowest part of the composite profile. Their average direction is quite similar to the
axial dipole field as expected. Thermomagnetic tests on several samples indicate that they are not very suitable
for paleointensity studies.
INTRODUCTION
Geology
The thick sequences of subaerial lava flows (mostly
of basalt) and minor sediments, which are exposed
above sea level in Iceland, are generally thought to
have been erupted in an active zone of rifting and vol-
canism. This zone which belongs to the Mid–Atlantic
rift system, trends roughly from south–west to north–
east through the island (Figure 1; see review by Sæ-
mundsson, 1986). The rift activity in the southwest
has been suggested to have moved to a new location
some tens of km to the east about 15 M.y. ago (Harðar-
son et al., 1997) and again 5–7 M.y. ago, with a third
ridge jump being in progress in the Late Quaternary.
Eruptive activity at any time is mostly confined to
central–volcano complexes within the volcanic zones.
The zones of volcanism are constantly subsiding as a
result of the extrusive volcanism, so that the exposed
lava pile tilts towards them (Walker, 1959, 1965). In
South–Western Iceland this regional dip is of the or-
der of 5–10 degrees to the south–east, decreasing up-
wards; considerable local variations in dip occur, es-
pecially near the central volcanoes. Some regional hy-
drothermal alteration has occurred in the lava pile, in-
creasing with depth. In the Akrafjall mountain (Figure
1) for instance, an alteration zone characterized by the
zeolites mesolite and scolecite is apparent from sea
level and up to about 100 m elevation, where it is fol-
lowed by an analcime zone, a chabazite–thomsonite
zone and then from 500 m altitude to the top at 550 m
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