Jökull - 01.06.2000, Blaðsíða 48

Aeromagnetic measurements over Mýrdalsjökull and vicinity
Geirfinnur Jónsson and Leó Kristjánsson
Science Institute, University of Iceland, Haga, Hofsvallagötu 53, 107 Reykjavík, Iceland
Abstract — Aeromagnetic survey results from the Mýrdalsjökull glacier region, acquired by Th. Sigurgeirsson
around 1970, have been reprocessed in combination with data from a new survey. The main feature of the
residual magnetic field over Mýrdalsjökull is a localized 8 x 12 km negative anomaly, of around -2000 nT
amplitude relative to the regional field. Models of the crust indicate that it is caused partly by the subglacial
topography and partly by a thermal anomaly associated with a possible magma chamber beneath the Katla
caldera.
INTRODUCTION
Local aeromagnetic anomalies in and around Iceland
are typically on the order of 1000 nT in amplitude at
1000 m altitude (Jónsson et al., 1991). They tend to be
circular rather than elongated and are generally less
than 20 km in size. Wide flight line spacing and gaps
in data limit the resolution of magnetic-field maps
based on aeromagnetic surveys. Some anomalies are
observed on only one or two flight lines and their
detailed shapes are, therefore, not well defined. The
causes of the anomalies vary, but can sometimes be
inferred from their geological setting. Some anom-
alies are clearly connected to central volcanoes, ot-
hers are due to topography or to subaquatic eruptions
which produce a complex structure of nonmagnetized
tuff and breccia with inclusions of highly magnetized
pillow lava (Kristjánsson, 1970).
Two, at least, of these anomalies show simil-
ar characteristics where a broad ( 10 km) negative
anomaly on the order of 2000 nT occurs in positi-
ve fields over an active volcano. These are Askja in
Dyngjufjöll and Katla in Mýrdalsjökull. A similar,
but less clear anomaly is found over Bárðarbunga in
the Vatnajökull glacier. Due to the position of these
volcanoes within the active spreading zones in Ice-
land, a fragment of reversely magnetized material can
be ruled out as their source. In this paper we focus on
the Katla anomaly, display the magnetic field of the
Mýrdalsjökull area and interpret it.
PREVIOUS WORK
The U.S. Naval Oceanographic Office Project Magnet
carried out total-field aeromagnetic surveys in the
South Iceland region in Feb. 1964, at varying altitu-
des. Results in the form of draft contour maps were
presented to the Science Institute, one of which was
published as Figure 14 of Kristjánsson et al. (1989).
Another map (1800 m a.s.l.) includes three flight
lines which cross the central part of the Mýrdalsjök-
ull icecap (Figure 1) and record a negative anomaly
of 2000 - 2500 nT amplitude. Compared to later data,
however, the positions of this survey seem to be un-
reliable, and it is not included in the present study.
Detailed aeromagnetic measurements over Mýr-
dalsjökull were made by Th. Sigurgeirsson during
his 1969-72 survey of South Iceland (Figure 2). The
flight lines were oriented east-west with 4 km spac-
ing. Positioning was based on visual identification of
landscape features. The nominal flight altitude was
1200+ m above sea level, indicating that the plane
ascended when approaching higher topography and
descended again when appropriate. The icecap altitu-
de reaches 1500 m. The magnetometer was a cont-
inuously recording proton precession instrument from
which field values, averaged over 10 s (5-600 m), were
obtained (Sigurgeirsson, 1970). Processing of the data
included correction for diurnal and secular variati-
on, subtraction of a regional field, and manual plott-
JÖKULL No. 49 47