Jökull - 01.12.1978, Page 34

The Hvannadalur and Geitafell intrusions, SE Iceland:
Magnetic anomalies, structure and the magnetic layer
problem
G. SCHÖNHARTING
Institut for almen Geologi, 0stervoldgade 10 1350 Kebenhavn K, Danmark
K. STRAND PETERSEN
Danmarks geol. Unders., Thoravej 31, 2400 Kebenhavn NV, Danmark
ABSTRACT
Field magnetic measurements revealed large mag-
netic anomalies over portions of the Hvannadalur and
Geitafell gabbros. Paleomagnetic directions were
determmed from sun-compass oriented samples from
the gabbros and associated basic to acid dykes and
microgramtes. These directions suggest a tilt of
30—40° towards NWfor almost all the investigated
intrusives, conforming with the maximum dips of
plateau basalts found in the flexure zone of SE Ice-
land. Strong remanent magnetization values and suf-
ficient stabilities make such gabbros potential con-
tnbutors to the magnetic anomalies over Iceland —
and perhaps also over the oceans. The structural set-
ting, however, indicates that these gabbros may not be
typical for rift zone instrusives.
INTRODUCTION
In the summer of 1976 magnetic and
geological field investigations were conducted
in two areas in SE Iceland, the Hvannadalur
and the Hoffellsjökull-Geitafell areas respec-
tively (Fig. 1). Cargill et al. (1928), Johnson
(1954), Annels (1967) and Newman (1967)
described various intrusions here, consisting of
gabbros, granophyres and associated basic to
acid dykes and sheets. Walker (1975) suggested
that basaltic sheet and gabbro assemblages
might constitute seismic layer 3 in Iceland,
which has been found by Palmason (1971) to be
at relatively shallow depths in the neighbour-
hood of several volcanic centres in Iceland.
Close to the Geitafell area, depths to layer 3 of
about 2 km have been measured.
Palmason (1971) interpreted layer 3 (with
seismic wave velocities similar to that of oce-
anic layer 3) as metamorphosed basaltic rocks,
probably amphibolite.
The problem of identifying the “magnetic
layer”, the portion of the crust that is mainly
responsible for the magnetic anomalies ob-
served over the oceans, has so far not been
solved by ocean bottom drilling. On the con-
trary, results of the Deep Sea Drilling Project
Fig. 1. Sketch map of intrusions in SE Ice-
land (after Walker, 1975), black areas: gabbro
and granophyre, stippled: flexure zone.
Mynd 1. Helstu innskot á Suðausturlandi (eftir
Walker, 1975); svart er gabbró og granófýr, punktar
sýna svceði með miklum jarðlagahalla.
32 JÖKULL 28. ÁR