Jökull - 01.12.1989, Blaðsíða 43

Gravity, Aero-Magnetism and Earthquakes in SW-Iceland
FREYR ÞÓRARINSSON1, STEFAN G. MAGNUSSON2,
PÁLL EINARSSON3, LEÓ KRISTJÁNSSON3,
GUÐMUNDUR PÁLMASON2 and AXEL BJÖRNSSON2
ABSTRACT
Gravity, magnetic and earthquake data from an
area in SW-Iceland containing a constructive rifting
zone are processed, compared and analyzed.
After a predominant bowl-like regional trend has
been removed, the residual gravity data are charac-
terized by a negative anomaly of 6-10 milligals that
follows the axis of the rifting zone. It is suggested
that this anomaly is caused by Pleistocene volcanics
buried in the crust and possibly a zone of partial
tnelt at subcrustal depths. Also revealed in the
analysis is a clustering of earthquakes in the South
Iceland seismic zone around a positive gravity ano-
maly.
The magnetic signature of the rifting zone is
enhanced by directional filtering of the aero-
magnetic data. This processing delineates two dis-
tinct areas of crustal accretion during the Brunhes
epoch, one coinciding with the presently most active
part of the plate houndary, the other some 20 kilom-
eters east of it. This is interpreted as a westward
shift of the axis of maximum activity in the rifting
zone during the last several hundred thousand
years.
Directional filtering of the gravity and magnetic
data also reveals linear anomalies trending
transversely to the rifting zone, but at a 30-40°
angle to the estimated direction ofplate motion. It is
suggested that these cross-grain structures may be
the result of magmatism related to contraction
cracks developing within the cooling plates as they
move awayfrom the plate boundary.
INTRODU CTION
Gravity and airborne magnetic data from the
southwest comer of Iceland have been processed
and analyzed by newly developed techniques and
computer software, partly as a test for these
methods. Using these processing techniques, it is
possible to isolate and emphasize lineaments in the
data. Due to the tectonic origin of the lineaments,
the available earthquake data were an ideal supple-
ment to the gravity and magnetic data, as shown by
the synergism in their joint interpretation.
The location of the test area is shown in Figure 1.
The size of the area was set somewhat arbitrarily at
126x126 km2, corresponding to a grid of 64x64
points. This covers most of the active volcanic zone
in SW-Iceland, henceforth called the westem vol-
canic or rifting zone, with the exclusion of the
westem part of the Reykjanes peninsula.
THE GRAVITY DATA
The acquisition of the gravity data under discus-
sion was started in the late sixties. Most of it was
collected in the early seventies but some dates from
1. School of Computer Studies, Commercial College of Iceland (Tölvuháskóli Verzlunarskóla íslands), Ofanleiti 1, 103
Reykjavík, Iceland.
2. National Energy Authority (Orkustofnun), Grensávegi 9, 108 Reykjavík, Iceland.
3. Science Institute, University of Iceland (Raunvísindastofnun Háskólans), Dunhaga 5, 107 Reykjavík, Iceland.
JÖKULL, No. 39, 1989 41