Jökull - 01.12.1989, Page 54

Fig. 12. Locations of earth-
quakes from 1974 to 1987
superimposed on the gravity
map. Gravity contours are 2
milligals apart.
Mynd 12. Staðsetning jarð-
skjálfta frá árunum 1974 til
1987 merkt á þyngdarkort-
ið. Milli jafngildislínanna
eru 2 milligal.
can be separated into two groups: those that occur in
the volcanic areas and those that occur in the South
Iceland seismic zone.
The two prominent magnetic anomalies presum-
ably show where magmatism has been most active
during the Brunhes magnetic epoch (the last 0.7 mil-
lion years). The southem anomaly is aligned with
the gravity low and seismically very active. The
anomaly and its continuation to the northeast
through the gravity low, as well as the earthquake
swarms at Skjaldbreiður and the glacier Langjökull,
trace the presently active rifting zone (Sæmundsson,
1979). The northem anomaly is located some 20
kilometers east of the center of the rift and is only
moderately seismically active. The last volcanic
episode in that area was probably the one that pro-
duced the Grímsnes lavas 5-6000 years ago (Jakobs-
The arrangement of the two magnetic anomalies
relative to the presently active rifting zone shows a
westward shift in the activity of the plate boundary
on a time scale of hundreds of thousands of years. A
gradual westward movement of activity has also
been demonstrated in the geological mapping of the
Hengill volcanic system by Kristján Sæmundsson
(e.g. Bjömsson and others, 1974; Amason and oth-
ers, 1986). This pattem of westward movement has
also been noted by Þórarinsson and others (1988)
and Foulger (1988a).
ACKNOWLEDGEMENTS
The gravity data were collected in a joint project
of the National Energy Authority (Orkustofnun) and
the U.S. Army. The manager of the project was
Gunnar Þorbergsson at Orkustofnun. The bulk of the
52 JÖKULL, No. 39, 1989