Jökull - 01.12.1994, Page 20

Fig. 1 Schematic tectonic map of the Hengill triple junction
showing the Hengill, Hromundartindur (Hrom) and Grens-
dalur systems and GPS points of our network. Dashed lines
show the Hengill and Hromundartindur topographic highs
and the extinct Grensdalur system. Inset shows the regional
setting. The neovolcanic zones are shaded.
1. mynd. Yfirlitskort af sprungusveimum við þrjár eld-
stöðvar á Hengilssœðinu. Punktalínur eru umhverfis hœstu
fjöll í Hengils- og Hrómundartinds-eldstöðvunum og hina
kulnuðu Grensdalseldstöð. Punktar sýna mœlistöðvar fyrir
GPS-gervitunglastaðsetningar. Á litla kortinu eru gliðn-
unarsvœði landsins skyggð.
tivity. The interaction of the tectonic units that meet
there, and deformation within the area are not well
understood and little data on crustal movements are
available. The area therefore provided a prime target
for study using GPS geodesy.
In 1991 a 30 x 25 km network of 23 ground con-
trol points was established in the Hengill area and sur-
veyed using GPS. Data were collected during eight-
and twenty-hour sessions and the diurnal effect of
ocean loading on baseline lengths was assessed to in-
vestigate the magnitude of this possible error source.
Sub-centimetre location accuracies were achieved in
the horizontal for all points within the network. The
effect of ocean loading was found to be much smaller
than that of other errors for standard GPS field sur-
veys and lines up to 75 km long in South Iceland and
therefore to comprise a negligible error source in our
survey. The Hengill GPS network will be remeasured
after a few years or following a major tectonic event
such as an earthquake in the SISZ.
TECTONICS OF THE HENGILL AREA
The accretionary plate boundary comes onshore in
Iceland at the Reykjanes Peninsula and strikes N70°E
for 85 km, as far as Mt. Hengill (Fig. 1, inset). The
Hengill system comprises the southemmost part of the
N35° trending Western Volcanic Zone (WVZ). The
SISZ forms a 90 km long EW zone of historical de-
stmctive earthquakes connecting the southem ends of
the WVZ and the Eastem Volcanic Zone (EVZ). The
WVZ and the EVZ are thought to be dying and propa-
gating rifts respectively and the SISZ releases strain
that accumulates in the rotating microplate between
them by dextral strike-slip movement on NS faults
(Einarsson, 1988; Phipps-Morgan and Kleinrock,
1991; Foulger et al., 1993). The SISZ is thought to be
migrating south with the southerly propagation of the
EVZ. Propagating-dying rift systems on the sea-floor
are generally a few km broad whereas this analogous
structure in Iceland is approximately 90 km broad. The
Hengill area thus represents a very unusual kind of
triple junction.
Crustal accretion within the triple junction occurs
along the Hengill volcanic system, which is dominat-
ed topographically by Mt. Hengill (Fig. 1). Two older
systems also lie within the area, the Hromundartindur
and Grensdalur systems, which have been rendered
almost inactive by migration of volcanism and
spreading to the Hengill system. Six postglacial emp-
tions have occurred within the area. A continuously
seismically active high-temperature geothermal area
encompasses the whole volcanic complex, which is
thought to be fueled by volcanic intrusions and partial
melt (e.g., Foulger, 1988a,b). South of the volcanic
complex, in Ölfus, the westem end of the SISZ forms
a zone of destructive historic seismicity. Southerly
migration of the SISZ through the Grensdalur vol-
canic system a few hundred thousand years ago may
have triggered the migration of spreading within the
Hengill triple junction. Interaction of the discrete tec-
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