Jökull - 01.01.2013, Side 20

Þ. Björnsdóttir and P. Einarsson
canic Zone. However, Einarsson (2008) places the
Tungnafellsjökull volcanic system within the Central
Iceland Volcanic zone along with the Hofsjökull vol-
canic system and we favor that opinion. The younger
Bárðarbunga central volcano, one of the more active
volcanoes in Iceland, is SE of Tungnafellsjökull. It is
in a separate volcanic system (Figure 1) and clearly
belongs in the Eastern Volcanic Zone. Its northern
fissure swarm extends beyond the triple junction well
into the Northern Volcanic Zone.
The Tungnafellsjökull central volcano forms a
ridge-shaped mountain reaching elevation of 1500 m
and is slightly eroded. It has a radius of about 10
km but doesn’t rise as a perfect cone due to the ero-
sion that has occurred. The volcano has an elongated
summit caldera (Figure 1), covered to a large degree
by a glacier that caps the edifice. A separate, 8 km
wide caldera is found at the SE foot of the moun-
tain, called the Vonarskarð caldera, with an active
high-temperature field in the center. This caldera with
its eruptive units is sometimes considered as a sep-
arate central volcano (Friðleifsson and Jóhannesson,
2005). The eastern part of the volcano is covered with
subglacial volcanics from the neighbouring Bárðar-
bunga central volcano. Based on geological map-
ping the Vonarskarð caldera is slightly younger than
the Tungnafellsjökull volcano. No eruption is known
to have originated from the volcano itself in Post-
glacial time (i.e., the last 9000 years), although two
small lavas beyond the northeastern flank are associ-
ated with the system, the Dvergar lava and the Tungu-
hraun lava (Sæmundsson, 1982; Friðleifsson and Jó-
hannesson, 2005) (Figure 1).
METHODS
Fractures and fissures within the study area were
mapped both from aerial photographs and satellite im-
ages. Field observations were conducted in the sum-
mer of 2009 and 2010. In 2009, the focus was on the
area north of Tungnafellsjökull and in 2010 the main
task was to further observe rifting features detected
from aerial photographs. Our observations were com-
bined with data on seismic activity obtained from the
Icelandic Meteorological Office (IMO). InSAR im-
ages were also used to evaluate fault movements dur-
ing the previous couple of years. The aerial pho-
tographs used were contact images from Landmæl-
ingar Íslands (The National Land Survey of Iceland)
and digital images from Loftmyndir ehf. These im-
ages were taken at approximately 6,700 and 8,000 m
altitude in 1996 and 1999, respectively. The satel-
lite images were obtained from SpotImage© and the
ASTER archive. Care was taken not to confuse tec-
tonic lineaments with glacial striae, which are abun-
dant in the area (Kaldal and Víkingsson, 1990).
Earthquake data from September 1996 to August
2011 were used to evaluate recent faulting activity.
Earthquake locations with larger azimuthal gap than
180◦ were excluded as well as earthquakes with larger
RMS value than 0.25 s and events recorded at fewer
than four stations.
STRUCTURAL ARCHITECTURE
Overview
The mapped fractures are shown in Figure 2. The
area mapped may be divided into three areas ac-
cording to its physiographic and tectonic style: The
Ógöngur area (Figure 3) southwest of Tungnafells-
jökull (named after hyaloclastite ridges, tindars, in the
area), the Tómasarhagi area (Figure 4) northwest of
Tungnafellsjökull, and the Langadrag area (Figure 5)
northeast of Tungnafellsjökull.
Fractures located within the study area are mostly
normal faults, sometimes forming a graben. The ori-
entation of fractures and faults is different between
the three areas. From north to south the orientation
changes from various orientations in the Langadrag
area to NE in the Tómasarhagi area, to ENE in the
Ógöngur area. Hyaloclastite ridges, tindar in recent
terminology, the product of subglacial fissure erup-
tions (Kjartansson, 1943), are common in the Ógöng-
ur area. Features that indicate recent movements like
sinkholes and open fissures are common, especially
in the Tómasarhagi area. The fissure swarm is wider
towards the west than the fissure swarm drawn in pre-
vious structural maps by Sæmundsson. Many of the
faults and fissures of the fissure swarm lie west of the
volcano, thus bypassing it rather than transecting it
(Figure 2).
20 JÖKULL No. 63, 2013