Jökull - 01.01.2013, Side 25

Recent fault movements in the Tungnafellsjökull fissure swarm
The Ógöngur area
The Ógöngur area is characterized by tindars and pil-
low lava mountains (or ridges) in the southeastern part
grading into sand ridges and dunes toward the west
(Figures 3 and 6a). All exposed volcanic formations
have a normal magnetization and are thus younger
than 700,000 years (Piper, 1979) with no Holocene
lavas. Both tindars and normal faults are ENE ori-
ented. The normal faults are prominent on satellite
images and aerial photographs. However, few fissures
or sinkholes were found indicating that recent move-
ments have been minimal.
It has been suggested that Vonarskarð and
Tungnafellsjökull are two separate volcanic systems
with separate fissure swarms (Jóhannesson and Frið-
leifsson, 2006). Fissure swarms often have the struc-
ture of a shallow graben with boundary faults dipping
towards the center of the swarm. If Vonarskarð and
Tungnafellsjökull had separate fissures swarms, one
would assume that the normal faults in the Ógöng-
ur area would change dip direction from west to east
forming two parallel grabens with westward and east-
ward dipping normal faults between them. A surface
cross section, surveyed from west to east, southwest
of Ógöngur (Figure 3) revealed only westward dip-
ping faults. The fissure swarm thus has the form of a
half graben at this latitude which is rather exceptional
for Icelandic fissure swarms. The question of the ex-
istence of two separate fissure swarms remains open.
Figure 3 shows mapped faults and fractures in
the Ógöngur area. Fault throw was estimated in four
places by measuring cross sections over the faults by
GPS. Their offsets were within the range 5–15 m.
The Tómasarhagi area
The Tungnafellsjökull volcano with its glacier lies in
the southeast corner of Tómasarhagi area (Figure 4)
encompassed by alluvial and glacially formed sand
ridges and with sparse vegetation. Three glacial rivers
drain the Tungnafellsjökull glacier as well as several
smaller streams. Faults and fissures in this area are
NE oriented, lying parallel to, but offset from the ma-
jor axis of the Tungnafellsjökull central volcano (Fig-
ures 2 and 4). This is not the normal spatial relation-
ship for Icelandic fissure swarms, where the norm is
for the fissures to transect their central volcano. The
area closest to Tungnafellsjökull is characterized by
numerous normal faults and a graben with open fis-
sures and sinkholes providing evidence of Holocene
fault movements. Some of the fissures and sinkholes
are very deep and indicate movements as recent as
spring of 2010 (Figures 6d, e and f). A cross sec-
tion over the boundary faults of the graben, measured
with GPS, gives a subsidence of 2–6 m of the graben
floor. Movements detected by InSAR measurements
following the Gjálp eruption in 1996 were also located
in this area (red stars in Figures 2 and 4). Field ob-
servations in these places revealed normal faults with
fissures and fresh-looking sinkholes (yellow stars in
Figures 2 and 4).
The Langadrag area
Sand ridges and dunes characterize the Langadrag
area, although some hyaloclastite ridges and pillow
mounts can also be seen. Faults and fractures are
scarce, compared to the Ógöngur and Tómasarhagi ar-
eas. However, there is ample evidence of recent move-
ments in the form of sinkholes and fissures along the
Langadrag faults. Rows of sinkholes and scarps of 0-
2 m height marked the faults that were discovered and
mapped on foot during fieldwork in the summer of
2009 (yellow lines in Figure 5). Both Holocene lavas
associated with the Tungnafellsjökull volcanic system
are located in the Langadrag area. Tunguhraun from
the Bokki crater lies in the west and the Dvergar lava
in the southeast. Both lavas are small in volume, in-
dicating small eruptions. A warm pool called Hita-
laug is located about 2,5 km south of one of the sites
where movements were detected by InSAR measure-
ments. A ground check at this site revealed no fresh
faults or fractures. The warm pool is located on an old
fractures. Fractures and faults in the Langadrag area
have somewhat variable orientations (Figures 2 and
5). With one NE aligned fissure mapped in the south
and several NNE aligned fractures and fissures further
north. The Dvergar eruptive fissure in the southeast
strikes in an ENE direction (Figure 6). The connec-
tion between the faults and fractures of the Langadrag
area and the main edifice of the Tungnafellsjökull cen-
tral volcano is difficult to establish due to steep topog-
raphy and high degree of erosion.
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