Jökull - 01.01.2011, Page 65

Reviewed research article
Tectonics of the Þeistareykir fissure swarm
Sigríður Magnúsdóttir and Bryndís Brandsdóttir
Institute of Earth Sciences, Science Institute, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland
Corresponding author: sigrima@hi.is
Abstract — Tectonic activity within the Tjörnes Fracture Zone is characterized by strike-slip and exten-
sional movements within three tranform zones, the Húsavík-Flatey Fault System, the Grímsey Volcanic Zone,
connecting the offshore Kolbeinsey Ridge with the Northern Volcanic Zone on land, and the Dalvík linea-
ment. We mapped the Þeistareykir volcanic system at the northwestern margin of the Northern Volcanic
Zone using multibeam bathymetric maps, aerial photographs and satellite images. The 70–80 km long and
7–8 km wide Þeistareykir fissure swarm consists of large normal faults with maximum displacements of
200–300 meters along its western rim and rift fissures further east. A marked change in rift direction across
the Húsavík-Flatey Fault System reflects complex tectonics at the junction of a transform fault system and
a divergent volcanic zone. Normal faults on land and offshore have an average azimuth close to N25◦E
whereas the average azimuth of rift fissures changes from ∼N22◦E south of the Húsavík- Flatey Fault Sys-
tem to ∼N43◦E, in western Kelduhverfi. The 1618 and 1885 AD rifting events within the Þeistareykir fissure
swarm were most likely fed by lateral magma propagation from the Þeistareykir volcanic center, northwards
to the western part of Kelduhverfi. Although limited, historical reports indicate that the January 1885 rifting
event triggered a ∼M 6.3 earthquake near Lake Víkingavatn. The 1885 rifting event in western Kelduhverfi
was similar to the 1975–1976 rifting event in eastern Kelduhverfi, when a laterally propagating dike from
the Krafla caldera triggered the MS 6.4 Kópasker earthquake on January 13th, 1976. Seismic sequences
within the Húsavík- Flatey Fault System and the Þeistareykir fissure swarm in 1867–1868 and 1884–1885
indicate a tectonic relationship between the rift zone and the transform zone.
INTRODUCTION
The Tjörnes Fracture Zone (TFZ) is a complex,
150 km long (E-W) and 50–80 km wide (N-S) trans-
form zone, connecting the Northern Volcanic Zone
(NVZ) with the offshore Kolbeinsey Ridge (KR),
(Figure 1, inset). The TFZ was formed 6–7 million
years ago when volcanic activity was transferred
eastwards, from the Skagi Volcanic Zone to the NVZ
(Sæmundsson, 1974; 1978). The average spreading
velocity across the NVZ is ∼19 mm/year in a direc-
tion of ∼105◦ (NUVEL-1A, DeMets et al., 1994).
The TFZ is seismically very active, with frequent
earthquake sequences which are mostly concen-
trated on three northwest trending transform zones;
the Grímsey Volcanic Zone (GVZ, previously called
the Grímsey Lineament), the Húsavík-Flatey Fault
System (HFFS) and the Dalvík lineament (DL),
(Stefánsson, 1966; Tryggvason, 1973; Einarsson,
1976; Einarsson and Björnsson, 1979; Rögnvalds-
son et al., 1998; Jakobsdóttir et al., 2002; Þorbjarn-
ardóttir and Guðmundsson, 2003; Guðmundsson et
al., 2004; Þorbjarnardóttir et al., 2003; 2007; Jak-
obsdóttir, 2008). Recent tectonic activity within the
TFZ consists of strike-slip and extensional move-
ments within the northwest striking transforms and
N-S grabens of Eyjafjarðaráll, Skjálfandi and Öxar-
fjörður (Brandsdóttir et al., 2002; 2004; Einarsson,
2008).
The NVZ is a divergent rift zone which extends
from the center of the Iceland hotspot beneath the
Vatnajökull ice cap north into the Tjörnes penin-
sula and Öxarfjörður bay, continuing obliquely off-
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