Jökull - 01.01.2010, Blaðsíða 103

Reviewed research article
The Kerlingar fault, Northeast Iceland:
A Holocene normal fault east of the divergent plate boundary
Ásta Rut Hjartardóttir, Páll Einarsson and Bryndís Brandsdóttir
Institute of Earth Sciences, Science Institute, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland
astahj@hi.is, palli@raunvis.hi.is, bryndis@raunvis.hi.is
Abstract — The Kerlingar fault is a ∼30 km long fault located at the boundary between the Northern Volcanic
Rift Zone, and the Tertiary Eastern Fjords Block in Iceland. The fault has a throw of 2–9 m down to the
east and is most likely a normal fault. It probably ruptured in several earthquakes over extended time, but
assuming it ruptured in one event it would have a magnitude of about Mw=6.7. The Kerlingar fault forms
a sharp offset in a flat moraine, showing that the fault was active in the Holocene. Several characteristics
of the fault are different from that of the presently active fissure swarms of the NVZ. It is unusually long,
straight and continuous, and it is parallel with the boundary between the NVZ and EFB not perpendicular to
the plate spreading. We consider three possible explanations for the existence of the fault. It may be formed
in a rifting event, by stress transfer in relation to the Húsavík transform, or by a stress field caused by rapid
crustal unloading during the last deglaciation. We favour the third explanation but note that the other two
cannot be excluded. Differential movements at the NVZ-EFB boundary during deglaciations can occur as the
two crustal blocks have different density, Young’s modulus, thickness, and subcrustal viscosity. They therefore
respond differently to the unloading. This may explain why the fault is parallel with the NVZ-EFB boundary
and not with the Holocene fissure swarms in the NVZ. Other faults at the NVZ-EFB boundary may be formed in
a similar manner. Magma may have intruded some of them to form the distinct arcuate pattern of hyaloclastite
ridges at the boundary between the NVZ and the EFB. Future model calculations could constrain better the
effects this process has on the formation of faults.
INTRODUCTION
The Northern Volcanic Rift Zone (NVZ) marks the
mid-Atlantic plate boundary in Northern Iceland (e.g.
Einarsson, 2008). This zone is ∼40 km wide, with
5–6 active volcanic systems. The outer flanks of the
NVZ are asymmetric with respect to the plate bound-
ary. A 30 km wide zone on the flank east of the neo-
volcanic zone is characterized by hyaloclastite ridges,
indicating subglacial basaltic fissure eruptions (Kjar-
tansson 1943; Sæmundsson 1974), while Holocene
volcanism is absent. Vilmundardóttir (1997) has as-
signed these ridges late Pleistocene age based on their
appearance. No corresponding zone is found on the
west side of the plate boundary. Although the ma-
jority of Holocene fractures in the area are situated
within the neovolcanic zone, there are exceptions. In
this paper, we present the results of our survey on
one of the notable exceptions, the Holocene Kerlingar
fault (Figure 1). We find that the fault is not a part of
the Holocene fissure swarm of any of the 5–6 volcanic
systems, and that while it has a different orientation
than the fissure swarms at this latitude, it is parallel
both with the line of central volcanoes in the NVZ,
and with the boundary of the NVZ with the Eastern
Fjords block (EFB). Therefore, we suggest that the
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