Jökull - 01.01.2010, Blaðsíða 113

The Kerlingar fault, Northeast Iceland
Kerlingar fault. That is unlikely since the easternmost
visible part of the Húsavík fault is located 65 km west
of the Kerlingar fault. Secondly, the change in orien-
tation of fractures in the area should happen abruptly
where the fractures and the Húsavík transform fault
meet. That is not the case in the area near the Kerl-
ingar fault. There, the change of orientation is grad-
ual along the border of the rift zone, suggesting even
more regional-scale processes.
In the third point, it is suggested that the Kerling-
ar fault was formed (or reactivated) due to a landrise
during the last deglaciation. This can occur as the hot-
ter and less viscous NVZ should react differently to
unloading due to deglaciation than the colder, thicker
and more viscous EFB block.
The reaction to unloading is both elastic and vis-
cous, partly instantaneous and partly extending over
a period of time. Physical properties, i.e. elastic con-
stants, viscosity and density, may differ between the
NVZ and the EFB, increasing differential stress and
inducing faulting between the NVZ and the EFB.
a) Effective Young’s modulus
Sudden pressure change at the surface leads to instan-
taneous elastic reaction of the crust which depends
on the effective Young’s modulus, which again is in-
versely dependent on crustal thickness. The elastic
rebound of the crust happens relatively fast during
deglaciation. Variation in Young’s modulus should
lead to higher uplift of the EFB than the NVZ (on the
order of few tens of centimeters). If this effect was
the only effect to cause the formation of the Kerlingar
fault, the fault should therefore have a throw towards
the west and not towards the east as observed. There-
fore, this effect cannot be the only cause of formation
of the Kerlingar fault.
b) Effective viscosity
Lower viscosity material responds more rapidly to de-
loading than higher viscosity material with a linear
relation between crustal relaxation time and viscosity
(e.g. Cathles 1975). Assuming that the lower crustal
and uppermost mantle viscosity beneath the volcanic
zones of Iceland is lower than beneath older, Ter-
tiary areas, the NVZ crust should rebound faster than
the EFB during deglaciations. As variable response
rates may generate differential stress field across the
NVZ-EFB boundary during deglaciations, the Kerl-
ingar fault may thus be a remnant of faster rebound
of the NVZ crust than the thicker, more viscous EFB
crust. Viscous crustal relaxation occurs more slowly
and remains over a longer time period than the elastic
rebound of the crust. As an example, a region with
lower crustal viscosity of 1.5×1019 Pa s, has a relax-
ation time of 1000 years (Sigmundsson 2006), while
the relaxation time is only 500 years if the viscosity is
lowered to 0.75×1019 Pa s. Therefore, a slight differ-
ence in the viscosity can cause significantly different
relaxation times.
c) Density difference – buoyancy effects
As the uppermost mantle below the NVZ has a lower
density (3170 kg/m3) than beneath the EFB (3240
kg/m3) (Staples et al. 1997), the isostatic uplift of the
NVZ during deglaciations should be higher than the
uplift of the EFB. Using these mantle densities, an ice
density of 920 kg/m3, a 1500 m thick glacier and a
simple isostatic uplift equation;
u= hice × ρice/ρmantle
the uplift of the NVZ is close to 435 m and the uplift
of the EFB 426 m, which implies a 9 m excess uplift
of the NVZ with respect to the EFB.
In addition, flexure at the rift zone margin can
cause a different stress field there than in the cen-
ter (i.e. Clifton and Kattenhorn 2006). However,
marginal flexure should generate faults with a throw
down to the west in this area. Therefore, that process
cannot explain the existence of the Kerlingar fault,
which has a throw down to the east.
A differential stress field, produced at the bound-
ary between the NVZ and the EFB during deglacia-
tions (or glaciations), could explain why the Kerling-
ar fault is located at the boundary of the NVZ and the
EFB, and why it is parallel to the boundary and not
parallel to the fissure swarms in the central NVZ. The
differential stress field could form faults in two dif-
ferent ways: either directly, without the involvement
of magma, or indirectly, by producing a stress field
which governs the orientation of dike propagations in
the area during deglaciations. Therefore, dike intru-
sions could play a part in the scenario, even though
JÖKULL No. 60 113