Jökull - 01.12.1999, Síða 31

than normal faults, due to their markedly fewer num-
bers and relatively small displacements. It is unclear
whether these strike-slip faults formed in association
with transform faulting at a plate boundary or in an
intraplate setting. Regarding the scale of maximum
displacements (20-30 m), they form a separate class
between those found in the Tjörnes Fracture Zone,
where displacements of many kilometres have been
inferred (Sæmundsson, 1974; Younge/uZ., 1985), and
those in the South Iceland Seismic Zone, where dis-
placements of 1-2 m have been found (Einarsson and
Eiríksson 1982; Bjarnason et al., 1993).
Striated planes and associated minor structures
A total of 58 slickensided planes were analysed in the
Tertiary rocks along the profiles (Figure 4c). Of these,
18 are on fault planes (including one dyke margin)
and 40 are on small individual planes associated with
or independent from the fault planes. Strike and dip
of slickensides were measured, along with the plunge
and trend of 195 striae on the slickensided planes. The
slip sense of striated planes was mainly inferred from
the striae and, in a few cases, from other kinematic
indicators such as centimetre-scale Riedel fractures,
steps with rough and smooth faces and striating el-
ements on the deformed surfaces. However, cross-
cutting striae are rare and fault planes usually show
only one generation of striae, probably from the last
slip. Secondary fractures (with length on the order
of metres), anthitetic and oblique to the main faults,
were found in the hanging wall of a few normal faults.
Their throw is very small and their slip-sense may
conform or be opposite to that of the main normal
faults (Figure 6a).
Of 195 striae, 67% pitch between 90° and 61°
(dip-slip), 17% are between 60° and 31° (oblique-
slip), and 16% have their pitch between 30° and
0° (strike-slip). About 5% of dip-slip and 12% of
oblique-slip striae show a reverse polarity. Of 58 stri-
ated planes, 67% are normal faults (Table 1), mostly
rift-parallel with a peak at N20°-30°E. A few are
oblique to the NNE trend of the rift zones (N170°-
180°E, N140°-150°E, and N50°-60°E). Few striated
normal fault planes strike WNW (Figure 4c and Ta-
ble 1), while on a regional scale many of the WNW
faults show normal-slip (Figures 4b and 5). Dextral
strike-slip faults (14%) trend mainly N100°-120°E
and N5°-30°E, in agreement with horizontal offsets
along both trends. Sinistral polarity (14%) was only
found on scattered small planes striking N10°-80°E
and N140°-170°E. Reverse-slip striae (5%) were ob-
served on steeply dipping faults striking WNW, NW
and NE. A few of these striae were found locally on
two normal faults with a throw of 22 m and 5.5 m
respectively. Reverse motion along steeply dipping
planes may thus be only apparent and due to local
bending of the normal fault planes.
Slip analyses of large-scale (regional) and small-
scale (mesoscopic) faults, shows that at both scales
normal faulting predominates in Borgarfjörður, and
that oblique-slip and strike-slip faults are less com-
mon.
Normal faults and strike-slip faults
Aerial photographs at the scale of 1/36.000 were used
for maþping fracture traces for Figure 5. Undifferenti-
ated lineaments (joints, faults and dykes) >100 m long
are shown as single lines on the maps. Fault types
were determined using the field data from profiles 2
(river Beilá), 3 (Figure 5b) and 6 (Figure 5c), com-
bined with observations from aerial photographs.
The strongest fault pattern is created by WNW and
NNE faults forming a network of orthogonal horsts
and grabens in the Tertiary lava pile (Figures 2 and
5a). Normal and strike-slip movements occurred on
faults of both sets. Around lake Langavatn, most
dextral slip is on WNW faults (Figure 5a) but a pre-
existing narrow WNW horst is offset by a NNE dex-
tral strike-slip fault (~2 km NE of the northeastern
shore of Langavatn). In the field, sinistral polarity was
found only on small-scale planes (profile 2), along al-
most all strikes except WNW (Figure 4c and Table
1). West of lake Hreðavatn (Figure 5b) a major WNW
normal fault (Dyngjá fault) and two NW-WNW dykes
are offset about 30 m by a NNE right lateral strike-slip
fault. A similar pattern appears north of Hallarmúli
(Figure 5c). A WNW basaltic dyke and two older
WNW normal faults are displaced by a NNE dextral
strike-slip fault, but the amount of offset varies. To
the north, the WNW dyke is offset about 11 m by the
dextral motion of the NNE fault; a nearby WNW nor-
JÖKULL, No. 47 29