Jökull - 01.12.1999, Page 38

with only 50°-55° dip in their lower parts. The faults
below the unconformity (Figure 6c) have a maximum
throw of 0.40 m and were most likely formed by dip-
slip movement within the lava flow combined with
slip of the underlying scoria on a tilted basement. The
fault set above the uncónformity (Figure 6d), shows
conjugate listric normal faults bordering a half graben.
The half graben is tilted towards one of the faults
which has a throw of 0.80 m. However, the faults
merge into a steeply dipping single crack in the lower
part, without an apparent displacement as a tension
fracture.
Crustal tilting
The dipping rate of the lava pile due to burial tilt-
ing is estimated at 1° for every 100 m depth (Walker,
1960). The Borgarfjörður lavas dip generally from
2° to 14° and locally up to 20°-25° along faults on
tilted blocks, but irregularities in the eroded land-
scape preclude any definite correlation with previous
burial depth. The vertical exposures are not contin-
uous enough and the 6 m.y. gap of the Hreðavatn
unconformity during which the uppermost part of the
SRZ basement was partly eroded (Jóhannesson, 1975,
1994) prohibits direct correlation across it. The high
fault density also makes it difficult to distinguish be-
tween burial-induced tilts and fault induced tilts.
According to the model of Guðmundsson (1992,
1995, 1996), fault dip should decrease with crustal
depth and inclined faults should frequently be parallel
Figure 7. Histogram of the
fault dips (from the Figure
3b) divided in two groups,
below (a) and above (b) the
Hreðavatn unconformity.
In both groups the faults dip
from 90° to 50°-5 5° but the
majority of the faults are
steeply dipping. — Saman-
burður á halla misgengja
og þykkt bergganga neðan
(a) og ofan (b) Hreða-
vatnsmislœgisins.
to and along inclined columnar joints in the deepest
exposures of the lava pile. Furthermore, as the burial
tilting is a continuous process, a noticeable difference
should occur in the dip of the faults tilted with the
lava towards or away from the rift axis. The Borg-
arfjörður faults dip steeply at different erosional lev-
els (Figure 7). At the scale of single cracks, many
fault planes or tension fractures were observed along
pre-existing columnar joints in the lava pile but they
are subvertical, even along the deepest sections (pro-
files 1-3) (Figure 4a). It is not surprising that pre-
existing tension fractures and columnar joints are ex-
ploited during normal fault development. However,
tension fractures and faults were also observed cutting
columnar joints obliquely or perpendicularly (Figure
6e), indicating that normal faults may not necessarily
start by coalescence of smaller sets of tension frac-
tures or tilted columnar joints.
At the regional scale, the axis of the Borgames an-
ticline seems to have been consistently oriented NNE
with time. The westward tilting towards the southern
portion of the SRZ and subsequently eastward tilting
towards the RLRZ has thus mostly affected faults sub-
parallel to the rift axis. The NNE faults show a strong
symmetry on both flanks of the anticline. They have
roughly equal inclinations (Figure 4b) and dip in ap-
proximately equal proportions towards the respective
rift axes. This unexpected symmetry of the NNE fault
planes indicates that they were not rotated by NW tilt-
ing towards the SRZ or SE tilting towards the RLRZ.
36 JÖKULL, No. 47