Jökull - 01.12.1999, Blaðsíða 34

Figure 6. (c) View north on a steeply dipping normal fault and small-scale listric normal faults along profile
3. The planar fault strikes N170°E and dips 84°W, the curved faults strike N58°E, dip maximum 70° and
minimum 50°NW and have a maximum throw of 0.5 m. — (c) Horft til norðurs eftir sniði 3. Myndin sýnir
bratt siggengi og nokkur lítil sveigð siggengi (listric faults). Beina siggengið hefur stefnu N170A og hallar 84
gráður til vesturs en sveigðu siggengin stefna N58A með 50-70 gráðu halla.
Photolinear interpretation suggests that NNE and
WNW horsts and grabens in the Tertiary rocks are
mutually cross-cutting, which may demonstrate the
simultaneous activity of these dip-slip faults, as sug-
gested first by Jóhannesson (1975). The age of fault-
ing is unknown but, locally, some of these normal
faults are sealed by the volcanic products of the
Staðarhnúkur volcano dated from 0.78 Ma (Jóhann-
esson, 1994).
Although the age of the tectonic pattern is un-
known, some of the WNW fractures are older than
NNE strike-slip faults. Generally, strike-slip faults
are younger than normal faults. WNW and NNE nor-
mal faults may have been simultaneously active be-
fore 0.78 Ma.
Reverse faults
Reverse faults and striated planes have been found
in the Tertiary areas of Iceland. In southeast Iceland,
small-scale low-angle reverse faults with 0.5 m dis-
placements were generated by sheet injections (Torfa-
son, 1979). In this same region, a major reverse fault
with maximum vertical displacement around 100 m
was caused by a gabbro intrusion (Friðleifsson, 1983).
In western Iceland, Jóhannesson (1975) described two
major reverse faults from the Upper Borgarfjörður
area, but has since changed his interpretation and re-
gards them as normal faults (personal communication,
1997). Franzson (1978) did not mention any reverse
faults in the southern part of the Borgarfjörður area.
At the. mesoscopic scale, Passerini et al. (1997) re-
ported, from south of the lake Langavatn, a few low-
angle thrust faults clustered along WNW trend, im-
plying compression parallel to the rift zone. Other
mesoscopic reverse slickensides have been observed
along variable strike in Iceland, and interpreted as evi-
dence for compression perpendicular or parallel to the
rift axis (Bergerat et al., 1990; Villemin et al., 1994).
32- JÖKULL, No. 47