Jökull - 01.12.1999, Page 35

Figure 6. (d) View east on conjugate small-scale listric normal faults. Only the fracture to the right has a vertical
displacement (0.8 m) and the central “graben” is slightly tilted towards this fracture; (e) View ESE, columnar
joints in the lava pile cut at almost right angles by steeply dipping tectonic fractures. — (d) Horft til austurs á
nokkur smágerð sveigð siggengi. Lóðrétt fœrsla (0,8 m) var einungis mœlanleg á misgenginu sem liggur lengst
til hœgri á myndinni. (e) Horft ASA á stuðluð hraunlög sem hafa verið þverskorin af bröttum brotum.
However, in each area, only one or two sites show
such striae on low-angle planes which are clustered
along one strike.
In the study area, no major reverse fault was ob-
served. A few of the measured reverse slickensides
(Figure 4c) are on steeply dipping planes and corre-
spond to local bending along normal faults for which
the slip-sense was identified based on marker hori-
zons. One questionable case of reverse faulting is
shown on Figure 6b. The fault is associated with a
dyke and the displacement (around 2 m) is determined
from the contact of the same scoria and lava on both
sides of the fault, which is not ideal for this purpose.
This fault was generated by dyke injection. The dyke
(2.5 m thick) is located 5 m southwest of the fault,.
both strike NW and dip 12°-16° SW. Apparently, dur-
ing magma injection and opening of the dyke, the joint
northeast of the dyke is tilted, so that the block be-
tween joint and dyke moves upwards and acts as the
hanging wall of a reverse fault. The association of
reverse faults with dyke injection is a common phe-
nomenon that also occurs in continental regions (Kho-
dayar, 1992). The observed reverse movements ap-
pear thus of local significance.
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