Jökull - 01.12.1999, Page 44

depth of 1.5 km. Columnar joints and large-scale nor-
mal faults are steeply dipping down to 1.5 km depth
and neither listric faults nor associated deformation
such as major subsidence, vertical block rotation and
roll over were found.
Jóhannesson (1980) suggested that N-S, NNE and
NW to WNW fractures were already active before the
emplacement of the SVZ, because their density de-
creases upwards across the unconformity. Preliminary
results on fracture-dips, the frequency of N-S and NW
to WNW dykes below the unconformity, and first time
attempts to use cross-cuttings between fractures, con-
firm Jóhannesson's theory. Field data indicate that the
fracture pattem is not monogenetic as was previously
thought (Schafer, 1972), but results from polyphase
tectonics, and is inherited and reactivated from rifting
periods:
(1) In terms of regional tilting; the dip analy-
sis of fractures with respect to the unconformity and
roughly the axis of the Borgarnes anticline, shows that
NNE-trending faults were not rotated by tilting to-
wards the NE portion of the SRZ, and subsequently
towards the RLRZ, implying reactivation or continu-
ous activity of the faults through time. By contrast,
N-S and NNE dykes show evidence of the two tilting
histories, implying a diachronous age for the regional
dykes.
(2) In terms of fracture and stress patterns; al-
though the derivation of stress fields should be taken
as preliminary (Figure 9) and subject to further
improvement, four paleostress subsets are defined.
Striae cross-cuttings are rare, but cross-cuttings be-
tween faults and dykes show that:
A) Fluctuations occur in the directions of ex-
tension (0-3) and compression (<ti); rifting and non-
rifting conditions seem to coexist at times, resulting
in rift-parallel fractures being simultaneously active
with, or cut by, other trends.
B) Strike-slip faults cut pre-existing normal faults
and dykes, and may be explained by the model of
Bergerat et al. (1990). However, cross-cuttings be-
tween different trends of dykes and faults indicated
that strike-slip faulting is not a monogenetic phe-
nomenon but occurred repeatedly.
Plate boundary instability in Iceland may reflect
transitions in volcano-tectonic activity, propagating
rifts, lateral rift jumps and transient transform fault-
ing. The complexity shown by the preliminary results
of this study may be symptom of these processes. For
the full understanding of these processes, a qualitative
model on the evolution of the fracture and stress pat-
terns with respect to the plate boundaries in time is
needed.
ACKNOWLEDGEMENTS
I thank Haukur Jóhannesson (Icelandic Institute of
Natural History) for having introduced me to the
complexity of the geology of Western Iceland and
for his valuable suggestions, Páll Einarsson (Uni-
versity of Iceland) for his constructive remarks on
the tectonics of Iceland and his comments on vari-
ous versions of this paper, Kristján Sæmundsson (Na-
tional Energy Authority of Iceland-Orkustofnun) and
Pierre Masse (Elf-Aquitaine, France) for discussions
on the tectonics of rift zones and for their comments
at different stages of this paper, as well as Guðrún
Ólafsdóttir, Kristján Jónasson and Kristján Geirsson.
Data collection and most of the analyses was funded
by La Fondation P. Mercier, and partly by the Ice-
landic Research Council (RANNÍS). At Orkustofnun,
I would like to convey my gratitude to Ólafur G.
Flóvenz, Helga Tuliníus and Guðmundur Pálmason
for providing working facilities since 1996, and to
Jakob Björnsson, Þorkell Helgason and Ingvar Birgir
Friðleifsson for their permission to use these facilities.
Other staff members are thanked for their support.
Helga Sveinbjörnsdóttir finalised many of the figures.
Critical reviews of Mark Jancin, Haraldur Sigurðsson
and the editors improved this paper. Profiles 1-5 were
measured jointly with Ágúst Guðmundsson (now at
University of Bergen, Norway), whereas profile 6 was
measured by the author. Some results have been pre-
sented separately (Guðmundsson, 1996).
42 JÖKULL, No. 47