Jökull - 01.12.1984, Side 97

In northwestern Iceland, normal faults are
more common and tend to have larger throws
than in eastern Iceland. The number of dykes
with a significant associated vertical displacement
is, however, similar. While the total length of
profiles in northwestern and eastern Iceland is
roughly similar, only 19 normal faults were
observed in eastern Iceland, compared with 68
normal faults observed in northwestern Iceland.
Also, the throw of faults in eastern Iceland is
0.5—8.0 m with an average of 2.7 m, whereas the
throw of the faults in northwestern Iceland is
0.5—25.0 m with an average of 5.3 m.
Neither the average thickness nor the thickness
distribution of the dykes in northwestern Iceland
can be related to the altitude of the profiles
(Table 1, Fig. 9), nor to the location of the
profiles in the research areas (Fig. 2). I suggest
that the variation in average thickness and thick-
ness distribution may be explained by the diffe-
rent lithology of the country rock. Difference in
lithology may be reflected in difference in various
mechanical factors, the most important of which
is Youngs’s modulus, E. Dykes dissecting layers
of high E tend to be thin, but tend to be thick
when dissecting layers of low E. Thus the average
thickness of dykes dissecting layers of high E can
be much less than the average thickness of these
same dykes where they dissect layers of low E
(Gudmundsson 1984). Layers of high or low E
may be expected to be randomly distributed
throughout the lava pile, thereby explaining the
lack of correlation between thickness of dykes
and the altitude or location of the corresponding
dyke profiles.
In general, the dyke orientation obtained by
field measurements in the present study is similar
to the orientation that Sigurdsson (1967)
obtained from air photographs of the same part
of northwestern Iceland. The small difference
between the results can be explained by the diffe-
rent method used and by the different areas used
to make the rose diagrams. Both these factors
result in somewhat different “samples“ being
used in Sigurdsson’s study as compared with the
present study.
ACKNOWLEDGEMENTS
I thank NevilleJ. Price and Kristján Saemunds-
son for critical comments on an earlier version of
the manuscript. Financial support was provided
by the National Energy Authority and by the Ice-
landic Science Foundation.
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