Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 117

CRUSTAL STRUCTURE OF ICELAND
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in the volcanic activity or a period of strong erosion during which the
present upper boundary of layer 1 in the volcanic belt was formed.
11.8. Microearthquakes and transform faults.
It has been suggested by Ward et al. (1969), mainly on the basis
of microearthquake data, that a transform fault might he causing the
eastward displacement of the volcanic zones in southem Iceland rela-
tive to the crest of the Reykjanes Ridge. It is not clear whether such
a transform fault, if it exists, should influence the seismic structure.
If the 2—3 boundary is essentially controlled by present or past iso-
thermal surfaces, its depth contours might be expected to show some
relationship to such a transform fault (cf. Fig. 37). Rut most probably
a transform fault should show up in the surface tectonic features as
east-west lineations. If such features have been buried under recent
volcanic products (layer 0) they should show up as anomalies in the
travel times on profiles crossing such features. No clear anomahes of
this kind have been found, as for instance shown by the arc profile
from lake Graenavatn (see sect. 6.3).
The only indication of an east-west structural feature on the
Reykjanes peninsula is an increased thickness of layer 0 on the south
coast of the peninsula, and perhaps an increased depth to layer 3
also. This is indicated by the depth contours in Fig. 37. This change
in depth to layer 3 probably occurs mainly south of the volcanic and
epicenter zone on the Reykjanes peninsula and it is therefore doubt-
ful whether it has any relation to a possible transform fault on the
Reykjanes peninsula.
The depth to layer 4 appears to be fairly uniform in SW-Iceland,
as seen in Fig. 38. No irregularities that could be related to a trans-
form fault, are visible on this map.
It is thus concluded that the seismic refraction data fail to show
any definite relationship to a possible transform fault in SW-Iceland.
It is not implied, however, that this is sufficient evidence to disprove
the hypothesis that such a transform fault may exist.
11.9. The effect of temperature and pressure
on seismic velocities.
Laboratory measurements show that the temperature coefficient
of seismic velocities is negative, i.e. the velocities decrease with in-
creasing temperattue (Hughes and Maurette, 1957; Rirch, 1958;