Rit (Vísindafélag Íslendinga) - 01.06.1971, Side 130

130
GUÐMUNDUR PÁLMASON
The seisxnic crustal structure of Iceland appears to be consistent
with the hypothesis that ocean-floor spreading may be taking place
on the Reykjanes Ridge, extending into the Icelandic plateau. At the
same time the data indicate much less active crustal spreading in
NE-Iceland.
It is not immediately clear how the seismic crustal structure
would he affected if the mid-ocean ridges were expressions of shear
faulting. Since relative vertical movements would probably be small,
it is likely that the crustal structure would not be affected by the
ridges. A symmetric structure about the ridges would not be expected,
and the seismic boundaries would not be expected to be shallower
under the ridges than away from them. It is hardly possible to say,
however, that the seismic structure is inconsistent with the hypo-
thesis of shear faulting, but it appears at least as consistent with the
ocean-floor spreading hypothesis as with the shear faulting hypo-
thesis.
14. DENSITY OF CRUSTAL SEISMIC LAYERS AND
CONSIDERATIONS OF ISOSTASY
Even though the available gravity data in Iceland are not suf-
ficient for a detailed comparison of the gravity anomalies and the
seismic structure, it is still possible to make some predictions of the
average densities to be expected in the crust. This will be done by
comparing laboratory measurements of density and seismic velocity
with density differences deduced from seismic profiles, where gravity
data are available.
Data from laboratory measurements of bulk densities and P-wave
velocities of crustal rocks have been compiled by Nafe and Drake
(in Talwani et al., 1959b), Woollard (1959, 1962, 1968), Birch
(1961), Kanamori and Mizutani (1965) and Manghnani and
Woollard (1968). The average curves given by these authors are
shown in Fig. 48. The scatter in the data, which is considerable, is
not shown, and also not the dependence on mean atomic weight
found by Birch (cf. sect. 12). Curve 5 is for Hawaiian basalts with
porosity less than 20%. The curves in Fig. 48 are in relatively good
agreement with each other for velocities above about 7 km/sec, but
for lower velocities the agreement is not as good.
It appears from Fig. 48 that the most likely density of layer 4 is