Rit (Vísindafélag Íslendinga) - 01.06.1971, Blaðsíða 121

CRUSTAL STRUCTURE OF ICELAND
121
tively. A simple correlation between seismic velocities and densities
is not readily apparent here.
Some of the variation in apparent velocities in layers 1 and 2,
which is seen on the histogram in Fig. 5 is probahly real although a
part of it is no doubt due to dipping interfaces. In several locahties,
where layer 1 is the surface layer, have relatively high velocity
values, near 4.5 km/sec, been found. The boundary between layers 1
and 2 may then in some cases be less clear than the overall results indi-
cate and it is also possible that intermediate velocity layers may in
some cases exist between layers 1 and 2. The boundary between
layers 1 and 2 seems on the whole to be not as well defined as the
boundary between layers 2 and 3. This does, however, not signi-
ficantly affect the depth calculations to layer 3.
Layer 3 and its upper boundary represent in many ways the most
interesting problem in the refraction interpretation. Its most probahle
true P-wave velocity is 6.50 km/sec on the basis of available profiles
in Iceland (cf. sect. 5.5). Where accurate velocity determinations
have been made from reversed profiles, values close to this are ob-
tained. Long profiles, 40-60 km or more, also usually give this velo-
city. Lower velocities are often foimd on shorter profiles, but here
the velocity determinations are less accurate and more affected by
dipping interfaces and possible misinterpretations of first arrivals.
Layer 3 in Iceland is probably to be equated to the oceanic layer
(Raitt, 1963), although the average velocity of the oceanic layer is
commonly given as 6.7-6.8 km/sec.
The most important evidence for the nature of layer 3 in Iceland
probably comes from the correlation of the depth to its upper boun-
dary with the temperature as deduced from borehole data (cf. sect.
11.3 and Fig. 43). The fact that the upper boundary of layer 3 in
southern and western Iceland appears to be at a temperature between
350 and 400°C, more or less independently of depth, strongly sug-
gests that layer 3 represents a metamorphic facies of basaltic rocks.
This would be in agreement with the suggestion of Cann (1968) that
the oceanic layer is formed hy a conversion of basaltic crust to amphi-
bolite. The estimated temperature required for this conversion is
about400°C (Ringwood and Green, 1966a, 1966b; Cann, 1968).
The fact that the temperature at the 2—3 boundary is lower in
northem and eastem Iceland (cf. Fig. 43) does not contradict this
hypothesis since amphibolite would remain metastable at lowered
temperatures, and the 2-3 boundary could here represent a frozen-in