Rit (Vísindafélag Íslendinga) - 01.06.1971, Side 125

CRUSTAL STRUCTURE OF ICELAND
125
distribution determines the seismic 2-3 boundary. In northern and
eastern Iceland on the other hand, the crust appears to have cooled
down since the creation of the 2-3 boundary sometime in the geologic
past. The volcanic zone in NE-Iceland may be an exception to this, as
no reliable temperature gradient data are available from that zone.
The interpretation of layer 3 and the nature of the 2—3 boundary,
which has been given here, is attractive apart from the evidence
presented above. The relative horizontahty of the 2—3 boundary com-
pared to the dips of the surface Tertiary flood basalts, which has been
somewhat of a puzzle (Einarsson, 1965; Gibson, 1966; Pálmason,
1967a), is explained. It furthermore gives space for the geological
estimates of the thickness of the flood basalts, which has been esti-
mated variously from a few km to over 10 km (Einarsson, 1954;
Bödvarsson and Walker, 1964; Walker, 1959, 1965).
The result that Poisson’s ratio for layer 3 is similar to that for the
overlying basaltic layers 1 and 2 (Pálmason, 1963) is in agreement
with the above hypothesis. Poisson’s ratio has been shown to depend
on quartz content (Birch, 1961; Lyakhovitsky, 1961). As far as
this property is concerned there is no difference to be expected be-
tween layer 3 and the overlying hasaltic layers 1 and 2.
The hypothesis of Hess (1962) that layer 3 (the oceanic layer)
is composed of serpentinized peridotite from the upper mantle is
contradicted by the subsurface temperatures in Iceland. According
to Hess (1962) the serpentinization cannot take place at tempera-
tures above about 500°C. Layer 3 in Iceland, however, extends down
to 8-16 km, where temperatures of 700-800°C, in some cases even
over 1000°C, are inferred from the near-surface temperature data.
This objection to the serpentinization hypothesis was pointed out by
Bott (1965b) and is confirmed by the more detailed data now avail-
able. Other objections to the serpentinization hypothesis have been
given by Cann (1968) and Oxburgh and Turootte (1968).
Layer 4 is found at a depth of 8—16 km in Iceland, shallower in
southwest Iceland (8-9 km) than in northern and southeastern Ice-
land (14—16 km). Data from the surrounding ocean areas (Báth and
Vogel, 1958; Ewing and Ewing, 1959; Tryggvason, 1962) indicate
a somewhat shallower depth to this layer, and perhaps that the depth
is greater north of Iceland than to the south (Báth and Vogel, 1958).
This is in a certain agreement with the change in depth to layer 4,
which is indicated by the refraction data, on going from southwest
Iceland to northem Iceland. Insufficient data are as yet availahle