Rit (Vísindafélag Íslendinga) - 01.06.1971, Blaðsíða 134

134
GUÐMUNDUR PÁLMASON
is at the surface in SE-Iceland and possibly in one or two other areas.
Layer 3 is nowhere found at the surface.
A fairly detailed map of the depth to layer 3 has been made from
the refraction data using the method of delay times. It shows very
large variations in depth ranging from less than 1 km to about 10
km. Little direct relationship to surface geological features is ohvious
on this map.
Layer 4 is found at a depth of 8-9 km in SW-Iceland. Its depth
increases to 14—15 km east of the volcanic zone in S-Iceland. A similar
increase in depth is indicated in N-Iceland and perhaps also to the
west of the Snaefellsnes peninsula.
A certain symmetry of the crustal structure appears to exist about
the Reykjanes volcanic zone in SW-Iceland, indicating that the
Reykjanes Ridge structure may extend into SW-Iceland.
Inferences about crustal temperatures have heen made on the basis
of available borehole data, and compared with the seismic structure.
The 2—3 boundary appears to be at a temperature of 350-400°C in
southem and westem Iceland, independently of depth. In northem
and eastem Iceland the 2-3 boundary is at a lower temperature, cor-
responding to a “normal” temperature gradient of 40—60°C/km. The
3-4 boundary appears to be at a temperature close to the melting
range of basalts in SW-Iceland and at a somewhat lower tempera-
ture in northern and southeastern Iceland.
The combined seismic-temperature data suggest that the 2—3
boundary is a boundary between metamorphic facies of basaltic
rocks, perhaps a greenshist-amphiholite boundary. To be consistent
with the data at hand the boundary must then remain fossilized upon
cooling of the crust.
Correlation of relatively shallow depths to layer 3 with major
Tertiary volcanic centres in eastern Iceland lends support to the
above interpretation of the 2—3 boundary.
The small scale features of the gravity field show a clear correla-
tion with the depth to layer 3. The density difference inferred he-
tween layer 3 and the overlying layer 2 in two locations is 0.18 and
0.19 g/cm3. The main bowl-shaped Bouguer anomaly has not been
explained by the seismic data. Its cause is probably in the upper
mantle.
It appears that the physical properties of the crust in Iceland, such
as seismic wave velocity and presumably density, are critically af-
fected by the temperature conditions in the crust.