Rit (Vísindafélag Íslendinga) - 01.06.1971, Qupperneq 133

CRUSTAL STRUCTURE OF ICELAND
133
the SW-Iceland area has a mass excess equivalent to some 100 milli-
gals compared to SE-Iceland (the area of profiles L8 and L9). The
N-Iceland area appears to he intermediate between the two, if the
depth to layer 4 is assumed to be 14 km. If isostatic equilibrium pre-
vails as concluded by Einarsson (1954), these mass excesses must be
compensated by mass deficiencies at a depth greater than 15 km, i.e.
in the upper mantle. When more detailed data on the depth to layer
4 have been obtained, especially in northern and eastem Iceland, it
should be possible to make corrections of the gravity data for varia-
tions in the structure of the crust and thus obtain a gravity map de-
pending solely on the density variations in the mantle.
15. SUMMARY AND CONCLUSIONS
The present work is mainly based on data from over 80 refraction
profiles that have been measured in the past 10 years. Most of the
profiles are not reversed but their number and geographical distri-
bution provide a consistency check on their interpretation. Ten of
the profiles are over 80 km long. Six of them are on the shallow shelf
off the south and west coasts.
A characteristic seismic layering of the crust has been found.
Calculating average velocities from apparent velocity values gives
the following P-wave velocities, where the error limits are the
standard deviations of the mean value, and s is the standard devia-
tion of a single determination.
Table 15.1.
Layer P-velocity km/sec s km/sec
0 2.75 ± 0.06 0.37
1 4.14 ± 0.03 0.24
2 5.08 ± 0.03 0.24
3 6.35 ± 0.03 0.21
Detailed studies in selected areas have indicated that a velocity
close to 6.50 km/sec is more representative for layer 3 than the
average value given above. Layer 4, which is found at a depth be-
tween 8 and 16 km, has a probable P-wave velocity of 7.2 km/sec.
Layer 0 is a surface layer in the Neovolcanic zone, and has a
thickness of 0-1.0 km. Layer 1 is usually a surface layer in the
Tertiary basalt districts on both sides of the volcanic zone. Layer 2