Rit (Vísindafélag Íslendinga) - 01.06.1971, Side 107

CRUSTAL STRUCTURE OF ICELAND
107
the present work. Details in the structure are, however, not revealed
by the earthquake data.
The results of Báth and Vogel (1958) mentioned above indicate
that the change in depth to layer 4, which was inferred in the reinter-
pretation of profile WP of Báth (1960), extends far heyond the
limits of the Iceland plateau, both to the southwest and to the north.
A major structural change may thus occur in Iceland near approxi-
mately the 65°N latitude.
11.2. Gravity.
The gravity survey of Iceland by Einarsson (1954) outlined the
main features of the gravity field in Iceland. The Bouguer anomaly
is a mirror image of the average topography, with a minimum of
ahout -35 mgals in the central part, and an average of about 40 mgals
in the coastal areas. Superimposed on this main anomaly are smaller
wavelength anomalies, a few kilometers or a few tens of kilometers
in horizontal extent. These are seen clearly on Einarsson’s map in
southwest Iceland where the density of gravity stations is larger than
in other parts of the country.
The refraction data have not as yet given an indication of the
cause of the main bowl-shaped anomaly. It could be due to an ano-
malously low density in the mantle to a depth of 100-200 km, as sug-
gested by Bott (1965a). Its cause is probably not lying at a depth of
less than about 15 km. That it is caused by a downward thickening of
layer 3, as suggested by Bödvarsson and Walker (1964), does not
seem likely on the basis of presently available refraction data from
southwest Iceland (profile L7).
The small wavelength gravity anomalies, on the other hand, have
in many cases been shown to be related to anomalies in the delay time
of the P3-wave. As examples may be mentioned the profiles Hrúta-
fjördur, Vatnsdalur and some of the Snaefellsnes profiles. The Skaga-
fjördur profiles may also be mentioned. Gravity highs are always
accompanied by low delay times. This is explained by a density dif-
ference mainly between layer 3 and the overlying layers. An estimate
of this density contrast on the Hrútafjördur profile gives a value of
0.18 g/cm3, and in Vatnsdalur a value of 0.19 g/cm3 was obtained.
Similar values appear to be of the right magnitude for other areas also.
In westem Iceland the area with relatively shallow depth to layer
3 extends approximately from Hengill to Mýrar. This area is probably
also characterized by relatively high values of gravity superimposed