Greinar (Vísindafélag Íslendinga) - 01.01.1976, Side 75

73
and Ewing (2); the Iceland-Faeroe Ridge by Rott et al. (3); the
Faeroe Island by Pálmason (4).
A summary of the data on Iceland was published by Pálmason
and Saemundsson in 1974 (5). The lowest velocities of the seismic
waves (2.0-3.0 km/sec) were obtained for the neovolcanic deposits
within the zones of active volcanism and rifting, the thickness of
the layer not exceeding 1 km (layer “0”). The underlaying seismic
layer (layer “1”), yielding velocities averaging 4.1 km/sec is as-
sociated with the Tertiary and Quatemary plateau-basalts and is
0.5 to 2 km in thickness. They are underlain by layer “2” with a
velocity approximating 5.1 km/sec, it is likely to be composed of
the old plateau-basalts, and ranges in thickness from 1 to 3 km.
A still deeper layer “3” with velocity 6.5 km/sec in average hes
at a depth more than 1-5 km and never emerges on the surface.
Its thickness is of an order of 4-5 km. In velocity and depth of
occurence layer “3” is related by Pabnason to the main layer of
the ocean crustal structure. The last 7.2 km/sec layer “4” lies in
Iceland at a depth varying from 8-9 to 14-15 km. Palmason con-
siders it to he associated with the upper mantle. Similar velocities
have been recorded by Ewing in the Atlandc Ocean south of Ice-
land and by Rott south-east of Iceland and by Talwani on the sub-
marine Reykjanes Ridge (6). Recently, on the basis of these data
there have been smnmarized ideas on the particular anomalous
oceanic crust of the Iceland type with lower velocities at the base-
ment (7). It should be noted here that the above seismic data were
obtained in the course of the seismic wave observations at short
distances from the shot point, the length of the travel time curves
being 70-80 km off shore (2), less than 130 km in the Iceland-
Faeroe Ridge (3) and less than 100-120 km in Iceland (1). There-
fore the data on layer 4 appears not sufficiently reliable.
A longer (up to 250 km) travel time curve was recorded by
M. Ráth in western Iceland as far back as in 1958. This author
came to the conclusion that the reflected wave evidence indicated
then the M boundary was 28 km and the velocities at the bottom
of the crust (18-28 km) were 7.4 km/sec (8).
Seismic evidence for the upper mantle structure was reported by
Francis (9), who used the records of earthquakes with foci at the
Mid-Oceanic Ridge north and south of Iceland. Having analysed