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tion). For the Rhinegraben the velocities of 7.6— 7.7 km/sec cor-
respond to 25-28 km depth and 8.2 km/sec velocities to approx.
40 km (fig. 9.3).
In the Afar area the mantle with normal velocity of approx.
8 km/sec is supposed to lie as deep as 60-70 km (21, Berkhemer,
personal communication), while the velocities of 7.3-7.6 km/sec
have been revealed at depths of 20-25 km (fig. 9.1) (21).
Thus in Iceland as well as in the area of the continental and
possibly oceanic rifts the crust bottom contains thick high-velocity
layers forming a kind of axial body or underbedding. This body
referred by many seismologists to the anomalous mantle lies in
the depressions of the normal mantle, which is analogous to the
root of the mountains normally characteristic of the folded zones
of the continents. The section in Fig. 8.1 demonstrates the varia-
tions in thickness and compositíon of the crust in the profile stu-
died. The layers with the velocities of 6.0-6.5 km/sec are gradu-
ally replaced by the ones with higher velocities in a direction from
the typical continental block of the Faeroes to Iceland. The total
thickness of the crust decreases too, minimizing at a distance
about 100 km from the Iceland coast.
Approaching the island the deep layers (isolines 7.0; 7.5 and
the M discontinuity) abruptly plunge towards the middle of Ice-
land. The dip angles on the seismic cross-section are 6°—8°. It is
notable that in eastern Iceland the geological evidence indicates
the plateau-basalts to dip north — north-west (i.e. towards our
seismic line), the surface beds striking comparatively gently and
dip angles increasing with depth locally up to 8° (22). In other
words the dip of the plateau-basalts approximates the dip of the
deep boundaries of the earth crust.
The layer with velocities 6.5 km/sec at this locality shows a
contrasting behavior; it rises and approaches the surface in the
island. From Pálmason’s data the seismic boundary with a velo-
city of 6.5 km/sec intersects the boundaries of the major geologic
structures of the island which gives a ground for believing it to
be superimposed and to reflect alterations in the principal proper-
ties of the substance of the earth’s crust. Having taken into account
a high heat flow, the thicker layer with velocities of 6.5 up to
7.0 km/sec is probably composed of basic and ultrabasic rocks.
L