Fróðskaparrit - 01.01.1976, Page 19

Seismic Refraction Measurements around the Faeroe Islands 27
closer to the Faeroes, thus indicating a basin of low velocities.
Across the Faeroes from line B to line A (e. g. from F4 to F2)
the Moho time terms seem to decrease corresponding to a higher
crustal velocity or a more shallow Moho, but the slight de-
crease of the gravity field in the same direction points towards
a light »high velocity« material (e. g. granite).
To some extent line C (south) resembles line B but the very
weak signals from shots south of C45 make the interpretation
highly uncertain. The weak signals may be an indication of a
relatively thin basalt layer underlain by a low velocity mate-
rial to the north of the gravity high on line C (south), but
other explanations are possible. In connection with the gravity
high an apparent velocity of 6.6 km/s and time terms about
2/3 s have been found, and it can be mentioned that gravity
highs on the British side of the Faeroe-Shetland channel are
connected with a refractor of 6.48’+ 0.06 km/s (Smith &
Bott 1975), but this is, of course, no proof of a similar struc-
ture on the Faeroese side.
By use of time term analyses on data for line A on the Ice-
land-Faeroe ridge Bott et al. (1976) found a three layer model
for the ridge. The velocities were found to be appr. 5.0 km/s
for the top layer and 6.73 T 0.12 km/s and 7.79 + 0.10
km/s for the refractor below, and the depths of these were
found to be appr. 5 km and 27 km assuming layers of con-
stant velocities.
The north-south striking area of relatively low free air ano-
malies on the ridge has been interpreted as a trough filled
with light material (Fleischer et al. 1974), and this corresponds
to relatively high time terms for the 6.73 km/s refractor at
the station MIR(D), and the shots A8 and A10 (Bott et al.
1971, 1976). The crustal structure of the ridge differs thus
significantly from that of the Faeroe Block and is more like
the structure of Iceland, although the Moho velocity on the
ridge is higher than the velocity in Iceland (appr. 7.2 km/s,
see Pálmason & Sæmundsson 1974).