Greinar (Vísindafélag Íslendinga) - 01.01.1976, Side 89

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The following are the principal features of the cross-sections
constructed:
1. A boundary with velocities 7.8-8.2 km/sec has been traced
ahnost continuously over the whole line of observations. This boun-
dary is referred by us to the M discontinuity i.e. to tihe mantle/
surface. In the Iceland area tfae velocities determined for the M
discontinuity are lower than those for the Faeroes and Shetland
Islands.
The greatest depth (the thickness of the earth crust) of the
M discontinuity (40-60 km) has been registered under Iceland,
the average (about 40 km) under the Faeroe Islands and the
thinnest (16-18 km) under the Faeroe-Shetland ohannel.
2. The earth’s crust in the NASP lines A, Bx and B2 in velo-
city composition, thickness and relief form of the M discontinuity
falls into the blocks of a typically continental structure, and the
anomalous structures: high-velocity crust and the crust with de-
creased thickness.
A typically continental crust with the velocities at the top rang-
ing from 5.5 to 6.0 km/sec has been observed in the shelf areas of
the Faeroes and Shetland Islands. At the bottom the velocities ap-
proximate 7.0 km/sec and the higher ones, up to 7.5 km/sec, are
likely to exist in the thin layers only.
The higher velocities are characteristic of the Iceland-Faeroe
Ridge. Here the top of the crust contains a thin (less than 10 km)
layer with a velocity of 6.0 km/sec. It is underlain by a thicker
layer with velocities 6.5-7.0 km/sec. The bottom layer is about
15 km thick and has a velocity more than 7.0 km/sec.
The highest velocity rock sequence is shown by the Iceland
block additionally characterized by the thickest crust. Here the
velocities of 6.5 km/sec almost approach the surface (depth 3-5
km). Deeper lies a thick succession with velocities 6.5—7.0 km/sec.
The velocities ranging from 7.5 to 8.0 km/sec are confined to the
stratum lying deeper than 40 km.
The thinner crust with a peculiar velocity structure has been
detected in the block of the Faeroe-Shetland channel. Here a sedi-
mentary sequence of more than 4 km in thickness shows velocities
3 to 5 km/sec. It is underlain by a consolidated crust with velo-