Jökull - 01.12.1971, Page 53

region. Since basement has been reported to
be near the surface farther south (Stride et al.,
1967) a graben-type structure is probably pre-
sent. The center is filled with transparent sedi-
ments.
The upper 500 meters of sediment on the
ridge crest are characterized by a number of
highly reflective layers. It is suggested these
may represent ash layers from the various near-
by volcanic centers. Ash layers, like sands, form
opaque layers to seismic reflection techniques
(Worzel, 1959; Ninkovich, 1968). An alterna-
tive suggestion would be the opaque layers are
lag deposits of sand and gravel deposited dur-
ing times of high current velocity.
Fig. 4, profiles B left end, C center ancl D
center have anomalous areas of sea floor. In B
and C the bottom is corrugated causing a dif-
fraction of the seismic energy. In profile D
this appears to be an erosional feature which
has either eroded or prevented deposition in
the soutli side of the basement high. The
sculpting effect of bottom corrents to create a
corrugated sea floor (abyssal anti-dunes) is well
documented as is the erosional power of bottom
currents (Fox et al., 1968 and Johnson and
Schneider 1969). Jones et al. (1970) suggested
that the large sediment accumulation at the
northern flank of Iceland—Faeroe Ridge was
due to a more tranquil environment. These
profiles dispute this statement as it appears
that bottom currents are certainly at least local-
ly active between 1200—1600 fathoms in the
Norwegian Basin.
ORIGIN
The origin of the Iceland—Faeroe Ridge is
uncertain. Ample outpourings of lava along a
broad linear belt between Greenland and Scot-
land produced and are still builcling the Green-
land—Iceland—Faeroe Plateau. Menard (1969)
suggested that a local “hot spot” may exist at
depth over a long periocl of time. As a parcel
of crust in response to sea floor spreacling
travels over this center it is injected with
magma. This will result in an elongated rela-
tively smootli ridge íormed perpendicular to
the axis of spreading. This is a possible ex-
planation for this rather unique ridge. One
weakness, however, is that the ridge should
shift its strike in response to the shifting axial
direction of the Reykjanes Ridge as reported
by Avery et al. (1969). The Faeroe Ridge
section does strike more south-easterly than
the Iceland-Greenland Ridge (Denmark Strait);
however one would expect symmetry with dist-
ance from the axis of the Mid-Oceanic Ridge
which is not apparent on these two massive
ridges flanking Iceland.
Morgan (1971) has proposed that aseismic
ridges such as the Iceland—Faeroe are con-
structed by ascending deep mantle plumes.
This is an expansion of the idea originally
proposed by Wilson (1963). Morgan, however,
prefers to use the deep mantle as a source
rather than the upper mantle. The idea is as
yet unproven: for instance Knopoff (1970)
states that lower mantle fluids are too viscous
to readily flow. Under this scheme horizontal
currents in the asthenosphere will flow radially
away from each of these plumes with the
motion of the plates recorded by the aseismic
ridges. Since Iceland probably at one time ex-
tended north to Jan Mayen to include the
Iceland Plateau (Johnson and Heezen, 1967)
it would appear that the hot spot may be
diminishing in thermal energy or else is cur-
rently expending it along the recent Mid-
Oceanic Ridge on the western edge of the Ice-
land Plateau and southward along the Reykja-
nes Ridge.
ACKN OWLEDGEMENTS
Data tuas reduced by Miss Elleyn Johnson.
R. Higgs of the U. S. Naval Oceanographic
Office reviewed the manuscript and offered
many helpful suggestions.
REFERENCES
Avery, O. E., G. D. Burton, J. R. Heirtzler.
1968: An aeromagnetic survey of the Nor-
wegian Sea. ./. Geophys. Res. 73, 4583—
4600.
Avery, O. E., P. R. Vogt and R. H. Higgs. 1969:
Morphology, magnetic anomalies and evo-
lution of the northeast Atlantic and Labra-
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