Jökull - 01.12.1972, Blaðsíða 67

The Mid-Oceanic Ridge in the Greenland Sea
G. L. JOHNSON,
U. .S. NAVAL OCEANOGRAPHIC OFFICE, WASHINGTON D. C. 20390
INTRODUCTION
The continuation o£ the Mid-Oceanic Ridge
through the Greenland Sea (Knipovitch Ridge)
has been postulated by numerous investigators
on the basis of both earthquake epicenters and
topography (Heezen and Ewing 1961, Johnson
and Eckhoff 1966, Demenitskaya and Dibner
1966). Morphologically the Ridge is atypical in
that in addition to the expected rough arch
its eastern flank is bordered by a continuous
linear trench (Fig. 1). This trench has generally
been accepted as an axial rift valley or/and a
fracture zone Johnson and Heezen (1967a) or
trench Vogt et al., 1970. The earthquake epi-
centers tend to lie in or very near to the axis
(Fig. 1). The rift is narrow, generally 2 or 3
rniles wide at the 1800 fm isobath (Fig. 2), and
has well developed structural benches on its
walls. The rift mountains generally lie at
depths less than 1200 fms and to the east, are
barely discernible, due to burial. The trench
abruptly terminates at 78° 30' N as it impinges
upon the continental rise of Spitsbergen.
ORIGIN
A recently acquired seismic reflection line
strongly suggests a rather intriguing possibility.
It is well known that on a world-wide scale the
crest of the Mid-Oceanic Ridge is characterized
by an absence or almost total lack o£ sediment
cover, Schneicler and Vogt, 1968, Ewing and
Ewing, 1967. Knipovitch Ridge (Fig. 2) has
basement rock cropping out or at least sedi-
ment cover thinner than the resolution o£ the
seismic system front 40—60 km west of the
trench. Magnetic profiles across the Ridge do
not show the characteristically high amplitude
Brunclies normal positive anomaly 50 km west
of the trench. A negative anomaly is however
present flanked by small positive anomalies
and does possess apparent symmetry (Fig. 1),
profiles 1—3. Possibilities for the lack of well
defined magnetic anomalies over either the
trench or ridge crest are:
1. Oblique spreading parallel to the Spits-
bergen and Greenland Fracture Zones must
occur in the Greenland Sea. Oblique spreading
at a rate of probably less than 1 cm/yr with
concommitant reduction in width of the norm-
ally and reversely magnetized strips of crust
would significantly decrease the amplitudes of
the magnetic anomalies (Vine, 1966).
2. If the sediment has free access to the axial
rift valley the extrusion of highly magnetized
pillow basalts will be restricted and buried
coarser grained less highly magnetic rocks will
tend to be created in the axial zone (Johnson
and Heezen, 1967b, Vogt et al., 1970). Bottom
photographs and dredges have shown that pil-
low basalts are by far the most common rock
type in the axial valley of the Mid-Oceanic
Ridge (Heezen et al., 1959, Schilling et al.,
1968, Heezen and Ewing, 1963). Schilling et al.
(1968) demonstrated that the pillow basalts of
the Reykjanes Ridge have a high value of
magnetization (J = 0.03 e.m.u.) and that values
away from the axial zone are smaller (J =
0.012). Recent studies by Talwani et al. (1971)
support these findings and suggest the causi-
tive layer of highly magnetized basalt is about
400 m thick. The decrease in magnetic intensity
away from the ridge axis has been explainecl
by Haggerty and Irving (1970) as being due to
alteration of titanomagnetite to maghemite pre-
sumably by the subaqueous weathering of the
basalt leaving the residual value at 0.012 e.m.u.
3. The Ridge became inactive during Matu-
yama reversed epoch and the negative anomaly
in profiles 1—3 50 km west of the axis re-
present a relic axis with perhaps a very thin
sediment cover which has accumulated in the
last 700,000 years.
JÖKULL 22. ÁR 65