Jökull - 01.12.1972, Blaðsíða 69

Fig. 2. Seismic reflection profile across Knipovitch Ridge and Bjornoya Trench. Note lack of
sediment cover about 50 km west of trench, and apparent step faults of sediment on walls of
trench. Record length is 140 km; west on the left. One second of travel time equals approxi-
mately one kilometer.
Mynd 2. Setpykktarmœling yfir Knipovitch hrygg og Bjarneyjar Irog. Setlög vantar um 50 km
vestan við trogið. I hlíðum trogsins virðast vera brotstallar i setlögum. Mcelilínan er um 140 km
löng, vestur til vinstri. Ein sekúnda svarar nokkurn veginn til eins kilómetra.
which has not yet been discovered and now is
in the initial process of descent into the trench.
A similar case has been noted in the northern
Pacific by Pitman and Hayes (1968) in which
the Aleutian Trench lias apparently swallowed
the ridge with only trace being the reverse
order of the magnetic anomalies. The lack of
intermediate earthquakes indicates that if the
trench is a site of crustal destruction it is
relatively youthful as no associated intermediate
earthquakes are found. Therefore number 2
above is probably untenable. Farther south the
Iceland—Jan Mayen Ridge apparently shifted
axis in the Pliocene (Vogt et al., 1970) and this
major crustal reorganization may well be re-
presented by an axial shift in the Greenland
Sea at the same time. The existence of a Mid-
Tertiary graben and Quaternary lava cones on
Svalbard (Harland, 1961) are indicative of recent
tension in the area further supporting the
existence of a newly created trench in the
Greenland Sea.
In the absence of fault plane solutions for
the earthquakes it is possible that Bjornoya
Trench within recent times has become the
site of crustal destruction and that the axis of
Knipovitch Ridge either active or recently in-
active is located 50 km west of the trench along
the sediment free crestal zone. However the
alternative that it is truly an axial valley as
previously shown witli a low magnetic signature
due to sediment fill cannot be discarded.
ACKNOWLEDGEMENTS
The seismic reflection data were obtained by
Mr. John Freitag. Thanks are due Dr. P. R.
Vogt who reviewed the manuscript and made
available the magnetic data.
REFERENCES
Demenitskaya, R. M. and V. D. Dibner. 1966;
Morphological structure and the Earth’s
crust of the North Atlantic region. In:
Continental Margins and Isla,nd Arcs (W.
H. Poole Ed.), Geol. Survey of Canada,
paper 66-15, pp. 62-79.
Ewing, J. and M. Ewing. 1967: Sediment distri-
bution on Mid-Ocean Ridges with respect
to spreading of the sea floor, Science, 156,
1590.
Haggerly, S. E. and E. Irving. 1970: On the
origin of the natural remanence (NRM) of
the Mid-Atlantic Ridge at 45° N. Trans.
Amer. Geophys. Un. 51, 273.
Harland, W. B. 1961: An outline structural
history of Spitsbergen. Geology of the Arc-
tic. G. O. Raasch, editor. Univ. Toronto
Press, 68—132.
Heezen, B. C. and M. Ewing. 1963: The Mid-
Oceanic Ridge, in the Sea, M. N. Hill, ed.
Interscience ,New York 3, 388—410.
JÖKULL 22. ÁR 67