Jökull - 01.12.1979, Blaðsíða 7

the Jan Mayen Ridge running south from Jan
Mayen at about 6°—10° W, has been confirmed by
geophysical work and drilling to contain sedimen-
tary sequences, reaching at least 40 m.y. in age.
The shelf
The insular shelf around Iceland is mostly at
depths of between 100 and 300 m, and its width
varies from less than 15 km to more than 120 km,
with 80 km being most representative. The shelf is
easily distinguishable from the transverse ridges to
the east and west, but less so from the active
mid-ocean ridges where these join it. The area of the
shelf is larger than that of Iceland itself.
The character of the shelf and slope shows con-
siderable variation around Iceland, as described in
three examples below.
Off southern and southeastern Iceland, the shelf
is narrow, 12—70 km, and the slope is very steep
(up to about 15°) out to ocean depths of 1000—
1200 m. According to magnetic results, the outer-
most 10 km or so of the shelf consist of sediments,
prograding beyond an even steeper edge in the
basement. On the inner part of the shelf, sediments
are thin. Shallow valleys, cut into the sediments,
probably channel large quantities of volcanic ash
and glacial erosion products towards deeper ocean
basins, and sedimentary drifts (the Katla ridges)
occur off the shelf.
Off northern Iceland, the major fjords continue
as north-trending depressions on the shelf for tens
of km. They have probably been shaped by large
glaciers, and are now being gradually filled with
sediment from land. The deepest of these
depressions (Eyjafjördur Deep) is due south of the
Kolbeinsey Ridge and appears to originate partly
by faulting.
West of southwestern Iceland, the shelf is over
120 km wide, and the insular slope is relatively
gentle (less than 2°). The insular margin has been
extensively modified here by glacial erosion and
morainal deposition, with 10—30 km of shelf-edge
progradation. A steep basement edge, such as
occurs off southeastern Iceland, has not been iden-
tified. There is evidence of sediment redistribution
in elongated drifts, by the action of bottom cur-
rents.
SELECTED REFERENCES
Bott, M. H. P., 1974: Deep structure, evolution and
origin of the Iceland transverse ridge. In
Kristjansson, L. (Ed.): Geodynamics of Iceland
and the North Atlantic Area: 33—47. D. Reidel
Publ. Co.
Brooks, C. K., and S. Jakobsson, 1974: Petrochem-
istry of the volcanic rocks of the North Atlantic
ridge system. In Kristjansson, L. (Ed.): Geody-
namics of Iceland and the North Atlantic Area:
139—154. D. Reidel Publ. Co.
Egloff, J., and G. L. Johnson, 1978: Erosional and
depositional structures of the Southwest Iceland
insular margin: thirteen geophysical profiles. In
Geological and Geophysical Investigations of
Continental Margins, Amer. Assoc. Petrol
Geologists Memoir 29: 43—63.
Einarsson, P., 1976: Relative location of earth-
quakes in the Tjörnes Fracture Zone. Soc. Sci.
Icel. Greinar V: 45—60.
Fleischer, U., 1974: The Reykjanes Ridge — a
summary of geophysical data. In Kristjansson,
L. (Ed.): Geodynamics of Iceland and the North
Atlantic Area: 17—32. D. Reidel Publ. Co.
Fleischer, U., F. Holzkamm, K. Vollbrecht, and D.
Voppel, 1974: Die Struktur des Island-Fáröer-
Riickens aus geophysikalischen Messungen.
Deutsche Hydrogr. Zeitschr. 27: 97—113.
Icelandic Hydrographic Office, 1975: Bathymetric
maps of the Iceland area, scales 1:1 000 000 and
1:2 500 000.
Kristjansson, L., 1976: Central volcanoes on the
western Icelandic shelf. Marine Geophys. Res.
2: 285—289.
Kristjansson, L., 1976: A marine magnetic survey
off southem Iceland. Marine Geophys. Res. 2:
315-326.
Kristjansson, L., 1978: Magnetic trends around the
Reykjanes Peninsula, SW-Iceland. Report 78-
14, Science Institute, Univ. of Iceland, 11 p.
McMaster, R. L., J.-G. Schilling, and P. R. Pinet,
1977: Plate boundary within Tjörnes Fracture
Zone on northern Iceland’s insular margin.
Nature 269: 663—668.
Nilsen, T., 1978: Lower Tertiary laterite on the
Iceland-Faeroe Ridge and the Thulean land
bridge. Nature 274: 786—789.
Pálmason, G., 1974: Insular margins of Iceland. In
Burk, C. A., and C. L. Drake, (Eds.): The
Geology of Continental Margins: 375—379.
Springer Verlag.
Saemundsson, K., 1978: Fissure swarms and cen-
tral volcanoes of the neovolcanic zones of Ice-
land. In Bowes, D. R. and B. E. Leake, (Eds.):
Crustal Evolution in Northwestern Britain and
JÖKULL 29. ÁR 5