Jökull - 01.12.1971, Blaðsíða 47

Geophysical Observations on the Iceland-Faeroe Ridge
G. LEONARD JOHNSON AND BERTRAND TANNER
U. S. NAVAL OCEANOGRAPHIC OFFICE, WASHINGTON D. C. 20390
ABSTRACT
West of the Faeroes the Iceland—Faeroe
Ridge is a massive feaiure with very litlle sedi-
ment cover and high amplitude short. wave
length magnetic anomalies. However to the east
of the Faeroes the ridge has a greater sediment
cover (up to 500 meters) suggesting erosion pro-
cesses asre not as dominant here as to the west.
Prominent sub-bottom reflectors may represent
lag deposits or ash layers. This eastern portion
of the ridge appa,rently is i?i the form of a
graben (Fig. 4, profiles C & D). Bottom currents
are active at depths of 1200—1600 fathoms in
the Norwegian Basin and have locally eroded
tlie sediments.
The Iceland—Shetland Ridge and its counter-
parts to the west of Iceland are most likely the
result of a local hot spot in tlie earth’s mantle.
INTRODUCTION
Recently the US Naval Oceanographic Olfice
conducted a reconnaissance survey of portions
of the Iceland—Faeroe Ridge. Parameters mea-
sured were magnetics and seismic reflection. A
proton precession magnetometer (Varian 4931
DR) was employed to measure the absolute
value of the earth’s magnetic field. Analog
records of magnetic intensity were scalecl at 50
gamma intervals and at all highs and lows.
Seismic reflection data was collected with a
Bolt Associates air gun, Teledyne hydrophone
streamer and two Raytheon PSR recorders. The
air gun was fired at about 1800 lbs. pressure
every 8 seconds using a 120 cubic inch chamber
and the incoming signal was band pass filtered
from 40 to 76 Hertz. Navigation was by Loran
and satellite and was accurate with excellent
agreement between intersecting lines.
The Iceland—Faeroe—Shetland Ridge is a
large flat-topped ridge of uncertain origin
(Fig. 1). From the Faeroes through Iceland the
evidence strongly suggests it is basaltic in com-
position. Iceland, which straddles the crest of
the Mid-Oceanic Ridge, consists of Tertiary
and Quaternary plateau basalts (Thorarinsson,
1966). Tertiary basalts occur on the Faeroe Is-
lands, along the coast of Greenland west of
Iceland, and in local areas of northern Scot-
land and northern Ireland (Thorarinsson,
1966). Geologists in the past liave assumed
from broad petrological comparisons and from
general similarities in geologic setting that the
intrusive centers of Iceland were of the sarne
age as those of northwest Scotland. This view
is no longer tenable as it has been established
by isotopic age methods that the Tertiary in-
trusive centers of northwest Scotland are about
50—60 M.Y. old; i.e. Palaeocene and/or Eocene;
East Greenland 49 ± 2 M.Y. from the Tertiary
Kangerdlussuak alkaline intrusive; Iceland,
however, is 1.5-10.0 M.Y. It appears, there-
fore, that the igneous activity in the visible
parts of Iceland probably represents an al-
together much younger episode in the history
of the North Atlantic than that of the sur-
rounding continental regions (Gale et al., 1966).
Radiometric age determinations on basalt
samples from the Faeroe Islands were reported
by Rasmussen and Noe-Nygaard (1966) as 86 ±4
M.Y. This indicates at least an Upper Creta-
ceous age for basaltic lava flows of the Faeroe
Islands (,Noe-Nygaard, 1962). It does seeni
reasonable, however, to assume that the plateau
basalts of the Faeroes (Noe-Nygaard, 1962,
Palmason, 1965) and Iceland (Bodvarsso?! and
Walker, 1964) are of widespread occurrence and
generally similar on the intervening submarine
ridge between the Faeroes and Iceland. The
ridge basalts were probably extruded during
the widespread early Tertiary igneous activity
of the Brito-Arctic province.
Most investigators (Litvin, 1965; Thorarins-
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