Jökull - 01.12.1982, Page 112

0.2
Fig. 2. Seismic profiles to the west of Flatey show-
ing the steep contact between basement rocks
(shaded) and sediments. Shading on profile G
indicates submarine extension of Flatev volcanics.
Vertical scale in seconds of two-way travel (left).
and basaltic-rhyolitic rocks of the Flateyjardalur
central volcano. The age of the sequence on land
ranges from 12 to less than 7 million years (Aronson
and Sœmundsson 1975).
Sparker measurements were made in the area
between Eyjafjörður and Tjörnes in order to map
this basement/sediment contact. The profiles (Fig.
1) were positioned so as to cross the gravity con-
tours. The survey was carriedout on vessels belong-
ing to the Marine Research fnstitute, Reykjavík.
Navigation was based on radar and Loran C.
Some of the profiles overlap lines surveyed by Mc-
Master et al. (1977).
RESULTS
The FlateyJault
Several profiles measured to the west of Flatey
island (profiles A-F, Figure 2) cross the boundary
between crystalline rocks and sediments. This
boundary is found 2 nautical miles off Gjögurtá
and appears to merge with the coast near Flatey.
The profiles show the contact to be near vertical and
its straightness and orientation suggest that this is
indeed a fault in direct continuation of the Húsavík
faults, as predicted by Semundsson (1974) and Mc-
Masteretal. (1977). This feature will be referred toas
the Flatey fault in the following discussion.
The Flatey fault is very strongly reflected in the
topography: the sea-floor is seen in profde A to drop
from 60 milliseconds (44 meters) to 280 ms (204
meters) across the fault. Reflectors in the sediments
were recorded at 420 ms depth (minimum of 310
meters) and these are not thought to represent bas-
altic basement. The coastline in this area is mount-
ainous and rises steeply ffom sea-level to elevations
above 800 meters. These figures give an indication
of the magnitude of the discontinuity, i.e. a min-
imum vertical fault component of 1100 m.
A narrow platform has been cut in the basement
between the Flatey fault and shore. The edge of this
Approximate scale in metres also shown. Horizontal
scale indicated with divisions of one nautical mile.
Mynd 2. Endurvarpssnið vestan Flateyjar, sem syna skil
milli berggrunns úr storkubergi og setlaga. A sniði G
kemur fram storkuberg tengt hraunlögunum í Flatey. Lóð-
re'ttur kvarði er syndur sem endurvarpstími í sekúndum, en
einnig er sýnt dýpi t metrum (ónákvœmt). Láréttur kvarði
táknaður með strikum á einnar sjómílu bili.
108 JÖKULL 32. ÁR