Jökull - 01.12.1992, Page 15

SEISMIC CRUSTAL STRUCTURE ALONG THE
AXARFJÖRÐUR TROUGH AT THE EASTERN MARGIN OF
THE TJÖRNES FRACTURE ZONE, N-ICELAND
Erik Sturkell
Nordic Volcanological Institute,
University oflceland, 101 Reykjavík, lceland
Bryndís Brandsdóttir,
Science Institute, University oflceland
Dunhaga 5,107 Reykjavík
Hideki Shimamura and Masashi Mochizuki,
Laboratory for Ocean Bottom Seismology,
Hokkaido University, Sapporo 060, Japan
abstract
A refraction survey along the easternmost trough
within the Tjörnesfracture zone on the N-Iceland shelf
'eveals a major sedimentary sequence. The sedimen-
tary layers are horizontal and as thick as 2.3 km. The
sedimentary sequence consists of an approximately
l km thick upper part, separated from the lower part
by a clear interface. This interface can he traced along
the profile, but in the south-east part ofthe profile the
mterface is buried by higher velocity sediments. This
wedge thins outfrom the coast, and is interpreted as
toarse material of near-glacier origin. The sedimen-
tuiy pile is deposited on an igneous basement with a
T-wave velocity of4.3 kmts. The age ofthis basement
ls 22-4.4 Ma based on sedimentation rates. At the
t’ansitionfrom the sediments to the basement there is
a distinct velocity discontinuity, with a steep velocity
g’adient. Layer 3 is at depth of 5.5-6.0 km below sea
bed, along the profile. This is deeper thanfor the nor-
tnal oceanic crust. The uppermost part ofLayer 3 is
ln the depth range observed for the normal Icelandic
crust.
INTRODU CTION
Iceland is located at the junction between the
Reykjanes ridge in the south and the Kolbeinsey ridge
in the north (Figure 1). In Iceland, the Mid-Atlantic
plate boundary shifts to the east, towards the station-
ary hot spot located below the central eastem volcanic
zone. The transform motion is taken up by two major
fracture zones, the South-Iceland seismic zone and the
Tjömes fracture zone offshore northern Iceland. Both
these fracture zones are structurally complex and are
primarily delineated by seismic activity.
The Tjömes fracture zone (TFZ) is a broad zone
of transform faulting which shifts the axial rift zone
in northern Iceland to the west, where it continues
along the Kolbeinsey ridge. Its existence was orig-
inally inferred from earthquake distribution and fo-
cal mechanisms along the Mid-Atlantic ridge (Sykes
1967; Ward 1971). Geological and bathymetric data
show that the TFZ is more complex than its coun-
terparts on the Mid-Atlantic ridge, as it incorporates
active N-S trending troughs and volcanic ridges within
the E-W trending zone. The approximately 120 km
JÖKULL, No. 42, 1992 13