Jökull - 01.12.1979, Side 28

Severe earthquakes occur along the transverse
zone. The last major ones in 1784 and 1896 were of
magnitude 7—8, and included a sequence of shocks
occurring within a few weeks, with the epicenters
moving with time from east to west across the 60
km long E-W seismic zone (see chapter 5). The
South-Iceland seismic zone might be looked upon
as an early stage of opening of an oblique rift axis
that eventually will connect the Reykjanes volcanic
zone directly to the axial rift zone northeast of the
Hekla area.
The axial rift zone in northern Iceland connects
with the Kolbeinsey Ridge along the Tjörnes
Fracture Zone which is a complicated structure
made up of both NW-SE trending wrench faults
and N-S grabens and volcanic fissure swarms
arranged in a dextral en echelon pattern. The total
width of the zone from north to south as defined
from earthquake distribution is about 70 km.
However, geological evidence points to a much
narrower zone of true transform character. This is
the NW-SE trending Húsavík fault zone which
limits the Tjörnes Peninsula to the south. The
Húsavík faults are traceable on land for more than
25 km from near the western margin of the axial rift
zone to the sea just north of Húsavík, juxtaposing
rocks of widely contrasting dips and ages. The
Húsavik faults appear to continue towards WNW
at least beyond the southern limit of the Grímsey
shoal. The shoal platform there is marked by the
depth contours clearly tracing deep embayments in
the shoal platform close to the coast offsetting the
Grímsey shoal to the east relative to the nearest
mainland peninsula. Gravity measurements have
revealed a very marked low coinciding with the
trace of the suspected fault zone in this area indi-
cating a sediment filled graben. The rqcks exposed
on land nearest to the supposed transcurrent
Húsavik faults are tectonically sheared and miner-
alized to a degree unknown elsewhere in surface
exposure in Iceland. The Húsavík faults are prob-
ably associated with tens of km right lateral offset.
They formed part of an active transform fault zone
during a time interval when a spreading axis lay
west of the Grímsey shoal (Fig. 6). The transform
motion was greatly reduced about a million years
ago when the axial rift zone in northern Iceland
extended northwards beyond them along the Ax-
arfjördur depression. Young volcanism in the
Tjörnes Fracture Zone is unknown outside the
fianks of the Axarfjördur depression. An array of
volcanic fissure swarms and grabens has been
identified between Kolbeinsey and Axarfjördur
which presumably formed when spreading on the
Grímsey shoal began to wane.
The transverse E-W zone across Central Iceland
has also been referred to as having transform cha-
racter. It extends from Snaefellsjökull eastward
across the central ice caps to the Vatnajökull
region. The arrangement of fissure swarms and
volcanic systems in the area between Langjökull
and Snaefellsnes indicates dextral shear. The faults
and fissures themselves show that also a component
of extension is involved.
TECTONICS OF ICELAND AND MANTLE
PLUME/HOT SPOT ACTIVITY
Hot spots or mantle plumes have been invoked to
explain the unusual elevations found across the
North Atlantic in the Iceland region. These appear
to be transient features of limited and variable ex-
tent, and variable in their activity. The scattered
subaerial remnants of volcanic rocks defining the
Brito-Arctic volcanic province may be considered a
manifestation of the plume activity.
Several of the gross features of Icelandic geology
seem to find a logical explanation if one assumes
that a mantle plume exists under Iceland. In par-
ticular (1) the elevation of Iceland and the thick-
ness of its oceanic crust, (2) the change in strike of
the extensional features across Central Iceland, (3)
the decrease in intensity of volcanism along the
axial rift zones away from South Central Iceland,
and (4) the very pronounced offset of the axial rift
zones in Iceland relative to the Reykjanes and
Kolbeinsey ridges.
The elevation of Iceland is a consequence of an
anomalous mantle underneath Iceland with lower
average densities and higher temperature than the
surrounding region. The Iceland crust is more than
twice as thick as the normal oceanic crust. Its upper
part down to seismic layer 3 (the oceanic layer) is
in the order of 4—6 km for an uneroded lava pile
compared to about 2—3 km for the average oceanic
crust. Both are interpreted as extrusive in origin
which implies that the extrusion rate per unit area
was about two times greater in Iceland than in the
submerged oceanic ridges.
A general principle probably controls the change
in strike of tectonic elements across Central Iceland.
The dyke trend in the older formations shows that
this bend in the axial rift zone has persisted
throughout the geological history of Iceland. These
26 JÖKULL 29. ÁR