Jökull - 01.12.1979, Page 27

Fig. 18. Major intrusions in SE-Iceland unrelated
to central volcanoes of the lava pile may have
formed in the roots of later volcanoes superimposed
on the lava pile. Their relation to the axial rift zone
of their day was probably similar as that of the
Öraefajökull-Snaefell line of volcanoes to the cur-
rently active axial rift zone. Based on information
from Helgi Torfason.
graben formátion is lacking, indicating that crustal
separation along the volcanic axis is negligible, and
the extra load of the volcanic edifices is compen-
sated by isostatic subsidence on a more regional
scale.
The Snaefellsnes volcanic chain has a trend
transverse to the axial rift zones. The fault pattern
has been interpreted variously in terms of horizon-
tal north-southerly extension or right lateral shear
which tears up three parallel volcanic swarms (Fig.
17). A fourth subparallel fissure swarm connects
the Snaefellsnes transverse chain to the north-
Langjökull region.
In southern Iceland the flank zone continues
southwards from the intersection of the axial rift
zone with the South-Iceland seismic zone. From an
analysis of the tectonic fracture pattern associated
with the Surtsey and Heimaey eruptions, both
within the Vestmannaeyjar volcanic field, a conju-
gate shear set with faults trending NNE-SSW and
ENE-WSW has been inferred. The orientation of
the volcanic axes of Hekla, Tindfjallajökull and
Eyjafjallajökull (ENE-WSW or E-W) follows the
same general trepd. The southern part of the
eastern volcanic zone, here referred to as a flank
zone, may thus be the result of sinistral shear
movement with the axis of maximum compression
oriented nearly N 45° E, and the axis of maximum
tension acting NW-SE, subparallel to the spreading
direction.
The Öraefajökull-Snaefell line of strato-vol-
canoes is located in the eastern Vatnajökull region
some 40—50 km east of the main rift axis, and two
volcanoes on this line are partly (Esjufjöll) or
wholly (Breidabunga) buried by the ice sheet.
Snaefell is ridge-shaped sitting on a short volcanic
axis but Öraefajökull (2119 m), the only one active
in Postglacial time, is almost circular. Both lack the
fissure swarms characteristic of the axial rift zones.
These volcanoes overlie an older basement that has
been eroded before Öraefajökull started to form.
The rocks erupted are slightly transitional as far as
is known. It is possible that similar volcanoes
existed in this part of Iceland in Tertiary time, also
offset eastwardly relative to the main spreading
axis (Fig. 18). Evidence of this is seen in the large
gabbro and granophyre intrusions of southeastern
Iceland of up to 20 km2 in area. They bear no
obvious relation to the build up of the surrounding
lava pile having been intruded during a later
magmatic period, and have no dyke or sheet
swarms localized about them. They most likely
formed in the roots of stravo-volcanoes that grew
on top of the lava pile removed from the zone of
active spreading.
Tectonics of segments connecting the axial rift zones
The axial rift zones in Iceland connect with the
submerged ocean ridges along transverse structures
which were originally defined as fracture zones
from earthquake distribution patterns. They dis-
play a variety of tectonic features along their strike
including oblique segments of the neovolcanic
zones with pronounced en echelon features as well
as older fractured crust.
The connecting segment between the Reykjanes
Peninsula and the eastern volcanic zone lies across
an eroded pile of Plio-Pleistocene rocks dipping to
the northwest and north. The fault pattern indi-
cates a component of sinistral shear and the zone as
a whole has been referred to as the South-Iceland
seismic zone. It has a slightly more east-westerly
trend than that branch of the neovolcanic zone
which lies along the Reykjanes Peninsula. The
fault pattern in its western part is also very similar,
with SW-NE trending en echelon fissures. Farther
east N-S and ENE-WSW trending conjugate faults
predominate, some of them having lateral offset.
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