Rit (Vísindafélag Íslendinga) - 01.06.1967, Qupperneq 158

154
the Rx- and N2-groups. On Fig. 1 these have been connected
by a line. Here Rx is also much more eroded than farther
south, and east of the volcanic belt, where it is continuous.
Possibly this indicates a late shift of the young volcanism to-
wards the west in the northern part of the volcanic belt.
Structurally the zone “Selvogsheidi-Thingvallavatn-Lang-
jökull” forms the “backhone” of the westem hranch of the
so called central Icelandic graben. The main faulting and
volcanic activity occurs in the middle of that zone, but more
isolated eruptive centers also occur far outside on either side.
Volcanism within this zone has mainly produced basaltic
rocks from eruptive fissures and shield volcanoes resulting
in extensive lava flows and more restricted pillow lavas and
related hyaloclastic rocks depending on, whether the area
was glaciated or not.
Since the last interglacial the accumulation of volcanic
rocks has not been uniform over the length of the volcanic
belt. The depression occupied by lake Thingvallavatn is an
area where accumulation has been rather insignificant, at
least during the last glaciation, but the Hengill-area to the
SW and the Langjökull mountains and the Laugardalsfjöll
represent areas of intensive volcanic activity during the same
lapse of time. On tracing faults through formations of diffe-
rent age it is observed that the greatest amount of subsidence
is found in the oldest rocks. The Hengill-area and the Thing-
vallavatn depression provide good examples. Here the west-
ern limit of the graben is hounded by a swarm of normal
step faults (Saemundsson 1967). The downthrow amounts
to at least 240 m on the eastern slope of the Haedir shield
volcano (probably formed during the last interglacial), and
to about 100 m in the Háhryggur series (the oldest rock unit
in the area helonging to the Wiirm glaciation). In the some-
what younger Hengill complex the downthrow amounts to
about 80 m and in the still younger Stóra Skardsmýrarfjall
about 70 m. Young postglacial lava flows occurring in the
same area show vertical displacements of several meters only.