Greinar (Vísindafélag Íslendinga) - 01.01.1976, Side 128

126
single zone at this time. The northern part of the postglacial zone
is again displaced by 5-6 km to the east, relative to the southern
part, and is represented by the Thjófadalir postglacial crater rows
on a 12-15 km stretch. Thus, these two parts constitute a natural
continuation of the en echelon picture which characterizes the
Reykjanes peninsula. Only the large shieldvolcano Skjaldbreidur
does not conform with the rule; its isolated position and its shield-
volcano nature is suggested to be due to the crossing of a non-
volcanic continuation of the Sandey line with a conjugate fracture
through Thverfell, as can be better judged in the light of the dis-
cussion under b) below.
The Pleistocene activity of the Reykjanes peninsula continues
along the Thingvellir-section in much the same way as the post-
glacial activity, first as uplifted ridges of relatively young vol-
canics, having the southern part of the “Sandey line” for axis, and
then similar activity with the “Thjófadalir line” as axis. The width
of this echelon zone of ridges is characteristically about 5 km, and
the volcanics are largely subaquatic as on the peninsula; they are
nearly exclusively tuff-breccias. The magnetic polarity is in all
cases normal, as on the peninsula, but the absolute age has in
neither case been determined.
In spite of the lack of radiogenic dating (cf. also Part II of this
study), we seem to have in the mentioned facts a fairly clear evi-
dence that the same late Pleistocene volcanic and tectonic phase
occinred along the Reykjanes peninsula and the Thingvellir-Lang-
jökull section as far north as we have discussed sofar. In both sec-
tions this phase was isolated in time, not part of a continuous acti-
vity. The volcanism of the phase is possibly not older than 100,000
years, and in Part II it is suggested that uplifts took place just
before the beginning of the Last Glaciation, about 70,000 years ago.
b) Upper Pleistocene Volcanism in the western
Plateau Basalt region.
The Thingvellir depression is flanked on the west side by moun-
tains of plateau basalts, up to 900 m high. In this area, as in many
other Plateau Basalt areas in Iceland, we find remnants of a few