Greinar (Vísindafélag Íslendinga) - 01.01.1976, Side 122

120
tion at a stratigraphically high level, setting here a lower are liniit of 0.7 million
years. As these strata have been traced without a break right across the 8 km
interval, it can be demonstrated that “spreading” has not occurred, or has been
less than the often assumed 2 cm/y, by at least two orders of magnitude, during
the last 0.7 million years. The age of the underlying dolerites can hardly be
less than one million years which is three or four times the earlier estimates.
Just before the Last Glaciation, the uplift phase of the Reykjanes peninsula
was also active in this area, and a 17-18 km long SW-NE arch was fonned by
uplift, with a maximum height (800 m) in the Hengill mountain.
The volcanic and tectonic activity in the Median Zone is considered to be the
result of yield to an extensive crustal stress field, the phases of which express
themselves somewhat differently, as can be explained in classical mechanical
teims.
Land-based data lead to conclusions conceming the submarine Reykjanes ridge.
These and several other general problems in geoscience will be treated in Part II
of these studies.
By the Median Active Zone in Iceland is meant that zone whose
axis runs, first, along the E-W-striking Reykjanes peninsula, and
then to the NE across the Thingvellir and Langjökull areas (1, p.
7564). A continuation of the zone into the Skagafjord valley is
indicated by faint postglacial volcanism in the tributary Svartár-
dalur (2, p. 22), and hy the considerable volcanic activity about
one million years ago at the mouth of Skagafjord (3; 4; 5). The
same zone is further indicated by a somewhat older volcanism in
the easternmost part of Húnavatnssýsla (2, p. 27; 3), cf. Fig. I.1)
Here we shall consider some instructive features of the activity
in the southem part of this zone — summarizing the field work
of the author during the last dozen summer seasons. We divide this
southem part into two sections: A) The narrow zone along the
Reykjanes peninsula; B) The broad zone in the Thingvellir-Lang-
jökull section. We shall mainly be concemed with the latter.
The distinction between the two sections is natural and quite
clear: the boundary follows rather closely a straight line, drawn
between the mouth of ölfusá river and the northern part of Reykja-
1) Many place names and topographic features are mentioned in the present
paper. Readers who are not familiar with these are referred to the available
topographic maps in scales from 1:250,000 to 1:25,000, and to the geological maps
in scale 1:250,000.