Greinar (Vísindafélag Íslendinga) - 01.01.1976, Blaðsíða 121

UPPER PLEISTOCENE VOLCANISM AND TECTONISM
IN THE SOUTHERN PART OF THE MEDIAN ACTIVE
ZONE OF ICELAND
Problems in Geology and Geophysics
Part I
By TRAUSTI EINARSSON
Department of Engineering and Science, and Science Institute
University of Iceland
ABSTRACT
On the basis of extensive field studies, the Upper Pleistocene volcanic and
tectonic geology of the Median Active Zone is described in its main structure,
from Reykjanes to the latitude of Skjaldbreidur. The area is divided into a south-
em one, comprising the Reykjanes moimtain chain, in which the probably under-
lying older rocks are only occasionally exposed, and a northem part in which
the older volcanic and tectonic activity can be studied.
In the former part, subaquatic and/or submarine tuff-breccias were formed,
and the oldest volcanoes exposed are from the Last Interglacial. Just before the
Last Glaciation these rocks were uplifted in blocks, to form the present mountain
chain.
In the northern part, a 45 km wide zone, we consider first the situation and
age of the volcano ruins found in axial position in the NW-striking valleys of the
Tertiary plateau basalts west of the Median Zone. This volcanism is considered
to be due to the crossing of conjugate shear fractures in a crustal stress field,
on one side the now dominating SW-NE trend, and old fractures of SE-NW
trend, along which the valleys developed. The distances of these volcanoes from
the present Median Zone are unrelated to their ages.
Younger than these volcanoes are the dolerite shieldvolcanoes on each side of
the Thingvellir Lake. They may also be due to the crossing of conjugate fractures.
There followed subsidence in a SW-NE zone between these shields, and the
rocks fornied in the depression can be studied at the south end of the Thingvellir
lake. They form an uninterrupted series of strata, overlying the dolerites at both
sides of an 8 km wide gap. These rocks are largely due to sub-aquatic emptions,
they were much leveled by surf, and later heavily eroded by subaerial valley
formation. The denudation is on such a large scale as to suggest an age of many
hundred thousand years. This is verified by the occurrence of reverse magnetiza-