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conjugate maximum shear stresses expressed themselves alterna-
tely or simultaneously (Fig. 6).
Thus, the dykes of the plateau basalts between the Median Zone
and the Hvalfjördur area show that these basalts are due to frac-
turing by the SW-NE shear. Later, the conjugate shear formed
fractures in this area, along which the NW-running valleys de-
veloped — whereas signs of accompanying volcanic activity are
rare. But most likely it was at this time, or somewhat later, that
the SW-NE shear led to the foundation of a major active zone, the
Median Zone — a seam between stronger crustal blocks, Fig. 1 — in
the Second Main Tectonic Phase of Iceland (11, 9).
During certain morphologically graded erosional stages of the
above NW-running valleys, volcanic activity took place at the
crossing points of both sets of shear fractures, as still evidenced by
volcano remnants in the valleys in the western outskirts of a 45 km
wide Median Zone. These volcanoes demonstrate the absence of a
relationship between their age and distance from the recent active
zone. Later, it was probably still the crossing of these sets of frac-
tures which gave rise to large shield volcanoes, 10-13 km away
from the present axis of the Zone (and to the postglacial Skjald-
breidur shield). These so-called dolerite lavas were thereupon much
fractured and subsided in a 12 km wide SW-NE axial zone, and
intensive and varied volcanism was now (about 1 My ago) re-
stricted to this narrower zone, the axis of which proceeds in echelon
steps right to the axis of the sub-marine Reykjanes Ridge, as ac-
curately as that axis can at present be defined.
Continuous strata right across an 8 km broad part of this nar-
rower zone at the southern end of the Thingvellir lake prove that
“spreading” has not taken place in the zone — or was less than
1/100 of what has been assumed — during a time span which is
thought to cover about the last 1 My. Most of the visible volcanics
on the Reykjanes peninsula are, however, relatively young, as far
as exposed rocks reveal; they probably date from the last inter-
glacial time.
A little before the last glaciation, there was a state of the stress
field (cf. theoretical background in Fig. 6) leading to the squeez-
ing up of blocks, forming the cliain of echelon mountains from
Tindaskagi (south of Skjaldbreidur) out onto the shelf off Reykja-