Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 157

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the deposition of the rocks of age 0.7 My or more, in the depres-
sion between the western and eastem dolerites. If “spreading” has
taken place here dnring the last 0.7 My or more, it must be two
orders of magnitude less than the value of 2 cm/y usually assumed
for this zone.
After about 0.7 My ago, relatively very little volcanic activity
has taken place in the present area, and it was only some 70—100
thousand years ago that a major tectonic phase set in, expressed
in an uplift which formed a chain of mountains from the Jóru-
kleif area, to a maximum height in Hengill, and the gradually
lower blocks farther southwest. This chain, which is only a part
of the one along the Reykjanes peninsula, forms a clear 17-18 km
long arch, when seen from the side at some distance.
e) Correlation of the state of a crustal stress field witli the
volcanic and tectonic activity: A summary.
The volcanic and tectonic geology of the southern part of the
Median Active Zone in Iceland, studied in this paper, is best under-
stood as a result of yield to a crustal stress field. The principal
stresses were unequal, at least usually, the larger compression hav-
ing normally E-W direction. Hence, there was shear stress with
right-Iateral shear on SW-NE lines, and a conjugate left-lateral
one on SE-NW lines (6).1)
The stress field was variable in strength, and the above stated
1) This result, which is supported by many facts, is not in disagreement with
a study of two graben areas in postglacial lava fields (20), where a symmetry of
dextral and sinistral en echelon fractures was found. In these cases the vertical
displacements by many, even tens of metres overwhelm and ohliterate significant
signs of horizontal displacement of the order of decimetres or so. The symmetrical
arrangement of dextral and sinistral echelon fractures within such grabens (20)
is, on the contrary, per se an obvious proof of the influence of local conditions,
and not to be taken as a clue to the nature of the stress in a far more extensivc
area. One must study many areas where there is no vertical disturbance of the
clear and significant en echelon regularity in the horizontal arrangement of
open fissures in postglacial lava field, to find the detail expression of the regular
stress field, e.g. as shown in (6).