Greinar (Vísindafélag Íslendinga) - 01.01.1976, Side 154

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exposures of the eastern and western dolerites, with rocks older or
close to 0.7 million years, and there is certainly nothing in this
zone which could suggest such a process as “spreading” since the
deposition of these rocks. There are only very few and insignifi-
cant dykes to be found. Even if their cumulative width were taken
to mean spreading — which corresponds to ignoring the compres-
sibility of the country rocks — that movement has been at least
two orders of magnitude less than the figure (2 cm/y) for this
zone circulating in the geoscientific literature. If as much as 350 m
spreading were assumed in the 8 km zone in 0.7 My, to account
for dykes and displacement on not fully vertical faults, that would
correspond to 1/20 cm/y. A still lower and more accurate upper
limit for spreading is found for the 2 km wide axial zone covered
hy the Hengill Basal I.ayer,
As pointed out on p. 127—128, the Súlur-Lágafell rocks, of an age
greater than that of the valley topography in the plateau hasalts
west of Súlur, extend right to the Sandey-Skjaldbreidur axis of the
present Thingvellir zone. This leads to these altematives: 1) No
“spreading” took place after the deposition of these rocks — and
they are very much older than the Mosfellsheidi Old dolerites. 2)
The “spreading” was only to one side. 3) The “spreading axis”
jumped very recently to its presently postulated position, after hav-
ing moved uni-laterally for a long time. Finally, altemative 1)
could also be by-passing by an assuming an infinite series of com-
binations of the two others.
SUMMARY OF d).
The geological history of the zone between the westem and
eastem dolerites appears to be as follows, in the main outlines.
At the time of the initial faulting at Jómkleif, a shallow lake
probably covered the dolerite in the neighbourhood of the fault, but
subsidence increased at the same time towards the middle of the
depression. Thus a considerable thickness of unporphyritic sub-
aquatic volcanics, some southward transported sediments, and suh-
aerial volcanics were formed, producing the Hátindur plateau series
and the Háhryggur series, now exposed in the westem part of the