Rit (Vísindafélag Íslendinga) - 01.06.1967, Síða 38

34
ed a period of peneplanation. Finally block-faulting and dif-
ferential subsidence took place along faults which delineate
the present gravity field of the country.
Bemmelen and Rutten (1955) consider Iceland a horst in
the submerged Thule basaltic province, and suggest that the
Icelandic area is raised above sea level by virtue of its posi-
tion on the Mid-Atlantic Ridge. They suggest that the ele-
vated position of the plateau “created a stressfield with com-
pressive stresses at the foot of the edifice and tensional stres-
ses in the top part” — which — “created swarms of fissures,
which were the pathways for the effusion of basaltic lavas”.
In Pleistocene the horst collapsed into a number of blocks
which were subjected to antithetic rotation and, according to
Bemmelen and Rutten, somehow obtained a toe-hold on the
rocks flooring the surrounding ocean basins, thus escaping
subsidence.
A new hypothesis of the building of the Icelandic basalt
pile, incorporating crustal drift, was put forward by Bödvars-
son and Walker (1964), based on the geological observations
of the latter in eastern Iceland and interpretation of geophy-
sical data. The Tertiary basalts in the easternmost part of
Iceland is a 10 km thick formation, dipping on the average
6° westerly. Each group of lavas shows up-dip thinning, re-
sulting in somewhat lower dips at the mountain tops. NNE-
trending dykes are numerous in the Tertiary basalts and
many are probably feeders of the lavas in fissure eruptions.
In Reydarfjördur, in a 53 km long E—W profile, dykes have
an aggregate thickness of 3 km, or 6% of the length of the
profile, and in Berufjördur dykes are 2.3 km of the length of
the profile, also 6%. Dyke intrusions have thus increased
the horizontal width of the base of the profile by 6% in the
above areas. Assuming that most of the dykes are feeders of
lavas a lava pile with cross-sectional area of 80 km2 re-
quires dykes of 3 km total thickness. Normal faults can also
cause laternal extension, amounting to 1% in the Reydar-
fjördur area according to Bödvarsson and Walker.