Greinar (Vísindafélag Íslendinga) - 01.01.1976, Síða 108

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have the vertical displaœments that in some places reach many
hundreds of meters and even exceed a kilometer. The axial parts
of the rift zones are depressed as regards the uplift zones framing
them, hy, possibly, several kilometers. Besides block structures, the
transition zones also contain gentle folds. The Borgames anticline
to the north of Reykjavik is typical. Its gentle crest (with the dip
of layers 3-5°) is traced in the north-eastem direction along the
Nordurár-valley. It is disturbed along its axial part by a graben
and also by numerous longitudinal, transversal and diagonal faults
and the blocks limited by them are dislocated in relation to each
other in the vertical direction by the amplitude of a few tens of
meters.
Volcanic rocks of different age are irregularly distributed in the
rift zones. Though these irregularities are still inadequately studied,
it is now evident that in the eastem rift zone the total thickness
of the Pleistocene and Holocene rocks is larger in the middle of the
island and decreases to its northern and southern margins. An espe-
cially considerable reduction of the thickness of these rocks is found
in the south, where the strata of hyaloclastites, breccia and thin
basalts flows, stratigraphically referred to “ancient grey basalts”,
i.e. the Pliocene and Eopleistocene, is traced in the cliffs of the
southem coast from the east for a considerable distance into the
rift and serves as a basement for recent volcanic formations.
Geological history of Iceland
The most ancient rocks of Iceland are found, as was already men-
tioned, to be as old as 16 million years and belong to the Miocene.
Much more ancient plateau-basalts 60 million years old (Early
Eocene), but quite similar in composition and conditions of forma-
tion with those developed in Iceland, are known in Greenland and
on the Faroe Is. (33, 53). In both places the plateau-basalts are
tilted towards the sea. On the eastern coast of Greenland a steep
flexure is observed in the basalts. It can, therefore, be stated that
the Eocene plateau-basalts cover the bottom of the Denmark Strait
and the surface of the submarine Faroes-Iceland threshold. This
interpretation of the observed geological structure is presented in