Rit (Vísindafélag Íslendinga) - 01.06.1967, Qupperneq 134

130
N 50 km wide. It seems significant that
/t'> / this width is commensurate with the
thickness of solid crust, as would be ex-
pected for a wedge-like shear zone (top
of the wedge at base of crust), whereas
such coincidence would be quite mean-
ingless if these zones were the result of
drift normal to the strike of the zones.
Thus the idea presents itself (Trausti
Einarsson 1950 and 1963) that in the
second tectonic phase the country was
broken into blocks along shear-planes,
and that recent volcanic activity reflects
continued movement along the seams be-
tween certain blocks at that time.
The next step, and the aim of the
present contribution, is to consider more
closely individual fractures in the recent
zones. One then finds in a number of
cases certain significant details which
lead to a distinction between principal
fractures (P-fractures, P-directions) and
secondary fractures (S-fractures, S-directions) which are
genetically related to the first ones. The long principal frac-
tures that as a whole are straight and appear to be continu-
ous are in reality a chain of shorter stepped fractures with a
somewhat different direction.
Fig. 2 shows a principal fracture near farm Hóll, west of
Undirveggur in Kelduhverfi, NE-Iceland. It is a chain of
Fig. 2. Principal frac-
ture at Hóll in Keldu-
hverfi, composed of
stepped fissures. Ar-
rows indicate the in-
ferred movement
along the deep hidden
principal fracture.
q_______________________t______<«0m
Fig. 3. Schematic picture of stepped fissures in Hrafnagfá at Vatnsleysa.
a — principal direction; b — secondary direction.