Jökull - 01.12.1980, Page 55

Fig. 12. A slightly reversely
inclined fault. Next to the
observer the width is 6 m, but
about 2 m where the reverse
inclination is seen. Visible depth
is 6 to 8 m. — Mynd 12. Mis-
genginu hallar í öfuga átt við venjuleg
siggengi. Nœst athugandanum er
breidd sprungunnar 6 m, en 2 m fjœr.
Sýnilegt dýpi er 6 til 8 m.
Development of the fractures
When the fractures are observed in detail
(on air photos) some important features conre
to light. Each fracture can be divided into
separate parts, according to orientation and
length. The longer parts usually have the same
main orientation, and are arranged en eche-
lon. The shorter parts have slightly different
orientations, and form the connections bet-
ween the longer parts. The major fractures
have grown along these connections. The
connections can often be distinguished from
the main fractures by the minor fractures that
are commonly associated with them (Fig. 13).
These minor fractures are in fact the ends of
the longer parts, and have similar orientation
as they do.
The development of each fracture seems to
follow the same definite pattern as do tensile
joints in experiments (Beloussov 1962). The
main stages are as follows: (1) Intially very
small, unconnected primary fractures form.
They have an en echelon arrangement within
a relatively narrow zone (Fig. 13). (2) Because
of stress concentrations near the terminations,
these fractures grow at both ends, in the
direction of strike. (3) As they grow, the dis-
tance between the ends decreases, but, due to
I’ig. 13. Development of frac-
ture number 64. State 1 is oldest
and state 4 is youngest. Today
the fracture looks like state 4. —
Mynd 13. Þróun sprungu nútner 64.
Stig 1 er elsl og stig 4 er yngst. St.ig 4
sýnir hvernig sprungan lítur út í dag.
JÖKULL 30. ÁR 53