Jökull - 01.12.1980, Page 54

number 67 and 36 are very vvide, and they cut
profile 4, but not profile 1.
The fissure swarm dies out towards the east;
no fracture intersects profile 1 f (Fig. 4b), and
no fractures are visible in the lava east of the
Thráinsskjöldur lavas. The total dilation in
profile 10 is nought, notwithstanding the fact
that 6 fractures intersect it. This may seem
strange, but the explanation is that all the
intersection points are narrower than can be
measured from air photos, i.e. less than 30 cm
wide. The maximum possible dilation in this
profiie is therefore less than 2 m. Calculations
show that less than 1/1000 of the area on the
map (Fig. 2) is covered by gaping fractures.
Faults
Only 34 of the 141 fractures in the area are
faults. All measured points, where vertical
displacement is over 0.5 m, are marked by
perpendicular lines on the map (Fig. 2). The
biggest throw, in the Flolocene lava, is 10 m,
on fractures number 67, number 94 and
number 139. Fracture number 105 (Fig. 11),
which has up to 20 m throw, lies for the most
part in Pleistocene lava, hence it might be
older than 10000 years.
The mean of the maximum throw on all the
fractures is 1.2 m, but if only the 34 faults are
considered the corresponding figure is 4.7 m
(Table 1). The only reasonable linear corre-
lation is between maximum throw and
maximum width — as discussed before (Table
2).
As pointed out before, all the faults I have
observed in this area are vertical or slightly
inclined in the reverse trend (Fig. 12). None
has the typical normal fault inclination. All
the vertical faults are classified as normal
faults, as is traditionally done with faults of
this type (Dennis 1972). All the fissures are also
vertical, but none steeply reversely inclined as
some of the faults. The faults are commonly
closed, either along the whole length or on
parts of it (Fig. 11). I measured 616 points on
the faults; 386 or 63% of these are closed or less
than 0.5 m wide. Taking into account the
length and the throw of the faults, those nar-
rower than 0.5 m are considered essentially
closed. That value is also close to the reso-
lution limits on the air photos. Very com-
monly, there are minor fractures in the upth-
row block, and fallen blocks from it are
frequent. Nowhere have I found slickensides.
The walls are generally very irregular (Fig. 8).
Direct observations on the fractures do not
allow one to decide whether the fractures
formed suddenly or by a continuous process.
Flowever, other information is available, as
discussed in the next section.
Fig. 11. A closed fault with a
20 m throw. This fault is a part
of fracture number 105, but here
it dissects Pleistocene lava. The
lava next to the observer is of
Holocene age. — Mynd II. 20 m
misgengi. Misgengið er lokað og er
hluti af sprungu númer 105. Hér sker
misgengið s. k. Keflavíkurgrágrýti,
sem er frá ísöld, en hraunið næst at-
hugandanum erfrá nútíma.
52 JÖKULL 30. ÁR