Jökull - 01.12.1984, Page 94

12
2
- -
-=aa33-I t 1 b □ i
N S
-THTD U lUJ] Eb □ □ I

30
10
-fl rriT =JH JTn fl-rL n f 0 ! ? V c
_w 0 i E Ikm
■15,
Fig. 17. Cumulative thickness
in meters (T), number of
dykes (N), and percent dila-
tion (D) in four well-exposed
profiles, located in Fig. (2).
(d) means that the profile is
discontinuous. For some
dykes only the minimum
thickness could be measured.
Columns that include such
dykes have a zig-zag line at
the top. nm = no measure-
ments.
Mynd 17. Samanlögð þykkt
ganga í metrum (T), fjöldi
ganga (N) og gliðnun skorp-
unnar í hundraðshlutum (D) í
sniðum númer 12, 2, 9 og 10
(staðsetning þeirra er á mynd
2).
method is unreliable. Second, there is no obvious
reason why the unit chosen should be a kilometer
or a mile, especially since completely continuous
exposures are rarely more than several hundred
meters long. One could as well use 100 m or
whatever as a unit. Third, when the unit of a
kilometer or a mile is chosen, the width of the
swarm is only several units so all detail is lost. I
have therefore used a somewhat different
method.
The method is as follows. Every dyke mea-
sured in the field is located on an air photograph.
A line, approximately perpendicular to mean
orientation of the dykes, is drawn on the photo-
graph (or a transparent overlay) and the dykes
projected on to that line. Then the number and
cumulative thickness of all dykes in a particular
interval on the line are calculated. This interval
can be chosen at will, but I have used 0.5 cm on
the air photograph, which corresponds to about
185 m in the field.
Fig. 17 shows the number of dykes, their
cumulative thickness, and the percent dilation in
104 unit-sections of four widely separated, nearly
continously exposed, profiles. The greatest dila-
tion is 16.7% in one unit of profile 13. Clearly,
the dilation is variable along each profile, which
of course means that the dilation is also variable
across each dyke swarm. For comparison, the
average dilation in the above profiles is about 5—
6%. Because continuously open sections across
whole dyke swarms are lacking, the average dila-
tion of a whole swarm is unknown. But as the
profiles were usually taken where the exposures
were best and where the dykes were most conspi-
cuous, the average dilation across a whole swarm
is presumably much lower than the average dila-
tion in the profiles, probably only a few percent.
DYKES AND NORMAL FAULTS
Although both dykes and normal faults are
very common in the area, the normal faults are
almost never occupied by dykes. Of the 402
dykes observed, it was possible in 116 instances
92 JÖKULL 34. ÁR