Jökull - 01.12.1984, Page 73

Fig. 4. J110 as a function of
altitude above sea level. Data
grouped at various altitude
ranges in each region, as
shown by short bars above
abscissae.
4. mynd. Segulmögnunar-
styrkur hrauna sem fall af
hœð yfir sjávarmál á ýmsum
svœðum á íslandi.
N- ICELAND
PA - PG
I I
I
I
SW - ICELAND
FA - SC
I
52
113 91 65 38
53
72
62
Altituda, m
0-1----------1---------------------1----------1------------1--
200 400 600 800 1000
--------------'----r-1------1---------.----»
0 200 400 600 800 m
i
I
I
I í
NW-ICELAND
SK-JF, SR -SM
W-ICELAND
NP - NT
NW- ICELAND
BA-BX
intensity observed within some lava flows (Wil-
son et al. 1968), one may only be able to state at
present that the correct primary remanence value
to use in anomaly interpretation over exposed
Miocene to Lower Quaternary basalts in Iceland,
is likely to be between 2.5 and 5 A/m.
For comparison, magnetic data from 30 dikes
in various areas of Iceland (L. Kristjánsson, in
prep.), have been obtained. The mean reman-
ence intensity of these after 100 Oe treatment is
2.1 A/m, considerably lower than the value
quoted above for lavas. However, the relative
fraction of original remanence removed by this
treatment seems to be somewhat higher than in
lavas, so that for anomaly interpretation one
should probably use an estimated value of Jr =
2.5-3 A/m, over unknown dikes outside the
present volcanic zones.
The average susceptibility of the above 30
dikes is 2.4-10 3 cgs volume units, similar to that
for lavas (see Fig. 1). Viscous remanence in the
dikes may however be considerably higher than
in lavas, or 1—2 A/m on average; this would help
explaining why individual dikes tend to give posi-
tive magnetic anomalies, and why fluxgate polar-
ity measurements on dike samples seem to be less
reliable than those on lavas. Piper (1973) quotes
somewhat higher mean N.R.M. intensity and sus-
ceptibility values than these.
PROBLEMS IN CORRELATION
BETWEEN DIFFERENT AREAS USING
POLARITY STRATIGRAPHY
The uncertainties in polarity determination
using fluxgate measurements in the field are com-
pounded by additional uncertainties when it
comes to correlating the resultant polarity col-
umns between areas.
It must then be kept firmly in mind that a) no
particular polarity zone is in any way distinct
from others b) thickness of zones may well vary
laterally, e.g. as a result of general down-dip
thickening of lava groups, or because of inter-
fingering of lavas from different volcanic centers
of variable productivity c) given any two sets of
polarity zones of similar average thickness per
zone, it is generally possible to demonstrate a
convincing fit between these after shifting one of
them slightly up or down.
Fig. 5 illustrates the last of these pitfalls. Here
we have made use of the fact that lengths of
geomagnetic epochs have an approximately ex-
ponential frequency distribution, and thus resem-
JÖKULL 34. ÁR 71