Jökull - 01.12.1986, Síða 20

5 E2£]
6 iH ^ •
I--------------------- 9 ®
3.
4.
5.
6.
Faxaflói flowed south across the western part of the
peninsula. During the last glacial period the moun-
tains were step by step built up to 300—400 m asl.
(see fig. 8) and a glacial cap was formed on them, clos-
ing the way to the south for the Faxaflói glacier. Gla-
cial striae indicate that towards the end of the last gla-
cial period the peninsular glacier got the better off
against the Faxaflói glacier. The latter must earlier
have been dominant, as pieces of rhyolites, obsidian
and dark compact tholeiites are found in patches of
moraine on the Rosmhvalanes peninsula, probably
originated in the Hvalfjörður area, since such rocks
are not known on the Reykjanes peninsula. At the
bottom of those moraines small pieces of granite,
gneiss and quartzite have been found (described in de-
tail by Kuhtan 1943). They are possibly remnants
from polar ice drift into the Faxaflói bay at the begin-
ning of the last glacial period or stadial, the nearest
sources of such material being in Greenland or even
Spitzbergen.
The tectonic pattern resembles the volcanic one
very closely (figs. 3 and 5). Open fissures and recently
acticve faults occur on zones almost identical with the
volcanic systems. The direction of the fissures is
usually 30—40° E. Fissures, and other tectonic linea-
ments, with a more northerly direction, 0—20° E, are
found in the northern, seismically active part of the
Fig. 8. Glacial sequences on the
Reykjanes peninsula. Signs of glaci-
ation and icefree periods alternate in
the stratigraphic sequence (Sigurðs-
son 1985). Mynd 8. Ummerki jökul-
skeiöa og hlýskeiða. Víða má finna
merki um jökulskeið og þíðskeið í
jarðlagastaflanum (Freysteinn Sig-
urðsson 1985).
E x p I a nations/Skýringa r:
1. Postglacial lavas / Nútímahraun.
2. Coastal sediments / Strandset.
Lava caps and covers, older than
Holocene / Basalthraun og
-hettur, eldri en frá nútíma.
Erosion during ice free periods /
Þíðskeiðsrof.
Tillites / Jökulberg.
Hyaloclastites and pillow lavas /
Móberg.
7. Glacial striae / Jökulrákir.
8. Glacial erosion / Jökulrof.
9. Reverse polarization/öfug segul-
stefna.
inner volcanic zone, in small apparently isolated
areas. Where they are crossed by volcanic strips, sur-
face manifestations of geothermal activity have been
observed, except in the Fagradalsfjall area, where
geothermal activity is however indicated by the
geochemistry of the freshwater (Sigurðsson et al.
1978) and geoelectrical resistivity (Georgsson 1979,
Georgsson & Tulinius 1983). The fissures have a
more easterly trend in the border zones of the volcanic
zone. North of the volcanic zone discontinuous fissure
zones appear, orientated near to 60° E, and situated
on the extensions of the volcanic systems.
The displacements on the faults are usually small or
only a few metres. The faults are discontinuous, result-
ing in local grabens, that are often arranged as a chain
in the main tectonic direction. The extensions of the
fissures at the surface have been measured on some
profiles, the greatest width of a single fissure being
near to 2.5 m. Stones dropping from the fissure walls
tend to fill the fissures with loose blocks, so that only
very few of them remain open down to the ground-
water level so they make excellent flow channels and
provide a strong element of anisotropy in the perme-
ability of the bedrock. The rate of extension (dilation)
amounts to 1 — 6 m/km at the surface on the measured
profiles. An analysis of the width of the fissures shows
that it is one-sided normally distributed, at least in the
18