Náttúrufræðingurinn - 1985, Blaðsíða 30

SUMMARY
A Reconstruction of the
Evolutionary History
of the Crust Around
Reykjavík with Special
Emphasis on Fissures
by
Páll Imsland
Nordic Volcanological Institute
Reykjavík
An attempt is made in this article to recon-
struct the fissure swarm - central volcano activ-
ity and the crustal generation of the Quaternary
time on the western flank of the active rift zone
in southwestern Iceland. This is done in order
to find out about the breaking-up, fissuring and
faulting of the so-called gray basalts.
The crust of this area is composed of four
stratigraphical units: 1) Tertiary basalts under-
lying the area and exposed north of it, 2) Early
Quaternary rocks — lavas and intercalated
hyaloclastites, 3) Late Quaternary and postgla-
cial rocks — lavas and surface hyaloclastites, 4)
the Late Quaternary gray basalts which occupy
the stratigraphical position between the other
Late Quaternary rocks and the Early Qua-
ternary ones.
This crust is generated in seven known fissure
swarms. Two of these are from Early Quater-
nary time and both of which evolved into central
volcanoes, the Kjalarnes- and the Stardalur cen-
tral volcanoes. They are now extinct and have
drifted away from the site of active crustal
generation. Their rock formations are now
found in the northern part of the region. At the
time of their formation, crustal generation
further south is considered to have been off
shore and mainly submarine. The other five
fissure swarms are still active. They have not
evolved to the really advanced central volcano
stages. One of these, the Hengill fissure swarm,
shows most of the characteristics of central vol-
canoes while the other ones, those on the Reyk-
janes peninsula, only show some of them. There
is a time-gap between the Early Quaternary
fissure swarm - central volcano systems, which
became extinct 2.1 and 1.6 m.y.a. respectively
and the presently active systems which became
active less than 0.7 m.y.a.
The Early Quaternary systems now drift inac-
tive towards west „pushed“ by the presently
active Hengill system. Between these the third
and youngest extinct system might possibly be
located, a system that would account for the
time-gap mentioned. No such system is though
known, and if it exists it would be buried below
the so called gray basalts.
The Late Quaternary gray basalts are mostly
relatively primitive tholeiites, erupted by subae-
rial shield volcanoes, which are not directly
related to any fissure swarm. They are less than
0.7 m.y. old, but older than the last and up to
several of the latest glacial periods. The
apparently high productivity of these gray
basalts at this particular time, might be a total or
partial substitution for a fissure swarm that
failed to evolve during the time-gap mentioned,
or the result of its delay. At the time of forma-
tion these gray basalts flooded an area pre-
viously faulted and rifted by the, at the time,
extinct Early Quaternary fissure swarms, but
they left an unfaulted and unrifted ground-sur-
face, as they themselves are not produced by a
rifting volcanism.
At present, though, the gray basalts are both
heavily faulted and fissured. The question is
asked, when this did happen. The interpretation
is forwarded, that this is the work of the pre-
sently active fissure swarms on the Reykjanes
peninsula, the Reykjanes-, Svartsengi-, Krísu-
vík- and Brennisteinsfjöll fissure swarms. These
fissure swarms are situated to the south of the
area of the gray basalts and produce lavas that
tend to bury and overflow the gray basalts at
their southern end. These swarms have a NE-
SW orientation of fissures. As these fissure
swarms evolve, their fissures extend into the
gray basalts and break them up. They may even
reactivate some of the old fissures of the extinct
Early Quaternary fissure swarms.
Two postglacial, Cl4-dated lavas have flooded
the gray basalts and their fissures in favourable
places. In the extention of the Krísuvík fissurc
swarm, the southernmost of the lavas, Búr-
fellshraun 7200 yr. old, is broken and faulted by
about 7 m. Around 7 km further towards NE
the other lava, Elliðaárhraun 4600 yr. old, cros-
ses the Krísuvík fissure swarm unbroken and
unfaulted. Further towards E the same lava is
fissured by the Brennisteinsfjöll fissure warm.
This shows, that the gray basalts are being
faulted and broken by the presently active fis-
sure swarms. Whether the fissuring activity of
these active fissure swarms is increasing or has
already reached its maximunt is unknown. It is
thus difficult to use the present knowledge to
predict future fissuring of the ground in the
Reykjavík area.
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