Jökull - 01.12.1986, Page 9

the area the temperature in an aquifer at 957 m depth
is 157°C (Arnórsson et al. 1983). Thus, temperatures
overlap with those found in some low-temperature
fields. Alteration mineralogy as described by Sig-
valdason (1963) indicates higher formation tempera-
tures than measured in drillholes at present. Hydro-
thermal alteration at depth in the Laugardalur
(Reykjavík) and Reykir (Mosfellssveit) low-tempera-
ture fields studied by Sigvaldason (1963) and Gunn-
laugsson (1977) bears withness of temperatures of
around 250”C. Such temperatures may have prevailed
some 1 Mya as deduced from geology. At present
maximum drillhole temperatures in Reykjavík and at
Reykir are 160°C and 100°C, respectively.
The evolution of the Reykir system is complicated
by the fact that the fossil high-temperature system has
drifted into the northeast end of the presently active
Kn'suvík fissure swarm. Recent fracturing has, in all
likelihood, created secondary permeability. One may
speculate that magmatic heat .has been supplied to the
system in recent times through lateral dyke injection
along the fissure swarm. If this was the case the
Reykir low-temperature field would have a magmatic
heat source just as is suggested for Borgarfjörður and
Upper-Árnessýsla. This discussion on the Reykir
geothermal field shows that the history of low-tem-
perature systems can be quite complicated and that a
smgle model cannot be expected to be applicable to
all such systems.
In view of the vicinity of the Upper-Árnessýsla
geothermal system to the volcanic zone and the young
age of the surface formations, it is conceivable that
this system represents a cooling high-temperature
system. The interpretation of the high hydrogen sul-
phide concentrations in the hot springs at Laugarvatn
mdicate though that magma is at present replenishing
the heat source. The geothermal activity in Reyk-
holtsdalur could, on the other hand, hardly represent
a cooling high-temperature system due to its distance
from the volcanic zone.
SUMMARY
Review of various geological, geophysical and
geochemical studies in the foregoing sections has
shown that:
1- The heat output of the Reykholtsdalur and the
Upper-Árnessýsla geothermal systems is very
large and confined to relatively small areas
(50 — 100 km2). It is comparable with that of many
high-temperature fields.
2. Dilation seems to be vanishing in the TL-zone
and volcanic activity has been limited in post-
glacial times. The diminishing tectonic/volcanic
activity in the TL-zone is considered to coincide
with the southward propagation of the eastern
volcanic zone which started about 1 Mya.
3. Several volcanic eruptions have occurred in
Upper-Quaternary and Recent times outside the
Reykjanes-Langjökull volcanic zone.
4. Near surface thermal gradient on both sides of the
Reykjanes-Langjökull zone is higher than pre-
dicted by the kinematic volcanic rift zone model
of Pálmason (1973).
5. High-temperature geothermal activity is almost
absent at present in the TL-zone.
6. The high hydrogen sulphide concentrations in hot
spring discharges at Laugarvatn, Upper-Árnes-
sýsla, are taken to indicate a magmatic source to
the geothermal system.
7. Subsurface temperatures overlapping with those
found in some high-temperature fields are indi-
cated by chemical geothermometry for a few
geothermal systems located in Quaternary and
Tertiary rocks flanking the Reykjanes-Langjökull
volcanic zone.
8. Flow of water from the central highlands to the
low-temperature fields need not be by deep
ground water circulation. In some instances the
flow could be on the surface.
9. Drillhole data show that some of the low-tem-
perature systems are convection systems. The
convection is driven by the pressure difference
exerted by the hot water column of the upflow
and the denser cold water column in the down-
flow zone. Hydrostatic head is not required to
sustain flow.
10. Alteration mineralogy in a few geothermal
systems in SW-Iceland indicates that high-tem-
perature systems may develop into low-tempera-
ture ones conjuncture with the drifting of the
former out the volcanic zone.
Based on the above listed results the following con-
ceptual models are envisaged for the Reykholtsdalur
and the Upper-Árnessýsla geothermal systems: As a
consequence of diminishing dilation in the TL-zone
magma rising under this zone tends to be diverted into
older crust on both sides. Discrete events involving
migration of magma batches lead to eruptions or the
formation of intrusive bodies. Geothermal systems,
like Reykholtsdalur and Upper-Árnessýsla, develop
over the intrusions while cooling.
In the case of Reykholtsdalur development of strain
in the crust has led to relatively recent earth move-
ments on already existing faults. The secondary
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