Jökull - 01.12.1986, Síða 5

(1975) showed that the low-temperature activity in the
eastern part of Árnessýsla represented three major
systems. One of them covers the geothermal activity
in Upper-Árnessýsla (Laugardalur and Biskupstung-
ur). According to Árnason (1976) the recharge area
jies to the north in the mountainous area of the ad-
jacent volcanic zone. One high-tempeature field, the
Geysir field, is located in Upper-Árnessýsla on the
outskirts of the volcanic zone. Chemical geotherm-
ometry and mixing models indicate subsurface tem-
peratures of 260°C (Arnórsson 1985).
geological leatures
The geothermal systems in Reykholtsdalur and
Upper-Árnessýsla are located on either side of the
northern half of the Reykjanes-Langjökull volcanic
zone (fig. 1)5 here termed the Thingvellir-Langjökull
zone (TL-zone). As defined here, the TL-zone
stretches from Lake.Thingvallavatn in the south to the
northern end of the Langjökull ice sheet.
The rocks exposed on the surface in Reykholtsdalur
are Tertiary flood basalts, 4 — 5 million years old. In
Upper-Árnessýsla the rocks are Upper-Quaternary,
sub-glacial hyaloclastites and pillow lavas and inter-
glacial basalt lavas. Minor outcrops of acid volcanics
occur in the vicinity of the Geysir geothermal field.
The main zone of volcanism and rifting in Iceland
shifted from Snæfellsnes to the position of the
TL-zone 6 — 7 Mya (Jóhannesson 1980, Óskarsson et
al- 1985). Lava flows originating in the TL-zone rest
unconformably upon the flood basalts belonging to
the older Snæfellsnes zone. The Hredavatn sediments,
located about 10 km northwest of the Reykholtsdalur
field, separate the two flood basalt sequences.
Dilation in the TL-zone is vanishing conjuncture
with the propagation of the eastern volcanic zone in
southern Iceland (Óskarsson et al. 1985). Fissuring is
still active in the southern extreme of the TL-zone but
it seems to have terminated in the northern part.
Volcanic activity has been relatively limited within
the TL-zone during post-glacial times. The total
number of eruptions, or more precisely lava fields, is
eleven. The latest eruption occurred about 1200 years
ago and formed Hallmundarhraun (Sœmundsson
1966). In late Quaternary times several volcanic erup-
tions have occurred outside the TL-zone proper as
mdicated by young volcanic edifices resting uncon-
formably upon older volcanics. Eruptions of this type
have, for example, formed Hvalfell, Kvígindisfell and
Reydarfell (fig.2).
Normal faults with downthrow of hundreds of
Fig. 1. Location of high-temperature geothermal
fields in Iceland and distribution of low-temperature
activity in Borgarfjörður and the Southern Lowlands
(based on Sœmundsson and Fridleifsson 1980). Loca-
tions of the Reykholtsdalur and Upper-Árnessýsla
geothermal systems within these areas are indicated.
Figures for stored heat (in the uppermost 3 km) in
high-temperature fields are from Pálmason (1980). —
Staðsetning háhitasvœða á íslandi og útbreiðsla lág-
hita í Borgarfirði og á Suðurlandsundirlendi (byggt á
gögnum frq Kristjáni Sœmundssyni og Ingvari B.
Friðleifssyni 1980). Sýnd er sérstaklega lega jarðhita-
kerfanna í Reykholtsdal og í ofanverðri Árnessýslu.
Tölur um varmaforða (í efstu 3 km) í háhitasvœðun-
um eru fengnar frá Guðmundi Pálmasyni (1980).
meters towards east occur near the western margin of
the TL-zone (Kjartansson 1943, Scemundsson 1967,
Jóhannesson 1980). The sagging of the volcanic zone
as indicated by these normal faults and the young vol-
canism outside the zone could be explained by dis-
placement of material — magma — from underneath
the volcanic zone and into the older rocks flanking it.
Pálmason (1971, 1973) observed anomalously high
heat flow near the Reykjanes-Langjökull volcanic
zone in SW-Iceland; thermal gradients in shallow
holes were in the range 129 — 1657km. His data and
later data which fall within the same range (Sœ-
mundsson and Fridleifsson (1980) cover an area on
both sides of the southern half of the TL-zone and for
some 20 — 30 km farther south. High heat flows like
those observed on both sides of the volcanic zone in
SW-Iceland do not occur around the eastern volcanic
zone. Pálmason (1973) explained these observations
3