Jökull - 01.12.1984, Blaðsíða 108

dykes which acted as barriers to the regional flow
of hot water through the basalt lava pile from the
highlands in east. Einarsson (1942) proposed that
the thermal water of the low temperature systems
was of meteoric origin; precipitation which has
fallen on the highlands. The water percolates
deep into the bedrock where it is heated by
regional heat flow, before it ascends to the sur-
face. This model was later strongly supported by
the deuterium studies of Árnason (1976).
Thorkelsson (1940) measured the radon con-
tent of the thermal waters in the Reykholtsdalur
valley. The radon content increases from west to
east culminating in the hot springs at Reykholt.
He concluded that the Reykholt hot springs were
close to the main upflow of hot water but the
decreasing content of radon away from Reykholt
could be explained by longer flow distances from
the main upflow.
In the year 1964 a systematic mapping of the
hot spring distribution and geology was initiated
at the National Energy Authority (NEA). First
the thermal manifestations and the main geologi-
cal features were mapped and later geophysical
and chemical studies were carried out
(Saemundsson et al. 1966, Sigvaldason 1966)
Additional detailed geological mapping has
been conducted almost continuously since 1971
under the supervision of Kristján Saemundsson
at NEA, partly summarized by Saemundsson and
Noll 1974, Jóhannesson 1975 and Franzson 1978.
Geophysical and chemical studies have been car-
ried out since 1975, both on a regional scale
(Gunnlaugsson 1980, Jóhannesson et al. 1980)
and detailed studies on individual thermal fields
and systems (Georgsson et al. 1978, 1981a and
1981b, Jóhannesson et al. 1979). A considerable
part of the data accumulated is still unpublished.
In this paper we present an overview of the
Reykholt thermal system based on both the
results of regional studies and detailed studies of
individual thermal fields.
THE GEOLOGY OF THE
BORGARFJÖRDUR REGION
The basement of the Borgarfjördur region con-
sists of Late Tertiary basaltic lava flows. The axis
of the Borgarnes anticline (Saemundsson 1977)
runs NE-SW and marks the western margin of
geothermal manifestations (Fig. 1). East of the
anticline axis the lavas dip 6-10°SE, towards the
active Reykjanes-Langjökull rift zone. The anti-
cline axis plunges towards the northeast and
disappears underneath the Hredavatn uncon-
formity north of lake Hredavatn. The Hreda-
vatn unconformity represents a major gap in the
lava succession. The flows below the uncon-
formity are believed to be as old as 13 million
years and have been extruded within the former
Snaefellsnes rift zone (Jóhannesson 1980). These
older lavas were tilted and denuded before being
covered by the Hredavatn sediments which sub-
sequently were covered by younger lava flows
extruded within the Reykjanes-Langjökull rift
zone. The oldest flows above the Hredavatn sedi-
ments have been dated 6.5-7.0 m.y. old
(McDougall et al. 1977). The flows become gra-
dually younger on approaching the rift zone.
There is a great variety of faults and joints in
the Borgarfjördur region. Jóhannesson (1980)
suggested that they are of two different origins.
Firstly are NE-SW trending fault swarms corres-
ponding to the fissure swarms of the active rift
zone which are believed to have formed by crus-
tal tension inside the rift zone. The fault swarms
are accompanied by dyke swarms. Secondly,
there are faults formed in a stress field character-
ized by lateral shear forces. They belong to the
Snaefellsnes Fracture Zone which stretches from
the Snaefellsnes peninsula in the west to the
Borgarfjördur region in the east. The faults of the
fracture zone can be divided into 3 groups, based
on their trend and age: NW-SE, N-S and
NE-SW. The fracture zone was active at about
8-13 m.y. ago, but minor movements have con-
tinued up to Postglacial times. The dykes and
faults are usually subvertical and those oriented
parallel to the strike of the lavas usually transect
the lavas at right angles.
THE DISTRIBUTION OF HOT SPRINGS
The Reykholt thermal system comprises the
following major thermal fields: Klettur-Runnar,
Deildartunga-Kleppjárnsreykir, Hurdarbak-
Sídumúli, Vellir (including Sturlu-Reykir),
Sudda, Reykholt-Kópareykir, Haegindi, Nordur-
reykir-Háafell, Úlfsstadir, Stóriás, Brúarreykir,
Lundar and Helgavatn. The total natural dis-
charge has been measured to be about 425 1/s,
with thermal output equivalent to about 400 1/s of
boiling water.
The total natural discharge of all low tempera-
106 JÖKULL 34. ÁR