Jökull


Jökull - 01.12.1984, Side 108

Jökull - 01.12.1984, Side 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 A considerable part of the data accumulated is still unpublished.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
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