Geological and structural mapping of the Midfell-Flúdir low-temperature geothermal field, S-Iceland

Abstract

Geological, structural and surface alteration mapping was carried out in the Flúdir- Midfell low-temperature geothermal field in South Iceland. A Plio-Pleistocene interglacial and glacial bedrock succession is made of three units of hyaloclastite, and two units of basaltic lava flows, with intercalated sandstone and tillite. Fluvial silt and gravel was deposited during a marine transgression in early Holocene time. Structurally, the bedrock of the area is steeply tilted to the northwest and strikes about N 40° E. It is intersected by a number of normal faults of mostly NE-SW trend with downthrows of several tens of metres, most of them to the southeast in a step fault pattern. Fractures of similar strike control most of the geothermal manifestations at Flúdir and Grafarbakki. The low-temperature zeolitization of the bedrock indicates that at least 300-400 metres have been stripped off by erosion from the original surface of rock pile. From palaeomagnetic dating and available radiometric age dating in the area, the age of the rock is estimated to be about 2 m.y. old. A soil temperature survey was carried out in a sedimentary covered geothermal field at Grafarbakki garden. The geothermal mapping indicated lateral flow from a NESW fracture underneath a 20 m high river bank of sediment. The geothermal model of the drilled area is consistent with the existence of a structurally controlled lowtemperature geothermal reservoir at about 400 m depth.