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Maryam Khodayar, Iceland GeoSurvey (Iceland)
Páll Einarsson, Institute of Earth Sciences, University of Iceland (Iceland)
Hjalti Franzson, Iceland GeoSurvey (Iceland)
Sveinbjörn Björnsson, National Energy Authority (Iceland)
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Low-temperature geothermal activity in Iceland, with temperatures up to 150°C, occurs outside the active rift zones. Boreholes have been successfully drilled in these areas and the heat is harnessed extensively. We investigate the distribution of the geothermal occurrences with respect to the plate boundaries and relocation of the rift zones. We conducted surface geothermal explorations in the highly fractured crust of the South and West Iceland rift-jump blocks. These crustal blocks were formed at the divergent plate boundaries but shifted away during rift jumps and transform faulting between 15-6 Ma (West Iceland), and 3 Ma-present (South Iceland). Furthermore, intra-plate earthquakes (M 6) and earthquakes in the transform zone (M 7) occur in West and South Iceland, respectively. We used GPS to locate 244 springs in the Holocene soil and sediments, and 23 exploration and production wells in these two blocks, with temperatures ranging from 10.3°C to 96.8°C. A correlation between the alignment of these manifestations and the surrounding mapped faults, dykes, mineral veins, and surface ruptures during earthquakes shows that: (a) Contrary to the rift zones where extensional NNE rift-parallel fractures are the dominant upflow channels, former rifting structures do not seem to control the primary tectonic settings of the springs in either of the two rift-jump blocks; (b) In these two blocks, the springs are aligned along 6 fracture sets formed under simultaneous rifting and horizontal shearing within transform zones. In the younger South Iceland rift-jump block, the springs leak from fractures that are dominantly northerly and east-northeasterly, i.e. the main set of dextral and sinistral conjugate strike-slip faults that are responsible for the present earthquake activity in the South Iceland transform zone. In the older West Iceland rift-jump block, the springs are aligned on northerly and NW fractures, as well as on NNE and WNW fractures. However, near the site of the 1974 Borgarfjordur intra-plate earthquakes, the springs show a prominent ENE/E-W alignment similar to the basement fractures reactivated during those earthquakes; (c) Often, the hottest springs and the most discharge appear at the intersection of two or more fracture sets; (d) While basement fractures are reopened during earthquakes to host the springs, the heat source may be underlying dykes injected into the fractures assuming that they are sufficiently close to magmatic sources in the rift zones.
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