Structure and activity of the geothermal field of Hvalfjörour (Iceland) from brittle tectonic, geothermal and paleostress analysis

• Published in: Bulletin de la Société Géologique de France
• Main Authors: Bergerat, Françoise, Saemundsson, Kristjan, Fourel, Loïc, Angelier, Jacques
• Other Authors: Institut des Sciences de la Terre de Paris (iSTeP), Université Pierre et Marie Curie - Paris 6 (UPMC)-Centre National de la Recherche Scientifique (CNRS), Iceland GeoSurvey (ISOR), Géoazur (GEOAZUR 6526), Institut de Recherche pour le Développement (IRD)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Université Nice Sophia Antipolis (1965 - 2019) (UNS)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de la Côte d'Azur, Université Côte d'Azur (UniCA)-Université Côte d'Azur (UniCA)-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2013
• Subjects: [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; Hvalfjörður; Iceland
• Online Access: https://hal.science/hal-01452861; https://doi.org/10.2113/gssgfbull.184.4-5.451

International audience This paper presents the results of brittle tectonic, palaeostress inversion, and hydrothermal mineralisation studies of the Hvalfjörður low-temperature geothermal field in Southwest Iceland. This geothermal field (including two pronounced thermal anomalies) is located in the highly altered core area of an extinct and deeply eroded Tertiary central volcano. Most of the geothermal water appears to be conducted by vertical extension fractures. Palaeostress analysis indicates a rather complex stress history, with four major trends of extension involving normal and strike-slip faulting modes as well as dyke injection. Analysis of the data on the relative chronology indicates that these four regimes were closely intricate in time and space. The most important regime is a NW-SE, rift-perpendicular extension related to the oceanic rifting in Iceland. This trend partly controls the past (Pliocene-Early Pleistocene) hydrothermal activity in the Hvalfjörður area; however, an E-W rift-oblique extension also occurred with a N-S trending fracture system including normal faults, dykes and veins that show higher levels of hydrothermal mineralisation. Currently, there is strong hot-water convection, producing a thermal anomaly, in this N-S-trending fracture system. Our study highlights the paleostress evolution and the development of fracture systems in Hvalfjörður, including the past geothermal history; nevertheless the most efficient tool in geothermal prospection in such complex area remains the shallow geothermal survey. It has proved successful in many localities in Early Pleistocene to Miocene rocks where no surface indication of geothermal activity exists.