Geology and geothermal considerations of Krisuvik valley, Reykjanes peninsula, Iceland

Abstract

A geological and geothermal alteration mapping over about 20 km2 in the Krisuvik valley, Reykjanes Peninsula, revealed an Upper Pleistocene succession of interglacial basalts overlain by pillow lava and hyaloclastites. The Postglacial volcanic rocks are characterized by hydrothermal and phreatic explosion debris, pyroclastic scorias, welded lavas and basaltic lava flows. Some areas show an intense geothermal alteration; a zeolite zone, lacking in deep wells, was found at the surface. From the shape of the en echelon structural array on the Reykjanes Peninsula, a shear strain model is proposed. The trend of the seismic zone in the Reykjanes Peninsula (Az 80) leads to the formation of extensional fissures at 45 (Az 35), and to a maximum horizontal compressive axis perpendicular to them (Az 35). The maximum compression would produce a conjugate set of strike-slip faults on both sides at 30 with itself (Az 5 and 65). Normal to maximum compression minor jointing would also be expected. This model matches well with the surface geology in the Krisuvik area, where the main faults and fissure eruptions can be interpreted according to it. Also, almost all the focal mechanism solutions determined for the peninsula behave according to the trends predicted by the model, taking into account some local geological conditions. A hydrogeological interpretation is proposed for the widespread thermal reversal in the geothermal reservoir, assuming an extended cold water percolation from lake Kleifarvatn, through fissures and two possible maars in the lake.