Krýsuvík high temperature geothermal area in SW Iceland: Geological setting and 3D inversion of magnetotelluric (MT) resistivity data

Abstract

The Krýsuvík area in SW Iceland is characterized by a 40 km long and around 6–8 km wide fissure swarm and two NE-SW trending tindars complexes. Associated with the fissure swarms are fumarole fields located primarily on parallel postglacial eruptive fissure segments. There are indications that Krýsuvík hosts a buried caldera and an intrusion complex in its roots. The geothermal surface manifestations cover an area of around 50–60 km2 and has potential for future exploitation. Faulting accompanies the ridges and postglacial volcanic fissures, with normal faults and open fissures with a maximum throw of 10 m.A 3D inversion was performed of the static shift corrected off-diagonal impedance tensor elements of 102 MagnetoTelluric (MT) resistivity soundings from the Greater Krýsuvík geothermal area. In the inversion, 21 periods were used, evenly distributed from 0.01 to 100 s on a logarithmic scale. The robustness of the inversion was tested by using three different initial models: A model compiled from a joint 1D inversion of TEM and MT soundings, with a homogeneous Earth of resistivity 100 Ω m and a homogeneous Earth of resistivity 10 Ω m. A flat topographic surface was assumed in the 3D inversion. The resulting models were later elevation corrected. The results of the 1D inversion and the 3D inversion are strikingly similar, where the 1D inversion reproduces the main resistivity structures while the 3D inversion shows considerably more details. An analysis of the electrical strike directions based on the vertical magnetic field data is in fairly good agreement with the final resistivity model.The subsurface resistivity structure in Krýsuvík has the same main features as other high temperature areas in Iceland and in general where the host rocks are basaltic. Above a shallow conductive cap, a high resistivity zone is seen, reflecting unaltered rock. The conductive cap reflects smectite hydrothermal mineral alteration. Below the low-resistivity cap, a resistive core is found, reflecting chlorite-epidote alteration. Good correlation is observed in the Krýsuvík field between the subsurface resistivity structure and the hydrothermal alteration revealed by boreholes cuttings. Parts of the field have cooled down and therefore the resistivity structure indicates alteration mineralogy but not necessarily the present rock temperature. The geothermal up-flow zones are therefore most likely where the hydrothermal alteration and the resistive core reach the highest elevation.The 3D inversion shows an indication of a relatively deep-seated conductive body below the central part of the high temperature area. This body coincides horizontally with the centre of the 2010–2011 inflation source at 4–5 km depth. It has been suggested that the 2007–2016 inflation/deflation periods are linked to gas flux, as no signs of S-wave attenuation have been found. It is therefore unlikely that the deep-seated conductive body resolved by our resistivity models comprises partial melt. The body is probably connected to the heat source of the geothermal field and could be due to emission of gas or dehydration.