Two-dimensional magnetotelluric modelling of the Kos Island geothermal region (Greece)

Abstract

The magnetotelluric (MT) method in the period range 0.075–42 s was applied to determine the electrical structure (<8 km) of the Kos Island geothermal region and delineate the tectonic features at this part of the Hellenic Volcanic Arc (HVA). Nineteen MT soundings were conducted on the western half of the island and the collected MT data were found to be two-dimensional (2-D). The NNW—SSE (30°W) direction was identified as the dominant electrical strike. This direction indicates tectonic activity compatible with major extensional episodes occurred during the Pleistocene. 2-D forward calculations resulted in an electrical model of the upper crust of the area. This model resolves a low resistivity (3.5–7 ohm m) feature under the central part of the island at depths of 250–3000 m, bounded by two more resistive (20–100 ohm m) features. The conductor correlates with a low gravity anomaly observed in this area which corresponds to the Kos Graben. The dimensions of the graben are estimated to be those of the conductor. The relatively low resistivities (3.5-100 ohm m) observed on Kos support the superficial thermal manifestations and imply the existence of a geothermal field. The low resistivities of Kos Graben correspond to fractured zones with geothermal fluid circulation and to volcanic and sedimentary rocks which have probably underwent hydrothermal alteration due to the circulation of thermal fluids. The deeper electrical structure was found to be more resistive (100 ohm m) because of the metamorphic rock formations present.