2-DIMENSIONAL MAGNETOTELLURIC MODELING OF THE KOS ISLAND GEOTHERMAL REGION (GREECE)

The magnetotelluric (MT) method in the period range 0.075-42 s was app lied to determine the electrical structure (<8 km) of the Kos Island g eothermal region and delineate the tectonic features at this part of t he Hellenic Volcanic Are (HVA). Nineteen MT soundings were conducted o n the western half of the island and the collected MT data were found to be two-dimensional (2-D). The NNW-SSE (30 degrees 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 elect rical 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 observe d 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 superfic ial thermal manifestations and imply the existence of a geothermal hel d. The low resistivities of Kos Graben correspond to fractured zones w ith geothermal fluid circulation and to volcanic and sedimentary rocks which have probably underwent hydrothermal alteration due to the circ ulation of thermal fluids. The deeper electrical structure was found t o be more resistive (100 ohm m) because of the metamorphic rock format ions present. (C) 1998 Elsevier Science B.V. All rights reserved.