Geoelectric investigation of the Hellenic subduction zone using long period magnetotelluric data D. Galanopoulos a , V. Sakkas b , D. Kosmatos b , E. Lagios b, * a General Secretariat for Civil Protection, Ministry of Interior, 2 Evangelistrias Str, 10563 Athens, Greece b Department of Geophysics and Geothermics, University of Athens, Panepistimiopolis-Ilissia, 15784 Athens, Greece Received 21 February 2005; received in revised form 1 July 2005; accepted 7 August 2005 Available online 6 October 2005 Abstract The strongest evidence up to date for a subduction zone in the Hellenic region is a clearly identified Wadati-Benioff zone below the central Aegean Sea, to a maximum depth of 180 km. Alternative seismic tomography models suggest that subduction process continues deeper than the Wadati-Benioff zone to a maximum depth of at least 600 km. So far the lack of deep electrical studies in the region impeded scientists from imposing other control factors than seismic to the proposed models for the Hellenic Subduction Zone (HSZ). A Long Period Magnetotelluric (LMT) study was carried out in the southern part of the Greek mainland to study the deep electrical characteristics of the HSZ and examine whether prominent modelled features correlate with structures identified by the seismic methods. The study comprised collection, processing and modelling of magnetotelluric (MT) data in the period range 100–10000 s from ten sites located along a 250 km NE–SW trending profile. The dimensionality of the data was examined at a pre-modelling stage and it was found that they do not exhibit three-dimensional (3-D) features. The latter enabled to construct both one-dimensional (1-D) and two-dimensional (2-D) models. The proposed geoelectric model for HSZ was based on 2-D modelling, since it had better maximum depth resolution of about 400 km, and revealed structures not detected by 1-D modelling attempts. The model structure which was related to the African and Euro-Asian lithosphere is relatively resistive (N 800 V-m) and has an average thickness of 150–170 km. Although the bottom of the lithosphere is adequately resolved, the Wadati-Benioff zone that delineates the top of the subducting lithospheric slab is not identified by any electrical feature. The modelled structure associated with the subducting part of the African lithosphere penetrates a relatively conductive (b 200 V-m) asthenosphere with a dip angle of 428. Intermediate electrical resistivities (200–800 V-m) are attributed to the ascending melting part of the lithosphere below the region of the Hellenic Volcanic Arc (HVA) and to a dipping zone below the south-western part of the profile, at 170–220 km depths. D 2005 Elsevier B.V. All rights reserved. Keywords: Long period magnetotellurics; Geoelectric; Hellenic subduction zone; Hellenic volcanic arc 1. Introduction The subduction of the African Plate under the Eur- asian one is the dominant tectonic process (McKenzie, 1978; Le Pichon and Angelier, 1979; Angelier et al., 1982) to account for the intense seismicity and the past or present volcanism (Fytikas et al., 1984) of the Hel- lenic region. The first strong evidence for a subducting lithospheric slab was an identified Wadati-Benioff zone below the central Aegean Sea to a maximum depth of 180 km (Galanopoulos, 1963; Papazachos, 1973). The seismicity zone was later confirmed with additional and 0040-1951/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2005.08.010 * Corresponding author. E-mail addresses: galanopoulos@gscp.gr (D. Galanopoulos), vsakkas@geol.uoa.gr (V. Sakkas), lagios@geol.uoa.gr (E. Lagios). Tectonophysics 409 (2005) 73 – 84 www.elsevier.com/locate/tecto