Three-dimensional magnetotelluric modelling of the Vilariça depression (NE Portugal)

Abstract

The Vilariça depression is located astride the northern branch of a Late Variscan (NNE–SSW) left-handed strike–slip fault known as the Vilariça Fault Zone (NE Portugal), whose reactivation in Pliocene–Quaternary times is well documented. The magnetotelluric method was applied in order to evaluate the electrical structure of the northern part of this tectonic depression. Thirty magnetotelluric soundings were carried out and a three-dimensional model was constructed. Different segments of the Vilariça Fault Zone, as well as the earlier Variscan thrusts preserved in the studied area, are well displayed in the model. Low-resistivity (30–200 Ω m) structures were revealed in the central part of the depression up to a depth of 1 km. These shallow conductors are mainly associated with the sedimentary infillings of the basin and are bordered and/or interrupted by strongly weathered and fractured blocks of metasediments displaying variable electrical resistivity. Deeper domains show a relatively homogeneous high resistivity (3500 Ω m), being difficult to distinguish the nonfractured and nonweathered metamorphosed autochthonous metasediments (at depths above 2 km) from the gneissic basement. A conductive layer (500 Ω m) was detected at a depth ranging from 8 to 12 km; this feature is tentatively associated with an important crustal detachment that may correspond to the root domain of the major active strike–slip faults in NW Iberia.