Magnetotelluric study of a plio-quaternary tectonic depression: the Vilarica basin (NE Portugal)

The Vilarica basin, located northeast Portugal astride a major late-Varisca n NNE-SSW reactivated strike-slip fault, is an excellent example of interpl ate neotectonic activity whose development has been mainly interpreted as a result of left-lateral displacement. Thirty magnetotelluric soundings were carried out in the Sta Comba tla Vilarica/Sampaio region (northern of the tectonic basin) in order to investigate the internal structure of the basin and its relationship with the main tectonic faults. Distortions of the imp edance tensors were studied using Groom-Bailey decomposition technique. The predominant regional strike (N26E) is in good accordance with exposed geol ogy and can be explained for the reactivation of previous structures. Using two-dimensional inversion, three resistivity cross sections were obtained at north, center and south of the studied area. The graben is revealed as a low resistivity (20-100 Omega m) structure due to the sedimentary filling. The increasing electrical resistivity (from 400 to 4000 Omega m) at depths greater than 2 km is related to the Hesperian basement rocks. The main fau lts, which controlled the formation and evolution of the basin, are reveale d by resistivity gradients within the upper crust. (C) 2000 Elsevier Scienc e B.V. All rights reserved.