A magnetometer array study in the north-northeast of Brazil has revealed a roughly NE-SW-trending conductive structure in the southeastern part of the intracratonic Parnaíba Basin. The magnetovariational response functions of this structure are numerically modelled to constrain its geometry to facilitate its geological and tectonic interpretation. The 2-D numerical model that incorporates the ocean effect and can account for the spatial and period dependence of the observed response locates the source regions of enhanced conductivity in a graben structure in the basement as well as in a block confined to the central part of the basin with an embedded resistive body. The anomalous electrical character of the sediments in the central part of the basin is consistent with the magnetotelluric data, the graben structure in the basement is corroborated by the aeromagnetic data. The formation of the graben structure is considered to be a manifestation of the extensional tectonics associated either with the Brasiliano orogeny or with the Jurassic–Cretaceous magmatic events. The diabase dikes intruded in the basin in association with the Jurassic-Cretaceous magmatic activity are shown to be accountable for the mapped resistive body entrapped in the conducting Paleozoic sediments. The thermal effects associated with magmatic activities are invoked to produce enhanced conductivity by the generation of carbon through the pyrolysis of hydrocarbon-saturated sediments
