2-D geoelectrical model for the Parnaiba Basin conductivity anomaly of northeast Brazil and tectonic implications

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 Parnaiba Basin. The magnetovariational response functions of this structure are numerically modelled to constrain its geometry to facil itate its geological and tectonic interpretation. The 2-D numerical model t hat incorporates the ocean effect and can account for the spatial and perio d dependence of the observed response locates the source regions of enhance d 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 extensiona l tectonics associated either with the Brasiliano orogeny or with the Juras sic-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 Paleo zoic sediments. The thermal effects associated with magmatic activities are invoked to produce enhanced conductivity by the generation of carbon throu gh the pyrolysis of hydrocarbon-saturated sediments. (C) 1999 Elsevier Scie nce B.V. All rights reserved.