Abstract:
Nighttime transient geomagnetic variations recorded by an array of 29 magnetometers in the equatorial region of north-northeast Brazil have been subjected to robust regression analysis in order to derive transfer functions as a diagnostic indication of lateral conductivity variations. A thin sheet conductance model was developed to explain and interpret the variety of conductive anomalies evidenced by the induction arrow maps. The two main structures revealed by the present study are: a large NE-SW trending conductive anomaly with an embedded resistive zone in the central part of the Parnaíba Basin (Parnaíba Basin Conductive Anomaly-PBCA) and a relatively weaker anomaly branching off from the northwestern corner of the PBCA and extending towards the Marajo basin (LINK anomaly). The major PBCA anomaly is provisionally interpreted as a graben in the Precambrian basement filled with carbonaceous carbonates. The embedded resistive body, also characterized by a high density, is shown to be consistent with the presence of a diabase intrusive related to a magmatic event. The LINK anomaly is tentatively considered to be the relics of a sedimentary channel connecting the Parnaíba and Marajó basins.
